Selected Publications by Joel Wm. Parker

Titles with links to abstracts and (p)reprints.
Abstracts with links to (p)reprints.
Preprints are available via anonymous FTP at: boulder.swri.edu/pub/joel.
CV (with full publication list).
Editor of Distant EKOs, the Kuiper belt electronic newsletter.
ADS list of my papers. (or, you can get the sub-lists of papers in refereed journals or other papers like MPECs, conference proceedings, etc.).

Titles of Selected Publications

``Io Eclipse Observations During the New Horizons Jupiter Flyby: Volcanic Atmospheres Revealed by Aurorae'' K.D. Retherford, J.R. Spencer, S.A. Stern, A.F. Cheng, J. Saur, H.A. Weaver, D.F. Strobel, D.C. Reuter, A.J. Steffl, G.R. Gladstone, J.Wm. Parker, D.C. Slater, M.H. Versteeg, M.W. Davis, A. Lunsford, R.M.C. Lopes, F. Bagenal, H.B. Throop, L.A. Young, S.J. Conard, & J.M. Moore 2007, Science, submitted

``Jupiter's Nightside Airglow and Aurora as seen from New Horizons'' G.R. Gladstone, S.A. Stern, D.C. Slater, M. Versteeg, M.W. Davis, K.D. Retherford, L.A. Young, A.J. Steffl, H. Throop, J.Wm. Parker, H.A. Weaver, A.F. Cheng, G.S. Orton, J.T. Clarke, & J.D. Nichols, 2007, Science, submitted

``Discovering the True KBO Orbit Distribution'' JJ Kavelaars, L. Jones, B. Gladman, J.Wm. Parker, & J.-M. Petit, 2007, in Kuiper Belt (M.A. Barucci, H. Boehnhardt,D. Cruikshank, and A. Morbidelli, eds.; U. Arizona Press, Tucson), in press

``Pluto, Charon, and the Kuiper Belt Objects'' S.A. Stern,J.Wm. Parker, & C.B. Olkin 2007, in Treatise on Geophysics, Volume 10: Planets and Moons (T. Spohn, ed.; Elsivier), in press

``Ultraviolet Spectroscopy of Comet 9P/Tempel 1 with Alice/Rosetta during the Deep Impact Encounter'' P.D. Feldman, S.A. Stern, A.J. Steffl, J.Wm. Parker, D.C. Slater, A'Hearn, J.-L. Bertaux, & M.C. Festou 2007, Icarus, 187, 104

``Alice: The Rosetta Ultraviolet Imaging Spectrograph'' S.A. Stern, D.C. Slater, J. Scherrer, J. Stone, M. Versteeg, M.F. A'Hearn, J.-L. Bertaux, P.D. Feldman, M.C. Festou, J.Wm. Parker, & O.H.W. Siegmund 2007, Space Science Reviews, 128, 507

``The CFEPS Kuiper Belt Survey: Strategy and Pre-survey Results'' R.L. Jones, B. Gladman, J.-M. Petit, P. Rousselot, O. Moussis, JJ Kavelaars, A. Campo Bagatin, G. Bernabeu, P. Benavenidez, J.Wm. Parker, P. Nicholson, M. Holman, A. Doressoundiram, C. Veillet, H. Scholl, & G. Mars, 2006, Icarus, 185, 508

``Ceres: High-Resolution Imaging with HST and the Determination of Physical Properties'', J.Wm. Parker, L.A. McFadden, C.T. Russell, S.A. Stern, M.V. Sykes, P.C. Thomas, & E.F. Young 2006, Advances in Space Research, 38, 2039

``Discovery of a Low-Eccentricity, High-Inclination Kuiper Belt Object at 58 AU'' R.L. Allen, B. Gladman, JJ Kavelaars, J.-M. Petit, J.Wm. Parker, & P. Nicholson 2006, The Astrophysical Journal Letters, 684, 83L

``Differentiation of the Asteroid Ceres as Revealed by its Shape'' P.C. Thomas, J.Wm. Parker, L.A. McFadden, C.T. Russell, S.A. Stern, M.V. Sykes & E.F. Young 2005, Nature, 437, 224

``Massive Field Stars and the Stellar Clustering Law'', M.S. Oey, N.L. King, & J.Wm. Parker 2004, The Astronomical Journal, 127, 1632

``The Discovery of a Twelfth Wolf-Rayet Star in the Small Magellanic Cloud'', P. Massey, K.A.G. Olsen, & J.Wm. Parker 2003, Publications of the Astronomical Society of the Pacific, 115, 1265

``HST/STIS Observations of Comet 19P/Borrelly During the Deep Space 1 Encounter'', H. A. Weaver, S. A. Stern, & J. Wm. Parker 2003, The Astronomical Journal, 126, 444

``Probing the Solar System's Outermost Frontier: The Future of Kuiper Belt Studies'' (Solar System Decadal Survey community papel white paper) W.M. Grundy, H.A. Levison, J.Wm. Parker, R.L. Allen, L.C. Ball, J.F. Cooper, M.C. De Sanctis, T.L. Farnham, B. Gladman, J.M. Hahn, C.W. Hergenrother, JJ Kavelaars, H. Kruger, D.J. Lien, R. Malhotra, R.M.E. Mastrapa, A. Quillen, R. Srama, J.A. Stansberry, G. Strazzulla, R.J. Terrile, & C.A. Trujillo, 2002, in The Future of Solar System Exploration (ed. M.V. Sykes), Astronomical Society of the Pacific Conference Series, 272, 337

``A New Spectral Classification System for the Earliest O Stars: Definition of Type O2'', N. R. Walborn, I. D. Howarth, D. J. Lennon, P. Massey, M. S. Oey, A. F. J. Moffat, G. Skalkowski, N. I. Morrell, L. Drissen, & J. Wm. Parker, 2002, The Astronomical Journal, 123, 2754

``Magellanic Orphans: Massive Stars Ex Nihilo?'', J. Wm. Parker 2002, in Hot Star Workshop III: The Earliest Phases of Massive Star Birth, Astronomical Society of the Pacific Conference Series (ed. P.A. Crowther), 267, 401

``The Binary Kuiper Belt Object 1998 WW31'', C. Veillet, J. Wm. Parker, I. Griffin, B. Marsden, A. Doressoundiram, M. Buie, D. J. Tholen, M. Connelley, & M. J. Holman, 2002, Nature, 416, 711 (2002 April 18)

``Analysis of the First Disk-Resolved Images of Ceres from Ultraviolet Observations with the Hubble Space Telescope'', J. Wm. Parker, S. A. Stern, P. C. Thomas, M. C. Festou, W. J. Merline, E. F. Young, R. P. Binzel, & L. A. Lebofsky 2002, The Astronomical Journal, 123, 549

``Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud'', J. Wm. Parker, D. Zaritsky, T. P. Stecher, J. Harris, & P. Massey 2001, The Astronomical Journal, 121, 891

``The Discovery of Argon in Comet Hale-Bopp'', S. A. Stern, D. C. Slater, M. C. Festou, J. Wm. Parker, G. R. Gladstone, M. F. A'Hearn, & E. Wilkinson 2000, The Astrophysical Journal Letters, 544, L169

``Chiron'', J. Wm. Parker 2000, Encyclopedia of Astronomy and Astrophysics

``A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraphic Images'', D. D. Durda, S. A. Stern, W. B. Colwell, J. Wm. Parker, H. F. Levison, & D. M. Hassler 2000, Icarus, 148, 312

``The Ultraviolet and Optical Spectra of Metal-Deficient O Stars in the Small Magellanic Cloud'', N. R. Walborn, D. J. Lennon, S. R. Heap, D. J. Lindler, L. J. Smith, C. J. Evans, & J. Wm. Parker, 2000, Publications of the Astronomical Society of the Pacific, 112, 1243

``HST/FOS Spatially Resolved Spectral Classification of Compact OB Groups in the Large Magellanic Cloud'', N. R. Walborn, L. Drissen, J. Wm. Parker, A. Saha, J. W. MacKenty, & R. L. White 1999, The Astronomical Journal, 118, 1684

``Comet Hale-bopp (C/1995 O1) Near 2.3 AU Postperihelion: Southwest Ultraviolet Imaging System Measurements of the H₂O and Dust Production'' S. A. Stern, W. B. Colwell, M. C. Festou, P. M. Tamblyn, J. Wm. Parker, D. C. Slater, P. R. Weissman, & L. J. Paxton 1999, The Astronomical Journal, 118, 1120

``The Spectroscopic Detectability of Argon in the Lunar Atmosphere'' J. Wm. Parker, S. A. Stern, G. R. Gladstone, & J. M. Shull 1998, The Astrophysical Journal (Letters), 509, L61

``Ultraviolet Observations of Stars in The Magellanic Clouds: A Historical Bridge From The Sun to R 136 and Beyond'', J. Wm. Parker 1998, in Hot Stars in Open Clusters of the Galaxy and the Magellanic Clouds, RMxAA Conf. Series (ed. V. Niemela & N. Morrell), in press

``Ultraviolet Imaging Telescope Observations of the Magellanic Clouds'', J. Wm. Parker, J. K. Hill, R. H. Cornett, J. E. Hollis, E. Zamkoff, R. C. Bohlin, R. W. O'Connell, S. G. Neff, M. S. Roberts, A. M. Smith, & T. P. Stecher 1998, The Astronomical Journal, 116, 180.

``HST Mid-Ultraviolet Spectroscopy of 46P/Wirtanen During Its Approach to Perihelion in 1996-1997'', S. A. Stern, , J. Wm. Parker, M. C. Festou, M. F. A'Hearn, P. D. Feldman, G. Schwehm, R. Schulz, J.-L. Bertaux, & D. C. Slater 1998, Astronomy & Astrophysics, 335, L30

``The Spectral Variability of the Cool Hypergiant Rho Cassiopeiae'', A. Lobel, G. Israelian, C. de Jager, F. Musaev, J. Wm. Parker, & A. Mavrogiorgou 1998, Astronomy & Astrophysics, 330, 659

``Ultraviolet Spectral Morphology of the Stellar Core of Eta Carinae'' D. C. Ebbets, N. R. Walborn, & J. Wm. Parker, 1997, The Astrophysical Journal, 489, L161

``An HST Search for Magnesium in the Lunar Atmosphere'', S. A. Stern, J. Wm. Parker, T. H. Morgan, B. C. Flynn, D. M. Hunten, A. Sprague, M. Mendillo, & M. C. Festou 1997, Icarus, 127, 523

``The Ultraviolet Imaging Telescope: Instrument and Data Characteristics'', T. Stecher, R. H. Cornett, M. R. Greason, W. B. Landsman, J. K. Hill, R. S. Hill, R. C. Bohlin, P. C. Chen, N. R. Collins, M. N. Fanelli, J. I. Hollis, S. G. Neff, R. W. O'connell, J. D. Offenberg, R. A. Parise, J. Wm. Parker, M. S. Roberts, A. M. Smith, & W. H. Waller 1997, Publications of the Astronomical Society of The Pacific, 109, 584

``Ultraviolet Observations of Chiron with the HST/FOS: Examining a Centaur's Gray Matter'', J. Wm. Parker, S. A. Stern, M. C. Festou, M. F. A'Hearn, & D. Weintraub 1997, The Astronomical Journal, 113, 1899

``UIT Ultraviolet Observations of the Small Magellanic Cloud'', R. H. Cornett, M. R. Greason, J. K. Hill, J. Wm. Parker, W. H. Waller, R. C. Bohlin, S. G. Neff, R. W. O'Connell, M. S. Roberts, A. M. Smith, & T. P. Stecher 1997, The Astronomical Journal 113, 1011

``Ultraviolet Imaging Telescope Observations of the OB Stars in the N 11 Region of the LMC'', J. Wm. Parker, J. K. Hill, R. C. Bohlin, R. W. O'Connell, S. G. Neff, M. S. Roberts, A. M. Smith, & T. P. Stecher 1996, The Astrophysical Journal, 472, L29

``The variable Mass-loss of the Peculiar Supergiant P Cygni'', G. Israelian, M. de Groot, J. Wm. Parker, & C. Sterken 1996, Monthly Notices of the Royal Astronomical Society, 283, 119

``The Stellar Initial Mass Function in the Giant H II Region NGC 595'' E. M. Malumuth, W. Waller, & J. Wm. Parker 1995, The Astronomical Journal, 111, 1128

``Identification of Emission-Line Stars in 30 Doradus using HST Observations'', J. Wm. Parker, S. R. Heap, & E. M. Malumuth 1995, The Astrophysical Journal, 448, 705

``International Ultraviolet Explorer Atlas of B-Type Spectra From 1200 to 1900 Å'', N. R. Walborn, J. Wm. Parker, & J. S. Nichols 1995, NASA Reference Publication 1363

``Resolution of Massive Compact Clusters in the 30 Doradus Periphery With HST'', N. R. Walborn, J. W. MacKenty, A. Saha, R. L. White, & J. Wm. Parker 1995, The Astrophysical Journal, 439, L47

``The OB Association LH 58 in the LMC'', C. D. Garmany, P. Massey, & J. Wm. Parker 1994, The Astronomical Journal, 108, 1256

``Dynamical and Observational Constraints on Satellites in the Inner Pluto-Charon System'', S. A. Stern, J. Wm. Parker, M. J. Duncan, J. C. Snowdall, Jr., & H. F. Levison 1994, Icarus, 108, 234

``The OB Associations of 30 Doradus in the Large Magellanic Cloud. I. Stellar Observations and Data Reductions'', J. Wm. Parker 1993, The Astronomical Journal, 106, 560

``The OB Associations of 30 Doradus in the Large Magellanic Cloud. II. Stellar Content and Initial Mass Function'', J. Wm. Parker & C. D. Garmany 1993, The Astronomical Journal, 106, 1471

``A New Luminous Blue Variable: R 143 in 30 Doradus'', J. Wm. Parker, G. C. Clayton, C. Winge, & P. S. Conti 1993, Astrophysical Journal, 409, 770

``SN 1991bg: A Type Ia Supernova with a Difference'', B. Leibundgut, R. P. Kirshner, M. M. Phillips, L. A. Wells, N. B. Suntzeff, M. Hamuy, R. A. Schommer, A. R. Walker, L. Gonzalez, P. Ugarte, R. E. Williams, G. Williger, M. Gomez, R. Marzke, B. P. Schmidt, B. Whitney, N. Caldwell, J. Peters, F. H. Chaffee, C. B. Foltz, D. Rehner, L. Siciliano, T. G. Barnes, K. -P. Cheng, P. M. N. Hintzen, Y.-C. Kim, J. Maza, J. Wm. Parker, A. C. Porter, P. C. Schmidtke, & G. Sonneborn 1993, The Astronomical Journal, 105, 301

``Two-Stage Starbursts in The LMC: N 11 as a Once and Future 30 Doradus'', N. R. Walborn & J. Wm. Parker 1992, Astrophysical Journal, 399, L87

``Ultraviolet and Optical Spectral Morphology of Melnick 42 and Radcliffe 136a in 30 Doradus'', N. R. Walborn, D. C. Ebbets, J. Wm. Parker, J. Nichols-Bohlin, & R. L. White 1992, Astrophysical Journal, 393, L13

``30 Doradus in the Large Magellanic Cloud: The Stellar Content and Initial Mass Function'', J. Wm. Parker 1992, Ph.D. thesis, University of Colorado

``The Stellar Content of LH 9 and 10 in the LMC: A Case for Sequential Star Formation'', J. Wm. Parker, C. D. Garmany, P. Massey, & N. R. Walborn 1992, The Astronomical Journal, 103, 1205

``A Search for Distant Satellites of Pluto'', S. A. Stern, R. A. Fesen, E. S. Barker, J. Wm. Parker, L. M. Trafton 1991, Icarus, 94, 246

``Measuring the Direct Sky Brightness on CCD Images'', J. Wm. Parker 1991, Publications of the Astronomical Society of the Pacific, 103, 243

``The Stellar Content of NGC 346: A Plethora of O Stars in the SMC'', P. Massey, J. Wm. Parker, C. D. Garmany 1989, The Astronomical Journal, 98, 1305

Publication Abstracts and links to (p)reprints

Io Eclipse Observations During the New Horizons Jupiter Flyby: Volcanic Atmospheres Revealed by Aurorae

Authors: K.D. Retherford, J.R. Spencer, S.A. Stern, A.F. Cheng, J. Saur, H.A. Weaver, D.F. Strobel, D.C. Reuter, A.J. Steffl, G.R. Gladstone, J.Wm. Parker, D.C. Slater, M.H. Versteeg, M.W. Davis, A. Lunsford, R.M.C. Lopes, F. Bagenal, H.B. Throop, L.A. Young, S.J. Conard, & J.M. Moore
Status: 2007, Science, submitted

Abstract

Preprint: Not yet available. Contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Not yet available.
ADS Links: Not yet available.

Jupiter's Nightside Airglow and Aurora as seen from New Horizons

Authors: G.R. Gladstone, S.A. Stern, D.C. Slater, M. Versteeg, M.W. Davis, K.D. Retherford, L.A. Young, A.J. Steffl, H. Throop, J.Wm. Parker, H.A. Weaver, A.F. Cheng, G.S. Orton, J.T. Clarke, & J.D. Nichols
Status: 2007, Science, submitted

Abstract

Preprint: Not yet available. Contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Not yet available.
ADS Links: Not yet available.

Discovering the True KBO Orbit Distribution

Authors: JJ Kavelaars, L. Jones, B. Gladman, J.Wm. Parker, & J.-M. Petit
Status: 2007, in Kuiper Belt (M.A. Barucci, H. Boehnhardt,D. Cruikshank, and A. Morbidelli, eds.; U. Arizona Press, Tucson), in press

Abstract Models of the evolution of Neptunes' migration and the dynamical processes at work during the formation of the outer solar system can be con- strained by measuring the orbital distribution of the remnant planetesimals in the Kuiper belt. Determining the true orbit distribution is not simple because the detection and tracking of KBOs is highly biased process. In this chapter we detail the various biases that are present in any survey of the Kuiper belt. We then present observational and analysis strategies that can help to minimize the effects of these biases on the inferred orbital distributions.

Preprint: Not yet available. Contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Not yet available.
ADS Links: Not yet available.

Pluto, Charon, and the Kuiper Belt Objects

Authors: S.A. Stern, J.Wm. Parker, & C.B. Olkin
Status: 2007, in Treatise on Geophysics, Volume 10: Planets and Moons (T. Spohn, ed.; Elsivier), in press

Abstract This chapter provides a review of the current state of knowledge concerning Pluto, Charon, and other objects in the Kuiper Belt. Necessarily, some background information on the Kuiper Belt itself will also be provided, but the emphasis is not on the Kuiper Belt as a structure, but the objects in it. We begin by placing the discovery of Pluto and the Kuiper Belt in historical context. We then provide with overviews regarding the Pluto system, the Kuiper Belt, and KBOs themselves. We then review environment of the Kuiper Belt and the properties of Pluto and KBOs in more detail. This is followed by a discussion of scenarios for the origin of the KB and KBOs. We close by examining the goals and prospects for future progress in these areas over the next 10-15 years.

Preprint: Not yet available. Contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Not yet available.
ADS Links: Not yet available.

Ultraviolet Spectroscopy of Comet 9P/Tempel 1 with Alice/Rosetta during the Deep Impact Encounter

Authors: P.D. Feldman, S.A. Stern, A.J. Steffl, J.Wm. Parker, D.C. Slater, A'Hearn, J.-L. Bertaux, & M.C. Festou
Status: 2007, Icarus, 187, 104

Abstract We report on spectroscopic observations of periodic comet 9P/Tempel 1 by theAlice ultraviolet spectrograph on the Rosetta spacecraft in conjunction with NASAsDeep Impact mission. Our objectives were to measure an increase in atomic andmolecular emissions produced by the excavation of volatile sub-surface material. Wedetected atomic hydrogen and oxygen emission from the quiescent coma but no enhancementat the 10% (1-sigma) level following the impact. We derive a quiescent H2Oproduction rate of 9x10²⁷ molecules s^-1 with an estimated uncertainty of ~30%. Our upper limits to the volatiles produced by the impact are consistent with other estimates.

Preprint: Contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Available
ADS Links: Available

Alice: The Rosetta Ultraviolet Imaging Spectrograph

Authors: S.A. Stern, D.C. Slater, J. Scherrer, J. Stone, M. Versteeg, M.F. A'Hearn, J.-L. Bertaux, P.D. Feldman, M.C. Festou, J.Wm. Parker, & O.H.W. Siegmund
Status: 2007, Space Science Reviews, 128, 507

Abstract We describe the design, performance and scientific objectives of the NASA-funded ALICE instrument aboard the ESA Rosetta asteroid flyby/comet rendezvous mission. ALICE is a lightweight, low-power, and low-cost imaging spectrograph optimized for cometary far-ultraviolet (FUV) spectroscopy. It will be the first UV spectrograph to study a comet at close range. It is designed to obtain spatially-resolved spectra of Rosetta mission targets in the 700-2050 A spectral band with a spectral resolution between 8 A and 12 A for extended sources that fill its ~0.05 deg x 6.0 deg field-of-view. ALICE employs an off-axis telescope feeding a 0.15-m normal incidence Rowland circle spectrograph with a concave holographic reflection grating. The imaging microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr and CsI) and employs a 2 D delay-line readout array. The instrument is controlled by an internal microprocessor. During the prime Rosetta mission, ALICE will characterize comet 67P/Churyumov-Gerasimenko's coma, its nucleus, and the nucleus/coma coupling; during cruise to the comet, ALICE will make observations of the mission's two asteroid flyby targets and of Mars, its moons, and of Earth's moon. ALICE has already successfully completed the in-flight commissioning phase and is operating normally in flight. It has been characterized in flight with stellar flux calibrations, observations of the Moon during the first Earth fly-by, and observations of comet Linear T7 in 2004 and comet 9P/Tempel 1 during the 2005 Deep Impact comet-collision observing campaign

Preprint: Available at Available
Electronic Journal Article: Available at Available
ADS Links: Available

The CFEPS Kuiper Belt Survey: Strategy and Pre-survey Results

Authors: R.L. Jones, B. Gladman, J.-M. Petit, P. Rousselot, O. Moussis, JJ Kavelaars, A. Campo Bagatin, G. Bernabeu, P. Benavenidez, J.Wm. ParkerJ, P. Nicholson, M. Holman, A. Doressoundiram, C. Veillet, H. Scholl, & G. Mars,
Status: 2006, Icarus, 185, 508

Abstract We present the data acquisition strategy and characterization procedures for the Canada-France Ecliptic Plane Survey (CFEPS), a sub-component of the Canada-France-Hawaii Telescope Legacy Survey. The survey began in early 2003 and as of summer 2005 has covered 430 square degrees of sky within a few degrees of the ecliptic. Moving objects beyond the orbit of Uranus are detected to a magnitude limit of R=23-24 (depending on the image quality). To track as large a sample as possible and avoid introducing followup bias, we have developed a multi-epoch observing strategy that is spread over several years. We present the evolution of the uncertainties in ephemeris position and orbital elements as the objects progress through the epochs. We then present a small 10-object sample that was tracked in this manner as part of a preliminary survey starting a year before the main CFEPS project. We describe the CFEPS survey simulator, to be released in 2006, which allows theoretical models of the Kuiper Belt to be compared with the survey discoveries since CFEPS has a well-documented pointing history with characterized detection efficiencies as a function of magnitude and rate of motion on the sky. Using the pre-survey objects we illustrate the usage of the simulator in modeling the classical Kuiper Belt.

Preprint: Available
Electronic Journal Article: Available
ADS Links: Available

Ceres: High-Resolution Imaging with HST and the Determination of Physical Properties

Authors: J.Wm. Parker, L.A. McFadden, C.T. Russell, S.A. Stern, M.V. Sykes, P.C. Thomas, & E.F. Young
Status: 2006, Advances in Space Research, 38, 2039

Abstract Nine HST orbits have been used to image Ceres using the High Resolution Camera of the Advanced Camera for Surveys. Two hundred and fifty-nine images in three filters (F555W, F330W and F220W; roughly V, U, and mid-UV) were obtained covering the entire surface. Two visible features are readily apparent, a dark patch and a bright point. The bright spot was suitable for control point work enabling us to determine the spin axis of Ceres (RA=291 deg, Dec=59 deg) and the latitude (14 deg N) of the bright point. Using limb measurements with accuracy of typically 0.1�pixel, we were able to measure the shape of Ceres: it is rotationally symmetric with a short axis diameter of 909 km and a long axis diameter of 975 km. Analysis of these images is preliminary and continues to be refined.

Preprint: Contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Available
ADS Links: Available

Discovery of a Low-Eccentricity, High-Inclination Kuiper Belt Object at 58 AU

Authors: R.L. Allen, B. Gladman, JJ Kavelaars, J.-M. Petit, J.Wm. Parker, P. Nicholson
Status: 2006, The Astrophysical Journal Letters, 684, 83L

Abstract We report the discovery of the first trans-Neptunian object, designated 2004 XR190, with a low-eccentricity orbit beyond the 2:1 mean motion resonance. Fitting an orbit to 23 astrometric observations spread out over 12 months yields an orbit of a=57.2+/-0.4, e=0.08+/-0.04, and i=46.6d. All viable orbits have perihelia distances q>49 AU. The very high orbital inclination of this extended scattered disk object might be explained by several models, but its existence again points to a large as-yet undiscovered population of trans-Neptunian objects with large orbital perihelia and inclination.

Preprint: Available.
Electronic Journal Article: Available.
ADS Links: Available.

Differentiation of the Asteroid Ceres as Revealed by its Shape

Authors: P.C. Thomas, J.Wm. Parker, L.A. McFadden, C.T. Russell, S.A. Stern, M.V. Sykes & E.F. Young
Status: 2005, Nature, 437, 224

Abstract The accretion of bodies in the asteroid belt was halted nearly 4.6 billion years ago by the gravitational influence of the newly formed giant planet Jupiter. The asteroid belt therefore preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula. Spectral features in reflected sunlight indicate that some asteroids have experienced sufficient thermal evolution to differentiate into layered structures1. The second most massive asteroid -- 4 Vesta -- has differentiated to a crust, mantle and core. 1 Ceres, the largest and most massive asteroid, has in contrast been presumed to be homogeneous, in part because of its low density, low albedo and relatively featureless visible reflectance spectrum, similar to carbonaceous meteorites that have suffered minimal thermal processing. Here we show that Ceres has a shape and smoothness indicative of a gravitationally relaxed object. Its shape is significantly less flattened than that expected for a homogeneous object, but is consistent with a central mass concentration indicative of differentiation. Possible interior configurations include water-icerich mantles over a rocky core.

Preprint: Contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Available
ADS Links: Available

Massive Field Stars and the Stellar Clustering Law

Authors: M.S. Oey, N.L. King, & J.Wm. Parker
Status: 2004, The Astronomical Journal, 127, 1632

Abstract The distribution of N*, the number of OB stars per association or cluster, appears to follow a universal power-law form N*^-2 in the local universe. We evaluate the distribution of N* in the Small Magellanic Cloud using recent broadband optical and space-ultraviolet data, with special attention to the lowest values of N*. We find that the power-law distribution in N* continues smoothly down to N* = 1. This strongly suggests that the formation of field massive stars is a continuous process with those in associations and that the field stars do not originate from a different star formation mode. Our results are consistent with the model that field massive stars represent the most massive members in groups of smaller stars, as expected if the clustering law applies to much lower masses as is expected from the stellar initial mass function (IMF). These results are consistent with the simultaneous existence of a universal IMF and a universal clustering law. Jointly, these laws imply that the fraction of field OB stars typically ranges from about 35% to 7% for most astrophysical situations, with an inverse logarithmic dependence on the most populous cluster, and hence on galaxy size and/or star formation rate. There are important consequences for global feedback effects in galaxies: field stars should therefore contribute proportionately to the volume of the warm ionized medium, and equal relative contributions by superbubbles of all sizes to the interstellar porosity are expected.

Preprint: Contact Sally Oey [Sally.Oey@lowell.edu] for (p)reprints.
Electronic Journal Article: Available.
ADS Links: Available.

The Discovery of a Twelfth Wolf-Rayet Star in the Small Magellanic Cloud

Authors: P. Massey, K. A. G. Olsen, & J. Wm. Parker
Status: 2003, Publications of the Astronomical Society of the Pacific, 115, 1265

Abstract We report the discovery of a relatively faint (V=15.5) early-type WN star in the SMC. The line strength and width of He II lambda 4686 emission is similar to that of the other SMC WNs, and the presense of N V lambda 4603,19 emission (coupled with the lack of N III) suggests this star is of spectral type WN3-4.5, and thus is similar in type to the other SMC WRs. Also like the other SMC WN stars, an early-type absorption spectrum is weakly present. The absolute magnitude is comparable to that of other (single) Galactic early-type WNs. The star is located in the Hodge 53 OB association, which is also the home of two other SMC WNs. This star, which we designate SMC-WR12, was actually detected at a high significance level in an earlier interference-filter survey, but the wrong star was observed as part of a spectroscopic followup, and this case of mistaken identity resulted in its Wolf-Rayet nature not being recognized until now.

Preprint: Available. Also available at the LANL arXiv. For reprints, contact Phil Massey [Phil.Massey@lowell.edu] for (p)reprints.
Electronic Journal Article: Available.
ADS Links: Available.

HST/STIS Observations of Comet 19P/Borrelly During the Deep Space 1 Encounter

Authors: H. A. Weaver, S. A. Stern, & J. Wm. Parker
Status: 2003, The Astronomical Journal, 126, 444

Abstract In support of the NASA Deep Space 1 (DS1) mission to comet 19P/Borrelly, we obtained Hubble Space Telescope (HST) images and ultraviolet (UV) spectra of the comet near the time of the DS1 flyby in 2001 September. The HST data provide context information on 19P/Borrelly's circumnuclear dust environment, the rotational period and rotational phase of its nucleus, the H₂O and CS₂ production rates, the dust production rate, the dust reflectivity in the visible and mid-UV, and the time variability of these quantities around the time of the DS1 encounter. We derive average values of Q_H2O = (3.0±0.6) × 1028 molecules s^-1, [CS₂/H₂O] = (1.0±0.3) × 10^-3, and Q_dust ~ 240 kg s^-1. The corresponding dust-to-gas mass ratio is 0.24, but this is only a rough estimate because the dust production rate is uncertain by about an order of magnitude. The dust continuum was strongly reddened between 2400 and 3200 Å, and the Afrho value of 745 ± 15 cm near 6500 Å was ~2.5 times larger than the value near 2900 Å. The observed coma morphology consisted of a strong jet centered ~6° from the projected solar vector, one broad fan centered ~23° from the sunward direction, and another broad fan centered ~18° from the antisunward direction. The light curve of the optical continuum, as measured in target acquisition images, has an amplitude of ~40% in a square aperture that subtends 160 km at the comet; the rotational period could not be independently derived from the HST images but is consistent with the value of ~26 hr derived from HST observations in 1994 and ground-based images in 2000. The new HST data reveal a prominent offset in the emission peak of neutral gas molecules, and therefore in the peak column densities of gas in the coma, relative to the position of the cometary nucleus, which may be related to the offset in ion densities observed in situ by the DS1 Plasma Experiment for Planetary Exploration (PEPE) plasma spectrometer.

Preprint: Contact Hal Weaver (hal.weaver@jhuapl.edu)
Electronic Journal Article: Available.
ADS Links: Available.

A New Spectral Classification System for the Earliest O Stars: Definition of Type O2

Authors: N. R. Walborn, I. D. Howarth, D. J. Lennon, P. Massey, M. S. Oey, A. F. J. Moffat, G. Skalkowski, N. I. Morrell, L. Drissen, & J. Wm. Parker
Status: 2002, The Astronomical Journal, 123, 2754

Abstract High-quality, blue-violet spectroscopic data are collected for 24 stars that have been classified as type O3 and that display the hallmark N IV and N V lines. A new member of the class is presented; it is the second known in the Cygnus OB2 association, and only the second in the Northern Hemisphere. New digital data are also presented for several of the other stars. Although the data are inhomogeneous, the uniform plots by subcategory reveal some interesting new relationships. Several issues concerning the classification of the hottest O-type spectra are discussed, and new digital data are presented for the five original O3 dwarfs in the Carina Nebula, in which the N IV, N V features are very weak or absent. New spectral types O2 and O3.5 are introduced here as steps toward resolving these issues. The relationship between the derived absolute visual magnitudes and the spectroscopic luminosity classes of the O2-O3 stars shows more scatter than at later O types, at least partly because some overluminous dwarfs are unresolved multiple systems, and some close binary systems of relatively low luminosity and mass emulate O3 supergiant spectra. However, it also appears that the behavior of He II 4686, the primary luminosity criterion at later O types, responds to other phenomena in addition to luminosity at spectral types O2-O3. There is evidence that these spectral types may correspond to an immediate pre-WN phase, with a correspondingly large range of luminosities and masses. A complete census of spectra classified into the original O3 subcategories considered here (not including intermediate O3/WN types or O3 dwarfs without N IV, N V features) totals 45 stars; 34 of them belong to the Large Magellanic Cloud and 20 of the latter to 30 Doradus.

Preprint: Available here, or contact Nolan Walborn (nwalborn@stsci.edu) (p)reprints.
Electronic Journal Article: Available.
ADS Links: Available.

Magellanic Orphans: Massive Stars Ex Nihilo?

Authors: J. Wm. Parker
Status: 2002, in Hot Star Workshop III: The Earliest Phases of Massive Star Birth, Astronomical Society of the Pacific Conference Series (ed. P.A. Crowther), 267, 401

Abstract Is there a heretofore unrecognized population of early-type ``field'' stars in the Magellanic Clouds?

Preprint: Available via ftp as a gzipped PDF file (0.5 MB compressed, 0.6 MB uncompressed) or a gzipped PostScript file (0.3 MB compressed, 1.1 MB uncompressed). Or, contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Available
ADS Links: Available

The Binary Kuiper Belt Object 1998 WW31

Authors: C. Veillet, J. Wm. Parker, I. Griffin, B. Marsden, A. Doressoundiram, M. Buie, D. J. Tholen, M. Connelley, & M. J. Holman
Status: 2002, Nature, 416, 711 (2002 April 18)

Abstract The recent discovery of a binary asteroid during a spacecraft fly-by generated keen interest, because the orbital parameters of binaries can provide measures of the masses, and mutual eclipses could allow us to determine individual sizes and bulk densities. Several binary near-Earth, main-belt and Trojan asteroids have subsequently been discovered. The Kuiper belt -- the region of space extending from Neptune (at 30 astronomical units) to well over 100 AU and believed to be the source of new short-period comets -- has become a fascinating new window onto the formation of our Solar System since the first member object, not counting Pluto, was discovered in 1992. Here we report that the Kuiper-belt object 1998 WW31 is binary with a highly eccentric orbit (eccentricity e~0.8) and a long period (about 570 days), very different from the Pluto/Charon system, which was hitherto the only previously known binary in the Kuiper belt. Assuming a density in the range of 1 to 2 g cm^-3, the albedo of the binary components is between 0.05 and 0.08, close to the value of 0.04 generally assumed for Kuiper-belt objects.

Reprint: Available. Also contact Christian Veillet (veillet@cfht.hawaii.edu) for (p)reprints.
Electronic Journal Article: Available.
ADS Links: Not yet available.
Press Releases and Web Pages: STScI/HST

Analysis of the First Disk-Resolved Images of Ceres from Ultraviolet Observations with the Hubble Space Telescope

Authors: J. Wm. Parker, S. A. Stern, P. C. Thomas, M. C. Festou, W. J. Merline, E. F. Young, R. P. Binzel, & L. A. Lebofsky
Status: 2002, The Astronomical Journal, 123, 549

Abstract We present HST Faint Object Camera observations of the asteroid 1 Ceres at near-, mid-, and far-UV wavelengths (lambda = 3636, 2795, and 1621 Å, respectively) obtained on 1995 June 25. The disk of Ceres is well-resolved for the first time, at a scale of ~50 km. We report the detection of a large, ~250 km diameter surface feature for which we propose the name ``Piazzi''; however it is presently uncertain if this feature is due to a crater, albedo variegation, or other effect. From limb fits to the images, we obtain semi-major and semi-minor axes of R₁=484.8±5.1 km and R₂=466.4±5.9 km, respectively, for the illumination-corrected projected ellipsoid. Although albedo features are seen, they do not allow for a definitive determination of the rotation or pole positions of Ceres, particularly because of the sparse sampling (two epochs) of the 9 hour rotation period. From full-disk integrated albedo measurements, we find that Ceres has a red spectral slope from the mid- to near-UV, and a significant blue slope shortward of the mid-UV. In spite of the presence of Piazzi, we detect no significant global differences in the integrated albedo as a function of rotational phase for the two epochs of data we obtained. From Minnaert surface fits to the near- and mid-UV images, we find an unusually large Minnaert parameter of k~0.9, suggesting a more Lambertian than lunar-like surface.

Preprint: Available via ftp as a gzipped PDF file (0.2 MB compressed, 0.3 MB uncompressed) or a gzipped PostScript file (0.2 MB compressed, 2.2 MB uncompressed). Also available at the LANL arXiv. Or, contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Available. (It also made the cover).
ADS Links: Available.

Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud

Authors: J. Wm. Parker, D. Zaritsky, T. P. Stecher, J. Harris, & P. Massey
Status: 2001, The Astronomical Journal, 121, 891

Abstract Using ultraviolet photometry from the Ultraviolet Imaging Telescope (UIT) combined with photometry and spectroscopy from three ground-based optical datasets we have analyzed the stellar content of OB associations and field areas in and around the regions N 79, N 81, N 83, and N 94 in the Large Magellanic Cloud. In particular, we compare data for the OB association Lucke-Hodge 2 (LH 2) to determine how strongly the initial mass function (IMF) may depend on different photometric reductions and calibrations. Although the datasets exhibit median photometric differences of up to 30%, the resulting uncorrected IMFs are reasonably similar, typically Gamma ~ -1.6 in the 5-60 M_solar mass range. However, when we correct for the background contribution of field stars, the calculated IMF flattens to Gamma = -1.3±0.2 (similar to the Salpeter IMF slope). This change underlines the importance of correcting for field star contamination in determinations of the IMF of star formation regions. It is possible that even in the case of an universal IMF, the variability of the density of background stars could be the dominant factor creating the differences between calculated IMFs for OB associations. We have also combined the UIT data with the most extensive of these ground-based optical datasets -- the Magellanic Cloud Photometric Survey -- to study the distribution of the candidate O-type stars in the field. We find a significant fraction, roughly half, of the candidate O-type stars are found in field regions, far from any obvious OB associations [in accord with the suggestions of Garmany, Conti, & Chiosi (1982) for O-type stars in the solar neighborhood]. These stars are greater than 2 arcmin (30 pc) from the boundaries of existing OB associations in the region, which is a distance greater than most O-type stars with typical dispersion velocities will travel in their lifetimes. The origin of these massive field stars (either as runaways, members of low-density star-forming regions, or examples of isolated massive star formation) will have to be determined by further observations and analysis.

Preprint: Available via ftp as a PDF file (5.3 MB) or a gzipped PostScript file (5.0 MB compressed, 13.5 MB uncompressed). Also available at the LANL arXiv. Or, contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Available.
ADS Links: Available.

Chiron

Authors: J. Wm. Parker
Status: 2000, Encyclopedia of Astronomy and Astrophysics

Preprint: Available via ftp as a gzipped PostScript file (227Kb compressed, 513Kb uncompressed), or contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Not available.
ADS Links: Not available.

The Discovery of Argon in Comet Hale-Bopp

Authors: S. A. Stern, D. C. Slater, M. C. Festou, J. Wm. Parker, G. R. Gladstone, M. F. A'Hearn, & E. Wilkinson
Status: 2000, The Astrophysical Journal Letters, 544, L169

Abstract On 30.14 March 1997 we observed the EUV spectrum of the bright comet C/1995 O1 (Hale-Bopp) at the time of its perihelion, using our EUVS sounding rocket telescope/spectrometer. The spectra reveal the presence H Lyman beta, O+, and, most notably, Argon. Modelling of the retrieved Ar production rates indicates that comet Hale-Bopp is enriched in Ar relative to cosmogonic expectations. This in turn indicates that Hale-Bopp's deep interior has never been exposed to the 35-40 K temperatures necessary to deplete the comet's primordial argon supply.

Preprint: Available via ftp as a gzipped PDF file (111Kb compressed, 191Kb uncompressed), or contact Alan Stern (alan@boulder.swri.edu) for (p)reprints.
Electronic Journal Article: Available.
ADS Links: Available.

A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraphic Images

Authors: D. D. Durda, S. A. Stern, W. B. Colwell, J. Wm. Parker, H. F. Levison, & D. M. Hassler
Status: 2000, Icarus, 148, 312

Abstract We examined SOHO/LASCO (Solar and Heliospheric Observatory/Large Angle Spectroscopic Coronagraph) C3 coronagraph images to search for the long-suspected population of small bodies, the vulcanoids, in heliocentric orbits interior to Mercury. We searched the entire vulcanoid region from 0.07-0.21 AU by visually blinking daily image averages co-registered on background stars. We found no vulcanoids to a moving object detection limiting magnitude of V=8.0, corresponding to objects 20 and 60 km in diameter (assuming Mercury-like albedos) at the inner and outer boundaries of the Vulcanoid Zone, respectively. This negative detection suggests that candidate objects apparently detected interior to 0.09 AU by Courten et al. (1976, Bull. Am. Astron. Soc. 8, 504) were not permanent residents of that region, if real at all. Extrapolating from largest objects of 20 and 60 km to smaller-size objects with a Dohnanyi power-law size distribution (1969, J. Geophys. Res. 74, 2531), this result implies that the present population of vulcanoids larger than 1 km in diameter is no greater than ~1800-42000 objects.

Preprint: Contact Dan Durda (durda@boulder.swri.edu) for reprints.
Electronic Journal Article: Available.
ADS Links: Available.

The Ultraviolet and Optical Spectra of Metal-Deficient O Stars in the Small Magellanic Cloud

Authors: N. R. Walborn, D. J. Lennon, S. R. Heap, D. J. Lindler, L. J. Smith, C. J. Evans, & J. Wm. Parker
Status: 2000, PASP, 112, 1243

Abstract An ultraviolet and optical spectral atlas of 15 O stars in the SMC is presented and described. The echelle data have resolving powers of order 10⁴; they were obtained with HST/STIS in the UV, and at the AAT or the ESO 3.6m in the optical. The ultimate objective is to develop metal-deficient templates for the interpretation of distant starbursts, but here we discuss interesting new properties of the SMC stars themselves, revealed by the high quality of these data. The SMC metal deficiency produces anomalously weak stellar-wind profiles along the entire O main sequence, as well as at intermediate luminosities; the first intermediate SiIV 1400 wind profile in the SMC is shown. The second known Of star in the SMC displays wind peculiarities that are identical to those of its spectral classmate, again likely due to the low systemic metallicity. Several objects display marked CNO anomalies, including the first cases of CIII 4650 emission without NIII 4640 in O-type spectra. The N/C ratio appears to increase with mass, extent of evolution away from the ZAMS, and/or rotational velocity in the young cluster NGC 346. In addition, the first examples of Onfp (Oef) and Of?p spectra in the SMC have been found (the latter being only the fourth member of its peculiar shell category known anywhere). The UV wind characteristics of these objects correlate with their optical peculiarities. All of these spectroscopic phenomena provide diagnostics of the evolutionary status of metal-deficient massive stars.

Preprint: Available via ftp as a gzipped PostScript file (311Kb compressed, 1.3Mb uncompressed), or contact Nolan Walborn (nwalborn@stsci.edu) for (p)reprints.
Electronic Journal Article: Available
ADS Links: Available.

HST/FOS Spatially Resolved Spectral Classification of Compact OB Groups in the Large Magellanic Cloud

Authors: N. R. Walborn, L. Drissen, J. Wm. Parker, A. Saha, J. W. MacKenty, & R. L. White
Status: 1999, The Astronomical Journal, 118, 1684

Abstract Blue-violet spectrograms of individual components in four compact OB groups of the Large Magellanic Cloud, obtained with the Hubble Space Telescope Faint Object Spectrograph, are presented and discussed. Two of the massive multiple systems are in the 30 Doradus periphery, while the other two represent the core and the peripheral, triggered associations in the giant shell HII region Henize N11. Uncontaminated spectrograms of three WR and two very early Of components have been obtained for the first time; they can be observed only as composites with their close companions from the ground. Many of the companions have also been observed separately with HST/FOS, and several are of special interest in their own right. In particular, one early O giant has twice the mass of the apparent turnoff in its compact group, and it is proposed as a possible massive stellar merger, perhaps providing a clue to the interpretation of the peculiar spectroscopic category to which it belongs. These observations provide information on the initial masses and ages of the peculiar objects, and on the evolutionary relationships among different spectral categories within the presumably coeval systems. The results are also relevant to the upper stellar mass limit and initial mass function.

Preprint: Available via ftp as a gzipped PostScript file (964 Kb compressed, 3.1Mb uncompressed), or contact Nolan Walborn (nwalborn@stsci.edu) for (p)reprints.
Electronic Journal Article: Available
ADS Links: Available.

Comet Hale-bopp (C/1995 O1) Near 2.3 AU Postperihelion: Southwest Ultraviolet Imaging System Measurements of the H₂O and Dust Production

Authors: S. A. Stern, W. B. Colwell, M. C. Festou, P. M. Tamblyn, J. Wm. Parker, D. C. Slater, P. R. Weissman, & L. J. Paxton
Status: 1999, The Astronomical Journal, 118, 1120

Abstract The Southwest Ultraviolet Imaging System (SWUIS) imaged comet C/1995 O1 (Hale-Bopp) in various bandpasses from the Space Shuttle on nine occasions during 1997 August 9-15. These observations occurred when the solar elongation of the comet was too small to permit HST and other UV observations. Here we present some first results of the continuum and gas emission measurements collected by SWUIS. We find that Hale-Bopp's dust production parameter, A-f-rho, was 2.0±0.8×10⁵ cm when the comet was 2.33 AU from the Sun. Further, we find that its water production rate, Q_H₂O, was 2.6±0.4×10²⁹ s^-1. Combining this result with both other published H₂O production rates and CO production rates, we find that our measurements were made at the begining of the period when the comet's activity was transitioning from a H₂O-dominated to a CO-dominated state. We also find that the average rate of decrease of the water production between perihelion and 2.33 AU post perihelion was very close to r_h^-4.0±0.6, but concerns over radio data indicate that it may have been shallower immediately post-perihelion and then considerably steeper beyond about 2 AU. Such a behavior could indicate a sharply declining H₂O production rate beyond 2 AU, but if this is the case, then the H₂O production curve's steepening and turn-off occurred ~1 AU closer to the Sun post-perihelion than did the H₂O turn-on pre-perihelion. An alternative explanation could be that a seasonal (i.e., obliquity-dependent shadowing) effect may have caused a reduction in illuminated area on Hale-Bopp's irregular nucleus between 1.5 and 2.3 AU outbound.

Preprint: Contact Alan Stern (alan@boulder.swri.edu) for (p)reprints.
Electronic Journal Article: Available.
ADS Links: Available.

The Spectroscopic Detectability of Argon in the Lunar Atmosphere

Authors: J. Wm. Parker, S. A. Stern, G. R. Gladstone, & J. M. Shull
Status: 1998, ApJ Letters, 509, L61

Abstract Direct measurements of the abundance of argon in the lunar atmosphere were made in 1973 by instruments placed on the Moon during the Apollo 17 mission, but the total daytime abundance is unknown due to instrument saturation effects; thus, until we are able to return to the Moon for improved direct measurements, we must use remote sensing to establish the daytime abundance. In this paper, we present a complete analysis of the potential for measuring argon in the lunar atmosphere via emission-line or absorption-line observations. We come to the surprising conclusion that the lower limit established by the in situ lunar argon measurements implies that any absorption-line measurement of argon in the lower, dayside lunar atmosphere requires analysis in the optically-thick regime. In light of this result, we present the results of our EUVS sounding rocket observations of the lunar occultation of Spica, which provide a new upper limit on the abundance of argon in the daytime lunar atmosphere. We also re-analyze a recently reported weak detection of lunar atmospheric Ar I 1048 in emission by the ORFEUS satellite, and show that those data are inconsistent with the emission being due to argon over a wide range of temperatures (up to at least 2000 K). This result is primarily due to our use of a more complete curve of growth analysis, and improved values for the argon fluorescent emission rates from radiation and solar wind interactions. We find that the detection reported by ORFEUS would imply an argon surface density significantly greater than the total surface density of the lunar atmosphere for argon accommodated to typical daytime surface temperatures (~400 K), and also is inconsistent with a high-density transient event. Therefore, we conclude that the reported argon detection is untenable.

Preprint: Available via ftp as a gzipped PostScript file (102 Kb compressed, 306 Kb uncompressed), or contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Available along with an Erratum
ADS Links: Available along with an Erratum

Ultraviolet Observations of Stars in The Magellanic Clouds: A Historical Bridge From The Sun to R 136 and Beyond

Author: J. Wm. Parker
Status: 1998, in Hot Stars in Open Clusters of the Galaxy and the Magellanic Clouds, Revista Mexicana de Astronomia y Astrofisica, Serie de Conferencia (ed. V. Niemela & N. Morrell), 8, 13

Abstract

The Magellanic Clouds have provided us with unique insights into the lives of massive stars and have served as test beds for star formation and evolutionary scenarios and for constraining stellar parameters. From an observational point of view, the LMC and SMC have a number of advantages over the Galaxy, with an almost ideal combination of comparatively small (but variable) line-of-sight reddening, different metallicities and reddening laws, rich populations of massive stars, a relatively well-known distance for each galaxy, and such a distance that allows us to make both global and small-scale observations of the stellar populations and their environments. In most cases, these properties make the Clouds ideal targets for UV observations.

I review the last few decades of UV observations of the Magellanic Clouds, and how these data have been instrumental in advancing our knowledge of massive stars. In particular, I present observations made by the Ultraviolet Imaging Telescope (UIT), which obtained photometric data over a wide field of view (images are 37 arcmin in diameter). These data are excellent at highlighting hot stars while suppressing the cooler, lower mass stars that often cause confusion in studies of crowded fields. Using the UIT data, I derive initial mass functions and other parameters for field stars as well as for stars in a collection of clusters. I discuss other uses of these UV data and their utility for identifying populations of massive stars in the Clouds for future studies.

Preprint: Available via ftp as a gzipped PostScript file (283 Kb compressed, 2.5 Mb uncompressed), or contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Not available.
ADS Links: Available.

Ultraviolet Imaging Telescope Observations of the Magellanic Clouds

Authors: J. Wm. Parker, J. K. Hill, R. H. Cornett, J. E. Hollis, E. Zamkoff, R. C. Bohlin, R. W. O'Connell, S. G. Neff, M. S. Roberts, A. M. Smith, & T. P. Stecher
Status: 1998, AJ, 116, 180

Abstract

We present wide-field far-ultraviolet (FUV: 1300-1800 Å) images of the Large and Small Magellanic Clouds (LMC and SMC). These data were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 (1990 December 1-10) and Astro-2 (1995 March 2-18) missions; the images provide an extensive FUV mosaic of the SMC (Cornett et al. 1997) and contain numerous regions in the LMC, covering a wide range of stellar densities and current star formation activity. A total of 47 LMC/Lucke-Hodge (1970) and 37 SMC/Hodge (1985) OB associations are completely or partially included in the observed fields. FUV data can identify the hottest OB stars more easily than optical photometry can, and these stars dominate the ionizing flux, which is correlated to the observed H-alpha flux of the associated HII regions. Of the HII regions in the DEM catalog (Davies, Elliott, & Meaburn 1976), the UIT fields completely or partially include 102 DEM regions in the LMC and 74 DEM regions in the SMC.

We present a catalog of FUV magnitudes derived from point spread function photometry for 37,333 stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMC were presented in the paper by Cornett et al. 1997), with a completeness limit of m_UV ~ 15 mag and a detection limit of m_UV ~ 17.5 mag. The average uncertainty in the photometry is ~ 0.1 mag. The full catalog with astrometric positions, photometry, and other information is available via the electronic journal as well as other publicly available astronomical data archives.

We divided the catalog into field stars and stars that are in DEM regions. We analyzed each of these two sets of stars independently, comparing the composite UV luminosity function of our data to UV magnitudes derived from stellar evolution and atmosphere models in order to derive the underlying stellar formation parameters. We find a most probable initial mass function slope for the LMC field stars of Gamma = -1.80±0.09. The statistical significance of this single slope for the LMC field stars is extremely high, though we also find some evidence for a field star initial mass function (IMF) slope of Gamma ~ -1.4, equal to the Salpeter (1955) slope. However, in the case of the stars in the DEM regions (the stars in all the regions were analyzed together as a single group), we find three IMF slopes of roughly equal likelihood: Gamma = -1.0, -1.6, and -2.0.

No typical age for the field stars is found in our data for time periods up to a continuous star formation age of 500 Myr, which is the maximum age consistent with the completeness limit magnitude of the catalog's luminosity function. The best age for the collection of cluster stars was found to be t₀=3.4±1.9 Myr; this is consistent with the age expected for a collection of OB stars from many different clusters.

Preprint: Available via ftp as a gzipped PostScript file (5 Mb compressed, 18 Mb uncompressed), or contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints. Note that the resolution of some of the figures in the preprint here has been reduced to make the PostScript file a manageable size. If possible, I recommend getting the electronic journal artical (below) instead.
Electronic Journal Article: Available.
ADS Links: Available.
Catalog: The catalog is available through NASA's NSSDC, and from here as a gzipped tar file (0.8 Mb compressed, 3.2 Mb uncompressed).

HST Mid-Ultraviolet Spectroscopy of 46P/Wirtanen During Its Approach to Perihelion in 1996-1997

Authors: S. A. Stern, , J. Wm. Parker, M. C. Festou, M. F. A'Hearn, P. D. Feldman, G. Schwehm, R. Schulz, J.-L. Bertaux, & D. C. Slater
Status: 1998, A&A, 335, L30

Abstract

We have used the Hubble Space Telescope Faint Object Spectrograph to observe the mid-UV spectrum of the Rosetta mission target comet, 46P/Wirtanen, on three dates during the comet's apparition in 1996 and early 1997. During this time the comet moved inward from a heliocentric distance near 2.7 AU to 1.3 AU. The first measurement (22.7 July 1996) detected only reflected solar continuum; based on these measurements, we set an upper limit on the nuclear radius of 1.45 km. A second measurement (25.9 Aug 1996) detected this reflected continuum, as well as weak OH (0-0) emission; the third measurement (15.2 Jan 1997) detected three OH bands at high signal-to-noise (the 0-0, the 1-0, and the 1-1), along with emission features from the CS radical and the CO2+ ion; upper limits on their emission brightness and production rate of C2 was set. Our key findings are as follows: The data indicate that Q(H2O) varies like R^(-4.9±0.25) inside 2.5 AU, and that the CS2/H2O production ratio at 1.3 AU is close to 3e-4. We also found that the mid-UV coma color slope is much more neutral (2-5%/1000 Å) than typical comets, and also much more neutral than Wirtanen's own visible-wavelength color slope. Wirtanen's CS2/H2O production ratio is near the low end of the range observed to date. A·f·rho was estimated from the data as a measure of dust production; we foundA·f·rho(2950 Å) values of 15, 17.5, and 15 cm, respectively, for the three FOS datasets, after correction for the estimated flux contribution of the nucleus. Interestingly, with regard to dust we found that production of small dust particles (to which the UV is sensitive) did not appear to vary by more than a factor of about two with heliocentric distance over the 2.7 to 1.3 AU range, despite a factor of 50 variation in the production of H2O. Wirtanen appears to be a small, but relatively normal comet. However, Wirtanen's ratio of dust (i.e., A·f·rho) to H2O production is smaller than in >90% of the 85 comets in A'Hearn et al.'s (1995) large sample.

Preprint: Not available here. Contact the lead author.
Electronic Journal Article: Available.
ADS Links: Available.

The Spectral Variability of the Cool Hypergiant Rho Cassiopeiae

Authors: A. Lobel, G. Israelian, C. de Jager, F. Musaev, J. Wm. Parker, & A. Mavrogiorgou
Status: 1998, Astronomy & Astrophysics, 330, 659

Abstract

We checked the change of the effective temperature with the atmospherical pulsation of rho Cas from combined analyses of optical spectra of 1993-95 and IUE spectra of 1979-81. We find an upper range for delta-T_eff ~ 750 K over a period of 16-17 months. We present a thorough study of the related absorption line profile changes from which a velocity stratification with excitation energy could not be detected for the metallic lines. The distinct evolution of Halpha displays variable distortions of filling-in by emission with the pulsation. Our theoretical line profile fits yield a value for vsini of 25 km/s.

We report the finding of two different causes for the splitting of absorption line cores in the spectra of rho Cas. For the phase near highest effective temperature we have analyzed the development of very far shortward extended line wings assuming velocities up to 150 km/s. These violet absorptions originate about 2.5 R_star above the photosphere in a cool and variable supersonic wind from which we determine an upper value of the mass-loss rate of 9.2e-5 M_sun/yr. We also report the finding of an increase of the supersonic wind velocity by ~ 15 km/s per electronvolt decrease, measured from the maximum velocities of these violet absorptions for FeI lines.

A separate analysis of the IUE spectra reveals tremendous changes of the Balmer continuum flux by a factor 4 to 5 in only 26 hours, clearly distinct from the long-term variations of Halpha .

Reprint: Available via ftp as a gzipped PostScript file (651 Kb), or contact the lead author.
Electronic Journal Article: Available.
ADS Links: Available.

Ultraviolet Spectral Morphology of the Stellar Core of Eta Carinae

Authors: D. C. Ebbets, N. R. Walborn, & J. Wm. Parker
Status: 1997, The Astrophysical Journal, 489, L161

Abstract We present high quality spectra of Eta Carinae between 1203 Å and 1765 Å obtained in October, 1995 with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. A square entrance slit 0.22 arcsec in diameter was used to isolate the brightest region of the core, ``component A'', from the nearby knots of ejecta and the nebulosity in which it is embedded. The spectrum is that of a dense stellar envelope, with a nearly flat continuum and numerous emission and absorption lines. Most lines exhibit the P Cygni type profiles characteristic of a stellar wind. Lines from a wide range of ionization stages, from N I through N V are visible in the spectrum of this unique object, most having very similar velocity structures. Wind lines from C, N and O are all clearly visible, as are numerous metallic species. Most of the lines have two discrete, well resolved absorption components, one centered near -500 km s¯¹ and the other near -1100 km s¯¹. The overall morphology does not correspond to any one normal spectral type, but suggests a composite of features seen in B-type supergiants in the range B2 Ia to B8 Ia, with additional lower temperature lines also present. The spectrum of Eta Carinae is shown to be qualitatively similar to that of P Cygni in this wavelength range.

Preprint: Available via ftp as a gzipped PostScript file (98 Kb), or contact the lead author.
Electronic Journal Article: Available.
ADS Links: Available.

An HST Search for Magnesium in the Lunar Atmosphere

Authors: S. A. Stern, J. Wm. Parker, T. H. Morgan, B. C. Flynn, D. M. Hunten, A. Sprague, M. Mendillo, & M. C. Festou
Status: 1997, Icarus, 127, 523

Abstract In October 1996 we used the Hubble Space Telescope's Faint Object Spectrograph to make the first-ever mid-ultraviolet spectroscopic search for emissions from the lunar atmosphere. This spectrum revealed no emission lines, despite the fact that strong resonance emission transitions from the Al, Si, and Mg neutrals, and Mg+, are present in the bandpass. We derive 5 sigma upper limits on each of these species, and OH (0-0) emission. The most constraining upper limit we obtained was for Mg, which we find to be depleted relative to model predictions by a factor of 9. These findings reinforce the negative findings of our previous, ground-based search for neutral atoms in the lunar atmosphere (Flynn & Stern 1996), and suggest that Na and K may be unique in their ability to sputter from the surface as atomic neutrals. Other species may sputter away as ions or in molecular fragments.

Preprint: Not available here. Contact the lead author.
Electronic Journal Article: Available.
ADS Links: Available.

The Ultraviolet Imaging Telescope: Instrument and Data Characteristics

Authors: T. Stecher, R. H. Cornett, M. R. Greason, W. B. Landsman, J. K. Hill, R. S. Hill, R. C. Bohlin, P. C. Chen, N. R. Collins, M. N. Fanelli, J. I. Hollis, S. G. Neff, R. W. O'connell, J. D. Offenberg, R. A. Parise, J. Wm. Parker, M. S. Roberts, A. M. Smith, & W. H. Waller
Status: 1997, Publications of the Astronomical Society of The Pacific, 109, 584

Abstract The Ultraviolet Imaging Telescope (UIT) was flown as part of the Astro observatory on the Space Shuttle Columbia in December 1990 and again on the Space Shuttle Endeavor in March 1995. Ultraviolet (1200-3300 Å) images of a variety of astronomical objects, with a 40 arcmin field of view and a resolution of about 3 arcsec, were recorded on photographic film. The data recorded during the first flight is available to the astronomical community through the National Space Science Data Center (NSSDC); the data recorded during the second flight will soon be available as well. This paper discusses in detail the design, operation, data reduction, and calibration of UIT, providing the user of the data with information for understanding and using the data. It also provides guidelines for analyzing other astronomical imagery made with image intensifiers and photographic film.

Preprint: Not available here. Contact the lead author. Electronic Journal Article: Not available.
ADS Links: Available as ADS abstract

Ultraviolet Observations of Chiron with the HST/FOS: Examining a Centaur's Gray Matter

Authors: J. Wm. Parker, S. A. Stern, M. C. Festou, M. F. A'Hearn, & D. Weintraub
Status: 1997, The Astronomical Journal, 113, 1899

Abstract We present ultraviolet (UV) observations of Chiron during two epochs using the Hubble Space Telescope / Faint Object Spectrograph (HST/FOS). The first observations (1996 Jan 23-24 UT) were made 3 weeks before Chiron's perihelion, and include a series of seven spectra obtained over 3.9 hours. A single follow-up observation was made 3 months later (1996 Apr 13 UT). We used these data to study flux variations and set upper limits on selected coma abundances.

The most intriguing result of our observations is that Chiron's UV continuum flux in April was 60% higher than expected from the January flux. The source of this significant increase is unknown, but may be tantalizing evidence of: a highly unusual phase function; recoating of the surface from an unobserved outburst; or a cold, persistent, near-surface ``fog'' (small scale height atmosphere) that experienced a peculiar, unresolved outburst.

Chiron was characteristically gray in the UV (2600-3300 Å) in January 1996, with an average geometric albedo of 0.076±0.003 (assuming a radius of 90 km) and displayed no detectable UV color variations with rotational phase. In the April 1996 data, the average UV geometric albedo was 0.109±0.007, and we detect evidence for a blue color, which may be connected to the observed flux increase. During the January of observations, Chiron's UV lightcurve had a peak-to-peak variation of 7%, similar to that of the visible lightcurve. We found no evidence of coma emissions, and use the April data to set 3-sigma upper limits to the column abundances of N(CS 2576) < 4.3e13 cm¯² and N(OH 3085) < 1.5e14 cm¯².

Preprint: Available via ftp as a gzipped PostScript file (167 Kb), or contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
ADS Links: Available.

UIT Ultraviolet Observations of the Small Magellanic Cloud

Authors: R. H. Cornett, M. R. Greason, J. K. Hill, J. Wm. Parker, W. H. Waller, R. C. Bohlin, S. G. Neff, R. W. O'Connell, M. S. Roberts, A. M. Smith, & T. P. Stecher
Status: 1997, The Astronomical Journal, 113, 1011

Abstract A mosaic of four UIT far-UV (FUV) (effective wavelength = 1620 Å) images, with derived stellar and H II region photometry, is presented for most of the Bar of the SMC. The UV morphology of the SMC's Bar shows that recent star formation there has left striking features including: a) four concentrations of UV-bright stars spread from northeast to southwest at nearly equal (~30 arcmin = 0.5 kpc) spacings; b) one of the concentrations, near DEM 55, comprises a well-defined 8-arcmin diameter ring surrounded by a larger H-alpha ring, suggestive of sequential star formation.

FUV PSF photometry is obtained for 11,306 stars in the FUV images, resulting in magnitudes m(B5). We present a FUV luminosity function for the SMC bar, complete to m(B5)~14.5. Detected objects are well correlated with other SMC Population I material; of 711 H-alpha emission-line stars and small nebulae within the UIT fields of view, 520 are identified with FUV sources. The FUV photometry is compared with available ground-based catalogs of supergiants, yielding 191 detections of 195 supergiants with spectral type earlier than F0 in the UIT fields. The m(B5)-V color for supergiants is a sensitive measure of spectral type. The bluest observed colors for each type agree well with colors computed from unreddened Galactic spectral atlas stars for types earlier than about A0; for later spectral types the observed SMC stars range significantly bluer, as predicted by comparison of low-metallicity and Galactic-composition models. Redder colors for some stars of all spectral types are attributed to the strong FUV extinction arising from even small amounts of SMC dust. Internal SMC reddenings are determined for all catalog stars. All stars with E(B-V) < 0.15 are within regions of visible H-alpha emission.

FUV photometry for 42 H-alpha selected H II regions in the SMC Bar is obtained for stars and for total emission (as measured in HII-region-sized apertures). The flux-weighted average ratio of total to stellar FUV flux is 2.15; consideration of the stellar FUV luminosity function indicates that most of the excess total flux is due to scattered FUV radiation, rather than stars fainter than m(B5)=14.5. Both stellar and total emission are well correlated with H-alpha fluxes measured by Kennicutt and Hodge (1986), yielding FUV/H-alpha flux ratios that are consistent with models of SMC metallicity, ages from 1-5 Myr, and moderate (E(B-V) = 0.0-0.1 mag) internal SMC extinction.

Preprint: Not available here. Contact the lead author.
ADS Links: Available as ADS abstract

Ultraviolet Imaging Telescope Observations of the OB Stars in the N 11 Region of the LMC

Authors: J. Wm. Parker, J. K. Hill, R. C. Bohlin, R. W. O'Connell, S. G. Neff, M. S. Roberts, A. M. Smith, & T. P. Stecher
Status: 1996, The Astrophysical Journal, 472, L29

Abstract We present an analysis of far-ultraviolet (FUV: 1300-1800 Å) and optical (U, B, and V) data of the stellar and nebular content of the OB associations LH 9, 10, and 13 in the Large Magellanic Cloud region N 11. The FUV images from The Ultraviolet Imaging Telescope strongly select the hot O and B stars; over 1900 stars were detected in the FUV to a limiting magnitude of m(152) = 17 mag. The resulting FUV photometry combined with optical ground-based data indicate there are approximately 88 confirmed or candidate O stars in the LH 9, 10, and 13 fields alone (in an area of ~41 arcmin²), and possibly as many as 170 to 240 O-type stars within the entire 40 arcmin diameter field of view.

Preprint: Available via ftp as a gzipped PostScript file (2.1 MB), or contact me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
Electronic Journal Article: Available.
ADS Links: Available.

The variable Mass-loss of the Peculiar Supergiant P Cygni

Authors: G. Israelian, M. de Groot, J. Wm. Parker, & C. Sterken
Status: 1996, Monthly Notices of the Royal Astronomical Society, 283, 119

Abstract The study of iron lines from 49 high-resolution IUE spectra observed in the period of 1985-1991 allowed us to find a repetition time between two successive shells of the order of six months. We found that absorption lines of Fe III often have two components while Fe II lines can have three. P Cygni is the only LBV which does not show photometric variability on the scale of 1-2 mag and the only one whose shell phenomena have been followed for some considerable time. The recent disappearence of shell components observed in Si, O and N lines during the last decades is possibly a new unique phenomenon.

Preprint: Available via ftp as a gzipped PostScript file (155 KB) or contact the first author or me [Joel Parker: joel@boulder.swri.edu] for (p)reprints.
ADS Links: Available.

The Stellar Initial Mass Function in the Giant H II Region NGC 595

Authors: E. M. Malumuth, W. Waller, & J. Wm. Parker
Status: 1996, The Astronomical Journal, 111, 1128

Abstract

Preprint: Not available here. Contact the lead author.
ADS Links: Available.

Identification of Emission-Line Stars in 30 Doradus using HST Observations

Authors: J. Wm. Parker, S. R. Heap, & E. M. Malumuth
Status: 1995, The Astrophysical Journal, 448, 705

Abstract Using broadband (F439W) and narrowband (F469N) Hubble Space Telescope archive images of the R 136 central cluster of 30 Doradus, we have identified candidate Wolf-Rayet stars from the strength of their He II 4686 emission lines. The number ratio of Wolf-Rayet to O stars found in our data is 0.11M0.03. Our results are more consistent with an instantaneous burst with an age of about 3 Myr than with a continuous star formation scenario. However, even in the starburst scenario, evolutionary model predictions for low-metallicity stellar evolution agree with our observations only if we also assume enhanced mass loss and/or a relatively flat slope (Gamma > -1.35) for the initial mass function with an upper mass limit > 80 solar masses.

Preprint: Not available here. Contact me [Joel Parker: joel@boulder.swri.edu] for reprints.
ADS Links: The ADS abstract and a copy of the full journal article are available.

International Ultraviolet Explorer Atlas of B-Type Spectra From 1200 to 1900 Å

Authors: N. R. Walborn, J. Wm. Parker, & J. S. Nichols
Status: 1995, NASA Reference Publication 1363, 439, L47

Abstract An Atlas of B-type spectra observed at high resolution by the International Ultraviolet Explorer is presented. It is designed to complement the widely used O-type Atlas (NASA RP-11 55). The purpose is to complete the OB natural group, i.e., to spectral type B3 on the main sequence and B8 at luminosity class Ia, in order to chart the gradual disappearance of the stellar-wind features in normal spectra as a function of spectral type and luminosity class. As in the O Atlas, the primary selection criterion is well-defined optical classifications to derive a consistent ultraviolet (UV) reference frame of normal objects. Some peculiar categories are also included, particularly types BN/BC and superluminous objects. Spectrograms of 86 stars are arranged in 20 spectral-type, luminosity-class, or peculiar-object sequences. The processing and presentation are similar to the O Atlas: the 1200-l900 Å range has been rectified, rebinned to a uniform resolution of 0.25Å, and plotted at 10 Å/cm.

The conclusions from the B Atlas are similar to those from the O Atlas, namely, that the UV stellar-wind features display strong systematic trends as a function of spectral type and luminosity class, and a high degree of correlation with the optical classifications as the winds decline toward the later types. A somewhat higher rate of exceptions to these correlations can be recognized among the B spectra (11%) than the O (2%), but they remain a small fraction of the total sample and do not prevent clear delineation of the normal behavior. Indeed, it is only as a result of the latter that the exceptions can be identified and described.

Preprint: Copies of the large-format atlas are available from me [Joel Parker: joel@boulder.swri.edu] or the lead author [Nolan Walborn: nwalborn@stsci.edu]. The data (spectra) are also directly available via the ADC archives.
ADS Links: Available.

Resolution of Massive Compact Clusters in the 30 Doradus Periphery With HST

Authors: N. R. Walborn, J. W. MacKenty, A. Saha, R. L. White, & J. Wm. Parker
Status: 1995, The Astrophysical Journal, 439, L47

Abstract Hubble Space Telescope Wide Field/Planetary Camera UBV images of three massive, compact multiple systems with the SNR 30 Dor B/NGC 2060 and 30 Dor C/NGC 2044 are discussed and illustrated. In two cases, WN+OB objects have been resolved into additional components to those previously known from ground-based observations, substantially reducing the luminosities of the WN stars and rendering them currently unidentified; in the third case, the components of a B+K composite-spectrum object have been clearly identified. The results are of significance for evolutionary interpretations of these massive stars and for determinations of the upper IMF in extragalactic systems.

Preprint: Not available here. Contact the lead author.
ADS Links: The abstract and a copy of the full journal article are available.

The OB Association LH 58 in the LMC

Authors: C. D. Garmany, P. Massey, & J. Wm. Parker
Status: 1994, The Astronomical Journal, 108, 1256

Abstract We present CCD photometry and spectroscopy for stars in Lucke-Hodge 58, an isolated OB association in the Large Magellanic Cloud northwest of 30 Doradus. The photometric catalog contains 839 stars with UBV magnitudes complete to V ~ 19. We have obtained spectra and classified 35 stars; combined with previous published spectral types, we find 22 O-type stars. The earliest type is O3-4 V, and there are three WR stars in the association. The slope of the initial mass function, Gamma = -1.7±0.3, is in good agreement with other LMC associations. The presence of several evolved supergiants with masses about 15-25 solar masses suggests that some star formation took place as early as 10 million years ago, but the majority of stars formed coevally within the past few million years.

Preprint: Not available here. Contact the lead author.
ADS Links: The abstract and a copy of the full journal article are available.

Dynamical and Observational Constraints on Satellites in the Inner Pluto-Charon System

Authors: S. A. Stern, J. Wm. Parker, M. J. Duncan, J. C. Snowdall, Jr., & H. F. Levison
Status: 1994, Icarus, 108, 234

Abstract It is not known if Pluto has other satellites besides its massive partner Charon. In the past, searches for additional satellites in the Pluto-Charon system have extended from the solar-tidal stability boundary (~ 90 arcsec from Pluto), inward to about one arcsec from Pluto. Here we further explore the inner (i.e., < 10 arcsec) region of the Pluto-Charon system to determine where additional satellites might lie. In particular, we report on (i) dynamical simulations to delineate the region where unstable orbits lie around Charon, (ii) dynamical simulations which use the low orbital eccentricity of Charon to constrain the mass of any third body near Pluto, and (iii) analyzing HST archival images to search for satellites in the inner Pluto-Charon system. Although no objects were found, significant new constraints on bodies orbiting in the inner Pluto-Charon system were obtained.

Preprint: Not available here. Contact the lead author.
ADS Links: Available.

The OB Associations of 30 Doradus in the Large Magellanic Cloud. I. Stellar Observations and Data Reductions

Author: J. Wm. Parker
Status: 1993, The Astronomical Journal, 106, 560

Abstract I present UBV photometry for 2400 stars in the OB associations of 30 Doradus in the Large Magellanic Cloud, and new spectroscopic observations for 54 stars including 24 O stars. The CCD fields cover an area of 50 arcmin² in the central region. The entire catalog (exclusive of the dense core cluster R 136) is photometrically complete to V = B = 18 mag and U = 17 mag, although the completeness magnitudes are fainter for regions with less nebular contamination.

Preprint: Not available here. Contact me [Joel Parker: joel@boulder.swri.edu] for reprints.
ADS Links: The abstract and a copy of the full journal article are available.

The OB Associations of 30 Doradus in the Large Magellanic Cloud. II. Stellar Content and Initial Mass Function

Authors: J. Wm. Parker & C. D. Garmany
Status: 1993, The Astronomical Journal, 106, 1471

Abstract From photometric and spectroscopic observations of the stars in 30 Doradus we determine the effective stellar temperatures and bolometric magnitudes. We construct the theoretical H-R diagram, and using stellar evolution models we then bin the stars by mass to obtain the initial mass function (IMF). The IMF shows marked curvature, flattening out to lower masses, even for masses above which the photometry is complete. Best estimates of the IMF slope yield values of Gamma = -1.5±0.2 for masses > 12 solar masses, where the Salpeter slope is Gamma = -1.35. However, there are indications that different regions of 30 Doradus have different IMF slopes, perhaps as the result of sequential star formation effects.

Preprint: Not available here. Contact me [Joel Parker: joel@boulder.swri.edu] for reprints.
ADS Links: The abstract and a copy of the full journal article are available.

A New Luminous Blue Variable: R 143 in 30 Doradus

Authors: J. Wm. Parker, G. C. Clayton, C. Winge, & P. S. Conti
Status: 1993, Astrophysical Journal, 409, 770

Abstract We have discovered that R 143 in the Large Magellanic Cloud is a Luminous Blue Variable (LBV), the first and perhaps the lone LBV in the central cluster of 30 Doradus, and only the sixth known LMC LBV. Photometric and spectroscopic observations over the past 40 years indicate that during that time R 143 moved redward (changing from an F5 to F8 supergiant), then blueward (possibly becoming as early as O9.5), and is now moving back to the red (currently appearing as a late B supergiant). Similarly, the V magnitude of the star has changed by at least 1.4 mag. Images of R 143 show very unusual filaments of nebulosity extending from the star to a shell at a distance of 3.5 pc, perhaps due to a similar ejection mechanism that created the spiral jets and shell associated with AG Car, another LBV.

Preprint: Not available here. Contact me [Joel Parker: joel@boulder.swri.edu] for reprints.
ADS Links: The abstract and a copy of the full journal article are available.

SN 1991bg: A Type Ia Supernova with a Difference

Authors: B. Leibundgut, R. P. Kirshner, M. M. Phillips, L. A. Wells, N. B. Suntzeff, M. Hamuy, R. A. Schommer, A. R. Walker, L. Gonzalez, P. Ugarte, R. E. Williams, G. Williger, M. Gomez, R. Marzke, B. P. Schmidt, B. Whitney, N. Caldwell, J. Peters, F. H. Chaffee, C. B. Foltz, D. Rehner, L. Siciliano, T. G. Barnes, K. -P. Cheng, P. M. N. Hintzen, Y.-C. Kim, J. Maza, J. Wm. Parker, A. C. Porter, P. C. Schmidtke, & G. Sonneborn
Status: 1993, The Astronomical Journal, 105, 301

Abstract While SN 1991bg is an unusual type Ia SN in such a feature as the brief duration of the photospheric phase, which ended only two weeks after maximum, it shares with other Ia SNs strong Si II and Ca II lines near maximum light. In addition, the light and color curve slopes are almost identical with the templates at late times. The spectral evolution of SN 1991bg is also unique but not unrecognizable; nevertheless, the peculiarities associated with this event complicate the fundamental question as to whether the Ia SNs make good standard candles.

Preprint: Not available here. Contact the lead author.
ADS Links: The abstract and a copy of the full journal article are available.

Two-Stage Starbursts in The LMC: N 11 as a Once and Future 30 Doradus

Authors: N. R. Walborn & J. Wm. Parker
Status: 1992, Astrophysical Journal, 399, L87

Abstract A recent, detailed study of the stellar content in the LMC giant shell H II region N 11 has revealed a distinct, dual structural morphology, which is remarkably analogous to that emerging from current IR imaging of 30 Doradus. In both regions, the energetic stellar activity from an initial, massive, centrally condensed starburst has apparently triggered a secondary burst around its periphery about 2 Myr later, but in N 11 the entire process is more advanced than in 30 Dor by 2 Myr. That is, N 11 appears to be an evolved 30 Dor. There is evidence that the two sequential star-formation mechanisms may produce different IMFs. In both regions, very early stages of massive stellar evolution are evident, including IR protostars, O stars embedded in dense nebular knots, candidate ZAMS O stars, and the O3 stars. Spectra of several recently discovered examples of the latter three categories in N 11 are illustrated, and previously unpublished 4 m photographs of the region demonstrate their spectacular interactions with the surrounding interstellar medium.

Preprint: Not available here. Contact me [Joel Parker: joel@boulder.swri.edu] for reprints.
ADS Links: The abstract and a copy of the full journal article are available.

Ultraviolet and Optical Spectral Morphology of Melnick 42 and Radcliffe 136a in 30 Doradus

Authors: N. R. Walborn, D. C. Ebbets, J. Wm. Parker, J. Nichols-Bohlin, & R. L. White
Status: 1992, Astrophysical Journal, 393, L13

Abstract HST/GHRS ultraviolet spectrograms of the individual O3 If*/WN6-A object Mk 42 in 30 Dor and the adjacent, central multiple system R136a are compared with each other and with an appropriate sequence of O3 If* and WN6-A standards from the IUE archive. The analogous spectral montages covering the blue-violet regino, based on new, homogeneous, digital observations of the same stars with the CTIO 4 m telescope, are also presented. These comparisons show clearly the intermediate O3/WN nature of the Mk 42 spectrum, in terms of both emission-line strength (increasing with envelope density) and stellar-wind velocity (decreasing with envelope density). It is also shown that R136a possesses stronger WN spectral characteristics than Mk 42, in agreement with HST narrow-band imaging by the WF/PC Team.

Preprint: Not available here. Contact the lead author.
ADS Links: The abstract and a copy of the full journal article are available.

30 Doradus in the Large Magellanic Cloud: The Stellar Content and Initial Mass Function

Author: J. Wm. Parker
Status: 1992, Ph.D. thesis, University of Colorado

Abstract UBV photometry for 2400 stars in the OB associations of 30 Doradus in the LMC and new spectroscopic classifications for 54 stars including 23 O stars are presented. The entire catalog (exclusive of the dense core cluster R 136) is photometrically complete to V = B = 18 mag and U = 17 mag, although the completeness magnitudes are fainter for regions with less nebular contamination. From these data, I have determined the effective temperatures and bolometric magnitudes of the stars and placed them on the theoretical H-R diagram. Using stellar evolution models, I then bin the stars by mass to obtain the initial mass function (IMF), which is the number distribution as a function of mass. The IMF shows marked curvature, flattening out to lower masses, even for masses above which the photometry is complete. Best estimates of the IMF slope yield values of Gamma = -1.3 to -1.5 for masses approximately greater than 12 solar mass, where the Salpeter slope is Gamma = -1.35. However, there are indications that different regions of 30 Doradus have different IMF slopes, perhaps as the result of sequential star formation effects. Calculations of the Lyman continuum photon luminosity, N(Ly), lead to the result that approximately 3.2e51 photons s¯¹ are being produced by the observed stars in 30 Doradus. Less than 60 percent of this luminosity comes from the 150 stars with spectroscopic classifications, implying that there still remain a large number of OB stars yet to be observed spectroscopically. This, along with stellar evolution effects, could explain the IMF curvature and the apparent deficiency of stars in the most massive bins of the IMF. For an ionization-bounded H II region, the H-alpha luminosity due to the Lyman continuum photons produced by the stars would be L(H-alpha) ~ 4.5e39 erg s¯¹. This value agrees with the luminosity determined from the observed H-alpha flux, but is a lower limit since the effects of W-R stars and the unresolved cluster of R 136 were not included.

Preprint: Not available here. Copies no longer available from me. Contact University Microfilms International for copies.
ADS Links: Available.

The Stellar Content of LH 9 and 10 (N 11) in the LMC: A Case for Sequential Star Formation

Authors: J. Wm. Parker, C. D. Garmany, P. Massey, & N. R. Walborn
Status: 1992, The Astronomical Journal, 103, 1205

Abstract We present CCD photometry and spectroscopy for stars in Lucke-Hodge 9 and 10, two adjacent OB associations in the northwest corner of the LMC. Our catalog contains UBV photometry (complete to ~18 mag in all three filters) for 795 stars and BV-only photometry (complete to ~19 mag in both filters) for an additional 434 stars. We have obtained spectra for and classified 43 O stars, 17 of which are earlier than O7, and 30 early B stars. LH 10 contains at least three and possibly as many as six O3 stars, and six stars whose ages appear to be quite young and are located near the ZAMS. We also include observations of the unusual objects HD 32228 - a multiple system containing a W-R + O composite - and N 11A, which is an extremely compact H II region/knot surrounding an early O star. We calculate the Lyman ionizing flux of the earliest OB stars to be N(Ly) = 4.9-7.2e50 photons s¯¹, in good agreement with the flux needed to produce the observed H-alpha luminosity of the surrounding H II region.

Preprint: Not available here. Reprints no longer available. Sorry.
ADS Links: The abstract and a copy of the full journal article are available.

A Search for Distant Satellites of Pluto

Authors: S. A. Stern, R. A. Fesen, E. S. Barker, J. Wm. Parker, L. M. Trafton
Status: 1991, Icarus, 94, 246

Abstract Results of a deep CCD-imaging search for satellites of Pluto, whose stability domain extends two orders of magnitude farther than the radius of Charon's orbit, are presented for a region extending from 6 arcsec from Pluto to beyond the edge of the planet's stability domain. Upper limits of 90 percent confidence are placed on the absence of potential satellites with a Charonlike albedo with a radius greater than 58±19 km in the region 6-10 arcsec from Pluto; this constitutes the strongest extant constraint on the possibility of Pluto satellites.

Preprint: Not available here. Contact the lead author.
ADS Links: Available.

Measuring the Direct Sky Brightness on CCD Images

Author: J. Wm. Parker
Status: 1991, Publications of the Astronomical Society of the Pacific, 103, 243

Abstract A new method of determining the background sky brightness for stellar photometric measurements with CCDs is described. By fitting and subtracting stellar profiles from a CCD image, one can measure the sky brightness directly at the position of each star after subtraction. These sky values can then be used for the next round of profile fitting and subtraction. The 'direct sky' brightness for a star is the convergent value of the sky after a few such iterations and is the most direct measurement of the line-of-sight contribution of the background to the observed magnitude of a star. Using this method, a decrease of the random photometric errors by more than a factor of two over standard methods is possible, with the most significant improvements for stars in very crowded regions and areas of strong and highly variable nebulosity.

Preprint: Not available here. Contact me [Joel Parker: joel@boulder.swri.edu] for reprints.
ADS Links: The abstract and a copy of the full journal article are available.

The Stellar Content of NGC 346: A Plethora of O Stars in the SMC

Authors: P. Massey, J. Wm. Parker, C. D. Garmany
Status: 1989, The Astronomical Journal, 98, 1305

Abstract The stellar content of NGC 346, the largest and brightest H II region in the SMC, was investigated using the results of CCD UBV photometry and spectroscopy. Spectra of 42 blue stars were classified, showing that 33 are of the O type, of which 11 are of type O6.5 or earlier, which is as many early-type O stars known in the rest of the SMC. The results identify 25-30 NGC 346 stars more massive than 25 solar masses, and six stars more massive than 40 solar masses, indicating that the upper-mass cutoff to the IMF is not lower in the SMC than in the Galaxy or the LMC. The presence of evolved 15 solar-mass stars in the NGC 346 indicates that some massive stars formed 15 Myr ago. The results of spatial distribution suggest that star formation began at the southwest side of the association and has spread to where the central cluster lies now, providing an example of sequential star formation in the SMC.

Preprint: Not available here. Contact the lead author.
ADS Links: The abstract and a copy of the full journal article are available.

Until one is committed, there is hesitancy, the chance to draw back, always ineffectiveness. Concerning all acts of initiative (and creation), there is one elementary truth, the ignorance of which kills countless ideas and splendid plans: that the moment one definitely commits oneself, then providence moves too. All sorts of things occur to help one that would never otherwise have occurred. A whole stream of events issues from the decision, raising in one's favor all manner of unforeseen incidents and meetings and material assistance, which no man could have

Selected Publications by Joel Wm. Parker

Titles of Selected Publications

Publication Abstracts and links to (p)reprints

Io Eclipse Observations During the New Horizons Jupiter Flyby: Volcanic Atmospheres Revealed by Aurorae

Jupiter's Nightside Airglow and Aurora as seen from New Horizons

Discovering the True KBO Orbit Distribution

Pluto, Charon, and the Kuiper Belt Objects

Ultraviolet Spectroscopy of Comet 9P/Tempel 1 with Alice/Rosetta during the Deep Impact Encounter

Alice: The Rosetta Ultraviolet Imaging Spectrograph

The CFEPS Kuiper Belt Survey: Strategy and Pre-survey Results

Ceres: High-Resolution Imaging with HST and the Determination of Physical Properties

Discovery of a Low-Eccentricity, High-Inclination Kuiper Belt Object at 58 AU

Differentiation of the Asteroid Ceres as Revealed by its Shape

Massive Field Stars and the Stellar Clustering Law

The Discovery of a Twelfth Wolf-Rayet Star in the Small Magellanic Cloud

HST/STIS Observations of Comet 19P/Borrelly During the Deep Space 1 Encounter

A New Spectral Classification System for the Earliest O Stars: Definition of Type O2

Magellanic Orphans: Massive Stars Ex Nihilo?

The Binary Kuiper Belt Object 1998 WW31

Analysis of the First Disk-Resolved Images of Ceres from Ultraviolet Observations with the Hubble Space Telescope

Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud

Chiron

The Discovery of Argon in Comet Hale-Bopp

A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraphic Images

The Ultraviolet and Optical Spectra of Metal-Deficient O Stars in the Small Magellanic Cloud

HST/FOS Spatially Resolved Spectral Classification of Compact OB Groups in the Large Magellanic Cloud

Comet Hale-bopp (C/1995 O1) Near 2.3 AU Postperihelion: Southwest Ultraviolet Imaging System Measurements of the H2O and Dust Production

The Spectroscopic Detectability of Argon in the Lunar Atmosphere

Ultraviolet Observations of Stars in The Magellanic Clouds: A Historical Bridge From The Sun to R 136 and Beyond

Ultraviolet Imaging Telescope Observations of the Magellanic Clouds

HST Mid-Ultraviolet Spectroscopy of 46P/Wirtanen During Its Approach to Perihelion in 1996-1997

The Spectral Variability of the Cool Hypergiant Rho Cassiopeiae

Ultraviolet Spectral Morphology of the Stellar Core of Eta Carinae

An HST Search for Magnesium in the Lunar Atmosphere

The Ultraviolet Imaging Telescope: Instrument and Data Characteristics

Ultraviolet Observations of Chiron with the HST/FOS: Examining a Centaur's Gray Matter

UIT Ultraviolet Observations of the Small Magellanic Cloud

Ultraviolet Imaging Telescope Observations of the OB Stars in the N 11 Region of the LMC

The variable Mass-loss of the Peculiar Supergiant P Cygni

The Stellar Initial Mass Function in the Giant H II Region NGC 595

Identification of Emission-Line Stars in 30 Doradus using HST Observations

International Ultraviolet Explorer Atlas of B-Type Spectra From 1200 to 1900 Å

Resolution of Massive Compact Clusters in the 30 Doradus Periphery With HST

The OB Association LH 58 in the LMC

Dynamical and Observational Constraints on Satellites in the Inner Pluto-Charon System

The OB Associations of 30 Doradus in the Large Magellanic Cloud. I. Stellar Observations and Data Reductions

The OB Associations of 30 Doradus in the Large Magellanic Cloud. II. Stellar Content and Initial Mass Function

A New Luminous Blue Variable: R 143 in 30 Doradus

SN 1991bg: A Type Ia Supernova with a Difference

Two-Stage Starbursts in The LMC: N 11 as a Once and Future 30 Doradus

Ultraviolet and Optical Spectral Morphology of Melnick 42 and Radcliffe 136a in 30 Doradus

30 Doradus in the Large Magellanic Cloud: The Stellar Content and Initial Mass Function

The Stellar Content of LH 9 and 10 (N 11) in the LMC: A Case for Sequential Star Formation

A Search for Distant Satellites of Pluto

Measuring the Direct Sky Brightness on CCD Images

The Stellar Content of NGC 346: A Plethora of O Stars in the SMC

Comet Hale-bopp (C/1995 O1) Near 2.3 AU Postperihelion: Southwest Ultraviolet Imaging System Measurements of the H₂O and Dust Production