Last update: 2008 Aug 14

Stellar occultations: Pluto, Aug 25, 2007


There is a potential occultation of the star P598.2 by Pluto on 2008 Aug 25 at ~4:46 UT. The star is faint but very red. Also, the location (US) and the slowness of the event makes this logistically easier than some. More information is at pages posted by Bruno Sicardy's group (Meudon) and Jim Elliot's group (MIT) .


Telescope           Approx. Location  Observer        Cameras
WIRO 2.3m           Laramie WY        Cathy Olkin     PHOT (possibly w/ dichroic)
Steward 90"         Tuscon AZ         Marc Buie       PHOT (possibly w/ dichroic)
Lick 36" (Crossley) San Jose CA       Leslie Young    PHOT
Lowell 42"          Flagstaff AZ      Larry Wasserman PHOT
Sommers Bausch 24"  Boulder CO        not yet decided PHOT
McDonald 36"        Fort Davis TX     Eliot Young     PHOT
UCF 20" (Robinson)  Orlando FL        Nate Lust       Facility CCD
Palomar 24"         Los Angeles CA    Mike Brown      Facility CCD
Table Mountain      Wrightwood CA     Gerbs Bauer     Facility CCD


Star Positions

Using the remeasurements of the Pluto path by UCAC in 2006 as our reference position, we find the following star positions:

Source          RA            Dec              dra     ddec errx erry
-------------- -------------- -------------   ----- ------- ---- ----
UCAC2_2006      17:53:27.1056 -17:15:27.517       0       0   64   42
Lowell31        17:53:27.0981 -17:15:27.528    -107     -12   88   73
Brazil          17:53:27.1032 -17:15:27.546     -34     -29   35   35
MIT             17:53:27.0975 -17:15:27.454    -116      63   10    9
RA and Dec are epoch of date (where proper motion is know), equinox J2000.
dra and ddec are the offsets from the reference position in mas.
errx and erry are the quoted errors in position (and therefore in dra, ddec) in mas.
UCAC2-2006: the position remeasured with the UCAC2 telescope in 2006, provided by Norbert Zacharias. This catalog is available here .

Brazil - Brazilian astrometric group that Bruno Sicardy and co. are working with, from email sent by Bruno 2008 July 28.

MIT prediction above uses the value as of 2008 07 28. The value has since been updated. See Jim Elliot's group's prediction

Lowell 31-in are from two nights on the 31 inch telescope at Lowell Observatory in August 2007.

The plot of the offsets is below. Notice that the MIT group is much farther north than the others.

With only four points (and rather large errors on the Lowell measurements), it's tough to say whether or not the positions fall into two camps in RA, two near the UCAC 2006 and Brazilian RA and two near the MIT and 31". On other events, a dichotomy was a warning of source confusion. For whatever reason (seeing? centroiding algorithm?) the UCAC positions are often skewed toward bright stars that are even a few arcsec away. But the Brazzilian and UCAC (2006) positions are close.

For this event, the difference in RA is important. For example, the Lowell 31" track is about a radius south of the UCAC2006 or Brazil track. MIT is a diameter south, getting about a radius from the RA difference and another from the dec.

Pluto offsets

Using the DE-413 and PLU-013 ephemerides from JPL as our reference ephemerides, the following offsets are reported

Source           dra     ddec (milliarcsec)
------------     -----   ----
DE-413+PLU-013    0      0
Bruno Sicardy   -22.6  114.0
PHOT            -22.2   37.7
MIT 2008-07-28   -1.3   71.0
------------     -----  ----
Bruno's offset is from the 22 June 2008 event.

PHOT's (our group's) offset is from the 31 July 2007 event.

The MIT offset is described at Jim Elliot's group's prediction


I've arranged the predictions with the northern-most track in the upper left. Each group's best prediction (at the time of the last edit of this page) is highlighted in red.
Brazil star position UCAC2_2006 star position MIT star position
Bruno's Pluto offest
MIT's Pluto offest
PHOT's Pluto offest
No Pluto offest from DE413

From Bruno Sicardy, 2008 July 27 star: R=15.2, I=14.0, JHK=12.0, 11.1, 10.7 Pluto: R=13.8, I=13.5, JHK=13.0, 12.9, 13.3

Observing Aids

Finder charts

Digital Sky Survey images with the location of Pluto at 4:30 UT on Aug 21 to Aug 26. Also marked are UCAC2 magnitudes (between V and R) of field stars. Sorry that the minus sign on the dec got cut off.

10 arcmin 5 arcmin 2 arcmin
Timing Notes

Go 12 or 14 sec per frame (0.5 points per scale height) if things are really dire (6 s readout, or heavy clouds). This will be good for constraining the geometry of where in Pluto's shadow we hit.

Go 6 or 7 sec per frame (1 point per scale height) if you have 3 sec readout. This will be good for constraining the geometry, and modelling the spatial variation of how Pluto's atmosphere.

Go 3 sec per frame (2 points per scale height) if you have < 1.5 s readout and if you're photon limited at 3 sec. This can be used for geometry, spatial variation, and to get the vertical structure of Pluto's atmosphere.

Go faster, up to 0.100 s per frame (~2 points per Fresnel scale), if you can have < 10% dead time and are photon limited at your rate. This can be used for geometry, spatial variation, vertical structure, and characterizing wave activity.

Reduction Aids

Reference stars The following stars have proved useful as on-chip standards. We letter them A through H for convenience (A being the occultation star). These are also in the USNO B1.0 catalog, with the following Imag and positions. Improved I-band photometry to follow ...

ID USNO-B1.0 number        I mag  RA (J2000)     Dec (J2000)
-- ----------------------  -----  -------------   ------------ 
A  USNO-B1.0 0727.0710748  14.49  17:53:27.1130  -17:15:27.340
B  USNO-B1.0 0727.0710722  14.13  17:53:26.7860  -17:15:46.920
C  USNO-B1.0 0727.0710712  13.49  17:53:26.7050  -17:15:51.540
D  USNO-B1.0 0727.0710670  14.25  17:53:26.1090  -17:15:53.340
E  USNO-B1.0 0727.0710725  13.97  17:53:26.8360  -17:14:56.310
F  USNO-B1.0 0727.0710772  13.06  17:53:27.5440  -17:16:25.280
G  USNO-B1.0 0727.0711158  13.64  17:53:32.5580  -17:15:35.010
H  USNO-B1.0 0727.0711152  12.95  17:53:32.4590  -17:14:58.830
Here is a finder chart with reference stars marked: