Stellar occultations

Charon, July 11, 2005


On 2005, July 11 (UT), there is a predicted occultation by Charon. This has the potential of beung the first Charon occultation observed by multiple sites, and the first Charon occultation observed at better than 2-second resolution. The only previous occultation was observed from a single site by Alister Walker (Walker 1980 (pdf) ). This event was reanalyzed by Elliot and Young (1991 (pdf) ) to place a lower limit on the radius of 601.5 km, and an upper limit on the atmosphere of 1-57 cm-A.


Pluto has a significant lightcurve. The latest paper with the seperate lightcurves of Pluto and Charon is from data taken 1992 to 1993 (Buie, Tholen and Wasserman 1997, Icarus 125, 233-244). At the time of the occultation (2005 July 11 03:36 UT), we have
r = heliocentric distance = 30.97 AU
delta = geocentric distance = 30.07 AU
Ob-lon = sub-observer longitude 98.12 deg for Pluto, 278.12 for Charon (IAU convention. Buie's Right-Hand-Rule longitude is approximately -Oblon).
alpha = solar phase angle = 0.87 deg.

For the star, USNO A2-0 gives B = 15.6, R = 14.6. At c313.2's dec, these are from the POSS-I O and E plates, not Johnson filters (or Cousin's, for that matter). Converting the Johnson B, V (link)
V = r + 0.375 * (b - r) = 15.0
B - V = 0.625 * (b - r) = 0.63, B=15.6
, but errors can be as large as 0.5 mag.

USNO B1-0 gives two measurements for B, B1 = 16.61 and B2 = 16.14. Also two for R, R1 = 14.82 and R2 = 14.70. I = 14.16. A discussion on the color conversion for the USNO B 2-0 (link) gives
Johnson B = USNO B1.0 B1 +/- 0.5 = 16.61
Johnson R = USNO B1.0 R1 +/- 0.5 = 14.81
V = 0.444R1 + 0.556B1 +/- 0.5 = 15.81

The UCAC2 magnitudes are through a filter midway between V and R, with a UCAC2 mag of 14.99.

The 2MASS catalog actually has magnitudes that people believe, with J=12.914 +/- 0.023, H = 12.314 +/- 0.022, and K = 12.155 +/- 0.023. Again, there is a color transformation link . Plus there are several IR system to convert into ... MKO, UKIRT, SAAO,... It happens that the 2MASS system is within a few tenths of a mag of the UKIRT system.

Finally, DENIS gives I = 13.9 +/- 0.04, J = 12.8 +/-0.11, K = 12.1 +/- 0.12, with the J and K consistent with 2MASS.

           B     V     R      I     J    H    K
Pluto    15.11 14.24 13.6?
Charon   16.96 16.25 15.7?       
C313.2   16.6  15.8  14.8    13.9   12.9 12.3 12.2


Predictions can be found at Bruno Sicardy's occultation page for this event, or at Jim Elliot's page As usual with occultations by small, outer-solar system bodies, the maps only show the current predicted location of the shadow. This can change dramatically depending on refinements to the star's position.


There are three groups observing this event, led by Jim Elliot (MIT and Williams), Bruno Sicady (Meudon, France), and Leslie Young (SwRI and Wellesley). The observers for the SwRI/Wellesley group are Leslie Young (SwRI), Eliot Young (SwRI), Kevin Shoemaker (Shoemaker Labs), Dick French (Wellesley), Brooke Gregory (NOAO), Cathy Olkin (SwRI), and Trin Ruhland (U Chicago).
Location           Observers               Instrument  
--------           ---------               ----------------------
CTIO 4-m (Blanco)  Olkin/Ruhland/L Youmg   Mosaic II camera 
CTIO 0.9-m         French/Gregory          Tek #3 CCD
SOAR 4.2-m         E Young/Shoemaker       PHOT (our new camera)


Three finder charts generated from the Digital Sky Survey via Vizier:
2 arcmin field of view (pdf) )
17 arcmin field of view (bmp) )
60 arcmin field of view (fits) )
At the event midtime, Charon will be 225 mas East and 865 mas South of Pluto (position angle of 165.4 degrees), according to the Plu013 ephemeris.


The PHOT group got data from all three telescopes.
Young, L. A., C. B. Olkin, E. F. Young and R. G. French 2005, Occultation by Pluto I (Charon), IAU Circular 8570. (text file) .

The MIT/Williams group got data from several sites. See their press release and their movie and description of the event from Las Campanas Obs. in Chile.

The Meudon group got data from at least VLT.

Last updated 2005 July 20 by Leslie Young, ,