A Re-investigation of the 1965 April 29 Stellar Appulse by the Pluto--Charon System.

Details A Re-investigation of the 1965 April 29 Stellar Appulse by the Pluto--Charon System. A Re-investigation of the 1965 April 29 Stellar Appulse by the Pluto--Charon System.

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0607m&link_type=abstract

American Astronomical Society, DPS meeting #28, #06.07; Bulletin of the American Astronomical Society, Vol. 28, p.1080

Astronomy and Astrophysics

Astronomy

Scientific paper

Although widely observed throughout N. America, no occultation of the star by Pluto was seen (Halliday et al. 1966, PASP 78, 113--124). Nevertheless, this event provided an upper limit on Pluto's diameter unsurpassed until the discovery of Charon in 1978. We now know that at the time of closest approach, Charon's position with respect to Pluto was (rho = 0.('') 79, theta = 137.(deg) 5). When the event is reanalyzed under the assumption that the appulse was with respect to the Pluto--Charon photocenter, and not the photocenter of a solitary planet, several new deductions can be made. To preclude an occultation at all observing sites, the Pluto/Charon light ratio had to exceed 6:1. Furthermore, the astrometry is consistent with the Halliday et al. result if one assumes Charon's absolute magnitude was identical to that recently measured by HST (Buie, 1996; personal communication), corresponding to a Pluto/Charon light ratio of ~ 8.2:1. (The current value at the same rotational phase is ~ 4.5:1.) Results are relatively insensitive to Pluto's intrinsic surface albedo distribution. There are several implications of a ``constant'' Charon. First, the albedo of Charon's S. hemisphere is likely very similar to the rest of its surface; only a small intrinsic light curve should be expected. Second, it provides the first observational evidence that the system's secular dimming (orbital light curve) is totally attributable to Pluto, heretofore a commonly-made assumption. This reinforces previous results regarding the temporally-variable nature of Pluto's surface albedo (Drish et al. 1995, Icarus 113, 360--386), to which mutual event- and HST-derived maps are insensitive, but consistent. Our updated interpretation of this classic data set provides additional constraints on the amount of volatile migration which has occurred since 1930, and shows the offset between the system's photocenter and barycenter historically was smaller than at present. This last result is important to improving theories of Pluto's heliocentric orbit.

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