DETERMINATION OF THE CHARON PLUTO MASS-RATIO FROM CENTER-OF-LIGHT ASTROMETRY/

The Charon/Pluto mass ratio is a fundamental but poorly known paramete r of the two-body system. Previous values for the mass ratio have rang ed from 0.0837 +/- 0.0147 (Null et al., 1993, Astron. J. 105, 2319-233 5) to 0.1566 +/- 0.0035 (Young et al, 1994, Icarus 108, 186-199). We r eport here a new determination of the Charon/Pluto mass ratio, using f ive sets of groundbased images taken at four sites in support of Pluto occultation predictions. Unlike the Null et al. and Young et al. dete rminations, where the centers of light for Pluto and Charon could be d etermined separately, this technique examines the motion of the center of light of the blended Pluto-Charon image. We compute the offsets of the observed center-of-light position of Pluto-Charon from the epheme ris position of the system and fit these offsets to a model of the Plu to-Charon system. The least-squares fits to the five data sets agree w ithin their errors, and the weighted mean mass ratio is 0.117 +/- 0.00 6. The effects of errors in the Charon light fraction, semimajor axis, and ephemeris have been examined and are equal to only a small fracti on of the formal error from the fit. This result is intermediate betwe en those of Null et al. and Young et at and matches a new value of 0.1 24 +/- 0.008 by Null and Owen (1996, Astron. J. 111, 1368-1381). The m ass ratio and resulting individual masses and densities of Pluto and C haron are consistent with a collisional origin for the Pluto-Charon sy stem. (C) 1997 Academic Press.