Long Term Observations of Chiron at Large Heliocentric Distances

J. M. Bauer, K. J. Meech (IfA), M. W. Buie (Lowell Observatory), O. R. Hainaut (IfA)

2060 Chiron is a unique object in that it is an uncommonly large comet nucleus, similar in size to many recently discovered Trans-Neptunian Objects, and has exhibited activity at extremely large heliocentric distances, r. Recent HST observations (Meech et al. 1997, AJ 113) have provided evidence for a two level structure to its coma, with an inner loosely bound ballistic atmosphere controlled by Chiron's gravity, and an outer exopause of unbound material. This ballistic atmosphere may consist of large dust particles ejected below the escape velocity. In order to interpret HST observations made of the inner coma of Chiron during Feb-July 96, we have undertaken an extensive ground-based observing campaign using the UH 2.2m and the KPNO 0.9m telescopes. Over 150 nights of broadband BVRI CCD images have been obtained to monitor the activity level of Chiron and to look for changes in the color and extent of the unbound coma to be used to model the grain size which is escaping (and thus infer the size of the trapped grains).

We will focus on observations made in April from the UH 2.2m telescope, taken about two months after Chiron's perihelion (r = 8.46AU, tex2html_wrap_inline20 AU). Over forty exposures of one minute duration were obtained in each filter. Preliminary analysis indicates a coma of tex2html_wrap_inline22 2.6'' (59,000 km) in extent, and a redder inner coma, indicative of a possibly larger average coma particle size closer to the nucleus. B-R values of tex2html_wrap_inline22 0.8 were observed, significantly bluer than solar color values of tex2html_wrap_inline30 , and consistent with previously reported observations. Furthermore, a slight ( tex2html_wrap_inline32 ), varying color difference is detectable between 2-5''. We will present two deep composite images, taken on adjacent nights, to illustrate the extent of short term variation in Chiron's coma. Finally, We will also report on the general light curve activity over the interval of the simultaneous HST observations, demonstrating how such a long-term investigation of Chiron's behavior will help us model the complex dynamics of Chiron's newly discovered ballistic atmosphere in the future.

This work is supported in part by NASA grant NAG5-4495.