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T. Sekiguchi, J. Watanabe, D. C. Boice (National Astronomical Observatory of Japan)
There has been much interest in (2060) P/Chiron since observations of comet-like activity and a resolved coma established it is a comet. The unusually large size of Chiron coupled with reports of sporadic outbursts at large heliocentric distances (> 12 AU), the identification of CN in the coma, and reports of the detection of CO in the coma, make Chiron a challenging object to model. Simple models of gas production and a dusty coma have been presented by several investigators but a general concensus on many basic features has not emerged.
Radio observations at the NRAO 12-m telescope claimed a detection of CO, but other observations at the IRAM 30-m telescope did not detect CO at almost the same time in June, 1995. We carried out post-perihelion observations of P/Chiron at the NRO 45-m telescope in March, 1998. The CO line (J=1-0, 115 GHz) was not detected and only a three-sigma upper limit on the production rate was derived, Q(CO) < 9E28 /s, due to poor observing conditions. This upper limit is higher than previous results. New observations are planned this winter to obtain conclusive evidence of CO and to estimate the distribution of CO in the coma of P/Chiron.
A fluid dynamics model of dusty comet comae with chemical kinetics and gravity is applied to P/Chiron using several dust size distributions and three dust-to-gas mass production ratios (1.0, 0.1, and 0). The dust coma is assumed to be produced from entrainment of dust particles by sublimating CO gas. The presence of dust in the inner coma has two important effects on the gas flow: 1) the gas is initially mass-loaded, reducing the gas speed to the subsonic range close to the surface and 2) the dust heats the gas in the near-nucleus region, resulting in terminal gas velocities almost twice as high as the dust-free model. In addition, dust could be an distributed source of CO in the coma. This model will be used to interpret observations searching for CO in the coma of P/Chiron. The goal is to provide a more complete framework for understanding Chiron and its surrounding environment.