The Evolution of the Outgassing of Comet C/1995 O1 (Hale-Bopp) from Radio Observations

Astronomy and Astrophysics – Astronomy

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Soon after its discovery at 7 AU from the Sun, the exceptionally bright comet C/1995 O1 (Hale-Bopp) was observed by means of radio spectroscopy to investigate gas species associated to its activity. Observations at the IRAM 30-m telescope revealed production of carbon monoxide with a rate as large as 2. x 10(28) s(-1) in September 1995. Since that time, the comet has been regularly observed with the IRAM 30-m telescope at mm wavelengths, with the JCMT 15-m telescope at sub-mm wavelengths and with the Nançay decimetric telescope in the OH lines at 18 cm. Up to now, the comet has been monitored when it approached from 7 to about 3.4 AU to the Sun. We observed the successive ``turning on'' of the CO, CH_3OH, HCN, OH, H_2S, CS, H_2CO and CH_3CN lines, revealing the progressive release of these various volatiles from the cometary ices. At mid-August, the following molecular production rates were determined: Q[OH] = 2.5 x 10(29) , Q[CO] = 1.5 x 10(29) , Q[CH_3OH] = 1.1 x 10(28) , Q[H_2S] = 8 x 10(27) , Q[HCN] = 9 x 10(26) ; Q[CS] = 3 x 10(26) , Q[H_2CO] = 1.5 x 10(26) , and Q[CH_3CN] = 1.1 x 10(26) molecules s(-1) . The line shapes reveal a preferred outgassing towards the Sun and the evolution of the expansion velocity as the comet approaches the Sun. The simultaneous observations of several transitions of the same species, either at different wavelengths (CO, HCN), or better, at the same wavelength (CH_3OH, CH_3CN), allows us to investigate the temperature of the coma and its variation with heliocentric distance.

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