Infrared Excitation of Ethane in Comet Hale-Bopp

Details Infrared Excitation of Ethane in Comet Hale-Bopp Infrared Excitation of Ethane in Comet Hale-Bopp

Sep 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998baas...30q1065k&link_type=abstract

American Astronomical Society, DPS meeting #30, #29.P13; Bulletin of the American Astronomical Society, Vol. 30, p.1065

Astronomy and Astrophysics

Astronomy

Scientific paper

Several sub-branches of the 3 micron band of C_2H_6 have been detected in comets Hyakutake (C/1996 B2) (Mumma et al., 1996), and Hale-Bopp (C/1995 O1) (Weaver et al., 1998). We have developed a line-by-line, time-dependent fluorescence model of this band and have calculated g-factors as a function of cometocentric distance. We find that the g-factors of the sub-branches near the nucleus are greater than those far from the nucleus by approximately a factor of two. Since the lifetime of C_2H_6 is long ( 91,000 seconds at a heliocentric distance of 1 AU), C_2H_6 molecules far from the nucleus should be close to fluorescence equilibrium, while molecules close to the nucleus should have a much colder rotational distribution due to collisional equilibration with the low temperature gas in that region. We analyzed a 3 micron spectrum of Hale-Bopp obtained at the IRTF with the CSHELL on 2 March 1997. Within a square aperture of 1" x 2" at a geocentric distance of 1.5 AU, we find that most of the C_2H_6 molecules are not in the fluorescence equilibrium condition. We constructed synthetic spectra of the 3 micron band of C_2H_6 within the aperture in order to compare the model spectra with the observation. We discuss how the infrared excitation processes affect the derivation of molecular production rates and the analysis of spatial brightness profiles. References Mumma, M.J. et al. 1996. Science, 272, 1310-1314. Weaver, H.A. et al. 1998. Accepted for publication in the Proceeding of the First International Conference on Comet, Tenerife, Canary Island, Feb. 2-5, 1998.

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