The Methane Abundance in Comet Hyakutake (C/1996 B2)

Details The Methane Abundance in Comet Hyakutake (C/1996 B2) The Methane Abundance in Comet Hyakutake (C/1996 B2)

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997dps....29.3405w&link_type=abstract

American Astronomical Society, DPS meeting #29, #34.05; Bulletin of the American Astronomical Society, Vol. 29, p.1041

Astronomy and Astrophysics

Astronomy

3

Scientific paper

We report on infrared observations of methane in comet Hyakutake (C/1996 B2) that were obtained during April 1996 using CSHELL at the NASA IRTF. The effective aperture size was 1'' x 2'' and the spectral resolving power was ~ 20,000. On 8-9 April we observed at r=0.73-0.71 AU and Delta =0.47-0.50 AU, while on 16 April r=0.54 AU and Delta =0.73 AU. Seven different lines in the nu_ {3} band near 3.3 mu m were clearly detected, and we have used their relative intensities to conduct the first detailed examination of the rotational excitation in cometary methane. We derive a rotational temperature (Trot) of ~ 110 K on 8-9 April and ~ 150 K on 16 April, which indicates that Trot varied approximately inversely with the heliocentric distance. We also find that the population distribution among the different spin species is consistent with the value expected in the high temperature limit (Tspin>=50 K). We estimate that the methane abundance relative to water was ~ 1% during the period of our observations, although this value is uncertain by at least a factor of two due to uncertainties in the water production rate. Methane is apparently an important trace constituent of both cometary nuclei and grain mantles in star-forming regions, which provides further evidence that comets retain a signature of their interstellar heritage.

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