The Methane Abundance in Comet Hyakutake (C/1996 B2)
H. A. Weaver (JHU), T. Y. Brooke (JPL), M. A. DiSanti, M. J. Mumma (GSFC), A. Tokunaga (UHawaii), G. Chin (GSFC), M. F. A'Hearn (UMD), T. C. Owen (UHawaii), C. M. Lisse (UMD)
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 and the spectral resolving power was 20,000. On 8-9 April we observed at r=0.73-0.71 AU and =0.47-0.50 AU, while on 16 April r=0.54 AU and =0.73 AU. Seven different lines in the band near 3.3 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 (T ) of 110 K on 8-9 April and 150 K on 16 April, which indicates that T 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 (T 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.