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 tex2html_wrap_inline20 and the spectral resolving power was tex2html_wrap_inline22 20,000. On 8-9 April we observed at r=0.73-0.71 AU and tex2html_wrap_inline32 =0.47-0.50 AU, while on 16 April r=0.54 AU and tex2html_wrap_inline32 =0.73 AU. Seven different lines in the tex2html_wrap_inline52 band near 3.3  tex2html_wrap_inline56 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 tex2html_wrap_inline58 ) of tex2html_wrap_inline22 110 K on 8-9 April and tex2html_wrap_inline22 150 K on 16 April, which indicates that T tex2html_wrap_inline58 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 tex2html_wrap_inline66 tex2html_wrap_inline68 50 K). We estimate that the methane abundance relative to water was tex2html_wrap_inline22 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.