Methane in Oort cloud comets

We detected CH4 in eight Oort cloud comets using high-dispersion (λ/Δλ∼2×104) infrared spectra acquired with CSHELL at NASAʹs IRTF and NIRSPEC at the W.M. Keck Observatory. The observed comets were C/1995 O1 (Hale–Bopp), C/1996 B2 (Hyakutake), C/1999 H1 (Lee), C/1999 T1 (McNaught–Hartley), C/1999 S4 (LINEAR), C/2000 WM1 (LINEAR), C/2001 A2 (LINEAR), and 153/P Ikeya–Zhang (C/2002 C1). We detected the R0 and R1 lines of the ν3 vibrational band of CH4 near 3.3 μm in each comet, with the exception of McNaught–Hartley where only the R0 line was measured. In order to obtain production rates, a fluorescence model has been developed for this band of CH4. We report g-factors for the R0 and R1 transitions at several rotational temperatures typically found in comet comae and relevant to our observations. Using g-factors appropriate to Trot as determined from HCN, CO and/or H2O and C2H6, CH4 production rates and mixing ratios are presented. Abundances of CH4/H2O are compared among our existing sample of comets, in the context of establishing their place of origin. In addition, CH4 is compared to native CO, another hypervolatile species, and no correlation is found among the comets observed.