DISCOVERY OF INTERSTELLAR METHANE - OBSERVATIONS OF GASEOUS AND SOLID CH4 ABSORPTION TOWARD YOUNG STARS IN MOLECULAR CLOUDS

We have searched in several molecular clouds for absorption at 7.6-mu-m due to gaseous and solid methane. Gaseous CH4 was detected toward NGC 7538 IRS 9 and probably OMC-1 IRc2 and W33 A. The abundance of gaseous CH4 is typically 10(-3) that of CO. Solid CH4 was probably detected toward NGC 7538 IRS 9 and possibly detected toward W33 A and NGC 7538 IRS 1. The abundance of solid CH4 is comparable to that of solid CO. The total CH4 abundance (predominantly in the solid phase) is 1%-4% of the total CO abundance (predominantly gaseous). The high fraction of CH4 in the solid state suggests that it is made in the grain mantles.