Detection of interstellar CH3

Observations with the Short Wavelength Spectrometer on board the Infrared S pace Observatory have led to the first detection of the methyl radical CH, in the interstellar medium. The nu(2) Q-branch at 16.5 mu m and the R(0) li ne at 16.0 mu m have been unambiguously detected toward the Galactic center Sagittarius A*. The analysis of the measured bands gives a column density of (8.0 +/- 2.4) x 10(14) cm(-2) and an excitation temperature of 17 +/- 2 K, Gaseous CO at a similarly low excitation temperature and C2H2 are detect ed for the same line of sight. Using constraints on the H-2 column density obtained from (CO)-O-18 and visual extinction, the inferred CH3 abundance i s (1.3(-0.7)(+2.2)) x 10(-8). The chemically related CH4 molecule is not de tected, but the pure rotational lines of CH are seen with the Long Waveleng th Spectrometer. The absolute abundances and the CH3/CH4 and CH3/CH ratios are inconsistent with published pure gas-phase models of dense clouds. The data require a mix of Jiff use and translucent clouds with different densit ies and extinctions, and/or the development of translucent models in which gas-grain chemistry, freeze-out, and reactions of H with polycyclic aromati c hydrocarbons and solid aliphatic material are included.