Detection of Interstellar CH3.

Observations with the Short Wavelength Spectrometer on board the Infrared Space Observatory have led to the first detection of the methyl radical CH(3) in the interstellar medium. The nu(2) Q-branch at 16.5 µm and the R(0) line at 16.0 µm have been unambiguously detected toward the Galactic center Sagittarius A*. The analysis of the measured bands gives a column density of &parl0;8.0+/-2.4&parr0;x1014 cm(-2) and an excitation temperature of 17+/-2 K. Gaseous CO at a similarly low excitation temperature and C(2)H(2) are detected for the same line of sight. Using constraints on the H(2) column density obtained from C(18)O and visual extinction, the inferred CH(3) abundance is &parl0;1.3+2.2-0.7&parr0;x10-8. The chemically related CH(4) molecule is not detected, but the pure rotational lines of CH are seen with the Long Wavelength Spectrometer. The absolute abundances and the CH(3)/CH(4) and CH(3)/CH ratios are inconsistent with published pure gas-phase models of dense clouds. The data require a mix of diffuse and translucent clouds with different densities and extinctions, and/or the development of translucent models in which gas-grain chemistry, freeze-out, and reactions of H with polycyclic aromatic hydrocarbons and solid aliphatic material are included.