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 CH₃ in the interstellar medium. The ν₂ 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 (8.0 ± 2.4) × 10¹⁴ cm^-2 and an excitation temperature of 17 ± 2 K. Gaseous CO at a similarly low excitation temperature and C₂H₂ are detected for the same line of sight. Using constraints on the H₂ column density obtained from C¹⁸O and visual extinction, the inferred CH₃ abundance is (1.3) × 10^-8. The chemically related CH₄ molecule is not detected, but the pure rotational lines of CH are seen with the Long Wavelength 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 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.