Observations with the Short Wavelength Spectrometer (SWS) onboard the { \it Infrared Space Observatory} (ISO) have led to the first detection of the methyl radical ${ \rm CH_3}$ in the interstellar medium. The $\nu_2$ $Q-$branch at 16.5 $\mu$m and the $R$(0) line at 16.0 $\mu$m have been unambiguously detected toward the Galactic center SgrA$^*$. The analysis of the measured bands gives a column density of (8.0$\pm$2.4)$\times10^{ 14}$ cm$^{ -2}$ and an excitation temperature of $(17\pm 2)$ K. Gaseous ${ \rm CO}$ at a similarly low excitation temperature and ${ \rm C_2H_2}$ are detected for the same line of sight. Using constraints on the ${ \rm H_2}$ column density obtained from ${ \rm C^{ 18}O}$ and visual extinction, the inferred ${ \rm CH_3}$ abundance is $(1.3{ { +2.2}\atop{ -0.7}}) \times 10^{ -8}$. The chemically related ${ \rm CH_4}$ molecule is not detected, but the pure rotational lines of ${ \rm CH}$ are seen with the Long Wavelength Spectrometer (LWS). The absolute abundances and the ${ \rm CH_3/CH_4}$ and ${ \rm CH_3/CH}$ 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 ${ \rm H}$ with polycyclic aromatic hydrocarbons and solid aliphatic material are included.
Categories: astro-ph