Detection of interstellar CH_3

Abstract

Observations with the Short Wavelength Spectrometer (SWS) onboard the {\it Infrared Space Observatory} (ISO) have led to the first detection of the methyl radical $\mathrm{C}{\mathrm{H}}_3$ 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${}^{\ast }$. The analysis of the measured bands gives a column density of (8.0$\pm$2.4)$\times 10^{14}$ cm${}^{-2}$ and an excitation temperature of $(17\pm 2)$ K. Gaseous $\mathrm{CO}$ at a similarly low excitation temperature and ${\mathrm{C}}_2{\mathrm{H}}_2$ are detected for the same line of sight. Using constraints on the ${\mathrm{H}}_2$ column density obtained from ${\mathrm{C}}^{18}\mathrm{O}$ and visual extinction, the inferred $\mathrm{C}{\mathrm{H}}_3$ abundance is $(1.3\genfrac{}{}{0ex}{}{+2.2}{-0.7})\times 10^{-8}$. The chemically related $\mathrm{C}{\mathrm{H}}_4$ molecule is not detected, but the pure rotational lines of $\mathrm{CH}$ are seen with the Long Wavelength Spectrometer (LWS). The absolute abundances and the $\mathrm{C}{\mathrm{H}}_3/C{\mathrm{H}}_4$ and $\mathrm{C}{\mathrm{H}}_3/CH$ 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 $\mathrm{H}$ with polycyclic aromatic hydrocarbons and solid aliphatic material are included.

2 figures. ApJL, Accepted

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