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Abstract
Methylidyne (CH) is one of the most common molecules in the interstellar medium, and is an excellent tracer of molecular hydrogen using optical observations.
Unfortunately, radio observations of CH at $3.3$ GHz require long integration times and, for analysis, the excitation
temperature ($T_{\rm{ex}}$) must be known or assumed.
The excitation temperature of the ground state, hyperfine, main line transition of CH at $3335$ MHz is often assumed to be
$-15$ K (Rydbeck et al. $1976$).
Based on comparisons of radio and optical/uv observations from the literature, using $T_{\rm{ex}}$ = $-15$ K for the $3335$ MHz line may be overestimating the CH column density on average. By comparing the N(CH)/N(H$_2$) ratio and N(CH)/E(B-V) ratio for both data sets, we determine $T_{\rm{ex}}$ may be approximately $-4$ K on average. We calculated the excitation temperature of the CH main line at $3335$ MHz for $16$ lines of sight with optical/UV and radio observations of CH, and we found $T_{\rm{ex}}$ varies significantly between the lines of sight. For example, for HD $24534$, $T_{\rm{ex}}$ = $-3.1$ K, while for HD $24398$ $T_{\rm{ex}}$ = $-27$ K.
CH production in the ISM is closely linked to the production of H$_2$, so we compare CH observations with CO in the diffuse clouds MBM $16$ and MBM $53$. In MBM $16$, CH is able to trace the CO-faint gas which surrounds a CO-bright clump. In CH the molecular gas in the sampled region is roughly uniform, but the CO observations suggests there is a compact clump surrounded by far less dense gas, a suggestion which is not supported by HI and extinction observations. CH and CO observations in MBM $53$ seem to also support that CH is able to trace CO-faint gas, but due to the faint CH $3335$ MHz signal, CH is able to trace gas surrounding CO-bright gas only if the column density of H$_2$ is greater than $\sim 10^{20}$ cm$^{-2}$. In very diffuse regions of MBM $53$, CO was able to be detected, but the signal was too faint for CH to be detected with the integration times used.
Based on comparisons of radio and optical/uv observations from the literature, using $T_{\rm{ex}}$ = $-15$ K for the $3335$ MHz line may be overestimating the CH column density on average. By comparing the N(CH)/N(H$_2$) ratio and N(CH)/E(B-V) ratio for both data sets, we determine $T_{\rm{ex}}$ may be approximately $-4$ K on average. We calculated the excitation temperature of the CH main line at $3335$ MHz for $16$ lines of sight with optical/UV and radio observations of CH, and we found $T_{\rm{ex}}$ varies significantly between the lines of sight. For example, for HD $24534$, $T_{\rm{ex}}$ = $-3.1$ K, while for HD $24398$ $T_{\rm{ex}}$ = $-27$ K.
CH production in the ISM is closely linked to the production of H$_2$, so we compare CH observations with CO in the diffuse clouds MBM $16$ and MBM $53$. In MBM $16$, CH is able to trace the CO-faint gas which surrounds a CO-bright clump. In CH the molecular gas in the sampled region is roughly uniform, but the CO observations suggests there is a compact clump surrounded by far less dense gas, a suggestion which is not supported by HI and extinction observations. CH and CO observations in MBM $53$ seem to also support that CH is able to trace CO-faint gas, but due to the faint CH $3335$ MHz signal, CH is able to trace gas surrounding CO-bright gas only if the column density of H$_2$ is greater than $\sim 10^{20}$ cm$^{-2}$. In very diffuse regions of MBM $53$, CO was able to be detected, but the signal was too faint for CH to be detected with the integration times used.