2020MNRAS.495..510D

Molecular hydrogen is the main constituent of dense molecular clouds, but is expected to also be a dominant constituent in many environments where CO can no longer be seen, the so-called 'CO-dark molecular gas'. Based on comparisons of ultraviolet spectroscopy of H₂ and optical line observations (4300 Å), CH is a prime candidate to trace H₂. Since the optical line (and the UV lines at 3143, 3890, and 3878 Å) require bright background sources (and the CH N = 2-1 ground state rotation line at 149 um requires space-based, or stratospheric, observations), the hyperfine structure transition at 3335 MHz is a potentially important tool for probing the CO-dark molecular gas. However, the excitation of this transition is complicated, and has often been found to be inverted, making column density determinations uncertain. To clarify the potential use of the 3.3-GHz line as a proxy for H₂, we have observed the CH 3335-MHz line with the Arecibo 305-m radio telescope along 16 lines of sight towards stars with existing measurements of the 4300-Å line. By comparing the CH column densities from optical and UV absorption lines to the CH radio emission line, we can derive the excitation temperature (T_ex) of the 3335-MHz transition. We obtain a wide range of excitation temperatures for nine lines of sight, including some with |T_ex| < 5 K. The common assumption that T_ex for the 3335-MHz line is always much larger than the background temperature (T_bg) is not always warranted and can lead to significant errors in the value of N(CH).