2018ApJ...856..151L

We present Jansky Very Large Array observations of 20-37 GHz absorption lines from nearby Galactic diffuse molecular gas seen against four cosmologically distant compact radio continuum sources. The main new observational results are that l-C₃H and CH₃CN are ubiqitous in the local diffuse molecular interstellar medium at A_V ≲ 1, while HC₃N was seen only toward B0415 at A_V > 4 mag. The linear/cyclic ratio is much larger in C₃H than in C₃H₂ and the ratio CH₃CN/HCN is enhanced compared to TMC-1, although not as much as toward the Horsehead Nebula. More consequentially, this work completes a long-term program assessing the abundances of small hydrocarbons (CH, C₂H, linear and cyclic C₃H and C₃H₂, and C₄H and C₄H^–) and the CN-bearing species (CN, HCN, HNC, HC₃N, HC₅N, and CH₃CN): their systematics in diffuse molecular gas are presented in detail here. We also observed but did not strongly constrain the abundances of a few oxygen-bearing species, most prominently HNCO. We set limits on the column density of CH₂CN, such that the anion CH₂CN^– is only viable as a carrier of diffuse interstellar bands if the N(CH₂CN)/N(CH₂CN^–) abundance ratio is much smaller in this species than in any others for which the anion has been observed. We argue that complex organic molecules (COMS) are not present in clouds meeting a reasonable definition of diffuse molecular gas, i.e., A_V ≲ 1 mag.