Chemical complexity in local diffuse and translucent clouds: ubiquitous l-C3H and CH3CN, a detection of HC3N and an upper limit on the abundance of CH2CN

TL;DR

This study uses VLA observations to analyze the chemical complexity of diffuse interstellar clouds, detecting key molecules and establishing their abundance patterns, which enhances understanding of molecular composition in such environments.

Contribution

It provides the first comprehensive survey of small hydrocarbons and CN-bearing species in diffuse clouds, revealing their ubiquity and abundance ratios, and sets limits on complex organic molecules in these regions.

Findings

 C3H and CH3CN are ubiquitous in diffuse clouds.

HC3N detected only in regions with AV > 4 mag.

The abundance ratios differ from dense cloud environments.

Abstract

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 \linearC3H\ and \methCN\ are ubiqitous in the local diffuse molecular interstellar medium at \AV\ $\la 1$ while HC$_3$N was seen only toward B0415 at \AV\ $>$ 4 mag. The linear/cyclic ratio is much larger in C$_3$H than in C$_3$\HH\ and the ratio \methCN/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, \cch, linear and cyclic C$_3$H and C$_3$\HH, and \cfh\ and \cfhm) and the CN-bearing species (CN, HCN, HNC, HC$_3$N, HC$_5$N and CH$_3$CN): 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 C\HH CN, such that the anion C\HH CN\m\ is only viable as a carrier of diffuse interstellar bands if the N(C\HH CN)/N(C\HH CN\m) 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 are not present in clouds meeting a reasonable definition of diffuse molecular gas, ie \AV\ $\la 1$ mag.