CO, CS, HCO, HCO+, C2H, And HCN In The Diffuse Interstellar Medium

Context. Radio frequency molecular absorption lines appear along sight lines with AV well below 1 mag, revealing the presence of H₂ in diffuse gas even when 2.6mm CO emission is absent.

Aims. We discuss absorption lines of HCO+, C₂H, HCN, CS, and HCO in a larger sample (88 sight lines) than was available before.

Methods. We observed millimeter-wave absorption at the IRAM and ALMA interferometers over the past 30 years and gathered the results for to compare with observations of HCO+ and CO emission taken at the ARO KP12m and IRAM 30m telescopes.

Results: We detected HCO+ along 72 of 86 sightlines where it was observed, C₂H along 53 of 76 sightlines and HCN along 38 of 57 sightlines. C₂H is ubiquitous and N(C₂H)/N(HCO+) increases at smaller EB-V and smaller N(HCO+) but C₂H absorption is intrinsically weaker, decreasing the number of sightlines with low column density along which it was this http URL dense-gas tracer HCN was uniformly detected down to N(H₂) = 10^20 cm-2 with little change in the relative abundance N(HCN)/N(HCO+) = 1.25.

HCO was detected along only 4 of 46 sight lines that were newly observed at ALMA because its lines are intrinsically weak, but HCO is ubiquitous in the interstellar medium with N(HCO)/N(HCO+) = 1/3 or N(HCO)/N(H₂) = 10^-9. The line widths of HCN features are (like those of CO) narrower than those of matching features observed in HCO+, and those of C₂H are broader. HCO+ emission is commonly observed at log levels -2+/-0.3 dex with respect to CO emission.

Harvey Liszt, Maryvonne Gerin

Comments: Accepted for A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2506.12518 [astro-ph.GA] (or arXiv:2506.12518v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2506.12518
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Submission history
From: Harvey S. Liszt
[v1] Sat, 14 Jun 2025 14:19:42 UTC (627 KB)
https://arxiv.org/abs/2506.12518
Astrobiology, Astrochemistry,