Astrochemistry

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

By Keith Cowing

Status Report

astro-ph.GA

June 18, 2025

Filed under astro-ph.GA, astrochemistry, biochemistry, Interstellar, interstellar medium, organic chemistry, Spectroscopy

CO, CS, HCO, HCO+, C2H, And HCN In The Diffuse Interstellar Medium — Galactic-scale properties of the sample. We plot against sin(|b|) EB−V (top left), N(HI) (top right), N(H₂) = N(HCO+ )/3 × 10^{−9^{(bottom left), and W_CO, the line profile integral of the CO J=1-0 emission. The sight lines that were not detected in HCO+ absorption are shown as smaller green symbols in the upper two panels. The smaller green symbols in the lower panels are the 3σ upper limits on the plotted quantity. The solid line in each panel is a regression line: A plane-parallel vertically stratified Galactic gas layer would exhibit a power-law slope of -1. We also show averages at intervals of 0.01 in sin(|b|) in blue at the top. A legend marking the values of the Galactic latitude is inset near the top of the upper two panels. The values for the statistical parameters and the regression fits are given in Table 1. — astro-ph.GA}}

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,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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