Issue |
A&A
Volume 699, July 2025
|
|
---|---|---|
Article Number | A17 | |
Number of page(s) | 19 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202453182 | |
Published online | 27 June 2025 |
CO, CS, HCO, HCO+, C2H, and HCN in the diffuse interstellar medium
1
National Radio Astronomy Observatory,
520 Edgemont Road,
Charlottesville,
VA
22903,
USA
2
LERMA, Observatoire de Paris, PSL Research University CNRS Sorbonne Université,
France
★ Corresponding authors: hliszt@nrao.edu; maryvonne.gerin@observatoiredeparis.psl.eu
Received:
26
November
2024
Accepted:
16
May
2025
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 λ2.6mm 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 Institut de radioastronomie millimetrique (IRAM) and Atacama Large Millimeter/submillimeter (ALMA) interferometers over the past 30 years and gathered the results for to compare with observations of HCO+ and CO emission taken at the Arizona Radio Observatory (ARO) KP12m and IRAM 30m telescopes.
Results. We detected HCO+ along 72 of 86 sight lines where it was observed, C2H along 53 of 76 sight lines, and HCN along 38 of 57 sight lines. C2H is ubiquitous, and N(C2H)/N(HCO+) increases at smaller EB−V and smaller N(HCO+), but C2H absorption is intrinsically weaker, which lowers the number of sight lines with a low column density along which it was detected. The dense-gas tracer HCN was uniformly detected down to N(H2) = 1020 cm−2, with little change in the relative abundance of 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. CS was not seen in the 13 directions where it was newly observed, which are all in Chamaeleon. 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. The comparison of HCO+ emission and absorption along 7 sight lines yielded a typical HCO+ excitation temperature of 2.76 K and total neutral hydrogen number densities n(H) of ≈50–200 cm−3.
Key words: ISM: clouds / ISM: lines and bands / ISM: molecules / ISM: structure
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.