2025A&A...699A..17L

Context. Radio frequency molecular absorption lines appear along sight lines with A_V 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 E_B–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.