Molecular hydrogen and its proxies HCO$^+$ and CO in the diffuse interstellar medium

TL;DR

This study evaluates the use of HCO+ and CO as proxies for molecular hydrogen in diffuse interstellar gas, revealing their effectiveness and variability in different observational regimes and conditions.

Contribution

It demonstrates the reliability of HCO+ as a proxy for H2 and explores the complexities of using CO emission to estimate molecular hydrogen in the diffuse interstellar medium.

Findings

HCO+ is a consistent proxy for H2 in diffuse gas.

CO emission varies significantly, affecting the CO-H2 conversion factor.

Gas/reddening ratios are lower than the Galactic mean at low reddening.

Abstract

There is a robust polyatomic chemistry in diffuse, partially-molecular interstellar gas that is readily accessible in absorption at radio/mm/sub-mm wavelengths. Accurate column densities are derived owing to the weak internal excitation, so relative molecular abundances are well known with respect to each other but not with respect to H2. Here we consider the use of proxies for hydrogen column densities N(H2) and N(H) = N(HI)+2N(H2) based on measurements of HCO+ absorption and CO emission and absorption, and we compare these with results obtained by others when observing HI, H2 and CO toward stars and AGN. We consider the use of HCO+ as a proxy for H2 and show that the assumption of a relative abundance N(H2) = N(HCO+)/3x10^{-9} gives the same view of the atomic-molecular hydrogen transition that is seen in UV absorption toward stars. CO on the other hand shows differences between the radio and optical regimes because emission is always detected when N(\hcop) > 6x10^{11}\pcc or N(H2) > 2x10^20\pcc. Wide variations in the integrated CO {J=1-0} brightness W_CO and N(CO)/N(H2) imply equivalent variations in the CO-H2 conversion factor even while the ensemble mean is near the usual Galactic values. Gas/reddening ratios found in absorption toward stars, N(H)/E(B-V) = 6.2x10^21 H \pcc/mag overall or 6.8x10^21 H \pcc/mag for sightlines at E(B-V) <= 0.08 mag lacking H2 are well below the Galactic mean measured at low reddening and high Galactic latitude, 8.3x10^21 H \pcc/mag.