Molecular Hydrogen and its Proxies HCO⁺ and CO in the Diffuse Interstellar Medium

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