The Role of Neutral Hydrogen in Setting the Abundances of Molecular Species in the Milky Way’s Diffuse Interstellar Medium. I. Observational Constraints from ALMA and NOEMA

• Published in: The Astrophysical journal 2022-03, Vol.928 (1), p.79
• Main Authors: Rybarczyk, Daniel R., Stanimirović, Snežana, Gong, Munan, Babler, Brian, Murray, Claire E., Gerin, Maryvonne, Winters, Jan Martin, Luo, Gan, Dame, T. M., Steffes, Lucille
• Format: Article
• Language: English
• Subjects: Absorption; Arrays; Astrophysics; Diffuse molecular clouds; Diffusion; Hydrogen; Interstellar absorption; Interstellar atomic gas; Interstellar chemistry; Interstellar matter; Interstellar medium; Interstellar phases; Mach number; Metallicity; Milky Way; Molecular gases; Molecule formation; Optical analysis; Optical thickness; Radio telescopes; Sciences of the Universe; Solar neighborhood; Spin temperature
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac5035

We have complemented existing observations of H i absorption with new observations of HCO + , C 2 H, HCN, and HNC absorption from the Atacama Large Millimeter/submillimeter Array and the Northern Extended Millimeter Array in the directions of 20 background radio continuum sources with 4° ≤ ∣ b ∣ ≤ 81° to constrain the atomic gas conditions that are suitable for the formation of diffuse molecular gas. We find that these molecular species form along sightlines where A V ≳ 0.25, consistent with the threshold for the H i -to-H 2 transition at solar metallicity. Moreover, we find that molecular gas is associated only with structures that have an H i optical depth >0.1, a spin temperature