Cosmic-ray-induced H₂ line emission

Astrochemical modeling and implications for JWST observations

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
e-mail: gaches@ph1.uni-koeln.de
² Center of Planetary Systems Habitability, The University of Texas at Austin, Austin, TX, USA
³ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁴ Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 23 May 2022
Accepted: 26 June 2022

Abstract

Context. It has been proposed that H₂ near-infrared lines may be excited by cosmic rays and thus allow for a determination of the cosmic-ray ionization rate in dense gas. One-dimensional models show that measuring both the H₂ gas column density and H₂ line intensity enables a constraint on the cosmic-ray ionization rate as well as on the spectral slope of low-energy cosmic-ray protons in the interstellar medium.

Aims. We aim to investigate the impact of certain assumptions regarding the H₂ chemical models and interstellar medium density distributions on the emission of cosmic-ray-induced H₂ emission lines. This is of particular importance for utilizing observations of these lines with the James Webb Space Telescope to constrain the cosmic-ray ionization rate.

Methods. We compare the predicted emission from cosmic-ray-induced, rovibrationally excited H₂ emission lines for different one-and three-dimensional models with varying assumptions regarding the gas chemistry and density distribution.

Results. We find that the model predictions of the H₂ line intensities for the (1-0)S(0), (1-0)Q(2), (1-0)O(2), and (1-0)O(4) transitions at 2.22, 2.41, 2.63, and 3.00 μm, respectively, are relatively independent of the astro-chemical model and the gas density distribution when compared against the H₂ column density, making them robust tracers of the cosmic-ray ionization rate.

Conclusions. We recommend the use of rovibrational H₂ line emission in combination with estimations of the cloud’s H₂ column density to constrain the ionization rate and the spectrum of low-energy cosmic rays.