Cosmic rays in molecular clouds probed by H₂ rovibrational lines

Journal Article · 23 February 2022 · Astronomy and Astrophysics

DOI: https://doi.org/10.1051/0004-6361/202142560 · OSTI ID:2310316

Padovani, Marco  ^[1]; Bialy, Shmuel ^[2]; Galli, Daniele  ^[1]; Ivlev, Alexei V. ^[3]; Grassi, Tommaso ^[3]; Scarlett, Liam H.  ^[4]; Rehill, Una S. ^[4]; Zammit, Mark Christian  ^[5]; Fursa, Dmitry V. ^[4]; Bray, Igor  ^[4]

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1. Istituto Nazionale di Astrofisica (INAF), Firenze (Italy). Osservatorio Astrofisico di Arcetri
2. Univ. of Maryland, College Park, MD (United States)
3. Max Planck Inst. fuer Extraterrestrische Physik, Garching (Germany)
4. Curtin Univ., Perth, WA (Australia)
5. Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Context. Low-energy cosmic rays (<1 TeV) play a fundamental role in the chemical and dynamical evolution of molecular clouds, as they control the ionisation, dissociation, and excitation of H₂. Their characterisation is therefore important both for the interpretation of observations and for the development of theoretical models. However, the methods used so far for estimating the cosmic-ray ionisation rate in molecular clouds have several limitations due to uncertainties in the adopted chemical networks. Aims. Here, we refine and extend a previously proposed method to estimate the cosmic-ray ionisation rate in molecular clouds by observing rovibrational transitions of H₂ at near-infrared wavelengths, which are mainly excited by secondary cosmic-ray electrons. Methods. Combining models of interstellar cosmic-ray propagation and attenuation in molecular clouds with the rigorous calculation of the expected secondary electron spectrum and updated electron-H₂ excitation cross sections, we derive the intensity of the four H₂ rovibrational transitions observable in cold dense gas: (1—0)O(2), (1—0)Q(2), (1—0)S(0), and (1—0)O(4). Results. The proposed method allows the estimation of the cosmic-ray ionisation rate for a given observed line intensity and H₂ column density. We are also able to deduce the shape of the low-energy cosmic-ray proton spectrum impinging upon the molecular cloud. In addition, we present a look-up plot and a web-based application that can be used to constrain the low-energy spectral slope of the interstellar cosmic-ray proton spectrum. We finally comment on the capability of the James Webb Space Telescope to detect these near-infrared H₂ lines, which will make it possible to derive, for the first time, spatial variation in the cosmic-ray ionisation rate in dense gas. Besides the implications for the interpretation of the chemical-dynamic evolution of a molecular cloud, it will finally be possible to test competing models of cosmic-ray propagation and attenuation in the interstellar medium, as well as compare cosmic-ray spectra in different Galactic regions

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: 89233218CNA000001
• OSTI ID: 2310316
• Report Number(s): LA-UR--21-29186
• Journal Information: Astronomy and Astrophysics, Journal Name: Astronomy and Astrophysics Vol. 658; ISSN 0004-6361
• Publisher: EDP SciencesCopyright Statement
• Country of Publication: United States
• Language: English