Cosmic rays in molecular clouds probed by H$_{2}$ rovibrational lines — Perspectives for the James Webb Space Telescope [GA]

Cosmic rays (CRs) at sub-TeV energies play a fundamental role in the chemical and dynamical evolution of molecular clouds, as they control the ionisation, dissociation, and excitation of H${2}$. Their characterisation is important both for the interpretation of observations and for the development of theoretical models. The methods used so far for estimating the CR ionisation rate ($\zeta$) in molecular clouds have several limitations due to uncertainties in the adopted chemical networks. We refine and extend the method proposed by Bialy (2020) to estimate $\zeta$ by observing rovibrational transitions of H${2}$ at near-infrared wavelengths, which are mainly excited by secondary CR electrons. Combining models of interstellar CR propagation and attenuation with the calculation of the expected secondary electron spectrum and updated H${2}$ excitation cross sections by electron collisions, we derive the intensity of the four H${2}$ rovibrational transitions observable in dense, cold gas: (1-0)O(2), (1-0)Q(2), (1-0)S(0), and (1-0)O(4). The proposed method allows the estimation of $\zeta$ for a given observed line intensity and H${2}$ column density. We are also able to deduce the shape of the low-energy CR proton spectrum impinging upon the molecular cloud. 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 CR proton spectrum. We comment on the capability of the James Webb Space Telescope to detect these near-infrared H${2}$ lines, making it possible to derive for the first time spatial variation of $\zeta$ in dense gas. Besides the implications for the interpretation of the chemical-dynamic evolution of a molecular cloud, it will be possible to test competing models of CR propagation and attenuation in the interstellar medium, as well as compare CR spectra in different Galactic regions.

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M. Padovani, S. Bialy, D. Galli, et. al.
Mon, 24 Jan 22
32/59

Comments: 14 pages, 11 figures, accepted by A&A