Cosmic-ray-induced H2 line emission: Astrochemical modeling and implications for JWST observations
Journal
Date Issued
2022
Author(s)
Abstract
Context: It has been proposed that H$_2$ near-infrared lines may be excited
by cosmic rays and allow for a determination of the cosmic-ray ionization rate
in dense gas. One-dimensional models show that measuring both the H$_2$ gas
column density and H$_2$ line intensity enables a constraint on the cosmic-ray
ionization rate as well as the spectral slope of low-energy cosmic-ray protons
in the interstellar medium (ISM). Aims: We aim to investigate the impact of
certain assumptions regarding the H$_2$ chemical models and ISM density
distributions on the emission of cosmic-ray induced H$_2$ 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,
ro-vibrationally excited H$_2$ emission lines for different one- and
three-dimensional models with varying assumptions on the gas chemistry and
density distribution. Results: We find that the model predictions of the H$_2$
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 $\mu$m, respectively, are relatively
independent of the astro-chemical model and the gas density distribution when
compared against the H$_2$ column density, making them robust tracer of the
cosmic-ray ionization rate. Conclusions: We recommend the use of ro-vibrational
H$_2$ line emission in combination with estimation of the cloud's H$_2$ column
density, to constrain the ionization rate and the spectrum of low energy
cosmic-rays.
Volume
664
Start page
A150
Issn Identifier
0004-6361
Rights
open.access
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