Cosmic rays in molecular clouds probed by H₂ rovibrational lines

Abstract

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. 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.

Dependence of Chemical Abundance on the Cosmic-Ray Ionization Rate in IC 348

“…A notable trend from low A V to high A V is that the CRIR decreases from 6.6 2.3 3.4 -+ × 10 −16 to 2 × 10 −16 s −1 . The decreasing trend of the CRIR from low-to-high A V in IC 348 seems to support the hypothesis of the ζ cr gradient as a function of A V , as predicted by theoretical models (Padovani et al 2018(Padovani et al , 2022Gaches et al 2022). We note that Position IDs 8-10 have similar A V but significant ζ cr (an order of magnitude difference).…”

Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds

“…Our results are consistent with the model H by Padovani et al (2018), in which CRs (and consequently the CRIR) are attenuated as a function of A V (Padovani et al 2018(Padovani et al , 2022Gaches et al 2022). However, model L, which corresponds to a "low" CR spectrum based on Voyager 1 data, is found to underestimate the CRIR by almost an order of magnitude.…”

“…Brown crosses and gray stars are the high confidence fit and low confidence fit of ζ 2 toward nearby star-forming cloud-IC 348. Black dashed and black solid curves represent the polynomial fit to ζ 2 at different E(B − V ) for models L, H , and with low-energy spectral slope a = -1.2 by Padovani et al (2022).…”

“…In Figure 9, we plot the Luo et al (2023) ζ 2 toward IC 348 measured from HCO + and CO measurements, and the Indriolo & McCall (2012) ζ 2 from + H 3  measurements. For comparison, we also include the Padovani et al (2022) CR attenuation models L, H , and with low-energy spectral slope α = − 1.2. To plot ζ 2 as a function of E(B-V), we convert N H 2 to E(B-V) by assuming that the atomic fraction f atomic follows the power law: f atomic = 0.64/Av at Av > 1 mag (Luo et al 2023), and the f atomic = 0.64 17 at Av < 1 mag 18 .…”

Magnetically regulated collapse in the B335 protostar?

“…4.3). However, the range of derived values is much higher than those expected from Galactic CRs (see Appendix C in Padovani et al 2022, for an updated compilation of observational estimates of ζ CR ). Following McKee (1989), χ e = 1.3×10 −5 n(H 2 ) −1/2 , and for the volume densities derived in Sect.…”