Cosmic rays explain the origin of atomic hydrogen in dark clouds

The formation of molecular hydrogen occurs on dust grains in molecular clouds through the reaction between two hydrogen atoms. Because this is an exothermic process, H₂ is then released into the gas phase. Depending on the position in the cloud (or on the amount of visual extinction measured inward from the cloud edge), two processes determine the destruction of H₂ and the restoration of the atomic form: photodissociation that is due to interstellar UV photons, and dissociation due to cosmic rays. In the diffuse part of molecular clouds, UV photons regulate the abundance of atomic hydrogen by dissociating H₂, while in the densest parts, interstellar UV photons are blocked by dust absorption as well as by H₂ line absorption. In the deepest parts of the cloud, cosmic rays dominate the destruction of molecular hydrogen.

Small amounts of atomic hydrogen, detected as absorption dips in the 21 cm line spectrum, are a well-known characteristic of dark clouds.
A wealth of studies has been carried out to characterise the origin of the atomic hydrogen component in dense environments [1,2,3,4,5], but the rate of cosmic-ray dissociation was always assumed to be constant (i.e., independent of the position in the cloud) or was simply neglected.

It is crucial to accurately determine the abundance of atomic hydrogen since it is the most mobile reactive species on the surface of bare dust grains and icy mantles. As a result, the atomic hydrogen fraction determines the production of species such as formaldehyde (H₂CO) and methanol (CH₃OH) from the hydrogenation of CO and ammonia (NH₃) from the hydrogenation of nitrogen.

A group of researchers - led by the Astrofit2 fellow Marco Padovani and including Daniele Galli of the Osservatorio Astrofisico di Arcetri - explored the role of cosmic rays in great detail, especially after the latest data release of the Voyager 1 spacecraft [6], which showed that the measured proton and electron fluxes are not able to explain the values of the cosmic-ray ionisation rate estimated in diffuse clouds [7].

Padovani et al. (2018) modelled the attenuation of the interstellar cosmic rays that enter a cloud and computed the dissociation rate of molecular hydrogen. They found that the cosmic-ray dissociation rate is entirely determined by secondary electrons produced in primary ionisation collisions. These secondary particles constitute the only source of atomic hydrogen at column densities above about 10²¹ cm^-2.

From comparison with observations, they concluded that a relatively flat spectrum of interstellar cosmic-ray protons, such as suggested by the most recent Voyager 1 data, can only provide a lower bound for the observed atomic hydrogen fraction. An enhanced spectrum of low-energy protons is needed to explain most of the observations, as predicted in their previous works [8,9,10,11].

This paper shows that a careful description of H₂ dissociation by cosmic rays can explain the abundance of atomic hydrogen in dark clouds. An accurate characterisation of this process at high densities is crucial for understanding the chemical evolution of star-forming regions.

Figure: Atomic hydrogen fraction vs. the total column density of hydrogen (bottom scale) and visual extinction (top scale). Observations from [4] are shown as solid orange circles. Coloured stripes represent the models by Padovani et al. (2018) for the case of photodissociation only (purple), a Voyager-like cosmic-ray flux (model L, black) and an enhanced flux at low energies (model H, grey). Dashed lines refer to the average value of the total volume density of hydrogen (suggested by [4]).

This article has been selected as an A&A Highlight: https://www.aanda.org/2018-highlights/1576

References:

[1] McCutcheon et al. (1978)
[2] van der Werf et al. (1988)
[3] Montgomery et al. (1995)
[4] Li & Goldsmith (2003)
[5] Goldsmith & Li (2005)
[6] Cummings et al. (2016)
[7] Indriolo et al. (2015)
[8] Padovani et al. (2009)
[9] Padovani et al. (2013)
[10] Ivlev et al. (2015)
[11] Padovani et al. (2018)

Marco Padovani acknowledges funding from the European Unions Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 664931.