Can diffuse X-rays be important in driving photoionisation in molecular clouds?

Abstract

The ionisation balance in molecular clouds is regulated by several ionising sources, including cosmic rays, X-rays, and ultraviolet radiation. Their relative importance depends on the local physical conditions and on the shielding column density. We compute the contribution of the large-scale diffuse X-ray radiation field to the ionisation in molecular clouds in the absence of strong local X-ray sources, such as young stars. Our goal is to quantify its significance relative to the Galactic cosmic rays. Using measurements of diffuse X-ray emission from the first eROSITA all-sky survey, we estimate the depth-dependent X-ray ionisation rate in molecular clouds across different Galactic environments. We consider both the observed diffuse field and a deabsorbed field obtained by correcting for foreground Galactic attenuation using HI4PI line-of-sight column densities in a pixel-by-pixel framework. Diffuse Galactic X-rays are unlikely to dominate the ionisation balance in well-shielded molecular gas ($N_{\mathrm{H}}>5\times 10^{21}\mathrm{c}{\mathrm{m}}^{-2}$), where cosmic rays remain the primary ionising agent. At low column densities, the deabsorbed diffuse X-ray field produces ionisation rate of ${\zeta }_X\sim 10^{-18}{\mathrm{s}}^{-1}$ at $N_{\mathrm{H}}\sim 10^{19}\mathrm{c}{\mathrm{m}}^{-2}$, remaining below the lower end of Galactic cosmic ray ionisation rate ($\sim 10^{-17}{\mathrm{s}}^{-1}$). The X-ray contribution decreases rapidly with increasing shielding, falling to ${\zeta }_X\sim 10^{-19}{\mathrm{s}}^{-1}$ by $N_{\mathrm{H}}\sim 10^{21}\mathrm{c}{\mathrm{m}}^{-2}$ and to ${\zeta }_X\sim 5\times 10^{-21}{\mathrm{s}}^{-1}$ by $N_{\mathrm{H}}\sim 10^{22}\mathrm{c}{\mathrm{m}}^{-2}$. Diffuse X-ray may therefore contribute to the thermal and chemical structure of low-extinction material and should be considered when interpreting ionisation tracers in diffuse or cloud-envelope environments.

8 pages, 10 figures, accepted for publication in Astronomy & Astrophysics