Description
Low-energy cosmic rays (<1 TeV) are a pivotal source of ionisation of the interstellar medium, where they play a central role in determining the chemical gas composition and, in turn, in influencing the formation of stars and planets. Observations of H$_3^+$ absorption lines in diffuse clouds -- n(H$_2$)~10$^2$ cm$^{-3}$ -- have been used for decades to provide reliable estimates of the cosmic ray ionisation rate relative to molecular hydrogen (ζ$_{H2}$). However, in denser clouds where stars and planets form, this method is often inefficient since H$_3^+$, similar to H$_2$, does not emit rotational lines as it does not have a permanent electric dipole. The ζ$_{H2}$ estimates are, therefore, still provisional in this context, and represent one of the least understood ingredients when it comes to defining general models of star formation. Recently, a new analytical approach to estimate ζ$_{H2}$ in the densest regions of molecular clouds has been proposed by Bovino et al. (2020), based on observations of ortho-H$_2$D$^+$ as the main observational constraint to derive the amount of H$_3^+$. This has been applied by Sabatini et al. (2020) in a large sample of high-mass star-forming regions. Exploiting the exceptional observational capabilities of ALMA, in this talk I will present the first high-resolution maps of ζ$_{H2}$ in two massive clumps. I will present these results and the way they provide crucial constraints for the chemical/physical modelling of star-forming regions.
