Atlas of Cosmic ray-induced astrochemistry

Abstract: Cosmic-rays are the primary initiators of interstellar chemistry, and getting a better understanding of the varying impact they have on the chemistry of interstellar clouds throughout the Milky Way will not only expand our understanding of interstellar medium chemistry in our own galaxy, but also aid in extra-galactic studies. This work uses the ALCHEMIC astrochemical modeling code to perform numerical simulations of chemistry for a range of ionization rates. We study the impact of variations in the cosmic ray ionization rate on molecular abundances under idealized conditions, given by constant temperatures and a fixed density of 1e4 cm-3. As part of this study we examine whether observations of molecular abundances can be used to infer the cosmic ray ionization rate in such a simplified case. We find that intense cosmic-ray ionisation results in molecules, in particular the large and complex ones, being largely dissociated, and the medium becoming increasingly atomic. Individual species have limitations in their use as probes of the cosmic ray ionization rate. At early time (<1 Myrs) ions such as N2H+ and HOC+ make the best probes, while at later times, neutral species such as HNCO and SO stand out, in particular due to their large abundance variations. It is however by combining species into pairs that we find the best probes. Molecular ions such as N2H+ combined with different neutral species can provide probe candidates that outmatch individual species, in particular N2H+/C4H, N2H+/C2H, HOC+/O and HOC+/HNCO. These still have limitations to their functional range, but are more functional as probes than individual species previously used.