Atlas of Cosmic ray-induced astrochemistry
  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.
