Molecular ion abundances in the diffuse ISM : CF+, HCO+, HOC+, and C3H+
  The transition between atomic and molecular hydrogen is associated with
important changes in the structure of interstellar clouds, and marks the
beginning of interstellar chemistry. Because of the relatively simple networks
controlling their abundances, molecular ions are usually good probes of the
underlying physical conditions including for instance the fraction of gas in
molecular form or the fractional ionization. In this paper we focus on three
possible probes of the molecular hydrogen column density, HCO+, HOC+, and CF+.
We presented high sensitivity ALMA absorption data toward a sample of compact
HII regions and bright QSOs with prominent foreground absorption, in the ground
state transitions of the molecular ions HCO+, HOC+, and CF+ and the neutral
species HCN and HNC, and from the excited state transitions of C3H+(4-3) and
13CS(2-1). These data are compared with Herschel absorption spectra of the
ground state transition of HF and p-H2O. We show that the HCO+, HOC+, and CF+
column densities are well correlated with each other. HCO+ and HOC+ are tightly
correlated with p-H2O, while they exhibit a different correlation pattern with
HF depending on whether the absorbing matter is located in the Galactic disk or
in the central molecular zone. We report new detections of C3H+ confirming that
this ion is ubiquitous in the diffuse matter, with an abundance relative to H2
of ~7E-11. We confirm that the CF+ abundance is lower than predicted by simple
chemical models and propose that the rate of the main formation reaction is
lower by a factor of about 3 than usually assumed. In the absence of CH or HF
data, we recommend to use the ground state transitions of HCO+, CCH, and HOC+
to trace diffuse molecular hydrogen, with mean abundances relative to H2 of
3E-9, 4E-8 and 4E-11.
