Origin of CH+ in diffuse molecular clouds warm H2 and ion-neutral drift
  This paper assesses the roles of the presence of warm H2, and the increased
formation rate due to the ion-neutral drift. We performed ideal MHD simulations
that include the heating and cooling of the multiphase ISM, and where we treat
dynamically the formation of H2. In a post-processing step we compute the
abundances of species at chemical equilibrium. We show that CH+ is efficiently
formed at the edge of clumps, in regions where the H2 fraction is low, but
nevertheless higher than its equilibrium value, and where the gas temperature
is high. We show that warm and out of equilibrium H2 increases the integrated
column densities of CH+ by one order of magnitude, up to values still 3-10
times lower than those observed in the diffuse ISM. We balance the Lorentz
force with the ion-neutral drag to estimate the ion-drift velocities (vd). We
find that the vd distribution peaks around 0.04 km s-1, and that high vd are
too rare to have a significant statistical impact on the abundances of CH+.
Compared to previous works, our multiphase simulations reduce the spread in vd,
and our self-consistent treatment of the ionisation leads to much reduced vd.
Nevertheless, our resolution study shows that this velocity distribution is not
converged: the ion-neutral drift has a higher impact on CH+ at higher
resolution. On the other hand, our ideal MHD simulations do not include
ambipolar diffusion, which would yield lower drift velocities. Within these
limitations, we conclude that warm H2 is a key ingredient in the efficient
formation of CH+ and that the ambipolar diffusion has very little influence on
the abundance of CH+, mainly due to the small drift velocities obtained.
However, we point out that small-scale processes and other non-thermal
processes not included in our MHD simulation may be of crucial importance, and
higher resolution studies with better controlled dissipation processes are
needed.
