Carbon Recombination Lines toward the Riegel-Crutcher Cloud and other
  Cold HI Regions in the inner Galaxy
  We report here, for the first time, the association of low frequency CRRL
with \HI\ self-absorbing clouds in the inner Galaxy and that the CRRLs from the
innermost $\sim 10^{\circ}$ of the Galaxy arise in the Riegel-Crutcher (R-C)
cloud. The R-C cloud is amongst the most well known of \HI\ self-absorbing
(HISA) regions located at a distance of about 125 pc in the Galactic centre
direction. Taking the R-C cloud as an example, we demonstrate that the physical
properties of the HISA can be constrained by combining multi-frequency CRRL and
\HI\ observations. The derived physical properties of the HISA cloud are used
to determine the cooling and heating rates. The dominant cooling process is
emission of the \CII\ 158 \mum line whereas dominant heating process in the
cloud interior is photoelectric emission. Constraints on the FUV flux (G0
$\sim$ 4 to 7) falling on the R-C cloud are obtained by assuming thermal
balance between the dominant heating and cooling processes. The H$_2$ formation
rate per unit volume in the cloud interior is $\sim$ 10$^{-10}$ -- 10$^{-12}$
s$^{-1}$ \cmthree, which far exceeds the H$_2$ dissociation rate per unit
volume. We conclude that the self-absorbing cold \HI\ gas in the R-C cloud may
be in the process of converting to the molecular form. The cold \HI\ gas
observed as HISA features are ubiquitous in the inner Galaxy and form an
important part of the ISM. Our analysis shows that combining CRRL and \HI\ data
can give important insight into the nature of these cold gas. We also estimate
the integration times required to image the CRRL forming region with the
upcoming SKA pathfinders. Imaging with the MWA telescope is feasible with
reasonable observing times.
