Arecibo-Green Bank-LOFAR Carbon Radio Recombination Line observations
  toward cold HI Clouds
  We present results from a search for radio recombination lines in three HI
self-absorbing (HISA) clouds at 750 MHz and 321 MHz with the Robert C. Byrd
Green Bank Telescope (GBT), and in three Galactic Plane positions at 327 MHz
with the Arecibo Telescope. We detect Carbon Recombination Lines (CRRLs) in the
direction of DR4 and DR21, as well as in the galactic plane position
G34.94+0.0. We additionally detect Hydrogen Recombination Lines (HRRLs) in
emission in five of the six sightlines, and a Helium line at 750 MHz towards
DR21. Combining our new data with 150 MHz LOFAR detections of CRRL absorption
towards DR4 and DR21, we estimate the electron densities of the line forming
regions by modeling the line width as a function of frequency. The estimated
densities are in the range 1.4 $\rightarrow$ 6.5 cm$^{-3}$ towards DR4, for
electron temperatures 200 $\rightarrow$ 20 K. A dual line forming region with
densities between 3.5 $\rightarrow$ 24 cm$^{-3}$ and 0.008 $\rightarrow$ 0.3
cm$^{-3}$ could plausibly explain the observed line width as a function of
frequency on the DR21 sightline. The central velocities of the CRRLs compare
well with CO emission and HISA lines in these directions. The cloud densities
estimated from the CO lines are smaller (at least a factor of 5) than those of
the CRRL forming regions. It is likely that the CRRL forming and HI
self-absorbing gas is located in a denser, shocked region either at the
boundary of or within the CO emitting cloud.
