Arecibo-Green Bank-LOFAR Carbon Radio Recombination Line observations toward cold HI Clouds

TL;DR

This study detects carbon, hydrogen, and helium radio recombination lines in cold HI clouds using GBT, Arecibo, and LOFAR, estimating electron densities and suggesting CRRLs originate in denser, possibly shocked regions within molecular clouds.

Contribution

First detection of multiple recombination lines in cold HI clouds at various frequencies, providing new insights into the physical conditions and structure of these regions.

Findings

CRRLs detected in specific clouds and galactic plane positions.

Electron densities estimated between 1.4 and 24 cm$^{-3}$.

CRRL forming regions are denser than CO-emitting gas, likely in shocked boundary regions.

Abstract

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.