TUNA Lunch Talk:

Anish Roshi


Molecule formation in the Heeschen-Riegel-Crutcher Cloud

August 16

12:10PM, Room 230, NRAO, Edgemont Road


We report, for the first time, that the low frequency carbon recombination lines (CRRLs) from the innermost 10 degrees of the Galaxy arise in the Heeschan-Riegel-Crutcher (HRC) cloud. The HRC 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. 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 microns line whereas dominant heating process in the cloud interior is photoelectric emission. Constraints on the FUV flux (G0 4 to 7) falling on the HRC cloud are obtained by assuming thermal balance between the dominant heating and cooling processes. The H2 formation rate per unit volume in the cloud interior is 10-10 -- 10-12 s-1 cm-3, which far exceeds the H2 dissociation rate per unit volume. We conclude that the self-absorbing cold HI gas in the HRC 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 valuable insight into the nature of these cold gas.