Molecules in the Circumnuclear Disk of the Galactic Center

Abstract

Within a few parsecs around the central Black Hole Sgr A*, chemistry in the dense molecular cloud material of the circumnuclear disk (CND) can be affected by many energetic phenomena such as high UV-flux from the massive central star cluster, X-rays from Sgr A*, shock waves, and an enhanced cosmic-ray flux. Recently, spectroscopic surveys with the IRAM 30 meter and the APEX 12 meter telescopes of substantial parts of the 80--500 GHz frequency range were made toward selected positions in and near the CND. These datasets contain lines from the molecules HCN, HCO$^+$, HNC, CS, SO, SiO, CN, H$_2$CO, HC$_3$N, N$_2$H$^+$, H$_3$O$^+$ and others. We conduct Large Velocity Gradient analyses to obtain column densities and total hydrogen densities, $n$, for each species in molecular clouds located in the southwest lobe of CND. The data for the above mentioned molecules indicate 10$^5\,$cm$^{-3} \lesssim n <10^6\,$cm$^{-3}$, which shows that the CND is tidally unstable. The derived chemical composition is compared with a chemical model calculated using the UCL\_CHEM code that includes gas and grain reactions, and the effects of shock waves. Models are run for varying shock velocities, cosmic-ray ionization rates, and number densities. The resulting chemical composition is fitted best to an extremely high value of cosmic-ray ionization rate $\zeta \sim 10^{-14}\,$s$^{-1}$, 3 orders of magnitude higher than the value in regular Galactic molecular clouds, if the pre-shock density is $n=10^5\,$cm$^{-3}$.