An analysis of HCN observations of the galactic centre's Circumnuclear Disk

The Circumnuclear Disk (CND) is a torus of molecular dust and gas rotating about the galactic centre and extending from 1.6pc to 7pc from the central massive black hole SgrA_. Observations of the CND in a number of transitions of HCN have shown the gas to be clumpy. The HCN(1-0) transition has been interpreted as being optically thick with molecular hydrogen number densities ' 107 cm-3 implying that the cores are tidally stable. Given this stability a predicted life for the disk of millions of years would allow star formation to occur through core condensation. Large Velocity Gradient modelling of the intensity lines of a number of selected HCN transitions is used to infer hydrogen density and HCN optical depth. The selection of HCN cores for (LVG) modelling requires identification of three transitions that share common locations and velocity spaces for valid comparisons and predictions of relevant parameter values. The geometry of the CND is explored as the first step in the core selection process. The projected co-ordinates and deprojected distances from SgrA listed in Christopher et al. (2005) are used to establish the disk's attitude relative to the plane of the sky, and deprojected co-ordinates that when plotted reveal a circular pattern of cores about a central cavity. A flat rotational velocity model compares modelled with observed HCN(1-0) core radial velocities that indicate eighteen out of twenty-six cores could be considered part of the CND. Previous studies suggest that HCN(1-0) is optically thick (_= 4) whereas the LVG modelling in this study suggests that the HCN(1-0) and H13CN(1-0) emission is optically thin with weakly inverted populations and _ (H12CN) _ -0.2. The excitation temperatures for H12CN and H13CN are markedly different, undermining earlier arguments for optically thick HCN(1-0). The molecular hydrogen density is found to range from 0.1 to 2 Å~ 106 cm-3, about an order of magnitude less than the previous estimate. This implies that the cores are tidally unstable and that the total mass of the disk is about 105M_ which is an order of magnitude lower than previous estimates based on HCN data and consistent with thermal emission from dust and dynamical arguments. Star formation within the disk therefore is not expected to occur without some significant “triggering” event.