The CND appears as a large, asymmetric disk of warm molecular gas with a high CO J = 3-2/CO J = 1-0 intensity ratio exceeding 1.5. It would trigger a burst of star formation, and subsequent processes could enhance the mass accretion rate to SgrA*.Ībstract = "We report the results of millimeter and submillimeter molecular line mapping observations of the Galactic circumnuclear disk (CND). Toomre's Q parameter indicates that the CND is gravitationally stable now, but it will become unstable and fragment as gas accumulates at r ≃ 2pc. The progenitor GMC may have been formed recently inside the 120-pc ring, possibly by the encounter with the small inner bar of the Galaxy. Our results are consistent with the scenario that the CND has been formed by tidal capture and disruption of a giant molecular cloud (GMC). The bulk of the CND seems to have an overall, systematic infall motion, with a velocity of V infall ≃ 50kms -1. A one-zone large-velocity-gradient analysis finds that molecular gas in the CND has a typical kinetic temperature of T k ≃ 63K and H 2 density of n(H 2) ≃ 10 4.1cm -3. Line ratios suggest that the CND is chemically immature, having higher density and higher temperature than the ambient gas. These data confirm the entity of the CND, and the 2-pc ring is just a part of it. The CND can be clearly traced by the J = 1-0 lines of HCN, H 13CN, HCO +, and HNC, but it is barely traced by N 2H +, SiO, CCS, and HC 3N lines. It has a mass of (2-5) × 10 5 M ⊙ and a diameter of about 10pc, including a well-known 2-pc radius ring of dense molecular gas around the minispiral.
We report the results of millimeter and submillimeter molecular line mapping observations of the Galactic circumnuclear disk (CND).