GREAT confirms transient nature of the circum-nuclear disk

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: mrequena@mpifr-bonn.mpg.de
² Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón Ajalvir, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ I. Physikalisches Institut der Universität zu Köln, Zülpicher Strağe 77, 50937 Köln, Germany

Received: 17 February 2012
Accepted: 27 March 2012

Abstract

We report SOFIA/GREAT, Herschel/HIFI, and ground-based velocity-resolved spectroscopy of carbon monoxide (CO) rotational transitions from J = 2–1 to J = 16–15 toward two positions in the circum-nuclear disk (CND) in our Galactic center. Radiative transfer models were used to derive information on the physical state of the gas traced by CO. The excitation of the CO gas cannot be explained by a single physical component, but is clearly the superposition of various warm gas phases. In a two-component approach, our large velocity gradient (LVG) analysis suggests high temperatures of  ~200 K with moderate gas densities of only  ~10^4.5 cm^-3 for the bulk of the material. A higher excited phase, carrying  ~20–30% of the column densities, is warmer (~300–500 K) but only slightly denser (~10^5.3 cm^-3). These densities are too low to self-stabilize the clumps against their high internal turbulence and fall below the Roche density (>10⁷ cm^-3) at 1.5 pc galactocentric distance. We conclude that the bulk of the material in the CND is not organized by self-gravity nor stable against tidal disruption, and must be transient.