Session 97 - Galactic Structure, Galactic Center.
Oral session, Friday, January 09
Georgetown,

## [97.05] The 12CO/13CO Abundance Ratio in the Galactic Center's Circum-Nuclear Disk

A. I. Harris (U. Maryland), M. H. Heyer, J. E. Howe (U. Mass), R. Genzel (MPE), J. Zmuidzinas (Caltech)

We have measured the [^12CO]/[^13CO] abundance ratio in the Circumnuclear Disk, 1.5 pc from the center of the Galaxy. Observations of the CO J=6-5 short-submillimeter main and rarer isotopic lines are particularly clean probes of the isotope ratio in this molecular gas closest to the nucleus, gas deep within the central stellar cluster. Combined with millimeter and far-infrared CO line intensities, the new data constrain the [^12CO]/[^13CO] abundance ratio to be 24 \pm 2. Photodissociation and clumping make this a strict upper limit to the atomic [^12C]/[^13C] ratio. The ratio provides direct information on the connections between the molecular clouds within the Galaxy's innermost 200 pc and has implications for CO intensity to mass conversions in external galaxies.