Abstract

We have observed the Galactic center in 12CO(J = 2-1) and 13CO(J = 2-1) lines using the Tokyo-Onsala-ESO-Calán 60-cm telescope and compared the data with the 12CO(J = 1-0) Columbia survey (Bitran et al. 1997). The 12CO J = 2-1/J = 1-0 intensity ratio is very high (0.96) in the central 900 pc of the Galaxy, although it is about 0.6-0.7 for typical molecular clouds in the Galactic disk. The observed intensity ratios, 12CO J = 2-1/J = 1-0 and 13CO/ 12CO (J = 2-1), indicate that even the 12CO line is not very optically thick: ?12 CO(J=1-0) ? 1 or smaller in the Galactic center. This may be due to high temperature and large velocity dispersion. The large velocity dispersion is probably caused by large external pressure and tidal forces. © 1999 COSPAR. Published by Elsevier Science Ltd.