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: τ12 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.
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