The CO J=2-1/J=1-0 ratio in the Large Magellanic Cloud

We observed 34 positions throughout the disk of the Large Magellanic Cloud in the CO J = 2-1 emission line with the 60 cm radio telescope. Comparing the spectra with Tokyo-Onsala-ESO-Calan those of the J = 1-0 line at the same angular resolution (similar or equal to 9', or 130 pc at 50 kpc), we found that the CO J = 2-1/J = 1-0 intensity ratio (R2-1/1-0) scatters in a range of 0.5-1.3. The luminosity ratio aver aged for all observed points is 0.92 +/- 0.05. The ratio R2-1/1-0 is approximately unity (0.95 +/- 0.06) in 30 Dor, consistent with optically thick and thermalized emission, even in the southern part where massive star formation does not occur yet. This suggests that the high R2-1/1-0 is not primarily due to the UV radiation from young stars but rather to the intrinsic nature of the molecular gas that is relatively dense (greater than or similar to 10(3) cm(-3)) and may be ready to form stars. In addition to a cloud-to-cloud difference of R2-1/1-0, there exists a radial gradient of the ratio of 0.94 +/- 0.11 in the inner region (less than or similar to2 kpc from the kinematic center) and 0.69 +/- 0.11 in the outer region (greater than or similar to2 kpc from the center, excluding the 30 Doradus complex). The higher R2-1/1-0 in the inner galaxy might be due to relatively higher gas densities within CO clumps in molecular clouds and/or higher external heating in that region.