Revealing the physical properties of molecular gas in Orion with a large
  scale survey in J=2-1 lines of 12CO, 13CO and C18O
  We present fully sampled ~3' resolution images of the 12CO(J=2-1),
13CO(J=2-1), and C18O(J=2-1) emission taken with the newly developed 1.85-m
mm-submm telescope toward the entire area of the Orion A and B giant molecular
clouds. The data were compared with the J=1-0 of the 12CO, 13CO, and C18O data
taken with the Nagoya 4-m telescope and the NANTEN telescope at the same
angular resolution to derive the spatial distributions of the physical
properties of the molecular gas. We explore the large velocity gradient
formalism to determine the gas density and temperature by using the line
combinations of 12CO(J=2-1), 13CO(J=2-1), and 13CO(J=1-0) assuming uniform
velocity gradient and abundance ratio of CO. The derived gas density is in the
range of 500 to 5000 cm-3, and the derived gas temperature is mostly in the
range of 20 to 50 K along the cloud ridge with a temperature gradient depending
on the distance from the star forming region. We found the high-temperature
region at the cloud edge facing to the HII region, indicating that the
molecular gas is interacting with the stellar wind and radiation from the
massive stars. In addition, we compared the derived gas properties with the
Young Stellar Objects distribution obtained with the Spitzer telescope to
investigate the relationship between the gas properties and the star formation
activity therein. We found that the gas density and star formation efficiency
are well positively correlated, indicating that stars form effectively in the
dense gas region.
