Large-scale CO (J=4-3) Mapping toward the Orion-A Giant Molecular Cloud
  We have mapped the Orion-A Giant Molecular Cloud in the CO (J=4-3) line with
the Tsukuba 30-cm submillimeter telescope.The map covered a 7.125 deg^2 area
with a 9' resolution, including main components of the cloud such as Orion
Nebula, OMC-2/3, and L1641-N. The most intense emission was detected toward the
Orion KL region. The integrated intensity ratio between CO (J=4-3) and CO
(J=1-0) was derived using data from the Columbia-Univ. de Chile CO survey,
which was carried out with a comparable angular resolution. The ratio was
r_{4-3/1-0} ~ 0.2 in the southern region of the cloud and 0.4-0.8 at star
forming regions. We found a trend that the ratio shows higher value at edges of
the cloud. In particular the ratio at the north-eastern edge of the cloud at
(l, b) = (208.375 deg, -19.0 deg) shows the specific highest value of 1.1. The
physical condition of the molecular gas in the cloud was estimated by non-LTE
calculation. The result indicates that the kinetic temperature has a gradient
from north (Tkin=80 K) to south (20 K). The estimation shows that the gas
associated with the edge of the cloud is warm (Tkin~60 K), dense (n_{H_2}~10^4
cm^{-3}), and optically thin, which may be explained by heating and sweeping of
interstellar materials from OB clusters.
