Large-scale CO (J = 4–3) mapping toward the Orion-A giant molecular cloud

Abstract 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 deg2 area with a 9′ resolution, including main components of the cloud such as the 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–Universidad de Chile CO survey, which was carried out with a comparable angular resolution. The ratio was r4−3/1−0 ∼ 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 values at the edges of the cloud. In particular, the ratio at the northeastern edge of the cloud at (l, b) ≈ (208$_{.}^{\circ}$375, −19$_{.}^{\circ}$0) shows the highest value of 1.1. The physical condition of the molecular gas in the cloud was estimated by non-LTE calculation. The result indicates that the kinetic temperature has a gradient from north (Tkin = 80 K) to south (20 K). The estimation shows that the gas associated with the edge of the cloud is warm (Tkin ∼ 60 K), dense ($n_{\mathrm{H_2}}\sim 10^{4}\:$cm−3), and optically thin, which may be explained by heating and sweeping of interstellar materials from OB clusters.