Large-scale CO (J=4-3) Mapping toward the Orion-A Giant Molecular Cloud

TL;DR

This study mapped the Orion-A Giant Molecular Cloud in CO (J=4-3) line, revealing temperature, density, and optical depth variations across the cloud, especially at edges and star-forming regions, using large-scale submillimeter observations.

Contribution

First large-scale CO (J=4-3) map of Orion-A, providing detailed physical conditions and intensity ratios across the cloud with new insights into temperature and density gradients.

Findings

Higher CO (J=4-3)/(J=1-0) ratio at cloud edges

Temperature gradient from 80 K in the north to 20 K in the south

Edge regions are warm, dense, and optically thin

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 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.