Revealing the physical properties of molecular gas in Orion with a large scale survey in J=2-1 lines of 12CO, 13CO and C18O

TL;DR

This study maps and analyzes the physical properties of molecular gas in Orion's giant molecular clouds using high-resolution CO line observations, revealing temperature gradients, interactions with stellar winds, and a positive correlation between gas density and star formation activity.

Contribution

It provides the first large-scale, high-resolution survey of Orion's molecular gas using J=2-1 lines, combining data with J=1-0 lines to derive detailed physical conditions and their relation to star formation.

Findings

Gas density ranges from 500 to 5000 cm-3.

Gas temperature mostly between 20 to 50 K, with higher temperatures near HII regions.

Star formation efficiency correlates positively with gas density.

Abstract

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.