The sub-mm J=6-5 line of 13CO in Orion

TL;DR

This study maps the sub-millimeter J=6-5 line of 13CO in Orion, revealing detailed structure and physical conditions of the hot core and surrounding dense gas, with implications for molecular gas properties in star-forming regions.

Contribution

First fully sampled sub-millimeter map of 13CO J=6-5 in Orion, providing new insights into the distribution and physical state of dense molecular gas.

Findings

Identified a compact maximum near the Orion Hot Core

Derived CO/H2 ratio lower than the canonical value

Mapped warm, dense gas distinct from PDRs

Abstract

We present a fully sampled map covering the Orion Hot Core and dense molecular ridge, in the sub-millimeter J=6-5 rotational transition of 13CO, at 0.45 mm with a resolution of 13 arcsec and 0.5 km s^-1. The map covers 3 arc min by 2arc min . The profile centered on the Hot Core peaks at 8.5 km s^-1 and has a peak intensity of 40 K, corrected antenna temperature. It shows line wings from 30 km s^-1 to -20 km s^-1. The map of intensity, integrated from 0 to +18 km s^-1, shows a prominent maximum <5 arcsec from the center of the Orion Hot Core. The FWHP is 3 arcsec, larger than the regions containing complex molecules. Single dish measurements of lines from the J=2-1 or J=1-0 transitions of CO isotopes show no such distinct maximum. Correcting for optical depth 1.5 in the J=6-5 line of 13CO, and assuming that the level populations are thermalized at 150 K, the beam averaged column density between 0 to +18 km s^-1 is N(13CO )=6.8 10^17 cm^-2 and N(CO)=5.2 10^19 cm-2. When combined with published dust emission data, the CO/ H2 number ratio is 2 {\cdot} 10^-5, a factor of ~5 lower than the canonical value, 10^-4. For the Orion South and Orion Ridge region, the column density of CO is <25% of that found for the Hot Core but CO/H2 ratios are similar. Models of Photodissociation Regions, PDRs, predict that CO lines from PDRs are only marginally optically thick. Thus our map traces warm and dense molecular gas rather than PDRs.