Mid-J CO shock tracing observations of infrared dark clouds II Low-J CO constraints on excitation, depletion, and kinematics

TL;DR

This study uses low-J CO line observations to analyze the physical conditions, depletion, and complex kinematics of infrared dark clouds, revealing significant CO freeze-out and dynamic evolution within these star-forming regions.

Contribution

It provides new observational constraints on CO excitation, depletion, and kinematic complexity in IRDCs using multi-line CO mapping and single-point measurements.

Findings

CO is significantly frozen out within IRDCs.

Maximum depletion factors range from 5 to 31.

IRDCs exhibit complex velocity structures and multiple components.

Abstract

Infrared dark clouds are kinematically complex molecular structures in the interstellar medium that can host sites of massive star formation. We present 4 square arcminute maps of the 12CO, 13CO, and C18O J = 3 to 2 lines from selected locations within the C and F (G028.37+00.07 and G034.43+00.24) infrared dark clouds (IRDCs), as well as single pointing observations of the 13CO and C18O J = 2 to 1 lines towards three cores within these clouds. We derive CO gas temperatures throughout the maps and find that CO is significantly frozen out within these IRDCs. We find that the CO depletion tends to be the highest near column density peaks, with maximum depletion factors between 5 and 9 in IRDC F and between 16 and 31 in IRDC C. We also detect multiple velocity components and complex kinematic structure in both IRDCs. Therefore, the kinematics of IRDCs seem to point to dynamically evolving structures yielding dense cores with considerable depletion factors.