Presentation #135.03 in the session Molecular Cloud Chemistry.
Infrared Dark Clouds (IRDCs) are cold (T < 25K), dense (n(H2) > 1e4 cm-3) and highly extinguished (Av >10 mag) structures of the Interstellar Medium (ISM), known to host the initial conditions of high-mass star and star cluster formation. It is thus important to understand their global physical and chemical properties to better trace the early stages of such star formation. Study of IRDC physical properties is often carried out using emission from CO, the most abundant molecule in the ISM gas phase after H2, and its optically thin isotopologues. Due to the very low temperatures typically of IRDCs, however, CO can easily freeze-out onto dust grains, which leads to heavy depletion of this species. Such depletion will consequently affect the measurement of an IRDC’s global properties. Here, we study the level of CO depletion in three IRDCs and how this depletion is affected by varying density and temperature. We have used observations of the 13CO J=1-0 (FUGIN-NRO-45m) and J=2-1 emission (IRAM-30m) to infer the levels of CO-depletion across three IRDCs, known to have different density and dust temperature conditions. By assuming the emission to be in local thermodynamical equilibrium (LTE), we obtained CO-based mass surface density maps across the IRDCs. We then compared these maps to independent measurements of cloud mass surface density, obtained from mid infrared (Spitzer) extinction and far infrared (Herschel) emission. From this comparison, we record CO depletion factors in the range ~1-5, with high CO depletion factors corresponding to low dust temperatures. Our work provides quantitative measures of CO depletion across the critical temperature range near 20 K, with implications for astrochemical models and measurements of IRDC kinematics and dynamics.