2011ApJ...738...11H

Infrared Dark Clouds (IRDCs) are cold, high mass surface density and high density structures, likely to be representative of the initial conditions for massive star and star cluster formation. CO emission from IRDCs has the potential to be useful for tracing their dynamics, but may be affected by depleted gas phase abundances due to freeze out onto dust grains. Here we analyze C¹⁸O J = 1 {x2192} 0 and J = 2 {x2192} 1 emission line data, taken with the Instituto de Radioastronomia Milimetrica 30 m telescope, of the highly filamentary IRDC G035.39.-0033. We derive the excitation temperature as a function of position and velocity, with typical values of ∼7 K, and thus derive total mass surface densities, Σ_C18O, assuming standard gas phase abundances and accounting for optical depth in the line, which can reach values of ∼1. The mass surface densities reach values of ∼0.07 g/cm2. We compare these results to the mass surface densities derived from mid-infrared extinction mapping, Σ_SMF, by Butler & Tan, which are expected to be insensitive to the dust temperatures in the cloud. With a significance of ≳ 10σ, we find Σ_C18O/Σ_SMFdecreases by about a factor of five as Σ increases from ∼0.02 to ∼0.2 g/cm2, which we interpret as evidence for CO depletion. Several hundred solar masses are being affected, making this one of the most massive clouds in which CO depletion has been observed directly. We present a map of the depletion factor in the filament and discuss implications for the formation of the IRDC.