MAPPING LARGE-SCALE CO DEPLETION IN A FILAMENTARY INFRARED DARK CLOUD

Abstract

Infrared Dark Clouds (IRDCs) are cold, high mass surface density and high\ndensity structures, likely to be representative of the initial conditions for\nmassive star and star cluster formation. CO emission from IRDCs has the\npotential to be useful for tracing their dynamics, but may be affected by\ndepleted gas phase abundances due to freeze-out onto dust grains. Here we\nanalyze C18O J=1-0 and J=2-1 emission line data, taken with the IRAM 30m\ntelescope, of the highly filamentary IRDC G035.39.-0033. We derive the\nexcitation temperature as a function of position and velocity, with typical\nvalues of ~7K, and thus derive total mass surface densities, Sigma_C18O,\nassuming standard gas phase abundances and accounting for optical depth in the\nline, which can reach values of ~1. The mass surface densities reach values of\n~0.07 g/cm^2. We compare these results to the mass surface densities derived\nfrom mid-infrared (MIR) extinction mapping, Sigma_SMF, by Butler & Tan, which\nare expected to be insensitive to the dust temperatures in the cloud. With a\nsignificance of >10sigma, we find Sigma_C18O/Sigma_SMF decreases by about a\nfactor of 5 as Sigma increases from ~0.02 to ~0.2 g/cm^2, which we interpret as\nevidence for CO depletion. Several hundred solar masses are being affected,\nmaking this one of the most massive clouds in which CO depletion has been\nobserved directly. We present a map of the depletion factor in the filament and\ndiscuss implications for the formation of the IRDC.\n