A¹²CO,¹³CO, and C¹⁸O Survey of Infrared Dark Clouds

Infrared dark clouds (IRDCs) are extinction features against the Galactic infrared background, mainly in the mid-infrared band. Recently they were proposed to be potential sites of massive star formation. In this work we have made a¹²CO,¹³CO, and C¹⁸O (J = 1→ 0) survey of 61 IRDCs, 52 of which are in the first Galactic quadrant, selected from a catalog given by Simon and coworkers, while the others are in the outer Galaxy, selected by visually inspecting the MSX images. Detection rates in the three CO lines are 90%, 71%, and 62%, respectively. The distribution of IRDCs in the first Galactic quadrant is consistent with the 5 kpc molecular ring picture, while a slight trace of a spiral pattern is also noticeable, and needs to be further examined. The IRDCs have a typical excitation temperature of 10 K and typical column density of several 10²² cm⁻². Their typical physical size is estimated to be several parsecs using angular sizes from the Simon catalog. Typical volume density and typical LTE mass are ~5000 cm⁻³ and ~5000 M_☉, respectively. The IRDCs are in or near virial equilibrium. The properties of IRDCs are similar to those of star-forming molecular clumps, and they seem to be intermediate between giant molecular clouds and Bok globules; thus they may represent early stages of massive star formation.