KOSMA 12CO(2–1) and (3–2) observations toward Infrared Dark Clouds

Abstract

Infrared Dark Clouds (IRDCs) are a valuable sample to study the initial conditions and the very early stages of the evolution of massive stars. Using the KOSMA telescope, we performed simultaneous single point 12CO(2–1) and (3–2) observations toward 117 IRDCs with a detection rate of 74 % for 12CO(2–1) and 58 % for 12CO(3–2). The non-detected sources are found not to represent real dark clouds. The distances of the IRDCs have been calculated using a Bayesian distance estimator indicating that the sources are mainly concentrated in the Outer, Perseus, Local, Sagittarius and Scutum arms. The excitation temperature and the filling factor have been solved simultaneously for the two emission lines. The average excitation temperature is about 10 K. The filling factors are used to correct the integrated intensity ratio of 12CO(3–2) to (2–1) resulting in a typical value of the ratio of $0.69\pm0.47$ for the majority of sources. A small number of sources exhibit enhanced values for the ratio because of the presence of a nearby HII region. The ratios do not show significant differences for sources with embedded far-infrared sources.