The cooling of atomic and molecular gas in DR21

Abstract

Aims: We present an overview of a high-mass star formation region through the major (sub-)mm, and far-infrared cooling lines to gain insight into the physical conditions and the energy budget of the molecular cloud.
Methods: We used the KOSMA 3 m telescope to map the core (10'× 14') of the Galactic star-forming region DR21/DR21 (OH) in the Cygnus X region in the two fine structure lines of atomic carbon (C I ^3P_1-^3P₀ and ^3P_2-^3P_1), in four mid-J transitions of CO and ¹³CO, and in CS J=7-6. These observations were combined with FCRAO J=1-0 observations of ¹³CO and C¹⁸O. Five positions, including DR21, DR21 (OH), and DR21 FIR1, were observed with the ISO/LWS grating spectrometer in the [O I] 63 and 145 μm lines, the [C II] 158 μm line, and four high-J CO lines. We discuss the intensities and line ratios at these positions and apply the local thermal equilibrium (LTE) and non-LTE analysis methods in order to derive physical parameters such as mass, density and temperature. The CO line emission was modeled up to J=20.
Results: From non-LTE modeling of the low- to high-J CO lines, we identify two gas components, a cold one at temperatures of T_kin∼ 30-40 K and one with T_kin∼ 80-150 K at a local clump density of about n(H2) ~ 10^4-10⁶ cm^-3. While the cold quiescent component is massive, typically containing more than 94% of the mass, the warm, dense, and turbulent gas is dominated by mid- and high-J CO line emission and its large line widths. The medium must be clumpy with a volume-filling of a few percent. The CO lines are found to be important in cooling the cold molecular gas, e.g. at DR21 (OH). Near the outflow of the UV-heated source DR21, the gas cooling is dominated by line emission of atomic oxygen and of CO. Atomic and ionised carbon play a minor role.