2007A&A...461..999J

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. We used the KOSMA 3m telescope to map the core (10'x14') of the Galactic star-forming region DR21/DR21(OH) in the Cygnus X region in the two fine structure lines of atomic carbon (CI ³P₁-³P₀ and ³P₂-³P₁), 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 DR21FIR1, were observed with the ISO/LWS grating spectrometer in the [OI] 63 and 145µm lines, the [CII] 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. 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∼30-40K and one with T_kin∼80-150K at a local clump density of about n(H₂)∼10⁴-10⁶cm^–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.