Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

Abstract

The kinetic temperature structure of the massive filament DR21 within the Cygnus X molecular cloud complex has been mapped using the IRAM\,30\,m telescope. This mapping employed the para-H$_2$CO triplet K_aK_c $, and 3$_ $) on a scale of sim 0.1\,pc. By modeling the averaged line ratios of $ and 3$_ $ with RADEX under non local thermodynamic equilibrium (LTE) assumptions, the kinetic temperature of the dense gas was derived, which ranges from 24 to 114\,K, with an average temperature of 48.3\,pm \,0.5\,K at a density of $. In comparison to temperature measurements using NH$_3$\,(1,1)/(2,2) and far-infrared (FIR) wavelengths, the para-H$_2$CO\,(3--2) lines reveal significantly higher temperatures. The dense clumps in various regions appear to correlate with the notable kinetic temperature kin $\,gtrsim \,50\,K) of the dense gas traced by H$_2$CO. Conversely, the outskirts of the DR21 filament display lower temperature distributions kin $\,$<$\,50\,K). Among the four dense cores (N44, N46, N48, and N54), temperature gradients are observed on a scale of sim 0.1-0.3\,pc. This suggests that the warm dense gas traced by H$_2$CO is influenced by internal star formation activity. With the exception of the dense core N54, the temperature profiles of these cores were fitted with power-law indices ranging from $-$0.3 to $-$0.5, with a mean value of approximately $-$0.4. This indicates that the warm dense gas probed by H$_2$CO is heated by radiation emitted from internally embedded protostar(s) and/or clusters. While there is no direct evidence supporting the idea that the dense gas is heated by shocks resulting from a past explosive event in the DR21 region on a scale of sim 0.1\,pc, our measurements of H$_2$CO toward the DR21W1 region provide compelling evidence that the dense gas in this specific area is indeed heated by shocks originating from the western DR21 flow. Higher temperatures as traced by H$_2$CO appear to be associated with turbulence on a scale of sim 0.1\,pc. The physical parameters of the dense gas as determined from H$_2$CO lines in the DR21 filament exhibit a remarkable similarity to the results obtained in OMC-1 and N113, albeit on a scale of approximately 0.1-0.4\,pc. This may imply that the physical mechanisms governing the dynamics and thermodynamics of dense gas traced by $CO in diverse star formation regions may be dominated by common underlying principles despite variations in specific environmental conditions.