2017A&A...598A..30T

Context. For a general understanding of the physics involved in the star formation process, measurements of physical parameters such as temperature and density are indispensable. The chemical and physical properties of dense clumps of molecular clouds are strongly affected by the kinetic temperature. Therefore, this parameter is essential for a better understanding of the interstellar medium. Formaldehyde, a molecule which traces the entire dense molecular gas, appears to be the most reliable tracer to directly measure the gas kinetic temperature.
Aims. We aim to determine the kinetic temperature with spectral lines from formaldehyde and to compare the results with those obtained from ammonia lines for a large number of massive clumps.
Methods. Three 218GHz transitions (J_KA_KC=3₀₃-2₀₂, 3₂₂-2₂₁, and 3₂₁-2₂₀) of para-H₂CO were observed with the 15 m James Clerk Maxwell Telescope (JCMT) toward 30 massive clumps of the Galactic disk at various stages of high-mass star formation. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured para-H₂CO 3₂₂-2₂₁/3₀₃-2₀₂ and 3₂₁-2₂₀/3₀₃-2₀₂ ratios.
Results. The gas kinetic temperatures derived from the para-H₂CO (3₂₁-2₂₀/3₀₃-2₀₂) line ratios range from 30 to 61K with an average of 46±9K. A comparison of kinetic temperature derived from para-H₂CO, NH₃, and the dust emission indicates that in many cases para-H₂CO traces a similar kinetic temperature to the NH₃ (2, 2)/(1, 1) transitions and the dust associated with the HII regions. Distinctly higher temperatures are probed by para-H₂CO in the clumps associated with outflows/shocks. Kinetic temperatures obtained from para-H₂CO trace turbulence to a higher degree than NH₃ (2, 2)/(1, 1) in the massive clumps. The non-thermal velocity dispersions of para-H₂CO lines are positively correlated with the gas kinetic temperature. The massive clumps are significantly influenced by supersonic non-thermal motions.