2018A&A...611A...6T

Context. Formaldehyde (H₂CO) is a reliable tracer to accurately measure the physical parameters of dense gas in star-forming regions. Aim. We aim to determine directly the kinetic temperature and spatial density with formaldehyde for the ∼100 brightest ATLASGAL-selected clumps (the TOP100 sample) at 870µm representing various evolutionary stages of high-mass star formation.
Methods. Ten transitions (J=3-2 and 4-3) of ortho- and para-H₂CO near 211, 218, 225, and 291 GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12m telescope.
Results. Using non-LTE models with RADEX, we derived the gas kinetic temperature and spatial density with the measured para-H₂CO 3₂₁-2₂₀/3₀₃-2₀₂, 4₂₂-3₂₁/4₀₄-3₀₃, and 4₀₄-3₀₃/3₀₃-2₀₂ ratios. The gas kinetic temperatures derived from the para-H₂CO 3₂₁-2₂₀/3₀₃-2₀₂ and 4₂₂-3₂₁/4₀₄-3₀₃ line ratios are high, ranging from 43 to >300K with an unweighted average of 91±4K. Deduced T_kin values from the J=3-2 and 4-3 transitions are similar. Spatial densities of the gas derived from the para-H₂CO 4₀₄-3₀₃/3₀₃-2₀₂ line ratios yield 0.6-8.3x10⁶cm^–3 with an unweighted average of 1.5 (±0.1)x10⁶cm^–3. A comparison of kinetic temperatures derived from para-H₂CO, NH₃, and dust emission indicates that para-H₂CO traces a distinctly higher temperature than the NH₃ (2, 2)/(1, 1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H₂CO line widths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H₂CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H₂CO increase with time through the evolution of the clumps. The high temperature of the gas traced by H₂CO may be mainly caused by radiation from embedded young massive stars and the interaction of outflows with the ambient medium. For L_bol/M_clump≥10L_☉/M_☉, we find a rough correlation between gas kinetic temperature and this ratio, which is indicative of the evolutionary stage of the individual clumps. The strong relationship between H₂CO line luminosities and clump masses is apparently linear during the late evolutionary stages of the clumps, indicating that L(H₂CO) does reliably trace the mass of warm dense molecular gas. In our massive clumps H₂CO line luminosities are approximately linearly correlated with bolometric luminosities over about four orders of magnitude in L_bol, which suggests that the mass of dense molecular gas traced by the H₂CO line luminosity is well correlated with star formation.