2017A&A...600A..16T

Context. The kinetic temperature of molecular clouds is a fundamental physical parameter affecting star formation and the initial mass function. The Large Magellanic Cloud (LMC) is the closest star-forming galaxy with a low metallicity and provides an ideal laboratory for studying star formation in such an environment.
Aims. The classical dense molecular gas thermometer NH₃ is seldom available in a low-metallicity environment because of photoionization and a lack of nitrogen atoms. Our goal is to directly measure the gas kinetic temperature with formaldehyde toward six star-forming regions in the LMC.
Methods. Three rotational transitions (J_KA_KC=3₀₃-2₀₂, 3₂₂-2₂₁, and 3₂₁-2₂₀) of para-H₂CO near 218GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12m telescope toward six star-forming regions in the LMC. These data are complemented by C¹⁸O 2-1 spectra.
Results. Using non-local thermal equilibrium modeling with RADEX, we derive the gas kinetic temperature and spatial density, using as constraints the measured para-H₂CO 3₂₁-2₂₀/3₀₃-2₀₂ and para-H₂CO 3₀₃-2₀₂/C¹⁸O 2-1 ratios. Excluding the quiescent cloud N159S, where only one para-H₂CO line could be detected, the gas kinetic temperatures derived from the preferred para-H₂CO 3₂₁-2₂₀/3₀₃-2₀₂ line ratios range from 35 to 63K with an average of 47±5K (errors are unweighted standard deviations of the mean). Spatial densities of the gas derived from the para-H₂CO 3₀₃-2₀₂/C¹⁸O 2-1 line ratios yield 0.4-2.9x10⁵cm^–3 with an average of 1.5±0.4x10⁵cm^–3. Temperatures derived from the para-H₂CO line ratio are similar to those obtained with the same method from Galactic star-forming regions and agree with results derived from CO in the dense regions (n(H₂)>10³cm^–3) of the LMC. A comparison of kinetic temperatures derived from para-H₂CO with those from the dust also shows good agreement. This suggests that the dust and para-H₂CO are well mixed in the studied star-forming regions. A comparison of kinetic temperatures derived from para-H₂CO 3₂₁-2₂₀/3₀₃-2₀₂ and NH₃(2, 2)/(1, 1) shows a drastic difference, however. In the star-forming region N159W, the gas temperature derived from the NH₃(2, 2)/(1, 1) line ratio is ∼16K (Ott et al., 2010ApJ...710..105O), which is only half the temperature derived from para-H₂CO and the dust. Furthermore, ammonia shows a very low abundance in a 30'' beam. Apparently, ammonia only survives in the most shielded pockets of dense gas that are not yet irradiated by UV photons, while formaldehyde, less affected by photodissociation, is more widespread and also samples regions that are more exposed to the radiation of young massive stars. A correlation between the gas kinetic temperatures derived from para-H₂CO and infrared luminosity, represented by the 250µm flux, suggests that the kinetic temperatures traced by para-H₂CO are correlated with the ongoing massive star formation in the LMC.