Kinetic temperature of massive star forming molecular clumps measured with formaldehyde. II. The Large Magellanic Cloud.

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 NH 3 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 K A K C = 3 03 –2 02 , 3 22 –2 21 , and 3 21 –2 20 ) of para-H 2 CO near 218 GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12 m telescope toward six star-forming regions in the LMC. These data are complemented by C 18 O 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-H 2 CO 3 21 –2 20 /3 03 –2 02 and para-H 2 CO 3 03 –2 02 /C 18 O 2–1 ratios. Excluding the quiescent cloud N159S, where only one para-H 2 CO line could be detected, the gas kinetic temperatures derived from the preferred para-H 2 CO 3 21 –2 20 /3 03 –2 02 line ratios range from 35 to 63 K with an average of 47 ± 5 K (errors are unweighted standard deviations of the mean). Spatial densities of the gas derived from the para-H 2 CO 3 03 –2 02 /C 18 O 2–1 line ratios yield 0.4–2.9 × 10 5 cm -3 with an average of 1.5 ± 0.4 × 10 5 cm -3 . Temperatures derived from the para-H 2 CO 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 (H 2 ) > 10 3 cm -3 ) of the LMC. A comparison of kinetic temperatures derived from para-H 2 CO with those from the dust also shows good agreement. This suggests that the dust and para-H 2 CO are well mixed in the studied star-forming regions. A comparison of kinetic temperatures derived from para-H 2 CO 3 21 –2 20 /3 03 –2 02 and NH 3 (2, 2)/(1, 1) shows a drastic difference, however. In the star-forming region N159W, the gas temperature derived from the NH 3 (2, 2)/(1, 1) line ratio is ~16 K (, ApJ, 710, 105), which is only half the temperature derived from para-H 2 CO 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-H 2 CO and infrared luminosity, represented by the 250 μ m flux, suggests that the kinetic temperatures traced by para-H 2 CO are correlated with the ongoing massive star formation in the LMC.