Dense clumps in giant molecular clouds in the large magellanic cloud: Density and temperature derived from ¹³CO(J = 3-2) observations

In order to precisely determine the temperature and density of molecular gas in the Large Magellanic Cloud, we made observations of the optically thin ¹³CO(J = 3-2) transition using the ASTE 10 m telescope toward nine peaks where ¹²CO(J = 3-2) clumps were previously detected with the same telescope. The molecular clumps include those in giant molecular cloud (GMC) Types I (with no signs of massive star formation), II (with H II regions only), and III (with H II regions and young star clusters). We detected ¹³CO(J = 3-2) emission toward all the peaks and found that their intensities are 3-12 times lower than those of ¹²CO(J = 3-2). We determined the intensity ratios of ¹²CO(J = 3-2) to ¹³CO(J = 3-2), R ^12/13/ _3-2, and ¹³CO(J = 3-2) to ¹³CO(J = 1-0), R ¹³ _3-2/1-0, at 45″ resolution. These ratios were used in radiative transfer calculations in order to estimate the temperature and density of the clumps. The clumps have a kinetic temperature range of T _kin = 15-200K and a molecular hydrogen gas density range of n(H₂) = 8 × 10²-7 × 10 ³cm^-3. We confirmed that the higher density clumps have higher kinetic temperature and that the lower density clumps have lower kinetic temperature to better accuracy than in previous work. The kinetic temperature and density increase generally from a Type I GMC to a Type III GMC. We interpret that this difference reflects an evolutionary trend of star formation in molecular clumps. The R ¹³ _3-2/1-0 and kinetic temperature of the clumps are well correlated with the Hα flux, suggesting that the heating of molecular gas with density n(H₂) = 10³-10 ⁴cm^-3 can be explained by stellar far-ultravoilet photons.