Dense Clumps in Giant Molecular Clouds in the Large Magellanic Cloud: Density and Temperature Derived from ${}^{13}$CO($J=3-2$) Observations

Abstract

In order to precisely determine temperature and density of molecular gas in the Large Magellanic Cloud, we made observations of optically thin ${}^{13}$CO($J=3-2$) transition by using the ASTE 10m telescope toward 9 peaks where ${}^{12}$CO($J=3-2$) 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 HII regions only), and III (with HII regions and young star clusters). We detected ${}^{13}$CO($J=3-2$) emission toward all the peaks and found that their intensities are 3 -- 12 times lower than those of ${}^{12}$CO($J=3-2$). We determined the intensity ratios of ${}^{12}$CO($J=3-2$) to ${}^{13}$CO($J=3-2$), $R_{3-2}^{12/13}$, and ${}^{13}$CO($J=3-2$) to ${}^{13}$CO($J=1-0$), $R_{3-2/1-0}^{13}$, at 45$\arcsec$ resolution. These ratios were used for radiative transfer calculations in order to estimate temperature and density of the clumps. The parameters of these clumps range kinetic temperature $T{}_{\mathrm{kin}}$ = 15 -- 200 K, and molecular hydrogen gas density $n({\mathrm{H}}_2)$ = 8$\times 10^2$ -- 7$\times 10^3$ cm${}^{-3}$. We confirmed that the higher density clumps show higher kinetic temperature and that the lower density clumps lower kinetic temperature at a better accuracy than in the 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 $R_{3-2/1-0}^{13}$ and kinetic temperature of the clumps are well correlated with H$\alpha$ flux, suggesting that the heating of molecular gas $n({\mathrm{H}}_2)$ = $10^3$ -- $10^4$ cm${}^{-3}$ can be explained by stellar FUV photons.

39 pages, 7 figures, 4 tables. Accepted for publication in The Astronomical Journal

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