Warm and Dense Molecular Gas in the N159 Region: 12CO J=4–3 and 13CO J=3–2 Observations with NANTEN2 and ASTE

Abstract

New 12CO J=4–3 and 13CO J=3–2 observations of the N159 region, an active site of massive star formation in the Large Magellanic Cloud, have been made with the NANTEN2 and ASTE sub-mm telescopes, respectively. The 12CO J=4–3 distribution is separated into three clumps, each associated with N159W, N159E and N159S. These new measurements toward the three clumps are used in coupled calculations of molecular rotational excitation and line radiation transfer, along with other transitions of the 12CO J=1–0, J=2–1, J=3–2, and J=7–6 as well as the isotope transitions of 13CO J=1–0, J=2–1, J=3–2, and J=4–3. The 13CO J=3–2 data are newly taken for the present work. The temperatures and densities are determined to be 70-80K and 3×103 cm−3 in N159W and N159E and 30K and 1.6×103 cm−3 in N159S. Observed 12CO J=2–1 and 12CO J=1–0 intensities toward N159W and N159E are weaker than expected from calculations of uniform temperature and density, suggesting that low-excitation foreground gas causes self-absorption. These results are compared with the star formation activity based on the data of young stellar clusters and HII regions as well as the mid-infrared emission obtained with the Spitzer MIPS. The N159E clump is associated with embedded cluster(s) as observed at 24 μm by the Spitzer MIPS and the derived high temperature, 80K, is explained as due to the heating by these sources. The N159E clump is likely responsible for a dark lane in a large HII region by the dust extinction. On the other hand, the N159W clump is associated with embedded clusters mainly toward the eastern edge of the clump only. These clusters show offsets of 20′′ - 40′′ from the 12CO J=4–3 peak and are probably responsible for heating indicated by the derived high temperature, 70 K. The N159W clump exhibits no sign of star formation toward the 12CO J=4–3 peak position and its western region that shows enhanced R4−3/1−0 and R3−2/1−0 ratios. We therefore suggest that the N159W peak represents a pre-star-cluster core of 105M⊙ which deserves further detailed studies. We note that recent star formation took place between N159W and N159E as indicated by several star clusters and HII regions, while the natal molecular gas toward the stars have already been dissipated by the ionization and stellar winds of the OB stars. The N159S clump shows little sign of star formation as is consistent with the lower temperature, 30K, and somewhat lower density than N159W and N159E. The N159S clump is also a candidate for future star formation.