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arXiv 1801.01653astro-ph.GA
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ALMA揭示了大麦云星云中的分子云N55是大质量恒星形成的场所

ALMA reveals molecular cloud N55 in the Large Magellanic Cloud as a site of massive star formation

N Naslim, K. Tokuda, T. Onishi, F. Kemper, T. Wong, O. Morata, S. Takada, R. Harada, A. Kawamura, K. Saigo, R. Indebetouw, S. C. Madden, S. Hony, M. Meixner

摘要:

利用阿塔卡玛大毫米波阵的$^{12}$ CO(1-0)和$^{13}$ CO(1-0)观测资料,给出了大麦云云N55的分子云性质。我们对分子气体的性质进行了详细的研究,以了解N55的云性质与银河系云的不同。在N55中,大多数CO发射出现团簇,并且具有YSO的分子核显示出更大的线宽和质量。大质量团块与高质量和中等质量的YSO有关。团簇质量由局部热力学平衡和$^{12}$ CO和$^{13}$ CO排放的维里分析确定。这些质量估计得出的结论是:(a)团块处于自引力维里平衡,(b)$^{12}$ CO(1-0)到H $_2$的转换因子X $_{\rm CO}$为6.5 $\times$ 10 $^{20}$ cm $^{-2}$(K km s $^{-1}$)$^{-1}$。N55团簇的CO到H的$_2$转换因子在$\sim$ 0.67 pc的空间尺度下测量,这比猎户座云在类似空间尺度下的X $_{\rm CO}$值高出约两倍。N55的核心质量函数清楚地显示出低于200 M $_{\odot}$的周转,将低质量端与高质量端分开。$^{12}$ CO质谱的低质量端符合指数0.5 $\pm$ 0.1的幂律,而对于$^{13}$ CO,符合指数0.6 $\pm$ 0.2的幂律。在高质量端,核心质谱符合功率指数为2.0 $\pm$ 0.3的$^{12}$ CO,与2.5 $\pm$ 0.4的$^{13}$ CO。这种幂律行为的核心质量函数在N55是一致的许多银河系云。

英文摘要:

We present the molecular cloud properties of N55 in the Large Magellanic Cloud using $^{12}$CO(1-0) and $^{13}$CO(1-0) observations obtained with Atacama Large Millimeter Array. We have done a detailed study of molecular gas properties, to understand how the cloud properties of N55 differ from Galactic clouds. Most CO emission appears clumpy in N55, and molecular cores that have YSOs show larger linewidths and masses. The massive clumps are associated with high and intermediate mass YSOs. The clump masses are determined by local thermodynamic equilibrium and virial analysis of the $^{12}$CO and $^{13}$CO emissions. These mass estimates lead to the conclusion that, (a) the clumps are in self-gravitational virial equilibrium, and (b) the $^{12}$CO(1-0)-to-H$_2$ conversion factor, X$_{\rm CO}$, is 6.5$\times$10$^{20}$cm$^{-2}$(K km s$^{-1}$)$^{-1}$. This CO-to-H$_2$ conversion factor for N55 clumps is measured at a spatial scale of $\sim$0.67 pc, which is about two times higher than the X$_{\rm CO}$ value of Orion cloud at a similar spatial scale. The core mass function of N55 clearly show a turnover below 200M$_{\odot}$, separating the low-mass end from the high-mass end. The low-mass end of the $^{12}$CO mass spectrum is fitted with a power law of index 0.5$\pm$0.1, while for $^{13}$CO it is fitted with a power law index 0.6$\pm$0.2. In the high-mass end, the core mass spectrum is fitted with a power index of 2.0$\pm$0.3 for $^{12}$CO, and with 2.5$\pm$0.4 for $^{13}$CO. This power-law behavior of the core mass function in N55 is consistent with many Galactic clouds.

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