CO and CS in the Magellanic Clouds: a χ2-analysis of multitransitional data based on the MEP radiative transfer model
CO and CS in the Magellanic Clouds: a χ2-analysis of multitransitional data based on the MEP radiative transfer model
复制标题
麦哲伦星云中的 CO 和 CS：基于 MEP 辐射传输模型的多重跃迁数据的 χ2 分析
DOI：
10.1051/0004-6361:20067034
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发表时间：
2007
影响因子：
6.5
通讯作者：
L. Johansson
中科院分区：
文献类型：
--
作者：
S. Nikolić;G. Garay;M. Rubio;L. Johansson
目标。我们的目标是通过建模和模拟来确定位于SMC不同环境中的分子气体的物理性质-从热H区附近到冷，静止的云-并与LMC中类似环境中发现的分子气体的性质进行比较。方法。我们观测了12 CO（1 - 0）、（2-1）、（3-2）、13 CO（1 - 0）、（2-1）和CS（2-1）、（3-2）对SMC中6个分子云的辐射谱线：n66、n88、Lirs 36、Lirs 49、Hodge 15和SMC- b1 #1。这些数据，以及LMC的三个云：30dor -10、n159 - w和n159 - s的已发表数据，被用于分析气体动力学温度、柱密度和表面填充因子，使用辐射传输方程的平均逃逸概率近似值。解使用$\chi^2$方法进行限制。结果。假设两个星系的[12 CO/ 13 CO]丰度比相似，我们发现SMC云的CO和CS柱密度比LMC云小一个数量级，反映了金属丰度的差异。我们的分析表明，在这两个星系中，12co / 13co同位素比值的下限为50。SMC云中CO发射的表面填充因子比LMC小一些，并且似乎随着紫外线辐射场的增加而减少，即恒星形成活动更活跃。一个简单的模型，假设一个具有均匀物理参数的球形云沉浸在CMB辐射场中，可以很好地拟合（假定）静止云SMC-B1#1和n159 - s的观测特性。对于考虑的所有其他云，该模型给出了较大的$\chi^2$值，强烈表明需要更复杂的模型。我们提出了一些双组分模型的结果，例如，对于Lirs 36，具有高光学深度的20 K气体和温度为100 K的密度较小的气体的混合物很好地再现了CO数据的主要特征。
Aims. Our goal is to determine the physical properties of molecular gas located in different environments of the SMC – from near the vicinity of hot H II regions to cold, quiescent clouds – via modelling and simulations, and compare with the properties of molecular gas found in similar environments in the LMC. Methods. We present observations of the 12 CO (1–0), (2–1), (3–2), 13 CO (1–0), (2–1), and CS (2–1), (3–2) line emission toward six molecular clouds in the SMC: N 66, N 88, Lirs 36, Lirs 49, Hodge 15, and SMC-B1#1. These data, as well as published data on three clouds of the LMC: 30 Dor-10, N 159-W, and N 159-S, are analysed to estimate gas kinetic temperatures, column densities, and surface filling factors using a Mean Escape Probability approximation of the radiative transfer equations. The solutions are restricted using the $\chi^2$ approach. Results. Assuming that the [ 12 CO/ 13 CO] abundance ratio is similar in both galaxies, we find that the CO and CS column densities of SMC clouds are a magnitude smaller than those of LMC clouds, mirroring the metallicity differences. Our analysis suggests the existence of a lower limit for the 12 CO/ 13 CO isotope ratio of 50 in both galaxies. The surface filling factors of the CO emission in the SMC clouds are a factor of a few smaller than in the LMC and seem to decrease with increasing UV radiation fields, i.e., more vigorous star formation activity. A simple model, which assumes a spherical cloud with uniform physical parameters immersed in the CMB radiation field, provides a reasonably good fit to the observed properties of the (supposedly) quiescent clouds SMC-B1#1 and N 159-S. For all other clouds considered, this model gives large values of $\chi^2$, strongly indicating the need for a more complex model. We present some results from 2-component modelling, e.g., for Lirs 36 a mixture of 20 K gas with high optical depth and a less dense gas with temperatures of 100 K reproduces well the main features of the CO data.