Extinction and dust/gas ratio in LMC molecular clouds
Extinction and dust/gas ratio in LMC molecular clouds
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DOI：
10.1051/0004-6361:20079151
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发表时间：
2008-06
影响因子：
6.5
通讯作者：
K. Dobashi;J. Bernard;A. Hughes;D. Paradis;W. Reach;A. Kawamura
中科院分区：
文献类型：
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作者：
K. Dobashi;J. Bernard;A. Hughes;D. Paradis;W. Reach;A. Kawamura
目标。本文的目的是通过比较近红外波长的消光图与Hi和CO观测所追踪的中性气体和分子气体的空间分布，来测量大麦哲伦星云(LMC)中的尘埃含量和分布。方法：研究方法。为了得到LMC的消光图，我们开发了一种新的方法来测量暗云的颜色过剩，使用2MASS全天观测。如果云被未变红的前景恒星严重污染，则测量颜色过剩的经典方法(包括NICE方法)往往会低估真实颜色过剩，就像LMC中的情况一样。我们提出了一种新的方法，该方法使用X百分位最红恒星的颜色，并且对这种污染具有很强的鲁棒性。使用这种方法，通过将观测到的颜色过剩作为所用百分位数的函数与模型预测的颜色过剩进行比较，可以推断出暗云相对于恒星分布的位置。结果。在得到的消光图的基础上，我们对一组暗分子云进行了关联分析。假定尘埃在中性相和分子相具有相似的红外吸收特性，我们推导出了吸收柱密度比Av/NH和CO-to-H2转换因子X_(CO)。我们发现，从LMC的外围到30度或恒星形成区，V/NH增大。这既可以用尘埃丰度的系统性增加来解释，也可以用一种额外的气体成分的存在来解释，这种气体成分没有被Hi或CO追踪，但与Hi分布密切相关。如果允许尘埃丰度变化，所选区域的X_(CO)因子比由CO数据的维里分析得出的X_(CO)因子低好几倍。这可能表明LMC中的分子云不受引力约束，或者它们受到相当大的外部压力的约束。然而，假设尘埃丰度为常值且存在一个附加的看不见的气体成分，则从吸收得到的X_(CO)值可以与维里结果相一致。这些结果与从漫反射远红外辐射得到的LMC的结果是一致的。
Aims. The goal of this paper is to measure the dust content and distribution in the Large Magellanic Cloud (LMC) by comparing extinction maps produced in the near-infrared wavelengths and the spatial distribution of the neutral and molecular gas, as traced by Hi and CO observations. Methods. In order to derive an extinction map of the LMC, we have developed a new method to measure the color excess of dark clouds, using the 2MASS all-sky survey. Classical methods to measure the color excess (including the NICE method) tend to underestimate the true color excess if the clouds are significantly contaminated by unreddened foreground stars, as is the case in the LMC. We propose a new method that uses the color of the X percentile reddest stars and which is robust against such contamination. Using this method, it is possible to infer the positions of dark clouds with respect to the star distribution by comparing the observed color excess as a function of the percentile used and that predicted by a model. Results. On the basis of the resulting extinction map, we perform a correlation analysis for a set of dark molecular clouds. Assuming similar infrared absorption properties for the dust in the neutral and molecular phases, we derive the absorption-to-column density ratio AV/NH and the CO-to-H2 conversion factor X_(CO). We show that AV/NH increases from the outskirts of the LMC towards the 30 Dor star-forming region. This can be explained either by a systematic increase of the dust abundance, or by the presence of an additional gas component not traced by Hi or CO, but strongly correlated to the Hi distribution. If dust abundance is allowed to vary, the derived X_(CO) factors for the selected regions are several times lower than those derived from a virial analysis of the CO data. This could indicate that molecular clouds in the LMC are not gravitationally bound, or that they are bounded by substantial external pressure. However, the X_(CO) values derived from absorption can be reconciled with the virial results assuming a constant value for the dust abundance and the existence of an additional, unseen gas component. These results are in agreement with those derived for the LMC from diffuse far-infrared emission.