arXiv 1809.004822018-09-03astro-ph.SRastro-ph.GA

用$^{13}$ CO探测原行星盘气体面密度分布

Probing the protoplanetary disk gas surface density distribution with $^{13}$CO emission

Anna Miotello, Stefano Facchini, Ewine F. van Dishoeck, Simon Bruderer

备注：: 14 pages, 10 figures, A&A accepted
发表信息：: A&A 619, A113 (2018)
DOI：: 10.1051/0004-6361/201833595

摘要：

在原行星盘中，约束气体表面密度分布σ_gas作为盘半径的函数是关键。在这项工作中，我们调查，如果空间分辨观测罕见的CO同位素可能是良好的示踪剂的Sigma_gas。运行具有不同输入Sigma_gas（R）的物理化学模型。输入盘表面密度分布与模拟的13 CO强度径向分布进行比较，以检查两者是否以及在何处相互遵循。在圆盘中总是存在中间区域，其中13 CO径向排放轮廓的斜率与Sigma_gas（R）重合。在小半径处，线径向轮廓低估了Sigma_gas，因为13 CO发射变得光学厚。同样的情况发生在大半径处，其中柱密度变得太低并且13 CO不能有效地自屏蔽。如果气体表面密度分布是一个简单的幂律半径，输入的幂律指数可以检索在20%的不确定性，如果一个选择适当的径向范围。相反，如果Sigma_gas（R）遵循粘性演化圆盘的自相似解，则检索输入幂律指数变得具有挑战性，特别是对于小圆盘。然而，它被发现，在任何情况下，幂律指数可以可靠地拟合在一个给定的线强度轮廓约6公里/秒，这产生了一个实用的方法来约束的斜率σ_gas（R）。这种方法的应用程序中示出的案例研究的TW Hya磁盘。空间分辨的13 CO谱线径向分布是探测盘表面密度分布的有效手段，因为它们可以直接跟踪在ALMA可以很好分辨的半径处的σ_gas（R）分布。在那里，像冻结和同位素选择性光解这样的化学过程不会影响排放，并且，假设挥发性碳不随半径变化，在解释观测结果时不需要化学模型。

英文摘要：

It is key to constrain the gas surface density distribution, Sigma_gas, as function of disk radius in protoplanetary disks. In this work we investigate if spatially resolved observations of rarer CO isotopologues may be good tracers of Sigma_gas. Physical-chemical models with different input Sigma_gas(R) are run. The input disk surface density profiles are compared with the simulated 13CO intensity radial profiles to check if and where the two follow each other. There is always an intermediate region in the disk where the slope of the 13CO radial emission profile and Sigma_gas(R) coincide. At small radii the line radial profile underestimates Sigma_gas, as 13CO emission becomes optically thick. The same happens at large radii where the column densities become too low and 13CO is not able to efficiently self-shield. If the gas surface density profile is a simple power-law of the radius, the input power-law index can be retrieved within 20% uncertainty if one choses the proper radial range. If instead Sigma_gas(R) follows the self-similar solution for a viscously evolving disk, retrieving the input power-law index becomes challenging, in particular for small disks. Nevertheless, it is found that the power-law index can be in any case reliably fitted at a given line intensity contour around 6 K km/s, and this produces a practical method to constrain the slope of Sigma_gas(R). Application of such a method is shown in the case study of the TW Hya disk. Spatially resolved 13CO line radial profiles are promising to probe the disk surface density distribution, as they directly trace Sigma_gas(R)profile at radii well resolvable by ALMA. There, chemical processes like freeze-out and isotope selective photodissociation do not affect the emission, and, assuming that the volatile carbon does not change with radius, no chemical model is needed when interpreting the observations.

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