One Solution to the Mass Budget Problem for Planet Formation: Optically Thick Disks with Dust Scattering
One Solution to the Mass Budget Problem for Planet Formation: Optically Thick Disks with Dust Scattering
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DOI：
10.3847/2041-8213/ab1f8c
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发表时间：
2019-04
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影响因子：
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通讯作者：
Zhaohuan Zhu;Shangjia Zhang;Yan-Fei Jiang;A. Kataoka;T. Birnstiel;C. Dullemond;S. Andrews;Jane Huang;L. Pérez;J. Carpenter;X. Bai;D. Wilner;L. Ricci
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作者：
Zhaohuan Zhu;Shangjia Zhang;Yan-Fei Jiang;A. Kataoka;T. Birnstiel;C. Dullemond;S. Andrews;Jane Huang;L. Pérez;J. Carpenter;X. Bai;D. Wilner;L. Ricci
阿塔卡马大型毫米阵列（阿尔马）的调查表明，在假设毫米盘连续辐射是光学薄的情况下，II类盘中的尘埃可能不足以解释系外行星的平均固体质量。这种光学薄的假设似乎是支持最近的磁盘子结构在高角分辨率项目（DSHARP）的观测，其中测量的光学深度大多小于一。然而，我们指出，尘埃散射可以大大减少从光学厚区域的发射。如果忽略散射，则具有散射的光学厚盘可能被误认为光学薄盘。尘埃散射在更倾斜的圆盘上会进一步降低强度，使圆盘看起来更暗。在几个DSHARP盘中测量的光学深度为0.6，可以自然地用在1.25 mm处具有0.9的光学厚度的尘埃来解释。使用DSHARP不透明度，该光学厚度对应于具有0.1-1 mm的最大粒度（smax）的尘埃群。对于光学厚度的散射盘，测得的光谱指数α可以大于或小于2，这取决于尘埃的散射系数随波长增加还是减少。我们描述了这种光学厚散射的情况下，可以解释所观察到的缩放submm连续的大小和光度之间，并可能有助于缓解从偏振和尘埃连续测量的尘埃大小约束之间的紧张局势。我们认为，大量的盘质量可以隐藏在阿尔马观测和更长波长的观测（例如，甚大阵列或平方公里阵列），需要探测磁盘中的尘埃质量。
Atacama Large Millimeter Array (ALMA) surveys have suggested that the dust in Class II disks may not be enough to explain the averaged solid mass in exoplanets, under the assumption that the mm disk continuum emission is optically thin. This optically thin assumption seems to be supported by recent Disk Substructures at High Angular Resolution Project (DSHARP) observations where the measured optical depths are mostly less than one. However, we point out that dust scattering can considerably reduce the emission from an optically thick region. If that scattering is ignored, an optically thick disk with scattering can be misidentified as an optically thin disk. Dust scattering in more inclined disks can reduce the intensity even further, making the disk look even fainter. The measured optical depth of ∼0.6 in several DSHARP disks can be naturally explained by optically thick dust with an albedo of ∼0.9 at 1.25 mm. Using the DSHARP opacity, this albedo corresponds to a dust population with the maximum grain size (smax) of 0.1–1 mm. For optically thick scattering disks, the measured spectral index α can be either larger or smaller than 2 depending on whether the dust albedo increases or decreases with wavelength. We describe how this optically thick scattering scenario could explain the observed scaling between submm continuum sizes and luminosities, and might help ease the tension between the dust size constraints from polarization and dust continuum measurements. We suggest that a significant amount of disk mass can be hidden from ALMA observations and longer wavelength observations (e.g., Very Large Array or Square Kilometre Array) are desired to probe the dust mass in disks.