[论文解读] Evidence for a massive dust-trapping vortex connected to spirals

P. Cazzoletti, E. F. van Dishoeck+7 more|arXiv (Cornell University)|Sep 11, 2018

Astrophysics and Star Formation Studies参考文献 79被引用 35

一句话总结

本研究利用高分辨率ALMA观测对原行星盘HD 135344B进行研究，发现其在毫米波段存在一个质量巨大且不对称的尘埃涡旋。该涡旋通过波长依赖的方位角位移和大颗粒增强得到证实，可能作为小行星的制造工厂，并可能驱动在消光光下观测到的螺旋臂结构，从而无需假定存在外侧行星即可解释其形态。

ABSTRACT

Context. Spiral arms, rings and large scale asymmetries are structures observed in high resolution observations of protoplanetary disks, and it appears that some of the disks showing spiral arms in scattered light also show asymmetries in millimeter-sized dust. HD 135344B is one such disk. Planets are invoked as the origin of these structures, but no planet has been observed so far and upper limits are becoming more stringent with time. Aims. We want to investigate the nature of the asymmetric structure in the HD 135344B disk in order to understand the origin of the spirals and of the asymmetry seen in this disk. Ultimately, we aim to understand whether or not one or more planets are needed to explain such structures. Methods. We present new ALMA sub-0.1′′ resolution observations at optically thin wavelengths (λ = 2.8 and 1.9 mm) of the HD 135344B disk. The high spatial resolution allows us to unambiguously characterize the mm-dust morphology of the disk. The low optical depth of continuum emission probes the bulk of the dust content of the vortex. Moreover, we have combined the new observations with archival data at shorter wavelengths to perform a multi-wavelength analysis and to obtain information about the dust distribution and properties inside the observed asymmetry. Results. We resolve the asymmetric disk into a symmetric ring + asymmetric crescent, and observe that (1) the spectral index strongly decreases at the centre of the vortex, consistent with the presence of large grains; (2) for the first time, an azimuthal shift of the peak of the vortex with wavelength is observed; (3) the azimuthal width of the vortex decreases at longer wavelengths, as expected for dust traps. These features allow confirming the nature of the asymmetry as a vortex. Finally, under the assumption of optically thin emission, a lower limit to the total mass of the vortex is 0.3 M Jupiter . Considering the uncertainties involved in this estimate, it is possible that the actual mass of the vortex is higher and possibly within the required values (~4 M Jupiter ) to launch spiral arms similar to those observed in scattered light. If this is the case, then explaining the morphology does not require an outer planet.

研究动机与目标

• 研究HD 135344B原行星盘中螺旋臂和尘埃不对称性的起源。
• 确定观测到的不对称性是由涡旋还是由行星引发的结构所致。
• 评估该涡旋是否足以解释在消光光下观测到的螺旋形貌。
• 估算涡旋的尘埃质量和物理特性，以评估其在行星形成中的作用。
• 通过多波长连续成分分析，解决盘结构解释中的歧义。

提出的方法

• 利用亚0.1′′分辨率的新ALMA Band 3（2.8 mm）和Band 4（1.9 mm）观测，解析尘埃形态。
• 将新数据与档案中的ALMA Band 7（343 GHz）和Band 9数据结合，进行多波长分析。
• 使用对称内环加外侧不对称、共半径月牙形（涡旋）成分的模型拟合uv平面数据。
• 通过在径向和方位角方向使用双高斯分布拟合数据，量化波长依赖的形态特征。
• 计算盘中各区域的谱指数α，以推断颗粒大小分布和光学厚度。
• 在光学薄假设下推导涡旋质量的下限，假设气尘质量比为10。

实验结果

研究问题

• RQ1 HD 135344B中不对称的毫米波尘埃发射最合理的解释是涡旋还是行星引发的结构？
• RQ2 观测到的峰值发射位置波长依赖性位移是否可由涡旋中的尘埃捕获解释？
• RQ3 该尘埃涡旋的质量是多少？其质量是否足以驱动盘中的螺旋密度波？
• RQ4 发射的方位角宽度如何随波长变化？这对应于颗粒大小和捕获机制有何含义？
• RQ5 该涡旋是否足以单独解释在消光光下观测到的螺旋臂，从而无需假定存在外侧行星？

主要发现

• 该不对称结构最适宜建模为一个对称内环与一个外侧共半径月牙形结构，证实了尘埃捕获涡旋的存在。
• 在涡旋区域内，谱指数α降低至约2，表明存在大尺寸（厘米级）颗粒，与尘埃捕获一致。
• 首次观测到峰值发射位置随波长单调变化的方位角位移，该现象与开普勒旋转不符，但与涡旋中颗粒尺寸依赖的捕获机制一致。
• 在较长波长（2.8 mm）下的发射比在较短波长（1.9 mm）下更集中于方位角方向，证实了尘埃捕获模型的预测：较大颗粒与气体的耦合更弱。
• 在光学薄假设下，涡旋质量下限为0.3 M_Jupiter，但若尘埃发射处于弱光学厚状态，其质量可能高达约1.7 M_Jupiter，接近驱动螺旋臂所需的质量。
• 盘中约一半的总尘埃质量被俘获在涡旋中，使其成为小行星形成的关键区域，可能无需依赖外侧行星即可解释观测到的不对称性。