[论文解读] The protoplanetary system HD 100546 in H$α$ polarized light from SPHERE/ZIMPOL. A bar-like structure across the disk gap?

I. Mendigutía, R. D. Oudmaijer+8 more|White Rose Research Online (University of Leeds, The University of Sheffield, University of York)|Oct 31, 2017

Astrophysics and Star Formation Studies参考文献 84被引用 11

一句话总结

本研究利用SPHERE/ZIMPOL首次对HD 100546周围的原行星盘进行了高分辨率偏振Hα观测，揭示了在东北-西南方向穿过盘隙的一条20 au长的棒状结构。该结构被解释为外盘向内盘的尘埃和气体径向流入，可能由未见行星驱动，为年轻行星系统中小尺度吸积过程提供了罕见的直接视图。

ABSTRACT

HD 100546 is one of the few known pre-main-sequence stars that may host a planetary system in its disk. We analyze new VLT/SPHERE/ZIMPOL polarimetric images of HD 100546 with filters in H$α$ and the adjacent continuum. We have probed the disk gap and the surface layers of the outer disk, covering a region < 500 mas (< 55 au at 109 pc) from the star, at an angular resolution of ~ 20 mas. Our data show an asymmetry: the SE and NW regions of the outer disk are more polarized than the SW and NE. This can be explained from a preferential scattering angle close to 90$^o$, consistent with previous polarization images. The outer disk extends from 13 $\pm$ 2 to 45 $\pm$ 9 au, with a position angle and inclination of 137 $\pm$ 5$^o$ and 44 $\pm$ 8$^o$. The comparison with previous estimates suggests that the disk inclination could increase with the stellocentric distance, although the different measurements are still consistent within the error bars. In addition, no direct signature of the innermost candidate companion is detected from polarimetry, confirming recent results based on intensity imagery. We set an upper limit to its mass accretion rate < 10$^{-8}$ M$_{\odot}$/yr for a sub-stellar mass of 15M$_{Jup}$. Finally, we report the first detection (> 3$σ$) of a ~ 20 au bar-like structure that crosses the gap through the central region of HD 100546. It is tentatively suggested that the bar could be dust dragged by infalling gas that radially flows from the outer disk. This could represent an exceptional case in which a small-scale radial inflow is observed in a single system. If this scenario is confirmed, it could explain the presence of atomic gas in the inner disk that would otherwise accrete on to the central star on a timescale of a few months/years, as previously indicated from spectro-interferometric data, and could be related with additional (undetected) planets.

研究动机与目标

• 利用高空间分辨率偏振成像研究HD 100546外盘及盘隙区域的形态与偏振结构。
• 通过偏振成像评估最内层候选伴星HD 100546c的存在性及其吸积状态。
• 判断观测到的盘面不对称性与结构是否可由散射几何或动力学过程（如径向流入）解释。
• 评估所探测到的棒状结构对盘演化与行星形成的影响。

提出的方法

• 利用甚大望远镜（VLT）上的SPHERE/ZIMPOL仪器获取高空间分辨率的Hα及相邻连续谱偏振图像。
• 使用SPHERE/ZIMPOL数据处理管道进行数据还原，包括平场校正、天幕背景减除及偏振校准。
• 测量斯托克斯参数以推导线性偏振度，并推断散射几何。
• 从13 ± 2 au至45 ± 9 au映射盘面表层，确定位置角与倾角。
• 利用偏振约束为HD 100546c的吸积光度与质量吸积率设定上限。
• 分析盘隙区域的形态，以识别并表征棒状结构。

实验结果

研究问题

• RQ1 在高角分辨率下，HD 100546外盘及盘隙区域的形态与偏振结构如何？
• RQ2 最内层候选伴星HD 100546c是否可在偏振Hα光中被探测到？其吸积特性如何？
• RQ3 何种物理机制可解释观测到的穿越盘隙的棒状结构？
• RQ4 盘倾角如何随恒星距变化？这对盘几何结构有何含义？
• RQ5 该棒状结构是否代表了外盘向内盘的径向物质流入？这对吸积与行星形成有何影响？

主要发现

• 外盘从13 ± 2 au延伸至45 ± 9 au，位置角为137 ± 5°，倾角为44 ± 8°，与先前估计一致，但提示倾角可能随半径增加。
• 观测到显著的偏振不对称性，东南与西北区域的偏振度高于西南与东北区域，与约90°的优先散射一致。
• 未探测到最内层候选伴星HD 100546c的直接偏振信号，为其质量吸积率设定上限为< 10⁻⁸ M⊙ yr⁻¹（假设其为15 MJup的亚恒星质量）。
• 首次在偏振Hα光中探测到20 au长的棒状结构，其在东北-西南方向穿过盘隙，显著性超过3σ。
• 该棒状结构暂且被解释为外盘向内区的尘埃与气体径向流入，可能由未见行星驱动。
• 若该流入情景成立，可解释观测到的高恒星吸积率，并可能解释内盘中存在原子气体的现象，但需速度分辨观测以确认动力学特性。