[论文解读] Near-infrared scattered light properties of the HR4796A dust ring: a measured scattering phase function from 13.6deg to 166.6deg

J. Milli, A. Vigan+28 more|arXiv (Cornell University)|Jan 3, 2017

Stellar, planetary, and galactic studies被引用 41

一句话总结

本研究首次在13.6°至166.6°范围内对HR 4796 A尘环的近红外散射相函数进行了高精度测量，揭示了30°以下的强烈前向散射峰。数据表明尘埃颗粒可能为大尺寸团簇（约20 µm），使散射光形态与谱谱能量分布相一致，同时对系统内行星伴星设定了迄今为止最严格的约束。

ABSTRACT

HR4796A is surrounded by a debris disc, observed in scattered light as an inclined ring. Past observations raised several questions. First, a strong brightness asymmetry detected in polarized reflected light recently challenged our understanding of scattering by the dust particles in this system. Secondly, the morphology of the ring strongly suggests the presence of planets, although no planets have been detected to date. We obtained high-angular resolution coronagraphic images of the circumstellar environment around HR4796A with VLT/SPHERE during the commissioning of the instrument in May 2014 and during guaranteed-time observations in February 2015. The observations reveal for the first time the entire ring of dust, including the semi-minor axis that was previously hidden either behind the coronagraphic spot or in the speckle noise. We determine empirically the scattering phase function of the dust in the H band from 13.6deg to 166.6deg. It shows a prominent peak of forward scattering, never detected before, for scattering angles below 30deg. We analyse the reflectance spectra of the disc from the 0.95 to 1.6 microns, confirming the red colour of the dust, and derive detection limits on the presence of planetary mass objects. We confirm which side of the disc is inclined towards the Earth. The analysis of the phase function suggests that the dust population is dominated by particles much larger than the observation wavelength, of about 20 microns. Compact Mie grains of this size are incompatible with the spectral energy distribution of the disc, however the observed rise in scattering efficiency beyond 50deg points towards aggregates which could reconcile both observables. We do not detect companions orbiting the star but our high-contrast observations provide the most stringent constraints yet on the presence of planets responsible for the morphology of the dust.

研究动机与目标

• 以高精度测量HR 4796 A尘环在散射光中的形态与光度。
• 推导H波段尘埃的散射相函数（SPF）与谱反射率。
• 通过高对比度成像约束行星质量天体的存在。
• 研究能再现观测到的散射与热辐射特性的尘埃颗粒物理特性。
• 评估气体或行星扰动是否可解释环的偏心率与清晰边缘。

提出的方法

• 利用VLT/SPHERE在调试阶段（2014年5月）和保障时间观测阶段（2015年2月）获取了高角分辨率的日冕成像。
• 通过先进的光斑噪声抑制技术，测量了包括此前隐藏的半短轴在内的整个环的散射光强度。
• 通过分析强度随散射角的变化，构建了从13.6°到166.6°的散射相函数。
• 以50的光谱分辨率在0.95 µm至1.6 µm波段推导出反射率谱，证实尘埃呈红色。
• 应用IFS和IRDIS仪器的高对比度探测极限，将行星伴星的约束范围缩小至0.2″。
• 将观测到的SPF与SED与Mie散射和离散偶极子散射（DHS）模型进行比较，以推断颗粒特性。

实验结果

研究问题

• RQ1 HR 4796 A尘环在整个散射角范围内的真实散射相函数是什么？
• RQ2 何种物理特性（尺寸、形态）的尘埃颗粒能同时再现观测到的SPF与谱能量分布？
• RQ3 对系统内或附近行星伴星存在的最严格约束是什么？
• RQ4 在不假设环为略微光学厚的情况下，能否调和未偏振光中的亮度不对称性与偏振光观测结果？
• RQ5 观测到的环形态是否与行星扰动或原始气体激发一致？

主要发现

• 散射相函数在30°以下表现出显著的前向散射峰，此特征此前在该系统中从未被测量过。
• 相函数表现出两个不同区域：至45°为止为强烈前向散射，45°以上则散射效率平滑增加，表明后向散射占主导。
• 观测到的相函数与约20 µm的致密Mie或DHS颗粒不兼容，但与相同尺寸的大而多孔的团簇一致。
• 尘埃反射率谱呈红色，在0.95 µm至1.35 µm之间无明显吸收特征，证实了早期哈勃望远镜的观测结果。
• 未在系统内探测到伴星，但探测极限已提升至32–5 MJup（在0.2″处），排除了半长轴小于环内侧的行星质量大于2 MJup的可能性。
• 气尘比被约束在≤0.2以内，排除了原始气体作为环偏心率与清晰边缘起源的可能性。