[论文解读] An interferometric study of the post-AGB binary 89 Herculis. II Radiative transfer models of the circumbinary disk

M. Hillen, J. Menu+7 more|UvA-DARE (University of Amsterdam)|May 8, 2014

Astrophysics and Star Formation Studies参考文献 66被引用 22

一句话总结

本研究利用多波长干涉和测光数据的辐射转移建模，探究了后-AGB双星89 Herculis周围的原双星盘。结果表明，一个被动的、膨胀的盘，其尘埃颗粒为毫米级并已沉降，能够重现红外谱能分布（SED）和干涉测量数据，但无法解释观测到的光学散射光，暗示需要额外的散射成分——可能存在于双极喷流或晕中——以解释未分辨的光学通量。

ABSTRACT

The presence of disks and outflows is widespread among post-AGB binaries. In the first paper of this series, a surprisingly large fraction of optical light was found to be resolved in the 89 Her post-AGB system. The data showed this flux to arise from close to the central binary. Scattering off the inner rim of the circumbinary disk, or in a dusty outflow were suggested as two possible origins. With detailed dust radiative transfer models of the disk we aim to discriminate between these two configurations. By including Herschel/SPIRE photometry, we extend the SED such that it now fully covers UV to sub-mm wavelengths. The MCMax radiative transfer code is used to create a large grid of disk models. Our models include a self-consistent treatment of dust settling as well as of scattering. A Si-rich composition with two additional opacity sources, metallic Fe or amorphous C, are tested. The SED is fit together with mid-IR (MIDI) visibilities as well as the optical and near-IR visibilities of Paper I, to constrain the structure of the disk and in particular of its inner rim. The near-IR visibility data require a smooth inner rim, here obtained with a two-power-law parameterization of the radial surface density distribution. A model can be found that fits all the IR photometric and interferometric data well, with either of the two continuum opacity sources. Our best-fit passive models are characterized by a significant amount of mm-sized grains, which are settled to the midplane of the disk. Not a single disk model fits our data at optical wavelengths though, the reason being the opposing constraints imposed by the optical and near-IR interferometric data. A geometry in which a passive, dusty, and puffed-up circumbinary disk is present, can reproduce all the IR but not the optical observations of 89 Her. Another dusty, outflow or halo, component therefore needs to be added to the system.

研究动机与目标

• 确定被动的、膨胀的原双星盘是否能解释89 Herculis观测到的红外和光学干涉数据。
• 评估尘埃颗粒大小、成分和沉降在塑造谱能分布和可见度曲线中的作用。
• 确定论文I中检测到的异常大比例光学散射光（35%）的来源。
• 检验观测到的近红外和光学可见度是否能与单一盘几何结构相容。
• 约束解释光学通量所需额外成分（如喷流、晕）的必要性。

提出的方法

• 使用MCMax代码构建了包含自洽尘埃沉降和散射的3D尘埃辐射转移模型网格。
• 通过赫歇尔/SPIRE测光（200–600 μm）扩展谱能分布（SED），以约束颗粒大小分布。
• 将模型拟合至档案的MIDI中红外可见度、论文I中的光学和近红外可见度，以及宽带测光数据。
• 测试以富含硅的颗粒为基础，辅以金属铁或非晶碳作为额外消光源的尘埃成分。
• 采用双幂律径向表面密度分布以模拟内盘边缘，实现平滑的可见度曲线。
• 通过比较模拟的SED和可见度在紫外至亚毫米波段与观测数据的一致性，评估模型拟合效果。

实验结果

研究问题

• RQ1 具有沉降尘埃颗粒的被动、膨胀原双星盘是否能重现89 Herculis的完整SED和干涉测量数据？
• RQ2 为匹配观测到的红外发射和可见度曲线，需要何种颗粒大小和成分？
• RQ3 为何观测到的光学散射光超过仅由盘模型预测的通量？
• RQ4 近红外可见度曲线的平滑性是否与几何上厚实的、膨胀的盘一致？
• RQ5 为解释光学通量，需要何种额外成分（如喷流、晕），其位置可能在何处？

主要发现

• 具有毫米级沉降尘埃颗粒和双幂律表面密度分布的盘模型，成功重现了红外SED以及中红外至近红外的干涉测量数据。
• 近红外可见度数据要求内盘边缘平滑，这仅能通过双幂律径向分布实现。
• 没有任何被动盘模型能拟合光学干涉数据，因为它们仅预测约50%的观测光学散射通量。
• 最佳拟合模型要求大量毫米级颗粒沉降至盘中平面，与高效的颗粒增长和沉降一致。
• 光学通量的差异暗示必须存在额外的散射成分——可能位于双极喷流或晕中——且该成分必须位于视线方向上。
• 远红外发射主要由盘主导，而光学散射光可能由延伸的喷流或晕中的尘埃贡献，尽管确切几何结构仍无法确定。