[论文解读] The circumstellar envelope around the S-type AGB star W Aql

S. Ramstedt, S. Mohamed+10 more|arXiv (Cornell University)|Sep 21, 2017

Stellar, planetary, and galactic studies参考文献 43被引用 21

一句话总结

本研究利用ALMA对S型渐近巨星支星W Aquilae的原行星包层（CSE）进行高分辨率CO(J=3→2)发射观测，结合三维光滑粒子流体动力学（SPH）双星相互作用模型与辐射转移建模，表明主要的大尺度弧状结构（约10′′分离）与已知的遥远伴星（180 AU）的引力塑造一致；而靠近恒星的小尺度弧状结构（2–3′′）则呈不对称性，可能与未解析的内层结构或周期性质量流失有关，但其起源仍不确定。

ABSTRACT

Context. Recent observations at subarcsecond resolution, now possible also at submillimeter wavelengths, have shown intricate circumstellar structures around asymptotic giant branch (AGB) stars, mostly attributed to binary interaction. The results presented here are part of a larger project aimed at investigating the effects of a binary companion on the morphology of circumstellar envelopes (CSEs) of AGB stars. Aims. AGB stars are characterized by intense stellar winds that build CSEs around the stars. Here, the CO(J = 3 → 2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. Methods. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. Results. The estimated average mass-loss rate of W Aql is Ṁ = 3.0 × 10-6M⊙ yr-1 and agrees with previous results based on single-dish CO line emission observations. The size of the emitting region is consistent with photodissociation models. The inner 10′′ of the CSE is asymmetric with arc-like structures at separations of 2−3′′ scattered across the denser sections. Further out, weaker spiral structures at greater separations are found, but this is at the limit of the sensitivity and field of view of the ALMA observations. Conclusions. The CO(J = 3 → 2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of ~10′′ and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yr) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.

研究动机与目标

• 通过以W Aquilae为案例研究，理解双星伴星如何塑造S型渐近巨星支星的原行星包层（CSE）。
• 利用ALMA CO(J=3→2)数据与单天线约束，确定W Aql的平均质量流失率。
• 调查CSE中观测到的弧状结构是否由已知伴星引起，或由其他过程导致。
• 基于ALMA的灵敏度与分辨率，评估双星诱导的不对称性在CSE中的可探测性极限。
• 探讨轨道偏心率与质量流失变异性在塑造不对称CSE特征中的作用。

提出的方法

• 获取了针对W Aql的高分辨率ALMA亚角秒CO(J=3→2)发射观测数据。
• 利用一维辐射转移建模对CO谱线发射进行建模，结合ALMA的亮度分布与先前的单天线测量结果，约束平均质量流失率。
• 构建了与W Aql轨道参数一致的三维SPH双星相互作用模型（分离距离~180 AU，伴星质量~1.04–1.09 M⊙），测试两种轨道偏心率（e=0.2与e=0.6）。
• 对SPH模拟结果进行三维辐射转移建模，生成可观测的亮度图，并与ALMA数据进行比较。
• 模拟干涉响应，评估在不同轨道与质量流失条件下，CSE中密度对比的可探测性。
• 利用偏振尘埃散射光图像（来自PolCor）对比尘埃与气体的形态。

实验结果

研究问题

• RQ1 W Aql的平均质量流失率是多少？与以往单天线测量结果相比如何？
• RQ2 观测到的CO(J=3→2)发射中的弧状结构是否由已知双星伴星引起？哪些轨道参数与数据一致？
• RQ3 为何在不同分离距离处存在两种明显的弧状图案（2–3′′与~10′′）？何种物理过程可解释小尺度不对称性？
• RQ4 已知伴星的轨道偏心率如何影响ALMA观测中CSE特征的可探测性与形态？
• RQ5 ALMA在多大程度上能解析或探测到分离距离超过180 AU的双星伴星所诱导的密度对比？

主要发现

• W Aql的平均质量流失率估计为3.0×10⁻⁶ M⊙ yr⁻¹，与以往单天线CO谱线观测结果一致。
• CO(J=3→2)发射主要由平滑对称的成分主导，其上叠加了弱的弧状结构，分别位于约10′′与2–3′′的分离距离处。
• 大尺度弧状结构（~10′′分离）与假设低偏心率轨道（e≈0.2）的双星相互作用模型一致，表明其极有可能由180 AU外的已知伴星引起。
• 小尺度弧状结构（2–3′′分离）呈不对称性，且在空间上与此前在恒星西南侧映射到的尘埃增强区重合，但其形成 timescale（约200年）与系统中已知过程不匹配。
• 已知伴星的分离距离过大，无法产生强烈的密度对比，即使更远的分离距离也难以被ALMA探测，除非轨道偏心率极高或质量流失率显著增加。
• 内层CSE显示出不对称性与可能的结构破坏迹象，或暗示存在一颗更近的第三颗伴星，但此推测仍不明确，需更深入监测。