[论文解读] IRC+10216 mass loss properties through the study of $\lambda$3mm emission: Large spatial scale distribution of SiO, SiS, and CS

L. Velilla-Prieto, J. Cernicharo+5 more|arXiv (Cornell University)|Aug 15, 2019

Astrophysics and Star Formation Studies参考文献 62被引用 1

一句话总结

本研究利用ALMA和IRAM-30m在3 mm波段对IRC + 10°216的SiO、SiS和CS发射进行高分辨率观测，绘制了其包层中大尺度空间分布。结果揭示了具有振荡性分数丰度的团块状壳层结构，表明其在数百年间存在间歇性质量流失，且由于不均匀性导致紫外辐射穿透更深，使光致离解半径内移，挑战了均匀风模型。

ABSTRACT

The study of the gas in the envelopes surrounding asymptotic giant branch (AGB) stars through observations in the millimetre wavelength range provides information about the history and nature of these molecular factories. Here we present ALMA observations at subarsecond resolution, complemented with IRAM-30m data, of several lines of SiO, SiS, and CS towards the best-studied AGB circumstellar envelope, IRC+10216. We aim to characterise their spatial distribution and determine their fractional abundances mainly through radiative transfer and chemical modelling. The three species display extended emission with several enhanced emission shells. CS displays the most extended distribution reaching distances up to approximately 20''. SiS and SiO emission have similar sizes of approximately 11'', but SiS emission is slightly more compact. We have estimated fractional abundances relative to H$_2$, which on average are equal to f(SiO)$\sim$10$^{-7}$, f(SiS)$\sim$10$^{-6}$, and f(CS)$\sim$10$^{-6}$ up to the photo-dissociation region. The observations and analysis presented here show evidence that the circumstellar material displays clear deviations from an homogeneous spherical wind, with clumps and low density shells that may allow UV photons from the interstellar medium (ISM) to penetrate deep into the envelope, shifting the photo-dissociation radius inwards. Our chemical model predicts photo-dissociation radii compatible with those derived from the observations, although it is unable to predict abundance variations from the starting radius of the calculations ($\sim$10$R_{*}$), which may reflect the simplicity of the model. We conclude that the spatial distribution of the gas proves the episodic and variable nature of the mass loss mechanism of IRC+10216, on timescales of hundreds of years.

研究动机与目标

• 表征IRC + 10°216包层中SiO、SiS和CS的大尺度空间分布。
• 利用辐射转移和化学模型，确定这些分子相对于H₂的分数丰度。
• 研究由于团块结构和质量流失变化导致的偏离均匀球对称风模型的情况。
• 评估星际紫外辐射对包层中光致离解半径的影响。
• 评估当前气相化学模型在再现观测到的丰度振荡方面的局限性。

提出的方法

• 获取ALMA在3 mm波段对SiO、SiS和CS多个转动能级跃迁的亚角秒分辨率观测。
• 利用高灵敏度IRAM-30m单天线观测补充ALMA数据，以恢复延伸发射。
• 执行辐射转移建模，从观测亮度分布推导径向丰度分布和光致离解半径。
• 构建标准气相化学模型，包含光致引发和宇宙射线驱动的反应，以模拟分子丰度。
• 利用该模型预测光致离解半径，并与观测约束进行比较。
• 分析方位平均和通道图，以识别增强的发射壳层和非均匀结构。

实验结果

研究问题

• RQ1 IRC + 10°216包层中SiO、SiS和CS的大尺度空间分布如何？
• RQ2 SiO、SiS和CS的分数丰度如何随半径变化，它们揭示了怎样的质量流失历史？
• RQ3 团块和低密度壳层在多大程度上允许星际紫外光子穿透到包层更深处？
• RQ4 观测到的光致离解半径是否与标准化学模型的预测一致？
• RQ5 为何观测到的丰度分布显示出当前化学模型无法再现的振荡？

主要发现

• SiO、SiS和CS表现出延伸的壳层状发射结构，其中CS的分布最广，可达距离恒星约~20′′。
• SiO和SiS的发射更集中，尺寸约为~11′′，而SiS的发射比SiO略为集中。
• 平均分数丰度为f(SiO) ~ 10⁻⁷，f(SiS) ~ 10⁻⁶，f(CS) ~ 10⁻⁶（相对于H₂），直至光致离解区域。
• 光致离解半径顺序为SiS < SiO < CS，即SiS在最靠近恒星处离解，CS在最远处离解，与模型预测一致。
• 化学模型预测的光致离解半径比观测值大约1.5倍，表明模型过于简化。
• 观测到的丰度振荡和壳层结构表明存在数百年的间歇性质量流失，与恒定质量流失率模型不一致。