[论文解读] Non-stoichiometric amorphous magnesium-iron silicates in circumstellar dust shells. Dust growth in outflows from supergiants

H. P. Gail, Akemi Tamanai+2 more|arXiv (Cornell University)|Nov 11, 2020

Astrophysics and Star Formation Studies参考文献 123被引用 15

一句话总结

本研究通过建模九颗大质量超巨星周围星周尘埃壳层的红外辐射，探究富氧化合物喷流中尘埃的生长机制。利用观测光谱，研究发现非化学计量比的非晶态镁铁硅酸盐——连同刚玉和金属铁——最能重现观测数据。关键结果表明，硅酸盐尘埃在约920 K时于预先存在的刚玉（氧化铝）核上凝聚，且凝聚前沿附近的喷流速度较低，表明为亚声速膨胀，暗示对尘埃颗粒生长的定量理解正在形成。

ABSTRACT

We investigate the dust growth in oxygen-rich stellar outflows for a set of nine well-observed massive supergiants with optically thin dust shells. Models of the infrared emission from their circumstellar dust shells are compared to their observed infrared spectra so as to derive the essential parameters that govern dust formation in the extended envelope of these stars. The results obtained from the comparative study are also compared with the predictions of a model for silicate dust condensation solely based on laboratory data and basic stellar properties. The infrared emission in the wavelength range between 6 and 25 mu can be reproduced rather well by a mixture of non-stoichiometric iron-bearing silicates, alumina, and metallic iron dust particles for all nine objects. The observed spectra obtained from three objects, mu Cep, RW Cyg, and RS Per, can be reproduced by a stationary and (essentially) spherically symmetric outflow which enables a direct comparison with predictions from a theoretical dust growth model. The temperature at the onset of massive silicate dust growth is of the order of 920 K and the corresponding outflow velocity of the order of the sound velocity for these objects. The condensation temperature suggests that the silicate dust grows on the corundum dust grains that are formed well in the interior of the silicate dust shell at a much higher temperature. Our results propose that regarding the two major problems of dust formation in stellar outflows: (i) formation of seed nuclei; (ii) their growth to macroscopic dust grains, we are gradually coming close to a quantitative understanding of the second item.

研究动机与目标

• 理解大质量超巨星富氧化合物喷流中的物理条件与尘埃组成。
• 检验尘埃生长模型是否能定量重现观测到的红外发射光谱。
• 确定硅酸盐尘埃形成的起始冷凝温度与喷流速度。
• 评估种子核（如刚玉）在硅酸盐尘埃生长中的作用，以及瞬时冷凝近似法的有效性。
• 评估现有模型在多大程度上能定量解释恒星喷流中尘埃形成，尤其是颗粒生长过程。

提出的方法

• 使用九颗具有光学薄尘埃壳层的大质量超巨星的中红外光谱（6–25 µm）作为观测约束。
• 应用辐射转移模型，基于瞬时冷凝近似法，计算球对称、静止喷流中的尘埃发射。
• 引入实验室实验中新测得的非化学计量比非晶态镁铁硅酸盐（介于橄榄石与辉石之间）的光学常数。
• 通过混合尘埃组分（非化学计量比铁-bearing硅酸盐、刚玉（Al2O3）和金属铁）拟合模型光谱与观测光谱。
• 约束关键参数：冷凝温度、尘埃质量、颗粒大小及硅酸盐壳层内边界处的喷流速度。
• 通过独立观测数据（如双星系统α Sco中的紫外吸收线亏损因子）验证结果。

实验结果

研究问题

• RQ1 何种尘埃组成最能重现具有光学薄尘埃壳层的大质量超巨星的中红外发射光谱？
• RQ2 硅酸盐尘埃在喷流中于何种温度与速度下开始凝聚，且是否与理论预期一致？
• RQ3 瞬时冷凝近似法是否能充分描述这些喷流中的尘埃形成，特别是硅酸盐生长过程？
• RQ4 是否存在硅酸盐尘埃在预先存在的刚玉核上生长的证据，这对尘埃形成序列有何启示？
• RQ5 硅酸盐壳层内边界处的物理条件（温度、速度）与流体动力学预期相比如何？

主要发现

• 九颗大质量超巨星的中红外光谱（6–25 µm）可被非化学计量比非晶态镁铁硅酸盐、刚玉和金属铁的混合物良好拟合。
• 对于三颗天体（µ Cep、RW Cyg 和 RS Per），数据最佳符合在瞬时冷凝近似下的静止、球对称喷流模型。
• 大规模硅酸盐尘埃生长的起始温度约为920 K，略低于硅酸盐的蒸发温度。
• 硅酸盐凝聚前沿处的喷流速度接近声速，表明内壳层区域为亚声速膨胀。
• 较低的膨胀速度表明，尽管光谱线宽显示存在高速湍流，但硅酸盐壳层内部区域整体呈现平均亚声速流动。
• 结果表明，尘埃形成中的第二个主要难题——种子核生长为宏观颗粒——正趋于获得定量理解，特别是在氢富集环境中以中性物种为主导时。