[论文解读] Exploring wind-driving dust species in cool luminous giants III. Wind models for M-type AGB stars: dynamic and photometric properties

Sara Bladh, S. Höfner+2 more|arXiv (Cornell University)|Jan 30, 2015

Stellar, planetary, and galactic studies参考文献 25被引用 45

一句话总结

本研究首次提出了M型渐近巨星支（AGB）恒星的时间依赖性辐射流体动力学模型，表明镁橄榄石（Mg2SiO4）尘埃颗粒对光子的散射可驱动真实的恒星风。模型成功再现了观测到的质量流失率、风速以及光学/近红外波段的光度测量结果，但除非添加少量铁以提高尘埃颗粒温度，否则模型中不会出现10和18 μm处的硅酸盐特征。

ABSTRACT

Stellar winds observed in asymptotic giant branch (AGB) stars are usually attributed to a combination of stellar pulsations and radiation pressure on dust. Shock waves triggered by pulsations propagate through the atmosphere, compressing the gas and lifting it to cooler regions, which create favourable conditions for grain growth. If sufficient radiative acceleration is exerted on the newly formed grains through absorption or scattering of stellar photons, an outflow can be triggered. Strong candidates for wind-driving dust species in M-type AGB stars are magnesium silicates (Mg$_2$SiO$_4$ and MgSiO$_3$). Such grains can form close to the stellar surface, they consist of abundant materials and, if they grow to sizes comparable to the wavelength of the stellar flux maximum, they experience strong acceleration by photon scattering. We use a frequency-dependent radiation-hydrodynamics code with a detailed description for the growth of Mg$_2$SiO$_4$ grains to calculate the first extensive set of time-dependent wind models for M-type AGB stars. The resulting wind properties, visual and near-IR photometry and mid-IR spectra are compared with observations.We show that the models can produce outflows for a wide range of stellar parameters. We also demonstrate that they reproduce observed mass-loss rates and wind velocities, as well as visual and near-IR photometry. However, the current models do not show the characteristic silicate features at 10 and 18 $μ$m as a result of the cool temperature of Mg$_2$SiO$_4$ grains in the wind. Including a small amount of Fe in the grains further out in the circumstellar envelope will increase the grain temperature and result in pronounced silicate features, without significantly affecting the photometry in the visual and near-IR wavelength regions.

研究动机与目标

• 测试在广泛的恒星参数范围内，Mg2SiO4尘埃颗粒对光子的散射是否能驱动M型AGB恒星的质量流失。
• 解决观测到的10和18 μm处硅酸盐特征与模型预测之间的差异，后者因尘埃颗粒温度过低而未表现出这些特征。
• 确定基于Mg2SiO4的风是否能再现以分子特征为主导、而非尘埃消光主导的观测到的光学和近红外波段光度变化。
• 通过对比合成光度与观测数据，评估无铁镁硅酸盐作为风驱动机制的可行性。
• 探索微量铁杂质对颗粒温度及中红外光谱特征的影响，同时不破坏光学与近红外波段的光度一致性。

提出的方法

• 采用频率依赖的辐射流体动力学代码，模拟M型AGB恒星的时间依赖性大气与风。
• 整合了Mg2SiO4颗粒的详细尘埃生长模型，考虑其尺寸演化与波长相关的消光截面。
• 在139组模型中变化恒星参数：光度为5000、7000和10000 L⊙，有效温度范围为2600 K至3200 K，脉动振幅各异。
• 计算合成的光学与近红外波段光度以及中红外波谱，以与观测结果对比。
• 测试在Mg2SiO4颗粒表面添加一层薄薄的MgFeSiO4包覆层的效果，以提高颗粒温度并诱导出硅酸盐特征。
• 采用正弦拟合观测光变曲线，将合成光度变化与实测数据进行比较。

实验结果

研究问题

• RQ1 Mg2SiO4尘埃颗粒对光子的散射是否能在M型AGB恒星中产生与观测一致的质量流失率和风速？
• RQ2 为何当前基于Mg2SiO4的模型无法再现AGB恒星中观测到的10和18 μm硅酸盐特征？
• RQ3 根据光度变化推断，M型AGB恒星的包层在光学与近红外波段的透明度如何？
• RQ4 在Mg2SiO4颗粒中引入少量铁是否显著改变颗粒温度与中红外光谱特征，同时不影响光学与近红外波段的光度？
• RQ5 结合详细尘埃生长过程的时间依赖性辐射流体动力学模型，能否同时再现观测到的光度与风动力学？

主要发现

• 模型在广泛的恒星参数范围内成功产生流出，包括光度为5000、7000和10000 L⊙，有效温度范围为2600 K至3200 K的情况。
• Mg2SiO4驱动的风模型能良好再现观测到的质量流失率与风速。
• 合成的光学与近红外波段光度与观测结果一致，表明包层在这些波段是透明的，与分子主导的通量变化一致。
• 模型中10和18 μm处缺乏硅酸盐特征的原因是Mg2SiO4颗粒在风中温度过低，而非尘埃丰度不足。
• 在Mg2SiO4颗粒上添加一层薄薄的MgFeSiO4包覆层，可显著提高颗粒温度，从而在中红外波段产生强硅酸盐特征，同时对光学与近红外波段光度影响甚微。
• 结果支持无铁Mg2SiO4尘埃颗粒对光子的散射作为有效的风驱动机制，而微量铁杂质可使中红外光谱与观测保持一致。