论文标题
使用球体/Zimpol
Inner dusty envelope of the AGB stars W~Hydrae, SW\,Virginis, and R~Crateris using SPHERE/ZIMPOL
论文作者
论文摘要
最初质量在$ \ sim0.8 $和8〜 $ m_ \ odot $之间的明星在渐近巨型分支（AGB）期间出现了大量的群众损失。公认的质量损失机制需要辐射压力作用于在延长的AGB恒星大气中形成的粉尘颗粒上。但是，此过程的细节尚未得到很好的理解。使用极端自适应的光学成像仪和偏光仪球/Zimpol，我们观察到光周围的晶粒极化，Hya，hya，sw \，vir和r \，r \，crt，它们的质量较低速率在10 $^{ - 7} $之间，10 $^{-6} 〜M_ \ odot〜m_ \ odot〜〜m_ \ odot〜我们发现尘埃在三个目标周围是不对称的分布。 R CRT可以看到双色形态，其位置角度与Maser发射和中红外连续发射的干涉测量值非常相似。双色流出的原因不能直接从Zimpol数据中推断出来。 R \，CRT和SW \，VIR，dust晶粒在0.65〜 $μ$ m的效率上更有效，W \，hya的0.82〜 $ m $ m。这表明在观察时，sw \，vir和r \周围的谷物的尺寸$ <0.1〜μ $ m，而w \周围的hya周围的谷物较大，尺寸较大，尺寸为$ \ gtrsim 0.1〜μ $ m。然而，CRT周围灰尘的不对称分布使该恒星的解释更加不确定。我们发现偏振光是从w〜hya的可见光球内产生的，我们使用具有光学厚至散射的内部灰尘壳模型来重现。在W \周围观察到的偏光光的径向剖面，HYA揭示了陡峭的灰尘密度曲线。我们发现W \，HYA的风加速区域至少扩展到$ \ sim 7〜r_ \ star $，与高达$ \ sim 12〜r_ \ star $的理论预测一致。
Stars with initial masses between $\sim0.8$ and 8~$M_\odot$ present copious mass loss during the asymptotic giant branch (AGB) at the end of their lives. The accepted mass-loss mechanism requires radiation pressure acting on dust grains that form in the extended AGB stellar atmospheres. The details of this process are not yet well understood, however. Using the extreme-adaptive-optics imager and polarimeter SPHERE/ZIMPOL, we observed light polarised by grains around W\,Hya, SW\,Vir, and R\,Crt, which have mass-loss rates between 10$^{-7}$ and 10$^{-6}~M_\odot~{\rm yr^{-1}}$. We find the distribution of dust to be asymmetric around the three targets. A biconical morphology is seen for R Crt, with a position angle that is very similar to those inferred from interferometric observations of maser emission and of mid-infrared continuum emission. The cause of the biconical outflow cannot be directly inferred from the ZIMPOL data. The dust grains polarise light more efficiently at 0.65~$μ$m for R\,Crt and SW\,Vir and at 0.82~$μ$m for W\,Hya. This indicates that at the time of the observations, the grains around SW\,Vir and R\,Crt had sizes $< 0.1~μ$m, while those around W\,Hya were larger, with sizes $\gtrsim 0.1~μ$m. The asymmetric distribution of dust around R\,Crt makes the interpretation more uncertain for this star, however. We find that polarised light is produced already from within the visible photosphere of W~Hya, which we reproduce using models with an inner dust shell that is optically thick to scattering. The radial profile of the polarised light observed around W\,Hya reveal a steep dust density profile. We find the wind-acceleration region of W\,Hya to extend to at least $\sim 7~R_\star$, in agreement with theoretical predictions of acceleration up to $\sim 12~R_\star$.
