[论文解读] The long-period Galactic Cepheid RS Puppis - II. 3D structure and mass of the nebula from VLT/FORS polarimetry

P. Kervella, A. Mérand+8 more|arXiv (Cornell University)|Mar 12, 2012

Stellar, planetary, and galactic studies参考文献 34被引用 6

一句话总结

本研究利用VLT/FORS2偏振测量重建了长周期造父变星RS Pup周围尘埃星云的三维结构，揭示出一个不规则、几何上薄的尘埃层，其并非以恒星为中心。推导出的尘埃质量为$2.9 \pm 0.9\,M_\odot$，总质量为$290 \pm 120\,M_\odot$，表明星云起源于星际云而非恒星质量损失，造父变星可能通过辐射压力或恒星风塑造了星云结构。

ABSTRACT

The long-period Cepheid RS Pup is surrounded by a large dusty nebula reflecting the light from the central star. Due to the changing luminosity of the central source, light echoes propagate into the nebula. This remarkable phenomenon was the subject of Paper I.The origin and physical properties of the nebula are however uncertain: it may have been created through mass loss from the star itself, or it could be the remnant of a pre-existing interstellar cloud. Our goal is to determine the 3D structure of the nebula, and estimate its mass. Knowing the geometrical shape of the nebula will also allow us to retrieve the distance of RS Pup in an unambiguous manner using a model of its light echoes (in a forthcoming work). The scattering angle of the Cepheid light in the circumstellar nebula can be recovered from its degree of linear polarization. We thus observed the nebula surrounding RS Pup using the polarimetric imaging mode of the VLT/FORS instrument, and obtained a map of the degree and position angle of linear polarization. From our FORS observations, we derive a 3D map of the distribution of the dust, whose overall geometry is an irregular and thin layer. The nebula does not present a well-defined symmetry. Using a simple model, we derive a total dust mass of M(dust) = 2.9 +/- 0.9 Msun for the dust within 1.8 arcmin of the Cepheid. This translates into a total mass of M(gas+dust) = 290 +/- 120 Msun, assuming a dust-to-gas ratio of 1.0 +/- 0.3 %. The high mass of the dusty nebula excludes that it was created by mass-loss from the star. However, the thinness nebula is an indication that the Cepheid participated to its shaping, e.g. through its radiation pressure or stellar wind. RS Pup therefore appears as a regular long-period Cepheid located in an exceptionally dense interstellar environment.

研究动机与目标

• 确定RS Pup周围包敷星云的三维几何结构，这对于通过光回波法实现精确距离测量至关重要。
• 估算星云中尘埃和气体的总质量，以约束其起源。
• 评估星云是否源于RS Pup的质量损失，或源于预先存在的星际云。
• 研究造父变星通过辐射压力或恒星风在多大程度上塑造了星云结构。
• 为未来利用光回波建模实现高精度距离测定提供几何基础。

提出的方法

• 利用VLT/FORS2获取成像偏振测量，通过半波片和沃拉斯顿棱镜测量星云中线性偏振的度和位置角。
• 将偏振特性映射，以推断尘埃颗粒相对于视线方向和天球平面的散射几何关系。
• 结合NTT/EMMI的直接成像数据（来自论文I）以约束尘埃的三维分布。
• 使用简单的散射模型，从偏振图推导出柱密度和尘埃质量。
• 假设尘气质量比为$1.0 \pm 0.3\%$，估算总气体与尘埃质量。
• 利用档案中的CFHT图像验证星云特征在数十年间保持稳定，以排除显著演化。

实验结果

研究问题

• RQ1 围绕RS Pup的尘埃星云具有怎样的三维结构？其是否关于恒星对称？
• RQ2 星云中尘埃和气体的总质量是多少？是否超过RS Pup可能喷射的质量？
• RQ3 基于其质量和几何结构，星云的起源是星际还是恒星起源？
• RQ4 造父变星在多大程度上通过辐射压力或恒星风影响了星云的形态塑造？
• RQ5 星云的几何结构能否用于通过光回波建模精确测定RS Pup的距离？

主要发现

• 星云表现出不规则且几何上薄的尘埃层，无中心对称性，表明其并非球对称质量损失的遗迹。
• 在RS Pup周围1.8角分范围内的总尘埃质量为$2.9 \pm 0.9\,M_\odot$，由偏振图与散射模型推导得出。
• 星云中气体与尘埃的总质量估计为$290 \pm 120\,M_\odot$，假设尘气质量比为$1.0 \pm 0.3\%$。
• 星云的高总质量排除了其起源于恒星质量损失的可能性，强烈支持其主要物质来自星际云。
• 缺乏中心对称性以及尘埃层的薄度表明，造父变星可能通过辐射压力或恒星风塑造了星云，尽管其仅贡献了极少部分质量。
• 星云并非RS Pup质量损失的产物，而更可能是一个致密的星际云，可能是其诞生云的残余物或临时嵌入结构，解释了此类造父变星-星云联结的稀有性。