[论文解读] An edge-on translucent dust disk around the nearest AGB star L2 Puppis - VLT/NACO spectro-imaging from 1.04 to 4.05 microns and VLTI interferometry

P. Kervella, M. Montargès+7 more|arXiv (Cornell University)|Apr 11, 2014

Stellar, planetary, and galactic studies被引用 20

一句话总结

本研究利用甚大望远镜/NAco光谱成像（1.04–4.05 μm）和甚大望远镜干涉仪（VLTI）观测，首次直接成像了距离最近的渐近巨星支（AGB）星L2 Puppis周围的一条边缘朝向、半透明的尘埃盘。在J波段高光学厚度、长波段透明度逐渐增加——并伴随L波段热辐射——的特征，通过蒙特卡罗辐射转移建模（RADMC-3D）成功再现，揭示了一个几何上厚实、边缘朝向的尘埃盘，可解释该恒星长期的光度变化，挑战了双星伴星假说，表明更可能是不对称的包层物质上可变照明效应所致。

ABSTRACT

As the nearest known AGB star (d=64pc) and one of the brightest (mK-2), L2 Pup is a particularly interesting benchmark object to monitor the final stages of stellar evolution. We report new lucky imaging observations of this star with the VLT/NACO adaptive optics system in twelve narrow band filters covering the 1.0-4.0 microns wavelength range. These diffraction limited images reveal an extended circumstellar dust lane in front of the star, that exhibits a high opacity in the J band and becomes translucent in the H and K bands. In the L band, extended thermal emission from the dust is detected. We reproduce these observations using Monte-Carlo radiative transfer modeling of a dust disk with the RADMC-3D code. We also present new interferometric observations with the VLTI/VINCI and MIDI instruments. We measure in the K band an upper limit to the limb-darkened angular diameter of theta_LD = 17.9 +/- 1.6 mas, converting to a maximum linear radius of R = 123 +/- 14 Rsun. Considering the geometry of the extended K band emission in the NACO images, this upper limit is probably close to the actual angular diameter of the star. The position of L2 Pup in the Herzsprung-Russell diagram indicates that this star has a mass around 2 Msun and is probably experiencing an early stage of the asymptotic giant branch. We do not detect any stellar companion of L2 Pup in our adaptive optics and interferometric observations, and we attribute its apparent astrometric wobble in the Hipparcos data to variable lighting effects on its circumstellar material. We however do not exclude the presence of a binary companion, as the large loop structure extending to more than 10 AU to the North-East of the disk in our L band images may be the result of interaction between the stellar wind of L2 Pup and a hidden secondary object. The geometric configuration that we propose, with a large dust disk seen almost edge-on, appears particularly favorable to test and develop our understanding of the formation of bipolar nebulae.

研究动机与目标

• 解析L2 Puppis（距离最近的AGB星）的包层环境，以理解其质量流失过程与光度变化机制。
• 确定Hipparcos星历表中观测到的天体测量摆动是否由双星伴星引起，或源于包层效应。
• 研究红外成像中观测到的延伸尘埃发射的几何与物理特性。
• 检验L2 Puppis长期光度变化是否由盘面中可变的尘埃消光所致的假设。

提出的方法

• 利用甚大望远镜/NAco自适应光学系统，在1.04–4.05 μm波段获取衍射极限、窄带成像。
• 通过VLTI/VINCI和MIDI进行干涉观测，测量恒星的边缘暗化角直径。
• 采用RADMC-3D代码进行蒙特卡罗辐射转移建模，模拟几何上厚实、边缘朝向的盘中尘埃的散射、吸收与热辐射。
• 将模型拟合至观测到的光谱能量分布与空间分辨图像，以约束尘埃密度、温度与盘面几何结构。
• 利用测得的角直径（17.9 ± 1.6 mas）推导出线性半径为123 ± 14 R☉，与恒星演化状态一致。
• 通过分析L波段图像中的不对称性，特别是10 AU环状结构，评估隐藏双星伴星的可能性。

实验结果

研究问题

• RQ1 导致L2 Puppis长期光度变化的包层物质的真实性质是什么？
• RQ2 Hipparcos星历表中观测到的141天天体测量摆动是由恒星伴星引起，还是由不对称包层尘埃的可变照明所致？
• RQ3 L波段中L2 Puppis周围延伸红外发射的几何与物理结构如何，特别是其特征？
• RQ4 观测到的波长依赖性光学厚度与透明度是否可由一个具有变化光学深度的单一边缘朝向尘埃盘解释？
• RQ5 隐藏伴星在塑造观测到的盘面形态与驱动质量流失不对称性方面可能扮演何种角色？

主要发现

• 甚大望远镜/NAco图像显示J波段存在高光学厚度尘埃带，H与K波段透明度提高，L波段可探测到热辐射。
• L2 Puppis的边缘暗化角直径被约束为17.9 ± 1.6 mas，对应线性半径123 ± 14 R☉。
• 使用RADMC-3D的蒙特卡罗辐射转移建模成功再现了波长依赖的形态与光谱能量分布，证实存在边缘朝向、几何上厚实的尘埃盘。
• 自约1995年以来的长期光深度最小值，最合理的解释是盘中一个致密、成团的区域穿越视线方向，与可变尘埃消光一致。
• NAco与VINCI观测中未检测到恒星伴星，且Hipparcos天体测量摆动归因于不对称包层尘埃的可变照明效应，而非轨道运动。
• L波段图像中10 AU环状结构暗示可能与隐藏伴星存在相互作用，支持双星系统可能主导盘结构形成的假说。