[论文解读] First spectro-interferometric survey of Be stars I. Observations and constraints on the disks geometry and kinematics

A. Meilland, F. Millour+6 more|arXiv (Cornell University)|Nov 10, 2011

Astrophysics and Star Formation Studies参考文献 36被引用 37

一句话总结

利用VLTI/AMBER对八颗明亮Be星进行的光谱干涉测量调查表明，其周围的星周盘主要呈开普勒旋转，旋转速度达到临界旋转速度的95%（Ω/Ωc = 0.95 ± 0.02），且恒星参数与盘结构之间无明显相关性。数据证实旋转主导了盘的动力学，尽管κ CMa和α Col中存在轻微不对称性，提示可能存在其他机制。

ABSTRACT

Context. Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed infrared-excess and emission lines. The phenomena involved in the disk formation still remain highly debated. Aims. To progress in the understanding of the physical process or processes responsible for the mass ejections and test the hypothesis that they depend on the stellar parameters, we initiated a survey on the circumstellar environment of the brightest Be stars. Methods. To achieve this goal, we used spectro-interferometry, the only technique that combines high spectral (R=12000) and high spatial ($ heta_{ m min}$=4\,mas) resolutions. Observations were carried out at the Paranal observatory with the VLTI/AMBER instrument. We concentrated our observations on the Br$\gamma$ emission line to be able to study the kinematics within the circumstellar disk. Our sample is composed of eight bright classical Be stars : $\alpha$ Col, $\kappa$ CMa, $\omega$ Car, p Car, $\delta$ Cen, $\mu$ Cen, $\alpha$ Ara, and extit{o} Aqr. Results. We managed to determine the disk extension in the line and the nearby continuum for most targets. We also constrained the disk kinematics, showing that it is dominated by rotation with a rotation law close to the Keplerian one. Our survey also suggests that these stars are rotating at a mean velocity of V/V$_{ m c}$\,=\,0.82\,$\pm$\,0.08. This corresponds to a rotational rate of $\Omega/\Omega_{ m c}$\,=\,0.95\,$\pm$\,0.02 Conclusions. We did not detect any correlation between the stellar parameters and the structure of the circumstellar environment. Moreover, it seems that a simple model of a geometrically thin Keplerian disk can explain most of our spectrally resolved K-band data. Nevertheless, some small departures from this model have been detected for at least two objects (i.e, $\kappa$ CMa and $\alpha$ Col). Finally, our Be stars sample suggests that rotation is the main physical process driving the mass-ejection. Nevertheless, smaller effects from other mechanisms have to be taken into account to fully explain how the residual gravity is compensated.

研究动机与目标

• 利用高分辨率光谱干涉测量研究经典Be星周围星周盘的几何结构与动力学特性。
• 检验质量抛射机制是否依赖于有效温度、旋转速度或光谱型等恒星参数。
• 确定驱动盘形成与稳定性的主导机制是旋转、辐射压力、脉动还是双星相互作用。
• 评估在K波段中探测极向风与非开普勒特征的可行性。

提出的方法

• 在VLTI上利用AMBER仪器进行光谱干涉观测，光谱分辨率为R = 12,000，角分辨率为4 mas。
• 以Brγ发射线为目标，探测盘的动力学，利用可见度和相位测量来建模盘结构。
• 采用简单的运动学模型：假设为几何薄盘、旋转盘，且为开普勒旋转，无径向外流。
• 通过盘的投影椭圆度估算恒星的倾角，从而推导出真实的旋转速度。
• 利用多个基线和位置角的干涉数据进行模型拟合。
• 后续工作中应用了理论模型（SIMECA、BEDISK）以进一步约束盘的质量、温度和密度分布。

实验结果

研究问题

• RQ1 Be星的星周盘是否主要受开普勒旋转支配，还是非开普勒运动占主导？
• RQ2 盘的结构或动力学是否与恒星参数（如有效温度或旋转速度）相关？
• RQ3 能否利用光谱干涉测量在K波段探测到极向风，其对观测发射线的贡献如何？
• RQ4 是否存在显著偏离轴对称盘模型的偏离，若存在，何种物理机制（如非径向脉动、双星相互作用）可解释这些现象？
• RQ5 仅靠旋转是否足以提供足够能量，将物质从恒星表面抛射出去？

主要发现

• 所有八颗Be星的星周盘最适宜用几何薄盘、旋转速度接近开普勒律的模型来解释。
• 样本的平均旋转速率为Ω/Ωc = 0.95 ± 0.02，表明这些恒星以临界旋转速度的95%旋转。
• 相对于临界速度的旋转速度为V/Vc = 0.82 ± 0.08，表明在大多数情况下，旋转本身足以解释质量抛射。
• 未发现恒星参数（如Teff、v sin i）与盘的延伸范围或动力学之间存在显著相关性。
• 在κ CMa和p Car中检测到可见度和相位曲线的不对称性，表明可能存在不均匀性或非轴对称结构。
• 在α Col中通过短极向基线观测到潜在的极向风信号，但需进一步分析以确认其在K波段的存在。