[论文解读] Multitechnique testing of the viscous decretion disk model I. The stable and tenuous disk of the late-type Be star $\beta$ CMi

Robert Klement, A. C. Carciofi+11 more|arXiv (Cornell University)|Oct 5, 2015

Astrophysics and Star Formation Studies参考文献 60被引用 17

一句话总结

本研究利用迄今为止最大规模的多技术、多波段数据集，对晚期型Be星β CMi的黏性去积盘（VDD）模型进行了检验。结果表明，内盘需要更陡的密度衰减指数（n = 3.5），而外盘则偏好较平缓的分布（n = 3.0），提示可能存在由不可见双星伴星引起的外盘截断，其中射电观测将盘的范围限制在35+10−5倍恒星半径，偏振测量则表明恒星接近临界旋转速率（W ≳ 0.98）。

ABSTRACT

The viscous decretion disk (VDD) model is able to explain most of the currently observable properties of the circumstellar disks of Be stars. However, more stringent tests, focusing on reproducing multitechnique observations of individual targets via physical modeling, are needed to study the predictions of the VDD model under specific circumstances. In the case of nearby, bright Be star $\beta$ CMi, these circumstances are a very stable low-density disk and a late-type (B8Ve) central star. The aim is to test the VDD model thoroughly, exploiting the full diagnostic potential of individual types of observations, in particular, to constrain the poorly known structure of the outer disk if possible, and to test truncation effects caused by a possible binary companion using radio observations. We use the Monte Carlo radiative transfer code HDUST to produce model observables, which we compare with a very large set of multitechnique and multiwavelength observations that include ultraviolet and optical spectra, photometry covering the interval between optical and radio wavelengths, optical polarimetry, and optical and near-IR (spectro)interferometry. Due to the absence of large scale variability, data from different epochs can be combined into a single dataset. A parametric VDD model with radial density exponent of $n$ = 3.5, which is the canonical value for isothermal flaring disks, is found to explain observables typically formed in the inner disk, while observables originating in the more extended parts favor a shallower, $n$ = 3.0, density falloff. Modeling of radio observations allowed for the first determination of the physical extent of a Be disk (35$^{+10}_{-5}$ stellar radii), which might be caused by a binary companion. Finally, polarization data allowed for an indirect measurement of the rotation rate of the star, which was found to be $W \gtrsim 0.98$, i.e., very close to critical.

研究动机与目标

• 利用多种观测技术，对单一、研究充分的Be星严格测试黏性去积盘（VDD）模型。
• 约束星周盘的径向密度分布，特别是数据稀疏的外区。
• 研究潜在的盘截断机制，尤其是由双星伴星引起的截断。
• 利用偏振诊断方法确定中心恒星的旋转速率。
• 首次通过射电连续谱观测评估Be盘的物理尺度。

提出的方法

• 使用HDUST蒙特卡罗辐射转移代码模拟VDD模型下的可观测量。
• 将模型拟合至覆盖紫外至射电波段的综合数据集，包括SED、光谱、偏振和干涉测量。
• 采用多波段干涉测量（光学、近红外、中红外）探测盘的几何结构与动力学特性。
• 分析射电连续谱数据以估算外盘半径并检验是否存在截断。
• 利用巴尔末系连续谱偏振测量推断恒星旋转速率（W）。
• 通过多种技术对比模型预测与观测结果，以打破盘结构中的参数歧义。

实验结果

研究问题

• RQ1 VDD模型能否准确再现β CMi的全波段、多技术综合观测数据？
• RQ2 盘的径向密度分布为何？其在内区与外区是否存在差异？
• RQ3 外盘是否被截断？若是，何种机制（如双星伴星）可能造成此现象？
• RQ4 中心B8Ve型恒星的旋转速率是多少？能否通过偏振测量独立约束？
• RQ5 星周盘的物理尺度是多少？射电观测能否提供直接测量？

主要发现

• 内盘需要径向密度指数n = 3.5，与等温展平盘一致；而外盘则偏好更平缓的分布，n = 3.0。
• 该密度分布差异无法由非等温效应解释，提示存在物理原因，如潮汐截断。
• 射电观测将盘外半径约束在35+10−5倍恒星半径，表明存在物理截断尺度。
• 巴尔末系连续谱中观测到的正偏振斜率要求恒星旋转速率W ≳ 0.98，表明接近临界旋转。
• 利用AMBER光谱干涉测量，精确估计了倾角为i = 43+3°−2°。
• 光谱、测光与干涉数据的综合分析表明，双星伴星是最可能造成盘截断的原因，且更倾向于晚型或亚矮星伴星。