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

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

Astrophysics and Star Formation Studies参考文献 62被引用 16

一句话总结

本研究利用覆盖紫外至射电波段的综合性多技术数据集，对晚型Be星β CMi的黏性去积盘（VDD）模型进行了检验。结果表明，内盘需要更陡峭的密度剖面（n = 3.5），而外区则偏好较平缓的剖面（n = 3.0），提示可能存在未见的双星伴星导致外盘截断，盘面延伸至35⁺¹⁰₋₅ R☉，且恒星自转速度达到临界速度的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 $β$ 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

• 针对单一Be星的综合性多波段数据集，严格检验黏性去积盘（VDD）模型。
• 约束星周盘的径向密度剖面，特别是结构尚不明确的外区。
• 利用射电观测研究双星伴星可能导致的盘面截断。
• 从偏振数据推导β CMi的自转速率，并检验其与临界自转的一致性。
• 确定盘的物理尺度，并评估非等温效应在盘结构中的作用。

提出的方法

• 使用HDUST蒙特卡罗辐射转移代码模拟紫外、光学、红外及射电波段的可观测量。
• 调整模型参数（包括径向密度指数n、黏性系数及盘几何结构），以匹配观测到的谱能量分布（SED）、谱线轮廓及偏振特性。
• 利用高分辨率光谱干涉测量（AMBER）和宽带干涉测量（NPOI、VLT/AMBER）约束盘的倾角与尺寸。
• 利用射电与亚毫米波光度测量探测外盘并估算其物理尺度。
• 分析Paschen连续谱波段的偏振测量以推断恒星的自转速率。
• 通过比较观测到的截断半径与理论预测的双星诱导截断半径，结合Hα V/R振荡的弱周期，进行验证。

实验结果

研究问题

• RQ1 具有单一幂律密度剖面的VDD模型能否充分再现β CMi的全波段多波段数据？
• RQ2 基于长波段观测，盘的径向密度结构（尤其是外区）如何被约束？
• RQ3 射电观测能否探测并测量Be盘的物理尺度？该尺度是否提示存在双星伴星导致的截断？
• RQ4 β CMi的自转速率是多少？其是否如偏振数据所示接近临界自转？
• RQ5 密度剖面的变化最合理的解释是非等温效应，还是外部扰动（如双星伴星）？

主要发现

• 内盘的可观测特征（如偏振及高阶巴耳末线）要求径向密度剖面指数n = 3.5的更陡峭分布。
• 外区延伸成分（包括Hα、Brγ及中远红外光度）偏好更平缓的密度衰减，指数n = 3.0。
• 根据射电与亚毫米波观测，盘在物理半径35⁺¹⁰₋₅倍恒星半径处被截断。
• 观测到的射电截断半径与理论预测一致，即双星伴星的轨道周期与Hα的弱V/R振荡周期相匹配。
• 偏振数据表明恒星自转速度达到临界速度的W ≳ 0.98，与近临界自转一致。
• 中心恒星的极半径为2.8 R☉，光度为185 L☉，由其近乎纯光球的紫外谱推导得出。