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arXiv 1111.1240astro-ph.SR
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根据高分辨率分光偏振观测的贝叶斯分析的恒星磁场参数

Stellar magnetic field parameters from a Bayesian analysis of high-resolution spectropolarimetric observations

V. Petit, G. A. Wade

备注：
Accepted for publication in Monthly Notices of the Royal Astronomical Society
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摘要：

在本文中，我们描述了贝叶斯统计方法，旨在推断使用高分辨率圆偏振光谱仪在大调查的背景下观察到的恒星的磁特性。这种方法非常适合于分析恒星自转周期未知的恒星，因此观测的旋转相位是模糊的。该模型假设磁观测对应于偶极斜旋转体，这是中等质量和高质量恒星中常见的情况。使用关于模型参数先验概率密度分布的合理假设，贝叶斯算法通过在观测到的和计算出的斯托克斯V分布之间进行比较来确定对应于表面磁场几何形状和强度的后验概率密度。基于数值模拟的结果，我们得出结论，这种方法产生了一个有用的估计的表面偶极场强度的基础上少量（即1或2）的观测。另一方面，该方法只提供了弱约束的偶极子的几何形状。比值比是由该算法计算的参数，其量化了磁偶极子模型与非磁模型的相对适当性，提供了比传统技术更灵敏的嵌入噪声中的弱磁信号的存在的诊断。为了说明应用该技术的实际数据，我们分析了7个ESPaDOns和Narval观测早期B型磁星LP猎户座。没有足够的信息来确定恒星的旋转周期和数据的相位，因此传统的建模技术无法推断偶极子强度。相比之下，贝叶斯方法允许对偶极极性强度$B_d=911^{+138}_{-244}$ G的稳健确定。

英文摘要：

In this paper we describe a Bayesian statistical method designed to infer the magnetic properties of stars observed using high-resolution circular spectropolarimetry in the context of large surveys. This approach is well suited for analysing stars for which the stellar rotation period is not known, and therefore the rotational phases of the observations are ambiguous. The model assumes that the magnetic observations correspond to a dipole oblique rotator, a situation commonly encountered in intermediate and high-mass stars. Using reasonable assumptions regarding the model parameter prior probability density distributions, the Bayesian algorithm determines the posterior probability densities corresponding to the surface magnetic field geometry and strength by performing a comparison between the observed and computed Stokes V profiles. Based on the results of numerical simulations, we conclude that this method yields a useful estimate of the surface dipole field strength based on a small number (i.e. 1 or 2) of observations. On the other hand, the method provides only weak constraints on the dipole geometry. The odds ratio, a parameter computed by the algorithm that quantifies the relative appropriateness of the magnetic dipole model versus the non-magnetic model, provides a more sensitive diagnostic of the presence of weak magnetic signals embedded in noise than traditional techniques. To illustrate the application of the technique to real data, we analyse seven ESPaDOnS and Narval observations of the early B-type magnetic star LP Ori. Insufficient information is available to determine the rotational period of the star and therefore the phase of the data; hence traditional modelling techniques fail to infer the dipole strength. In contrast, the Bayesian method allows a robust determination of the dipole polar strength, $B_d=911^{+138}_{-244}$ G.

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