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arXiv 1702.07684astro-ph.SR
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Be星自转速度分布的临界研究; II：微分自转和干扰Vsin i参数解释的某些隐藏效应

Critical study of the distribution of rotational velocities of Be stars; II: Differential rotation and some hidden effects interfering with the interpretation of the Vsin i parameter

J. Zorec, Y. Frémat, A. Domiciano de Souza, F. Royer, L. Cidale, A. -M. Hubert, T. Semaan, C. Martayan, Y. R. Cochetti, M. L. Arias, Y. Aidelman, P. Stee

备注：
To appear in A&A
发表信息：
A&A 602, A83 (2017)
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摘要：

我们假设恒星可能会经历表面差异旋转，以研究其对$V\!\sin i$的解释和对观测到的真旋转速度比分布$Φ(u)$的影响$u=V/V_\rm c$（$V_\rm c$是赤道临界速度）。本文讨论了影响谱线形成和谱线展宽的一些现象，这些现象会使$V\!\sin i$所携带的有关实际恒星自转的信息消失。我们研究了几种微分转动定律产生的谱线展宽，但采用了蒙德的表达式$Ω(θ)=Ω_o(1+α\cos^2θ)$，试图用尽可能少的自由参数来解释所有这些定律。我们研究了微分旋转参数$α$对测量的$V\!\sin i$参数和比率$u=V/V_\rm c$的分布$Φ(u)$的影响。我们的结论是，推断的$V\!\sin i$小于暗示的实际赤道线旋转速度$V_\rm eq$，如果恒星旋转与$α<0$，但较大，如果恒星有$α>0$。对于给定的$|α|$，当$α<0$时，$V\!\sin i$的偏差较大。如果被研究的Be星平均有$α<0$，那么具有$V_\rm eq\simeq0.9V_\rm c$的旋转星的数量比从观测分布$Φ(u)$中预期的要大;如果这些星平均有$α>0$，那么这个数量比预期的要低。我们讨论了七种现象，有助于狭窄或扩大谱线，模糊的旋转信息进行$V\!\sin i$，特别是，以决定是否是现象主要依赖于临界旋转。我们表明，二维辐射传输计算需要在快速旋转更可靠地诊断恒星旋转。

英文摘要：

We assume that stars may undergo surface differential rotation to study its impact on the interpretation of $V\!\sin i$ and on the observed distribution $Φ(u)$ of ratios of true rotational velocities $u=V/V_\rm c$ ($V_\rm c$ is the equatorial critical velocity). We discuss some phenomena affecting the formation of spectral lines and their broadening, which can obliterate the information carried by $V\!\sin i$ concerning the actual stellar rotation. We studied the line broadening produced by several differential rotational laws, but adopted Maunder's expression $Ω(θ)=Ω_o(1+α\cos^2θ)$ as an attempt to account for all of these laws with the lowest possible number of free parameters. We studied the effect of the differential rotation parameter $α$ on the measured $V\!\sin i$ parameter and on the distribution $Φ(u)$ of ratios $u=V/V_\rm c$. We conclude that the inferred $V\!\sin i$ is smaller than implied by the actual equatorial linear rotation velocity $V_\rm eq$ if the stars rotate with $α<0$, but is larger if the stars have $α>0$. For a given $|α|$ the deviations of $V\!\sin i$ are larger when $α<0$. If the studied Be stars have on average $α<0$, the number of rotators with $V_\rm eq\simeq0.9V_\rm c$ is larger than expected from the observed distribution $Φ(u)$; if these stars have on average $α>0$, this number is lower than expected. We discuss seven phenomena that contribute either to narrow or broaden spectral lines, which blur the information on the rotation carried by $V\!\sin i$ and, in particular, to decide whether the Be phenomenon mostly rely on the critical rotation. We show that two-dimensional radiation transfer calculations are needed in rapid rotators to diagnose the stellar rotation more reliably.

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