Weak magnetic fields in Ap/Bp stars - Evidence for a dipole field lower limit and a tentative interp
Weak magnetic fields in Ap/Bp stars - Evidence for a dipole field lower limit and a tentative interp
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Ap/Bp 星中的弱磁场 - 偶极子场下限和暂定插值的证据
DOI：
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发表时间：
2007
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通讯作者：
N. Toqué
中科院分区：
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作者：
M. Aurière;G. Wade;J. Silvester;F. Lignières;S. Bagnulo;K. Bale;B. Dintrans;J. Donati;C. Folsom;M. Gruberbauer;A. Hoa;S. Jeffers;Neil E. Johnson;J. Landstreet;A. Lébre;T. Lueftinger;S. Marsden;D. Mouillet;S. Naseri;F. Paletou;P. Petit;J. Power;F. Rincon;S. Strasser;N. Toqué
目标。我们研究了28个著名的光谱鉴定的磁性Ap/Bp星的样本，这些恒星具有弱的，不确定的或以前未检测到的磁场。本研究的目的是探索Ap/Bp星磁场分布的弱部分。方法.利用法国南峰天文台Bernard Lyot望远镜上的MuSiCoS和NARVAL分光偏振仪，利用互相关技术最小二乘反卷积（LSD），我们获得了28颗样本星的282个LSD Stokes V特征，以高精度（中值σ = 40 G）探测磁场并推断其纵向分量。结果对于研究的28颗恒星，我们从MuSiCoS观测中获得了27个StokesV Zeeman签名检测。第28颗星星（HD 32650）的斯托克斯V特征探测是在南峰的新NARVAL分光偏振仪的科学演示期间获得的。这一结果清楚地表明，当以足够的精度观测时，所有被严格分类的Ap/Bp星都显示出可检测的表面磁场。此外，所有检测到的磁场对应于显著大于几十G的纵向场。为了更好地描述样品的表面磁场强度和几何形状，我们使用新的和以前公布的旋转周期对每个星星的纵向场测量进行了分阶段，并对其进行建模以推断偶极场强度（Bd，在磁极处测量）和磁饱和度（β）。这些恒星的偶极子强度分布在1 kG左右有一个平台，随着场强的增大和减小而下降。值得注意的是，在这个样本中，我们发现只有2颗恒星的偶极子强度低于300 G。我们解释这个“磁阈值”作为大规模磁场的稳定性所必需的临界值，并开发一个简单的定量模型，能够近似再现所观察到的阈值特性。这种情况导致一个自然的解释的一小部分中等质量的磁星。这也可以解释为什么质量更大的B和O型恒星几乎没有磁场。
Aims. We investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields. The aim of this study is to explore the weak part of the magnetic field distribution of Ap/Bp stars. Methods. Using the MuSiCoS and NARVAL spectropolarimeters at Telescope Bernard Lyot (Observatoire du Pic du Midi, France) and the cross-correlation technique Least Squares Deconvolution (LSD), we obtained 282 LSD Stokes V signatures of our 28 sample stars, in order to detect the magnetic field and to infer its longitudinal component with high precision (median σ = 40 G). Results. For the 28 studied stars, we obtained 27 detections of StokesV Zeeman signatures from the MuSiCoS observations. Detection of the Stokes V signature of the 28th star (HD 32650) was obtained during science demonstration time of the new NARVAL spectropolarimeter at Pic du Midi. This result clearly shows that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. Furthermore, all detected magnetic fields correspond to longitudinal fields which are significantly greater than some tens of G. To better characterise the surface magnetic field intensities and geometries of the sample, we phased the longitudinal field measurements of each star using new and previously-published rotational periods, and modeled them to infer the dipolar field intensity (Bd, measured at the magnetic pole) and the magnetic obliquity (β). The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this “magnetic threshold” as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.