B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type stars β CMa and ɛ CMa. Possible lack of a "magnetic desert" in massive stars
B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type stars β CMa and ɛ CMa. Possible lack of a "magnetic desert" in massive stars
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OB 星中的 B 场（BOB）：关于两颗早期 B 型恒星 β CMa 和 ɛ CMa 中弱磁场的探测，大质量恒星中可能缺乏“磁沙漠”。
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发表时间：
2014
期刊：
影响因子：
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通讯作者：
H. Sana
中科院分区：
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作者：
L. Fossati;N. Castro;T. Morel;N. Langer;M. Briquet;T. Carroll;S. Hubrig;M. Nieva;L. Oskinova;N. Przybilla;F. Schneider;M. Schöller;S. Simón;I. Ilyin;A. D. Koter;A. Reisenegger;H. Sana
只有一小部分大质量恒星似乎拥有可测量的结构磁场，其起源仍然未知，其对恒星演化的影响仍然需要评估。在“OB星中的B场（BO B）”合作的背景下，我们在2013年12月和2014年4月使用HARPSpol分光偏振仪观测了早期B型星β CMa（HD 44743; B1 II / III）和（cid：4）CMa（HD 52089; B1.5 II）。对于这两颗恒星，我们始终检测到弱（绝对值< 30 G）纵向磁场的特征，随时间近似恒定。我们确定了这两颗恒星的物理参数，并测量了它们的X射线光谱。对于β Cep星星β CMa，我们的模式识别分析导致确定旋转周期为13。6 ± 1。2天，旋转轴相对于视线的倾角为57.6 ± 1.7 °。在这些测量的基础上，并假设偶极场的几何形状，我们得出了一个最佳拟合的磁密度约为22 Ω，偶极磁场强度（B d）约为100 G（60 < B d < 230 G，在1 σ水平内），低于其他磁性大质量恒星的典型值。这一结论通过对星星X射线光谱的考虑而得到进一步的加强。对于（cid：4）CMa，我们只能确定偶极磁场强度的下限为13 G。对于这颗星星，我们确定其自转周期在1.3到24天之间。我们的结果意味着这两颗恒星都有一个动态的磁层，所以磁场不能支持星周盘。我们还得出结论，这两颗恒星最有可能是核心氢燃烧，它们已经度过了超过2 / 3的主序星寿命。迄今已知的大质量磁性恒星的偶极磁场强度分布直方图并没有显示出中等质量恒星的磁场“沙漠”。用目前可用的仪器和技术检测大质量恒星中的（弱）磁场所涉及的偏差意味着弱场可能比目前观察到的更常见。我们的研究结果表明，如果存在的话，即使是相对较弱的磁场也可以在大质量恒星中检测到，并且可能仍然需要更多的观测时间来正确地访问磁场入射。
Only a small fraction of massive stars seem to host a measurable structured magnetic field, whose origin is still unknown and whose implications for stellar evolution still need to be assessed. Within the context of the “B fields in OB stars (BOB)” collaboration, we used the HARPSpol spectropolarimeter to observe the early B-type stars β CMa (HD44743; B1 II / III) and (cid:4) CMa (HD52089; B1.5II) in December 2013 and April 2014. For both stars, we consistently detected the signature of a weak ( < 30 G in absolute value) longitudinal magnetic field, approximately constant with time. We determined the physical parameters of both stars and characterise their X-ray spectrum. For the β Cep star β CMa, our mode identification analysis led to determining a rotation period of 13 . 6 ± 1 . 2 days and of an inclination angle of the rotation axis of 57.6 ± 1.7 ◦ , with respect to the line of sight. On the basis of these measurements and assuming a dipolar field geometry, we derived a best fitting obliquity of about 22 ◦ and a dipolar magnetic field strength ( B d ) of about 100 G (60 < B d < 230 G within the 1 σ level), below what is typically found for other magnetic massive stars. This conclusion is strengthened further by considerations of the star’s X-ray spectrum. For (cid:4) CMa we could only determine a lower limit on the dipolar magnetic field strength of 13 G. For this star, we determine that the rotation period ranges between 1.3 and 24 days. Our results imply that both stars are expected to have a dynamical magnetosphere, so the magnetic field is not able to support a circumstellar disk. We also conclude that both stars are most likely core hydrogen burning and that they have spent more than 2 / 3 of their main sequence lifetime. A histogram of the distribution of the dipolar magnetic field strength for the magnetic massive stars known to date does not show the magnetic field “desert” observed instead for intermediate-mass stars. The biases involved in the detection of (weak) magnetic fields in massive stars with the currently available instrumentation and techniques imply that weak fields might be more common than currently observed. Our results show that, if present, even relatively weak magnetic fields are detectable in massive stars and that more observational e ff ort is probably still needed to properly access the magnetic field incidence.