Searching for links between magnetic fields and stellar evolution III. Measurement of magnetic fields in open cluster Ap stars with ESPaDOnS
Searching for links between magnetic fields and stellar evolution III. Measurement of magnetic fields in open cluster Ap stars with ESPaDOnS
复制标题
寻找磁场与恒星演化之间的联系 III。
DOI：
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发表时间：
2008
期刊：
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通讯作者：
G. Wade
中科院分区：
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作者：
J. Landstreet;J. Silaj;V. Andretta;Stefano Bagnulo;S. Berdyugina;S. Berdyugina;J. Donati;L. Fossati;P. Petit;J. Silvester;G. Wade
上下文。一小部分较高的主序恒星具有很强的高度结构化的磁场。这些油田的起源和演化还没有得到充分的了解。目标。我们正在对疏散星团中的AP星的磁场进行测量，以获得第一个具有精确年龄的磁性上主序恒星样本。这些数据将约束这些恒星的场演化理论。方法：研究方法。CFHT使用新的ESPADONS光谱偏振仪对候选疏散星团磁性AP星进行了调查。该仪器提供了一种替代FORS1型光谱偏振仪用于本次调查的仪器。结果。我们已经获得了23颗B6-A2星的平均纵场“BZ#”的44个测量值，这些星被确认为可能的AP星，可能是疏散星团的成员，平均不确定度约为45G。其中10颗星有明确的场探测。我们的样本中有九颗恒星被发现不是磁性AP星。与我们之前的样本相比，这些观测大大增加了关于TAMS附近低质量恒星的可用信息。结论。我们发现，ESPADONS提供了与我们以前使用FORS1获得的测量相当的现场测量，并且这些数据还包含大量从低分辨率光谱偏振测量中不易获得的有用信息。有了新的数据，我们能够扩展现有的低质量、相对演化的AP星的数据，并更有力地识别哪些观测到的恒星实际上是磁性AP星和星系团成员。对扩大的AP星团数据集的重新分析表明，这些质量在2-5M$范围内的恒星只有在靠近ZAMS时才显示出大于约1公斤的均方根场。这些大磁场消失的时间尺度随质量变化很大，2-3M星体的时间尺度约为250Myr，4-5M星体的时间尺度为15Myr。我们的数据要么与大多数(但不是所有)AP恒星的紧急通量守恒一致，要么与通量随年龄的适度下降相一致。
Context. A small fraction of upper main sequence stars have strong, highly structured magnetic fields. The origin and evolution of these fields are not adequately understood. Aims. We are carrying out a survey of magnetic fields in Ap stars in open clusters in order to obtain the first sample of magnetic upper main sequence stars with precisely known ages. These data will constrain theories of field evolution in these stars. Methods. A survey of candidate open cluster magnetic Ap stars was carried out using the new ESPaDOnS spectropolarimeter at the CFHT. This instrument provides an alternative to the FORS1 spectropolarimeter used up to now for this survey. Results. We have obtained 44 measurements of the mean longitudinal fields " Bz# of 23 B6 ‐ A2 stars that have been identified as possible Ap stars and that are possible members of open clusters, with a median uncertainty of about 45 G. Of these stars, 10 have definite field detections. Nine stars of our sample are found not to be magnetic Ap stars. These observations significantly increase the information available about low-mass stars near the TAMS compared to our previous sample. Conclusions. We find that ESPaDOnS provides field measurements comparable to those that we have previously obtained with FORS1, and that these data also contain a large amount of useful information not readily obtained from lower resolution spectropolarimetry. With the new data we are able to expand the available data on low-mass, relatively evolved Ap stars, and identify more robustly which observed stars are actually magnetic Ap stars and cluster members. Re-analysis of the enlarged data set of cluster Ap stars indicates that such stars with masses in the range of 2 ‐ 5 M$ show RMS fields larger than about 1 kG only when they are near the ZAMS. The time scale on which these large fields disappear varies strongly with mass, ranging from about 250 Myr for stars of 2 ‐3 M$ to 15 Myr for stars of 4 ‐ 5 M$ . Our data are consistent either with emergent flux conservation for most (but not all) Ap stars, or with modest decline in flux with age.