A Chandra Study: Are Dwarf Carbon Stars Spun Up and Rejuvenated by Mass Transfer?
A Chandra Study: Are Dwarf Carbon Stars Spun Up and Rejuvenated by Mass Transfer?
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DOI：
10.3847/1538-4357/ab2bf4
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发表时间：
2019-05
期刊：
影响因子：
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通讯作者：
P. Green;R. Montez;Fernando Mazzoni;J. Filippazzo;S. Anderson;O. Marco;J. Drake;J. Farihi;
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文献类型：
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作者：
P. Green;R. Montez;Fernando Mazzoni;J. Filippazzo;S. Anderson;O. Marco;J. Drake;J. Farihi;
长期以来，人们一直认为碳星(C/O>；1)都是巨星，因为只有渐近巨星(AGB)会将大量的碳排放到它们的大气层中。现在的情况几乎是确凿的，因为以前神秘的矮碳(DC)恒星实际上比C巨星更常见，并且从前AGB伴星那里吸积了富碳物质，产生了一颗白矮星(WD)和一颗DC星，获得了显著的质量和角动量。一些这样的DC系统经历了行星状星云阶段，有些可能演化成CH、CEMP或BA巨星。最近的研究表明，大多数DCs可能来自较年长的、缺乏金属的运动学人群。鉴于众所周知的年龄和活动的反相关关系，预计DCs不会显示与日冕活动有关的显著X射线发射。然而，吸积自旋可能会在这些后质量转移的双星系统中恢复磁性发电机的活力。我们描述了钱德拉对从SDSS中选择的六个用于Hα发射和/或热WD伴星的DC进行的钱德拉初步研究，以测试它们的X射线发射强度和光谱特性是否与恢复活力的发电机一致。我们在样本中检测到了所有六个DDC，它们的X射线亮度范围从log-29.7，初步证据表明，DCs可能在与自转周期短至几天或更短的恒星一致的水平上活跃。更确定的结果需要典型的分散控制系统样本，并进行更深入的X射线观测，以更好地约束它们的等离子体温度。
Carbon stars (with C/O > 1) were long assumed to all be giants, because only asymptotic giant branch (AGB) stars dredge up significant carbon into their atmospheres. The case is nearly ironclad now that the formerly mysterious dwarf carbon (dC) stars are actually far more common than C giants and have accreted carbon-rich material from a former AGB companion, yielding a white dwarf (WD) and a dC star that has gained both significant mass and angular momentum. Some such dC systems have undergone a planetary nebula phase, and some may evolve to become CH, CEMP, or Ba giants. Recent studies indicate that most dCs are likely from older, metal-poor kinematic populations. Given the well-known anticorrelation of age and activity, dCs would not be expected to show significant X-ray emission related to coronal activity. However, accretion spin-up might be expected to rejuvenate magnetic dynamos in these post-mass-transfer binary systems. We describe our Chandra pilot study of six dCs selected from the SDSS for Hα emission and/or a hot WD companion, to test whether their X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo. We detect all six dCs in the sample, which have X-ray luminosities ranging from log –29.7, preliminary evidence that dCs may be active at a level consistent with stars that have short rotation periods of several days or less. More definitive results require a sample of typical dCs with deeper X-ray observations to better constrain their plasma temperatures.