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arXiv 0903.2340astro-ph.SRastro-ph.EP
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两个强磁场冷矮星的短期光谱监测

Short-term spectroscopic monitoring of two cool dwarfs with strong magnetic fields

E. W. Guenther, M. R. Zapatero Osorio, A. Mehner, E. L. Martin

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7 pages, 9 figures. Astronomy and Astrophysics (accepted)
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摘要：

内容：现在有越来越多的证据表明，一些褐矮星（BD）具有非常强的磁场，但它们的表面温度非常低，以至于物质与大气中的磁场之间的耦合预计很小。然而，在更深的层中，耦合预计要强得多。目的：这就提出了一个问题，即磁场是否仍然导致光球层中结构的形成。研究方法：我们对两个具有强磁场的超冷矮星进行了光谱监测活动：BD LP 944 -20和2 MASSW J 0036159 +182110。LP 944 -20在光学和近红外区域同时被观察到，2 MASSW J 0036159 +182110仅在红外区域被观察到。结果：这两个侏儒被证明是非常恒定的。在LP 944 -20的情况下，Tef变化<50 K，并且H α的等效宽度的均方根变化很小。我们还发现光球谱线的等效宽度是相当恒定的。我们在2 MASSW J 0036159 +182110的情况下也没有发现任何显著变化。因此，最重要的结果是，在我们的观察时没有发现显著的变化。将我们的光谱与过去11年的光谱进行比较时，我们认识到这段时间发生了重大变化。结论：我们将这些结果解释为这些天体的光球层非常均匀的证据，尽管有很强的磁场，但其中只有很少的结构。因此，与活跃的恒星不同，这些物体上没有突出的斑点。

英文摘要：

Context: There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fields, and yet their surface temperatures are so low that the coupling is expected to be small between the matter and the magnetic field in the atmosphere. In the deeper layers, however, the coupling is expected to be much stronger. Aims: This raises the question of whether the magnetic field still leads to the formation of structures in the photosphere. Methods: We carried out a spectroscopic monitoring campaign of two ultracool dwarfs that have strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110. LP944-20 was observed simultaneously in the optical and in the near infrared regime, 2MASSW J0036159+182110 only in the infrared. Results: Both dwarfs turned out to be remarkably constant. In the case of LP944-20, the Teff-variations are <50K, and the rms-variations in the equivalent widths of Halpha small. We also find that the equivalent widths of photospheric lines are remarkably constant. We did not find any significant variations in the case of 2MASSW J0036159+182110 either. Thus the most important result is that no significant variability was found at the time of our observations. When comparing our spectra with spectra taken over the past 11 years, we recognize significant changes during this time. Conclusions: We interpret these results as evidence that the photosphere of these objects are remarkably homogeneous, with only little structure in them, and despite the strong magnetic fields. Thus, unlike active stars, there are no prominent spots on these objects.

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