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arXiv 0801.4586astro-ph
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特殊DQ白矮星中的C_2

C_2 in Peculiar DQ White Dwarfs

Patrick B. Hall, Aaron J. Maxwell

备注：
ApJ in press. 8 pages emulateapj style, 1 figure
发表信息：
Astrophysical Journal 678, 1292-1297 (2008)
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摘要：

在光谱中具有碳吸收特征的白矮星（WD）被称为DQ WD。DQWDs的子类是冷天体（T_eff<6000 K），它们的分子吸收带中心波长比C_2的Swan带头短100-300埃。这些“特殊的DQ带”被认为是一种碳氢化合物，如C_2H。我们指出C_2H没有显示出与DQ带波长相匹配的强吸收带，也没有任何其它可能存在于冷WD大气中的简单分子或离子。对这种特殊的DQ带最直接的解释是它们是C_2的压移Swan带。虽然目前的WD大气模型表明，在一般情况下，特殊的DQ WD没有比正常DQ WD更高的光球压力，这一发现需要通过WD大气和C_2在高压和高温下的行为的改进模型来证实。如果最后证明这种特殊的DQ带不能用压移的Swan带来解释，那么剩下的唯一解释似乎是它们来自于高转动激发的C_2（J_peak>45）。在这两种情况下，吸收带轮廓原则上都可以用来约束正常和特殊DQ WD大气谱线形成区中C_2的压力和转动温度，这将有助于与模型进行比较。最后，我们注意到，在理解磁DQ WD的进展可能需要模型，同时考虑磁场，高压和旋转激励的C_2。

英文摘要：

White dwarfs (WDs) with carbon absorption features in their optical spectra are known as DQ WDs. The subclass of peculiar DQ WDs are cool objects (T_eff<6000 K) which show molecular absorption bands that have centroid wavelengths ~100-300 Angstroms shortward of the bandheads of the C_2 Swan bands. These "peculiar DQ bands" have been attributed to a hydrocarbon such as C_2H. We point out that C_2H does not show strong absorption bands with wavelengths matching those of the peculiar DQ bands and neither does any other simple molecule or ion likely to be present in a cool WD atmosphere. The most straightforward explanation for the peculiar DQ bands is that they are pressure-shifted Swan bands of C_2. While current models of WD atmospheres suggest that, in general, peculiar DQ WDs do not have higher photospheric pressures than normal DQ WDs do, that finding requires confirmation by improved models of WD atmospheres and of the behavior of C_2 at high pressures and temperatures. If it is eventually shown that the peculiar DQ bands cannot be explained as pressure-shifted Swan bands, the only explanation remaining would seem to be that they arise from highly rotationally excited C_2 (J_peak>45). In either case, the absorption band profiles can in principle be used to constrain the pressure and the rotational temperature of C_2 in the line-forming regions of normal and peculiar DQ WD atmospheres, which will be useful for comparison with models. Finally, we note that progress in understanding magnetic DQ WDs may require models which simultaneously consider magnetic fields, high pressures and rotational excitation of C_2.

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