Bright Be-shell stars
Bright Be-shell stars
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明亮的贝壳星
DOI：
10.1051/0004-6361:20053008
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发表时间：
2006
影响因子：
6.5
通讯作者：
D. Baade
中科院分区：
文献类型：
--
作者：
T. Rivinius;S. Štefl;D. Baade
提出并讨论了 27 颗已知“正常”壳层恒星中 23 颗亮度超过 6.5 星等的阶梯状观测结果。除了这些典型情况外，样本中还添加了三颗已知发射与壳层和纯发射线外观之间转变的恒星，以及三颗没有线发射记录的快速旋转的 B 星（Bn 星）。在 75% 的正常壳层恒星中发现了光球线中的长期 V/R 发射线变化和中心准发射凸起 (CQE)。这有力地表明，大多数（如果不是全部）Be 星盘的速度定律大致遵循开普勒定律。这两种现象可能发生在同一颗恒星中，但不会同时发生。这与之前的结论一致，即 CQE 仅在视线速度可忽略不计的情况下形成，而长期 V/R 变化是由于全局盘振荡引起的非圆形气体粒子轨道造成的。与双星轨道相关的 V/R 变化不太明显。同样，仅在长期 V/R 变星中检测到不同谱线之间的相位滞后。仅三分之一的二元分数太低，无法支持二元假设作为 Be 现象的解释。在 19 Bn 星中的 3 颗中检测到的 CQE 揭示了在没有发射线的 B 星中存在盘状赤道物质浓度。因此，无盘 B 星和 Be 星之间似乎存在中间情况。先前关于存在低 v sin i 壳星的说法无法得到证实。壳星是从赤道上观察的Be星，其观测到的自转速度与赤道上的自转速度没有区别，赤道上的自转速度与Be星中的相同。临界旋转速度的平均分数为 81 ± 12%。标准差与观测不确定性相当，甚至更小。由于这需要恒星与恒星之间的差异可以忽略不计，这是不现实的，因此强谱线的宽度与恒星旋转速度之间的相关性可能在其极端处被截断或严重扭曲。还报道了一些关于个别恒星以前不为人知的事实。
Echelle observations are presented and discussed for 23 of the 27 known "normal" shell stars brighter than about 6.5 mag. In addition to those typical cases, three stars with known transitions between emission & shell and pure emission line appearance, and three rapidly rotating B stars without records of line emission (Bn stars) are added to the sample. Long-term V/R emission-line variability and central quasi emission bumps (CQEs) in photospheric lines were found in 75% of all normal shell stars. This strongly suggests that the velocity law in most, if not all, disks of Be stars is roughly Keplerian. Both phenomena may occur in the same star but not at the same time. This is in agreement with the previous conclusion that CQEs only form in the presence of negligible line-of-sight velocities while long-term V/R variations are due to non-circular gas particle orbits caused by global disk oscillations. V/R variations associated with binary orbits are much less pronounced. Similarly, phase lags between different lines were detected in long-term V/R variable stars only. A binary fraction of only one-third is too low to support binary hypotheses as an explanation of the Be phenomenon. CQEs detected in 3 out of 19 Bn stars reveal the presence of disk-like equatorial concentrations of matter in B stars without emission lines. Accordingly, there seem to be intermediate cases between disk-free B stars and Be stars. Previous claims of the existence of shell stars with low v sin i could not be confirmed. Shell stars are Be stars viewed equator-on, and their observed rotational velocities are indistinguishable from the equatorial ones which are the same as in Be stars. The mean fraction of the critical rotation velocity is 81 ± 12%. The standard deviation is comparable to, or even less than, the observational uncertainties. Since this would require star-to-star differences to be negligible, which is unrealistic, the correlation between the widths of strong spectral lines and the stellar rotation velocities may be truncated or severely distorted at its extreme end. A number of not previously known facts about individual stars is also reported.