铍壳层星EW Lac在V/R变周期的光谱变化
Spectroscopic Variations of the Be-shell Star EW Lac in the V/R Variation Periods
摘要：
EW Lac在1976 - 1986年表现出明显的V/R变化，在经历了一些准周期V/R变化之后，2007年左右又开始了类似的V/R变化。对于第一个V/R变化事件，我们分析了巴耳末系的发射线和壳层吸收线的光谱行为。H_α到H_δ谱线的V/R变化的特征在于各发射谱线在V/R相位的时间滞后和持续时间上的不同变化方式。在V/R变化与谱线轮廓参数的其它变化（如峰速度、发射谱线强度和峰间距）之间也存在弱相关性。我们分析了壳吸收线的巴耳末系的较高成员的中心深度和径向速度。H_α线的光学深度在2000 ~ 6000之间，其长期变化揭示了与V/R变化不同的行为。结合发射线和壳层吸收线的分析，把V/R变化看作是波的传播现象，我们发现1976 - 1986年事件的V/R变化是一种逆行结构，在该事件的后半期，盘内可能形成一种螺旋结构。物理参数之间的弱相关性表明圆盘在某个半径处被截断。观测结果表明，在事件的后半期，恒星发生了显著的增亮，并伴随着发射线强度的显著下降。对于2007年前后出现的V/R变化，我们发现盘的发展程度较低，没有明显的时间滞后，这看起来像是前一次事件的重演。
英文摘要：
EW Lac has shown remarkable V/R variations in 1976 - 1986, and a similar V/R variation has started again in around 2007, after some quasi-periodic V/R variation. For the first V/R variation event, we analyze the spectroscopic behaviors of emission lines and shell absorption lines of the Balmer series. The V/R variations of the H_alpha through H_delta lines are characterized by the different manner of variations of the individual emission lines in time lag and in duration of the V/R phases. Weak correlation is also notable between the V/R variations and other variations of the line-profile parameter such as the peak velocities, emission line intensities and peak separations. We analyze shell absorption lines for higher members of the Balmer series for their central depths and radial velocities. The optical depth of the H_alpha line is in a range of 2000 to 6000, and its long-term variation discloses different behaviors as compared to the V/R variations. Combining the analyses of emission and shell absorption lines, and regarding the V/R variation as wave propagation phenomena, we find for the 1976 - 1986 event that the V/R variation is of retrograde structure and a spiral structure is likely formed inside the disk in the latter half of this event. The weak correlations among physical parameters are suggestive of the disk truncated at some radius. It is noticed that remarkable stellar brightening has occurred in the latter half of the event, accompanying marked decrease of emission line intensities. As for the V/R variation appeared in around 2007, which looks like a recurrence of the previous event, we found less developed state of the disk without appreciable time lag.
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