早期B型星的弱金属发射线
Weak Metallic Emission Lines in Early B-Type Stars
摘要：
在缓慢旋转的早期B型恒星的红区光谱中检测到了以前未被识别的来自Si II（12.84 eV）和Al II（13.08 eV）高激发态的弱发射线。我们测量了35颗B型星的高分辨率光谱，覆盖了主序附近B1和B7之间的光谱子类型，并在所有13颗具有光谱子类型B2和B2.5的恒星中发现了位于6239.6 A的SiII发射线。B3亚型有17颗星，其中7颗星显示Si II的发射线。在一个较窄的温度范围内（Teff在19000K和23000K之间），在9颗恒星中探测到Al II的6243.4A发射线。在我们的样本中，这两条发射线都没有在较冷（Teff < 16000 K）的恒星中检测到。Si II在6239.6 A处的发射线显示出单峰和对称的轮廓，并且线中心相对于Si II的低激发吸收线的波长没有偏移。测量的发射线的半宽度是相同的旋转加宽的低激发吸收线的Si II。这些观测结果表明，发射气体不是星周起源，而是位于大气的最外层，覆盖了整个恒星表面，并与恒星共同旋转。
英文摘要：
Previously unrecognized weak emission lines originating from high excitation states of Si II (12.84 eV) and Al II (13.08 eV) are detected in the red region spectra of slowly rotating early B-type stars. We surveyed high resolution spectra of 35 B-type stars covering spectral sub-types between B1 and B7 near the main sequence and found the emission line of SiII at 6239.6 A in all 13 stars having spectral sub-types B2 and B2.5. There are 17 stars belonging to sub-type B3 and seven stars among them are found to show the emission line of Si II. The emission line of Al II at 6243.4 A is detected in a narrower temperature range (Teff between 19000K and 23000 K) in nine stars. Both of these emission lines are not detected in cooler (Teff < 16000 K) stars in our sample. The emission line of Si II at 6239.6 A shows a single-peaked and symmetric profile and the line center has no shift in wavelength with respect to those of low excitation absorption lines of Si II. Measured half width of the emission line is the same as those of rotationally broadened low excitation absorption lines of Si II. These observations imply that the emitting gas is not circumstellar origin, but is located at the outermost layer of the atmosphere, covering the whole stellar surface and co-rotates with the star.
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