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arXiv 1702.07609astro-ph.SR
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究竟有多热是$ω$半人马座极端水平分支减速器？

Just how hot are the $ω$ Centauri extreme horizontal branch pulsators?

M. Latour, S. K. Randall, P. Chayer, G. Fontaine, A. Calamida, J. Ely, T. M. Brown, W. Landsman

备注：
9 pages, accepted for publication in Astronomy and Astrophysics
发表信息：
A&A 600, A130 (2017)
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摘要：

过去基于光谱学的研究表明，五个$ω$ Cen恒星形成了一个相当均匀的富氢亚矮星O群，有效温度约为50 000 K。这将恒星置于$g$ $-$ Teff对数图中理论不稳定带的红边之下，在那里预计不会激发脉动模式。我们的目标是确定这种温度差异是否真实，或者恒星的有效温度是否被低估了。我们提出了一个光谱分析的两个快速脉动的极端水平分支（EHB）恒星发现的$ω$岑。我们获得了两个$ω$ Cenerator V1和V5的Hubble Space Telescope/COS UV光谱，并利用UV金属线的电离平衡来更好地约束它们的有效温度。作为一个副产品，我们还获得了FUV光曲线的两个发光体。使用N IV和N V线的相对强度作为温度指示器，产生接近60 000 K的Teff值，比先前导出的温度显著更热。根据FUV光变曲线，我们能够确认从光学数据中已知的主要脉动周期。紫外光谱显示的有效温度比之前假设的要高，sdO星现在应该在预测的不稳定带内被发现。这种较高的温度也提供了一致的光谱质量为我们以前研究的样品的冷和热EHB恒星。

英文摘要：

Past studies based on optical spectroscopy suggest that the five $ω$ Cen pulsators form a rather homogeneous group of hydrogen-rich subdwarf O stars with effective temperatures of around 50 000 K. This places the stars below the red edge of the theoretical instability strip in the log $g$ $-$ Teff diagram, where no pulsation modes are predicted to be excited. Our goal is to determine whether this temperature discrepancy is real, or whether the stars' effective temperatures were simply underestimated. We present a spectral analysis of two rapidly pulsating extreme horizontal branch (EHB) stars found in $ω$ Cen. We obtained Hubble Space Telescope/COS UV spectra of two $ω$ Cen pulsators, V1 and V5, and used the ionisation equilibrium of UV metallic lines to better constrain their effective temperatures. As a by-product we also obtained FUV lightcurves of the two pulsators. Using the relative strength of the N IV and N V lines as a temperature indicator yields Teff values close to 60 000 K, significantly hotter than the temperatures previously derived. From the FUV light curves we were able to confirm the main pulsation periods known from optical data. With the UV spectra indicating higher effective temperatures than previously assumed, the sdO stars would now be found within the predicted instability strip. Such higher temperatures also provide consistent spectroscopic masses for both the cool and hot EHB stars of our previously studied sample.

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