全色哈勃仙女座宝藏I：M31凸出部分的明亮PDKEEPAAA星
The Panchromatic Hubble Andromeda Treasury I: Bright UV Stars in the Bulge of M31
摘要：
作为全色哈勃仙女座宝藏（PHAT）多周期计划的一部分，我们用WFC 3/UVIS滤光片F275 W和F336 W观测了M31凸起的12' \times 6.5'区域。从这些数据中，我们收集了一个\sim 4000紫外线明亮的老恒星的样本，比以前可用的大得多。我们使用更新的Padova恒星演化轨迹将这些热星分为三类：后AGB星（P-AGB），后早期AGB星（PE-AGB）和AGB-manqué星。P-AGB星是渐近巨星分支（AGB）阶段的最终结果，预计在广泛的恒星星族中，而PE-AGB和AGB-manque（以下统称热后横支; HP-HB）星是由于包层质量不足以形成完整的AGB阶段，并且当RGB上的质量损失高时，预计在高氦或α丰度下特别突出。我们的数据支持以前的说法，即大多数在隆起的紫外线明亮的来源可能是热（极端）水平分支恒星（EHB）和他们的后代。我们构建了这些恒星星族的第一径向配置文件，并表明它们是高度集中的，甚至比集成的紫外线或可见光。然而，我们发现，这个紫外线明亮的人口并不占主导地位的总紫外线光度在任何半径，因为我们只检测的后代的EHB星的紫外线X的可能来源。我们计算出，只有百分之几的MS星在中央隆起可以通过HP-HB阶段，这一百分比强烈降低与中心的距离。我们还发现，热的紫外亮星的表面密度有相同的径向变化的低质量的X射线双星。我们讨论了年龄，金属丰度，和丰度的变化可能的解释所观察到的径向变化的紫外线明亮的人口。
英文摘要：
As part of the Panchromatic Hubble Andromeda Treasury (PHAT) multi-cycle program, we observed a 12' \times 6.5' area of the bulge of M31 with the WFC3/UVIS filters F275W and F336W. From these data we have assembled a sample of \sim4000 UV-bright, old stars, vastly larger than previously available. We use updated Padova stellar evolutionary tracks to classify these hot stars into three classes: Post-AGB stars (P-AGB), Post-Early AGB (PE-AGB) stars and AGB-manqué stars. P-AGB stars are the end result of the asymptotic giant branch (AGB) phase and are expected in a wide range of stellar populations, whereas PE-AGB and AGB-manqué (together referred to as the hot post-horizontal branch; HP-HB) stars are the result of insufficient envelope masses to allow a full AGB phase, and are expected to be particularly prominent at high helium or α abundances when the mass loss on the RGB is high. Our data support previous claims that most UV-bright sources in the bulge are likely hot (extreme) horizontal branch stars (EHB) and their progeny. We construct the first radial profiles of these stellar populations, and show that they are highly centrally concentrated, even more so than the integrated UV or optical light. However, we find that this UV-bright population does not dominate the total UV luminosity at any radius, as we are detecting only the progeny of the EHB stars that are the likely source of the UVX. We calculate that only a few percent of MS stars in the central bulge can have gone through the HP-HB phase and that this percentage decreases strongly with distance from the center. We also find that the surface density of hot UV-bright stars has the same radial variation as that of low-mass X-ray binaries. We discuss age, metallicity, and abundance variations as possible explanations for the observed radial variation in the UV-bright population.
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