AGB stars are not expected to be X-ray-emitters, yet a small fraction of them, the so-called X-AGBs, show X-ray emission that can be attributed to coronal activity of a companion or accretion onto it. By searching the recently released eROSITA-DE eRASS1 source catalog, we aim to increase the sample of known X-AGBs. So far, 36 X-AGBs have been reported, including 21 previous ROSAT RASS, Chandra and XMM detections and 15 more eROSITA eRASS1 recent detections. We cross-correlated the position of sources in the eROSITA-DE eRASS1 catalog with those of AGB stars to find possible X-ray counterparts. We carefully checked these by comparing X-ray and near-IR $K$ images, disregarding those affected by optical loading, diffuse sources, or unreliable positional associations. Seven high-confidence X-AGBs and another seven possible ones are found. Accounting for previous X-ray detections, the sample of X-AGBs is increased from 36 up to 47, of which eROSITA-DE has so far discovered 26 new X-AGBs, more than doubling the number of known X-AGBs. This demonstrates eROSITA's capability to detect X-AGBs despite the challenge posed by the optical loading caused by their near-IR brightness. The eRASS1 X-AGBs tend to have higher X-ray luminosity than previously detected X-AGBs, suggesting a bias toward brighter sources. A comparison of the X-ray and far-UV luminosity of X-AGBs with those of X-ray-emitter symbiotic stars (X-SySts) revealed an overlap in the X-ray luminosity range $10^{29.5}<L_X$ (erg s$^{-1}$) $<10^{33.0}$. The average higher X-ray luminosity of X-SySts AGBs ($\approx 10^{32}$ erg s$^{-1}$) can be interpreted as X-ray emission arising from a boundary layer between an accretion disk and a white dwarf, whereas the average lower X-ray luminosity ($\approx5\times10^{30}$ erg s$^{-1}$) of X-AGBs would arise from an accretion disk around main-sequence or subgiant F-K companion stars.
The X-ray catalogue of FGK stars within 10 pc: The coronal temperature-brightness relation explained with the RTV scaling law
Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen（蒂宾根大学天文与天体物理研究所）；INAF – Osservatorio Astronomico di Palermo “G.S. Vaiana”（意大利国家天体物理研究所巴勒莫天文台（G.S. Vaiana））；Hamburger Sternwarte, University of Hamburg（汉堡大学汉堡天文台）
Clean catalogues of blue horizontal-branch stars using Gaia EDR3
背景：蓝色水平分支恒星是非常古老的天体，可以用作研究银河系结构和形成历史的标记。为了创建一个蓝色水平分支恒星的干净天空目录，我们将Gaia数据发布2（DR 2）数据集与现有的参考目录进行交叉匹配，以根据Gaia DR 2参数定义选择标准。在盖亚早期数据发布3（EDR 3）发布后，这些方法得到了验证，并随后应用于最新版本。目的：该星表的目的是确定一组蓝色水平分支恒星候选者，这些候选者是通过光度和天体测量观测选出的，并且显示出低污染率。方法：我们将参考蓝色水平分支数据集与Gaia DR 2数据库进行交叉匹配，并定义了两套选择标准。首先，在Gaia DR 2-颜色和绝对G幅度空间中，其次，在Gaia DR 2-颜色和约化自行空间中。通过集中研究银河系的星系晕，在星表的两个子集中，主序星的污染被减少了，但这是以牺牲完整性为代价的，因为相对年轻的主序星不可能出现在那里。结果：我们提出了一个目录，基于盖亚EDR 3，57,377蓝色水平分支恒星。盖亚EDR 3视差用于选择16，794名候选人，自行用于识别另外40，583名候选人。
Chromospheric activity catalogue of 4454 cool stars. Questioning the active branch of stellar activity cycles
色球活动监测范围广泛的冷星可以提供有价值的信息，恒星的磁活动及其对基本恒星参数的依赖，如有效的温度和rotation.We汇编的档案HARPS光谱和多个其他调查，包括威尔逊山的数据，最近已被国家统计局公布的组合4454冷星的色球活动目录。我们探讨了不同有效温度的冷星色球活动沿主序星的变化。此外，我们还进行了一个活动周期搜索，并调查其与旋转的关系。从存档的高分辨率HARPS光谱中测量了304颗主序星的色球活动指数S指数。此外，测量和存档的S指数被转换成色球通量比log R 'HK。使用广义Lomb-Scargle周期图确定活动周期，以研究旋转活动周期平面上的活动和非活动分支。全球样本显示，色球活动的双峰性，即沃恩-普雷斯顿间隙，并不突出，有相当大比例的恒星处于中等活动水平，log R 'HK =-4.75左右。独立地，循环周期搜索表明恒星可以位于活动和非活动分支之间的中间区域，这意味着活动分支并不像以前认为的那样明显。沃恩-普雷斯顿间隙的减弱表明，冷恒星从较高的活动水平开始旋转，并在较低的活动水平上稳定下来，而不会在中等活动水平上突然中断。有些周期接近太阳活动支和非活动支之间的值，这表明太阳发电机很可能是恒星发电机的一种常见情况。
