论文标题
球状簇M19中两个发光后恒星的光谱证实
Spectroscopic Confirmation of Two Luminous Post-AGB Stars in the Globular Cluster M19
论文作者
论文摘要
球状簇（GC）中最亮的恒星是从渐近巨型分支（AGB）中演变出来的，并经过光谱类型F和A-“黄色”后AGB（YPAGB）恒星。 YPAGB恒星可能是测量外层面距离的出色“人口II”标准蜡烛。 A recent survey of the Galactic GC system, using uBVI photometry to identify stars of low surface gravities with large Balmer discontinuities, discovered a candidate luminous yPAGB star in the GC M19 (NGC 6273), designated ZNG 4. The same survey also identified a bright, hotter candidate blue PAGB star, ZNG 2, lying near the top of the white-dwarf cooling sequence.在最近的Gaia早期数据版本3中，两个PAGB候选者都有适当的动作和视差，与集群成员一致，但他们仍然缺乏光谱验证。在这里，我们提出了这两个恒星的中等分辨率光谱，将它们确认为极有可能是群集成员的低重力对象。通过与合成光谱库进行比较，我们对恒星的大气参数进行了近似估计。我们发现YPAGB恒星Zng 4的有效温度为teff〜6500 K，log g〜1的表面重力和[Fe/H]〜 -1.5的金属性，类似于宿主群集。蓝色PAGB星Zng 2具有teff〜18000 K，log g〜3，并且在[Fe/H]〜 -2.0至-2.5的范围内显然很低金属性。两颗恒星都是明亮的（分别为12.5和13.3）。我们敦促高分散光谱随访，以确定详细的大气参数和化学成分，并监测径向速度。
The visually brightest stars in globular clusters (GCs) are the ones evolving off the asymptotic giant branch (AGB) and passing through spectral types F and A--the "yellow" post-AGB (yPAGB) stars. yPAGB stars are potentially excellent "Population II" standard candles for measuring extragalactic distances. A recent survey of the Galactic GC system, using uBVI photometry to identify stars of low surface gravities with large Balmer discontinuities, discovered a candidate luminous yPAGB star in the GC M19 (NGC 6273), designated ZNG 4. The same survey also identified a bright, hotter candidate blue PAGB star, ZNG 2, lying near the top of the white-dwarf cooling sequence. Both PAGB candidates have proper motions and parallaxes in the recent Gaia Early Data Release 3 consistent with cluster membership, but they still lacked spectroscopic verification. Here we present moderate-resolution spectra of both stars, confirming them as low-gravity objects that are extremely likely to be cluster members. Through comparison with a library of synthetic spectra, we made approximate estimates of the stars' atmospheric parameters. We find that the yPAGB star ZNG 4 has an effective temperature of Teff ~ 6500 K, a surface gravity of log g ~ 1, and a metallicity of [Fe/H] ~ -1.5, similar to that of the host cluster. The blue PAGB star ZNG 2 has Teff ~ 18000 K, log g ~ 3, and an apparently low metallicity in the range of [Fe/H] ~ -2.0 to -2.5. Both stars are bright (V=12.5 and 13.3, respectively). We urge high-dispersion spectroscopic follow-up to determine detailed atmospheric parameters and chemical compositions, and to monitor radial velocities.
