Multiplicity of ancient Population II stars

恒星形成的关键是恒星的多样性。最详细的观测来自于太阳系附近的野星。在这里，我们基本上遇到了两个不同的种群：星族I（薄盘）恒星，年龄高达8 Gyr，星族II（厚盘）恒星- 12到13 Gyr的星暴种群，定义了银河系的形成。大多数（60%）类太阳星族I星被证明是非单一的，随着原始质量的增加而急剧增加，并且更高级别系统的比例很大（20%）。由于其极端的年龄，星族II主要是一个黑暗的恒星群，其中超过一个太阳质量的成员已经退化，主要是白矮星，只留下一小部分普通的太阳型恒星。关于它们的形成、初始质量函数和多样性，我们基本上对第二族一无所知，因为那里不存在已知成员的代表性样本。在我们的体积完整样本的基础上，在25pc内大约500颗太阳型恒星——其中只有30颗星族II的一小部分——我们打算观察、分析和描述一个更大的、统计上有意义的、无偏倚的、体积完整的样本，在42pc内超过100颗附近的星族II太阳型恒星。为此，我们过去五年的工作已经揭示了大量的星族II质量传递（蓝离散星）和合并系统，这些系统对恒星年龄至关重要。同样，我们发现暗简并伴星的证据具有前所未有的光谱精度，部分已经被直接成像和/或变径向速度证实，或者可能会被即将到来的盖亚DR3精确天体测量轨道解决方案探测到，从2022年开始。在完成42pc星族II的调查后，我们期望对银河系形成时期的古代太阳型恒星的质量、金属丰度和多样性分布有一个深刻而公正的看法。

A principal key of star formation is the multiplicity of stars. The mostdetailed observations originate from the field stars in the solarneighbourhood. Here we basically meet two distinct populations:Population I (thin-disc) stars, with ages up to 8 Gyr, and Population II(thick-disc) stars - the 12 to 13 Gyr old starburst population thatdefines the formation of the Milky Way Galaxy. The majority (>60%) ofthe solar-type Population I field stars proves to be non-single, with asteep increase as a function of primary mass and a large (>20%)fraction of higher level systems. On account of its extreme age,Population II is mostly a dark stellar population, whose membersexceeding around one solar mass have turned into degenerates,mostly white dwarfs, leaving only a small fraction of ordinary solar-type stars. With respect to their formation, initial mass function, andmultiplicities we know essentially nothing about Population II, for theredoes not exist a representative sample of known members. On thebase of our volume-complete sample of about 500 solar-type starswithin 25 pc - and with a small fraction of only 30 Population II starsamong them - we intend to observe, analyze, and characterize asignificantly larger, statistically meaningful, unbiased, and volume-complete sample of more than 100 nearby Population II solar-typestars within 42 pc. To this end, our work of the last five years hasalready disclosed a significant number of Population II mass transfer(blue straggler) and merger systems that are of key importance forstellar ages. Likewise, we find evidence fordark degenerate companions with unprecedented spectroscopicaccuracy that in part have already been confirmed by direct imagingand/or variable radial velocities, or will likely become detected with theupcoming Gaia DR3 precision astrometric orbital solutions, starting in2022. Upon completion of the 42 pc Population II survey, we expect tohave a first insightful and unbiased view on the mass, metallicity, andmultiplicity distributions of ancient solar-type stars from the formationepoch of the Milky Way.