Multiplicity of ancient Population II stars

星星形成的一个主要关键是恒星的多样性。最详细的观测来自太阳附近的场星。在这里，我们基本上遇到了两个不同的群体：人口I（薄盘）恒星，年龄高达8 Gyr，和人口II（厚盘）恒星-定义银河系形成的12至13 Gyr的星暴人口。大多数（>60%）的太阳型星族I场星被证明是非单一的，随着初始质量的增加而急剧增加，并且有很大一部分（>20%）的更高级别的系统。由于它的极端年龄，星族II主要是一个黑暗的恒星群体，其成员超过一个太阳质量左右已经变成退化，大多数是白色矮星，只留下一小部分普通的太阳型恒星。关于它们的形成、初始质量函数和多重性，我们对第二星族基本上一无所知，因为它不存在已知成员的代表性样本。基于我们在25秒差距内的大约500颗太阳型恒星的体积完整样本--以及其中只有30颗星族II恒星的一小部分--我们打算观察、分析和表征一个明显更大的、统计上有意义的、无偏的、体积完整的样本，其中包括100多颗附近42秒差距内的星族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质量转移（蓝离散星）和合并系统，这些系统对恒星年龄至关重要。同样，我们发现了具有前所未有的光谱精度的暗简并同伴的证据，部分已经通过直接成像和/或可变径向速度得到证实，或者可能会在即将到来的盖亚DR 3精确天体测量轨道解决方案中被发现，从2022年开始。在完成42 pc星族II巡天后，我们期望对银河系形成时期的古代太阳型恒星的质量、金属丰度和多重性分布有一个第一次有见地和公正的看法。

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.