Multiplicity of ancient Population II stars

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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.