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Multiplicity of stars as a function of mass

Applicant Dr. Klaus Fuhrmann
Subject Area Astrophysics and Astronomy
Term from 2010 to 2017
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 163865239
 
Final Report Year 2017

Final Report Abstract

A thorough assessment of the multiplicity of stars as a function of mass requires unbiased volume-complete samples and a detailed scrutiny of the existing companions. To this end, the solar neighbourhood is a natural laboratory, where in the mass range 0.90 ≤ M ≤ 1.70 M⊙ of the F- and G-type stars one can rely on a complete set of the primaries with reference to the accurate Hipparcos parallaxes. The inventory of the companion stars, however, is less well known and this observational circumstance will also not be solved with the upcoming Gaia precision astrometry. Yet, there is a continuous and remarkable progress in this field of research in the sense that, when this DFG project started in 2010, there was the general believe that a majority of the solar-type stars are single, whereas we know by now that at least about 60% of them are non-single. Actually, at least two out of three sources prove to be non-single if we restrict this census to the more massive F-type stars. Likewise, it was believed in 2010 that only about 10% of the solar-type stars are part of higher level systems, a percentage that has no less than doubled since then. Both results, to which the present project significantly contributes, have far-reaching consequences. In the first place, the enormous two-third percentage of non-single F-type stars is completely at odds with the inventory of their descendants – the white dwarfs – where only a one-third companion fraction is presently known. This inevitably means that there is a very insufficient census of the locally existing degenerate stars with important implications on the baryonic mass content and hence the dynamics of the Milky Way. The second point, the more than 20% fraction of triple and higher level systems, has the principal consequence that orbital evolution, including the formation of blue straggler stars, is a potentially important issue on all spatial and time scales for many systems, in particular with reference to stellar age estimates. Important aspects of the present work therefore arise from the explicit identification of the Population I and Population II disc stars, the former with stellar ages up to 8 Gyr, the latter at or above 12 Gyr. Here, for the Population II disc stars, upon which the Milky Way came into being, we find a significant fraction of blue straggler stars and white dwarf companions and also tentatively identify an even larger fraction of non-single stars compared to Population I. This may hint at important differences of the star-formation process in an early violent environment and potentially contradicts the notion of an invariant initial mass function.

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