On ancient solar-type stars – II

Abstract

ABSTRACT We report on the progress of our survey on ancient solar-type stars down to main-sequence effective temperatures Teff ≥ 5300 K and within 42 pc of the Sun. High signal-to-noise, high-resolution spectroscopy is presented for a second major subset of the Population II (τ ≥ 12 Gyr) and the intermediate-disc stars (τ ≃ 10 Gyr) within that volume. In conjunction with updates and the analyses of the single and composite sample spectra, we discuss evidence for new companions or candidates from their radial velocities, chromospheric activities, lithium and barium enrichments, and we also draw attention to related sources in the Gaia DR2 data. Among the Population II stars we note a substantial fraction of degenerates, mass transfer, and merger systems that possibly amount to about 20 per cent of that population, with inherently important consequences on the involved stellar ages and the baryonic mass budget. At the present stage, the survey has reached a two-thirds level of local volume-completeness. Key to that objective will be the forthcoming Gaia data, in terms of new companions, companion masses, and precision parallaxes from orbital solutions, in particular at the sample periphery, where many of the sources inevitably reside. In an appendix we describe a subset of about fifty a priori survey candidates, whose analyses discard them as Population I stars.