Local subgiants and time-scales of disc formation

Abstract

Detailed evolutionary tracks and basic stellar parameters for the nearby subgiants 104 Tau and HD 168443 are presented and discussed. Both serve as invaluable key stars for accurate age-datings, especially as they possess the chemical signatures intermediate to the stars of the thick- and thin-disc population. We derive τ= 9.7−0.6+0.8 Gyr for 104 Tau and τ= 9.8−0.8+1.0 Gyr for HD 168443, very much in line with their local sibling HR 7569 at τ= 9.1 ± 1.0 Gyr that was formerly presented in Bernkopf, Fiedler & Fuhrmann. Both, 104 Tau and HD 168443, thus add to our local sample of the rare but very relevant subgiants for the disc formation time-scales and support the notion of a thick disc as an extremely old τ≥ 12 Gyr single-burst population, and a thin disc that has a local age of τ∼ 8 Gyr.