Galactic Archaeology with asteroseismic ages: Evidence for delayed gas infall in the formation of the Milky Way disc

¹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: spitoni@phys.au.dk
² Dipartimento di Fisica, Sezione di Astronomia, Università di Trieste, Via G.B. Tiepolo 11, 34131 Trieste, Italy
³ I.N.A.F. – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste, Italy
⁴ I.N.F.N. – Sezione di Trieste, Via Valerio 2, 34100 Trieste, Italy
⁵ I.N.A.F. – Osservatorio Astronomico di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy

Received: 4 September 2018
Accepted: 28 January 2019

Abstract

Context. Precise stellar ages from asteroseismology have become available and can help to set stronger constraints on the evolution of the Galactic disc components. Recently, asteroseismology has confirmed a clear age difference in the solar annulus between two distinct sequences in the [α/Fe] versus [Fe/H] abundance ratios relation: the high-α and low-α stellar populations.

Aims. We aim to reproduce these new data with chemical evolution models including different assumptions for the history and number of accretion events.

Methods. We tested two different approaches: a revised version of the “two-infall” model where the high-α phase forms by a fast gas accretion episode and the low-α sequence follows later from a slower gas infall rate, and the parallel formation scenario where the two disc sequences form coevally and independently.

Results. The revised two-infall model including uncertainties in age and metallicity is capable of reproducing: i) the [α/Fe] versus [Fe/H] abundance relation at different Galactic epochs, ii) the age−metallicity relation and the time evolution [α/Fe]; iii) the age distribution of the high-α and low-α stellar populations, iv) the metallicity distribution function. The parallel approach is not capable of properly reproducing the stellar age distribution, in particular at old ages.

Conclusions. The best chemical evolution model is the revised two-infall one, where a consistent delay of ∼4.3 Gyr in the beginning of the second gas accretion episode is a crucial assumption to reproduce stellar abundances and ages.