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

Abstract

Context. Precise stellar ages from asteroseismology have become available and can help setting 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 at reproducing 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] vs. [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 reproduce 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.

The age–chemical abundance structure of the Galactic disc – II. α-dichotomy and thick disc formation

“…These two starburst phases are bridged by a prolonged phase of low-level star formation. In the traditional two-infall model, instead, the second accretion occcurs at earlier epochs, and on longer timescales (Chiappini, Matteucci & Gratton 1997;Spitoni et al 2019).…”

“…The chemical bimodality in the MW, found over two decades ago (Fuhrmann 1998), remains challenging, not only for phenomenological chemical evolution models (Spitoni et al 2019), but also cosmological simulations (Loebman et Interestingly, in some recent simulations signatures of the α-dichotomy begin to arise, albeit with different origins. Calura & Menci (2009) presents the first chemical evolution model, embedded in a hierarchical semi-analytical model, that qualitatively reproduce the observed bimodal distribution in [α/Fe]-[Fe/H] plane.…”

“…To explain observations from modern spectroscopic surveys, the original'two-infall' model by Chiappini, Matteucci & Gratton (1997) has recently been modified (Grisoni et al 2017;Spitoni et al 2019). Grisoni et al (2017) propose a model in which the two phases of gas infall evolve independently and coevally in order to explain a group of αenhanced, metal-rich stars.…”

“…Grisoni et al (2017) propose a model in which the two phases of gas infall evolve independently and coevally in order to explain a group of αenhanced, metal-rich stars. However, this model fails to produce the age distribution, especially for the low-α sequence, as shown in Spitoni et al (2019). To solve this problem, Spitoni et al (2019) propose a further revision, in which the second phase of gas infall is delayed by 4.3 Gyr.…”

Spatial variations in the Milky Way disc metallicity–age relation

“…Such a gradient implies that the metal-poor, low-α stars formed 10 kpc outwards of the metal-rich stars. Spitoni et al (2019) recently presented a chemical evolution model that suggests a two infall model is sufficient to reproduce the [α/Fe]-[Fe/H]-age distribution of the solar neighborhood using asteroseismic ages. Figure 7 is qualitatively in agreement, but the mean age of the metal-rich stars in this work is 5-6 Gyr compared to 8-10 Gyr as predicted by Spitoni et al (2019).…”