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.

We study the chemical evolution of the thick and thin discs of the Galaxy by comparing detailed chemical evolution models with recent data from the AMBRE Project. The data suggest that the stars in the thick and thin discs form two distinct sequences with the thick disc stars showing higher [α/Fe] ratios. We adopt two different approaches to model the evolution of thick and thin discs. In particular, we adopt: i) a two-infall approach where the thick disc forms fast and before the thin disc and by means of a fast gas accretion episode, whereas the thin disc forms by means of a second accretion episode on a longer timescale; ii) a parallel approach, where the two discs form in parallel but at different rates. By comparing our model results with the observed [Mg/Fe] vs. [Fe/H] and the metallicity distribution functions in the two Galactic components, we conclude that the parallel approach can account for a group of α-enhanced metal rich stars present in the data, whereas the two-infall approach cannot explain these stars unless they are the result of stellar migration. In both approaches, the thick disc has formed on a timescale of accretion of 0.1 Gyr, whereas the thin disc formed on a timescale of 7 Gyr in the solar region. In the two-infall approach a gap in star formation between the thick and thin disc formation of several hundreds of Myr should be present, at variance with the parallel approach where no gap is present.

Context. The majority of chemical evolution models assume that the Galactic disk forms by means of infall of gas and divide the disk into several independent rings without exchange of matter between them. However, if gas infall is important, radial gas flows should be taken into account as a dynamical consequence of infall. Aims. We test the effects of radial gas flows on detailed chemical evolution models (one-infall and two-infall) of the Milky Way disk with different prescriptions for the infall law and star formation rate. Methods. We modified the equation of chemical evolution to include radial gas flows according to the method described in Portinari & Chiosi (2000, A&A, 355, 929). Results. We found that with a gas radial inflow of constant speed the metallicity gradient tends to steepen. Taking into account a constant timescale for the infall rate along the Galaxy disk and radial flows with a constant speed, we obtained too flat a gradient, at variance with data, implying that an inside-out formation and/or a variable gas flow speed are required. To explain the observed gradients, the gas flow should increase in modulus with the galactocentric distance, in both the one-infall and two-infall models. However, the inside-out disk formation coupled with a threshold in the gas density (only in the two-infall model) for star formation and/or a variable efficiency of star formation with galactocentric distance can also reproduce the observed gradients without radial flows. Conclusions. We show that the radial flows can be the most important process in reproducing abundance gradients but only with a variable gas speed. Finally, one should consider that uncertainties in the data concerning gradients prevent us from drawing firm conclusions. Future more detailed data will help us to ascertain whether the radial flows are a necessary ingredient in the formation and evolution of the Galactic disk and disks in general.

Aims. Our aim is to show how different hypotheses about type Ia supernova progenitors can affect Galactic chemical evolution. Supernovae Ia are believed to be the main producers of Fe and the timescale with which Fe is restored into the interstellar medium depends on the assumed supernova progenitor model. This is a way of selecting the most appropriate progenitor model for supernovae Ia, a still debated issue. Methods. We include different type Ia SN progenitor models, identified by their distribution of time delays, in a very detailed chemical evolution model for the Milky Way which follows the evolution of several chemical species. We test the single degenerate and the double degenerate models for supernova Ia progenitors, as well as other more empirical models based on differences in the time delay distributions. Results. We find that assuming the single degenerate or the double degenerate scenario produces negligible differences in the predicted [O/Fe] vs. [Fe/H] relation. On the other hand, assuming a percentage of prompt (exploding in the first 100 Myr) type Ia supernovae of 50%, or that the maximum type Ia rate is reached after 3-4 Gyr from the beginning of star formation, as suggested by several authors, produces more noticeable effects on the [O/Fe] trend. However, given the spread still existing in the observational data, no model can be firmly excluded on the basis of only the [O/Fe] ratios. On the other hand, when the predictions of the different models are compared with the G-dwarf metallicity distribution, the scenarios with very few prompt type Ia supernovae can be excluded. Conclusions. Models including the single degenerate or double degenerate scenario with a percentage of 10-13% of prompt type Ia supernovae produce results in very good agreement with the observations. A fraction of prompt type Ia supernovae larger than 30% worsens the agreement with observations and the same occurs if no prompt type Ia supernovae are allowed. In particular, two empirical models for the type Ia SN progenitors can be excluded: the one without prompt type Ia supernovae and the one assuming a delay time distribution that is ∝t −0.5 . We conclude that the typical timescale for the Fe enrichment in the Milky Way is around 1-1.5 Gyr and that type Ia supernovae already should appear during the halo phase.

In this paper, we study the formation and chemical evolution of the Milky Way disc with particular focus on the abundance patterns ([α/Fe] vs. [Fe/H]) at different Galactocentric distances, the present-time abundance gradients along the disc and the time evolution of abundance gradients. We consider the chemical evolution models for the Galactic disc developed by Grisoni et al. (2017) for the solar neighborhood, both the two-infall and the one-infall ones, and we extend our analysis to the other Galactocentric distances. In particular, we examine the processes which mainly influence the formation of the abundance gradients: the inside-out scenario, a variable star formation efficiency, and radial gas flows. We compare our model results with recent abundance patterns obtained along the Galactic disc from the APOGEE survey and with abundance gradients observed from Cepheids, open clusters, HII regions and PNe. We conclude that the inside-out scenario is a key ingredient, but cannot be the only one to explain abundance patterns at different Galactocentric distances and abundance gradients. Further ingredients, such as radial gas flows and variable star formation efficiency, are needed to reproduce the observed features in the thin disc. The evolution of abundance gradients with time is also shown, although firm conclusions cannot still be drawn.