On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios

TL;DR

This paper uses a semi-analytical model to explain how secular evolution can produce the observed properties of the Milky Way's thin and thick discs, including abundance patterns and gradients, without requiring external events.

Contribution

It demonstrates that secular evolution alone can account for key disc features and introduces a classification scheme based on element yields and source lifetimes to interpret abundance ratios.

Findings

Secular evolution can produce the double-branch behavior of [alpha/Fe] vs metallicity.

The non-monotonic stellar abundance gradient can be explained by internal disc evolution.

Abundance ratios with different source timescales serve as diagnostics for galactic evolution.

Abstract

Using a semi-analytical model of the evolution of the Milky Way, we show how secular evolution can create distinct overdensities in the phase space of various properties (e.g. age vs metallicity or abundance ratios vs age) corresponding to the thin and thick discs. In particular, we show how key properties of the Solar vicinity can be obtained by secular evolution, with no need for external or special events, like galaxy mergers or paucity in star formation. This concerns the long established double-branch behaviour of [alpha/Fe] vs metallicity and the recently found non-monotonic evolution of the stellar abundance gradient, evaluated at the birth radii of stars. We extend the discussion to other abundance ratios and we suggest a classification scheme, based on the nature of the corresponding yields (primary vs secondary or odd elements) and on the lifetimes of their sources (short-lived vs long-lived ones). The latter property is critical in determining the single- or double- branch behavior of an elementary abundance ratio in the Solar neighborhood. We underline the high diagnostic potential of this finding, which can help to separate clearly elements with sources evolving on different timescales and help determining the site of e.g. the r-process(es). We define the "abundance distance" between the thin and thick disc sequences as an important element for such a separation. We also show how the inside-out evolution of the Milky Way disc leads rather to a single-branch behavior in other disc regions.