The heavy-elements heritage of the falling sky

Abstract

Recent dynamical analysis based on Gaia data have revealed major accretion events in Milky Way's history. Nevertheless, our understanding of the primordial Galaxy is hindered because the bona fide identification of the most metal-poor and correspondently oldest accreted stars remains challenging. Contrary to alpha-elements, neutron-capture elements present unexplained large abundance spreads for low metallicity stars, that could result from a mixture of formation sites. We have analysed the abundances of yttrium, europium, magnesium and iron in Milky Way satellite galaxies, field halo stars and globular clusters. The chemical information has been complemented with orbital parameters based on Gaia data. In particular, the orbit's average inclination has been considered. The [Y/Eu] abundance behaviour with respect to the [Mg/Fe] turnovers for satellite galaxies of different masses reveals that higher luminosity systems, for which the [Mg/Fe] abundance declines at higher metallicities, present enhanced [Y/Eu] abundances, particularly in the [Fe/H] regime between -2.25 and -1.25 dex. In addition, the analysis has uncovered a chemo-dynamical correlation for both globular clusters and field stars of the Galactic halo, accounting for about half of the [Y/Eu] abundance spread. [Y/Eu] under-abundances typical of protracted chemical evolutions, are preferentially observed in polar-like orbits, pointing to a possible anisotropy in the accretion processes. Our results strongly suggest that the observed [Y/Eu] abundance spread in the Milky Way halo could result from a mixture of systems with different masses. They also highlight that both nature and nurture are relevant to the Milky Way formation, since its primordial epochs, opening new pathways for chemical diagnostics of our Galaxy building up.

Submitted to A&A Letters, comments welcome

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• Gaia Data Release 3: Chemical cartography of the Milky Way
• The Pristine survey -- XVI. The metallicity of 26 stellar streams around the Milky Way detected with the STREAMFINDER in Gaia EDR3
• The cerium content of the Milky Way as revealed by Gaia DR3 GSP-Spec abundances
• Exploring the dependence of chemical traits on metallicity: chemical trends for red giant stars with asteroseismic ages