Atypical Mg-poor Milky Way field stars with globular cluster second-generation like chemical patterns

TL;DR

This paper reports the discovery of eleven Milky Way field stars with unique chemical patterns resembling second-generation globular cluster stars, challenging current understanding of Galactic formation and suggesting possible extragalactic origins.

Contribution

It identifies a new class of atypical stars with unusual chemical signatures, providing insights into Galactic evolution and globular cluster dissolution.

Findings

Eleven stars exhibit strong Al and N enhancements with Mg depletions.

Majority of these stars are metal-rich with peculiar chemical patterns.

The stars may have extragalactic origins or result from exotic binary systems.

Abstract

We report the peculiar chemical abundance patterns of eleven atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low-Mg abundances ([Mg/Fe]$<$0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H]$\gtrsim - 1.0$) sample stars, which is at odds with actual observations of SG stars in Galactic CGs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy; a fundamental step forward to understand the Galactic formation and evolution.