A Tale of Two Origins: In-Situ versus Accreted Nitrogen-Rich Field Stars in the MW

TL;DR

This study analyzes nitrogen-rich field stars in the Milky Way to determine their origins, revealing that some originated from accreted dwarf galaxies while others formed in-situ, using chemodynamical properties and orbital analysis.

Contribution

It provides the first detailed chemodynamical characterization of a sample of N-rich stars, distinguishing their origins from different progenitor systems.

Findings

High-energy N-rich stars likely originated from accreted dwarf galaxies.

Low-energy N-rich stars probably formed in-situ within the Milky Way.

Chemical patterns of N-rich stars resemble high-redshift N-emitters.

Abstract

Spectroscopic surveys have identified significant numbers of metal-poor nitrogen-rich (N-rich) field stars. These stars are strong candidates for escapees from globular clusters (GCs), as their distinctive nitrogen enhancement mirrors the chemical patterns observed in some of the members of GCs. As part of the effort to characterize their chemodynamical properties, we derived abundances for up to 25 elements in a sample of 33 N-rich field giant stars (18 of them are studied for the first time) using high-resolution optical spectroscopy. We confirm their elevated abundances of N, Na, and Al, strongly supporting a GC origin. Given that Galactic GCs themselves formed within diverse progenitor galaxies, we sought to identify the ancestral systems of these N-rich field stars. By analyzing their dynamical parameters, we separated the sample into high-energy (HE) and low-energy (LE) groups. The HE group exhibits lower [{\alpha}/Fe] and enhanced r-process abundances compared to the LE group. This indicates that the HE stars likely escaped from GCs accreted from massive dwarf galaxies (e.g., Gaia-Sausage-Enceladus), while the LE stars probably originated from in-situ GCs. We also find that the chemical pattern of these N-rich stars with [Fe/H] {\lessapprox} -1.0 are similar to the high-redshift ''N-emitters''. Furthermore, orbital integrations revealed a close encounter between one N-rich field star and the globular cluster NGC 6235. Our work demonstrates the potential of using chemodynamical analyses to trace Galactic assembly through chemical peculiar stars, while highlighting that larger samples and more precise data in the future are crucial to establish definitive origins.