High-resolution spectroscopy of extremely metal-poor stars in the least evolved galaxies: Bootes II

TL;DR

This study provides high-resolution spectroscopic data of four metal-poor stars in Bootes II, revealing their chemical patterns and suggesting limited stellar enrichment, with implications for understanding galaxy formation and neutron-capture element origins.

Contribution

First detailed high-resolution chemical abundance analysis of stars in Bootes II, highlighting unique neutron-capture element patterns and binary star presence in an ultra-faint dwarf galaxy.

Findings

Stars have chemical patterns similar to the Galactic halo.

Neutron-capture elements Sr and Ba are undetectable in all stars.

One star shows radial velocity variations indicating a binary system.

Abstract

We present high-resolution Magellan/MIKE spectra of the four brightest confirmed red giant stars in the ultra-faint dwarf galaxy Bootes II (Boo II). These stars all inhabit the metal-poor tail of the Boo II metallicity distribution function. The chemical abundance pattern of all detectable elements in these stars is consistent with that of the Galactic halo. However, all four stars have undetectable amounts of neutron-capture elements Sr and Ba, with upper limits comparable to the lowest ever detected in the halo or in other dwarf galaxies. One star exhibits significant radial velocity variations over time, suggesting it to be in a binary system. Its variable velocity has likely increased past determinations of the Boo II velocity dispersion. Our four stars span a limited metallicity range, but their enhanced {\alpha}-abundances and low neutron-capture abundances are consistent with the interpretation that Boo II has been enriched by very few generations of stars. The chemical abundance pattern in Boo II confirms the emerging trend that the faintest dwarf galaxies have neutron-capture abundances distinct from the halo, suggesting the dominant source of neutron-capture elements in halo stars may be different than in ultra-faint dwarfs.