Elemental Abundances and their Implications for the Chemical Enrichment of the Bo\"{o}tes~I Ultra-Faint Galaxy

TL;DR

This study analyzes the chemical abundances of stars in the Boötes I galaxy to understand its star formation history and chemical evolution, revealing a halo-like distribution and insights into CEMP-no stars and enrichment processes.

Contribution

It provides a detailed chemical abundance analysis of Boötes I stars, demonstrating its self-enrichment, halo-like abundance patterns, and the existence of two enrichment pathways at low metallicity.

Findings

Boötes I shows halo-like elemental abundance patterns.

Evidence of self-enrichment from primordial conditions.

Presence of two enrichment paths: CEMP-no and carbon-normal.

Abstract

We present a double-blind analysis of high-dispersion spectra of seven red giant members of the Bo\"{o}tes I ultra-faint dwarf spheroidal galaxy, complemented with re-analysis of a similar spectrum of an eighth member star. The stars cover [Fe/H] from -3.7 to -1.9, and include a CEMP-no star with [Fe/H] = -3.33. We conclude from our chemical abundance data that Bo\"{o}tes I has evolved as a self-enriching star-forming system, from essentially primordial initial abundances. This allows us uniquely to investigate the place of CEMP-no stars in a chemically evolving system, in addition to limiting the timescale of star formation. The elemental abundances are formally consistent with a halo-like distribution, with enhanced mean [alpha/Fe] and small scatter about the mean. This is in accord with the high-mass stellar IMF in this low-stellar-density, low-metallicity system being indistinguishable from the present-day solar neighborhood value. There is a non-significant hint of a decline in [alpha/Fe] with [Fe/H]; together with the low scatter, this requires low star formation rates, allowing time for SNe ejecta to be mixed over the large spatial scales of interest. One star has very high [Ti/Fe], but we do not confirm a previously published high value of [Mg/Fe] for another star. We discuss the existence of CEMP-no stars, and the absence of any stars with lower CEMP-no enhancements at higher [Fe/H], a situation which is consistent with knowledge of CEMP-no stars in the Galactic field. We show that this observation requires there be two enrichment paths at very low metallicities: CEMP-no and "carbon-normal".