Chemical Signatures of the First Supernovae in the Sculptor Dwarf Spheroidal Galaxy

TL;DR

This study analyzes the chemical compositions of the most metal-poor stars in the Sculptor dwarf galaxy, revealing unique abundance patterns and suggesting enrichment by a few early supernovae, including the galaxy's most metal-poor star.

Contribution

First detailed homogeneous chemical abundance analysis of Sculptor's extremely metal-poor stars, identifying the lowest metallicity star in an external galaxy and revealing diverse enrichment histories.

Findings

Two new extremely metal-poor stars discovered in Sculptor.

The lowest metallicity star in an external galaxy identified at [Fe/H] = -4.1.

Significant star-to-star abundance scatter indicating enrichment by few supernovae.

Abstract

We present a homogeneous chemical abundance analysis of five of the most metal-poor stars in the Sculptor dwarf spheroidal galaxy. We analyze new and archival high resolution spectroscopy from Magellan/MIKE and VLT/UVES and determine stellar parameters and abundances in a consistent way for each star. Two of the stars in our sample, at [Fe/H] = -3.5 and [Fe/H] = -3.8, are new discoveries from our Ca K survey of Sculptor, while the other three were known in the literature. We confirm that Scl 07-50 is the lowest metallicity star identified in an external galaxy, at [Fe/H] = -4.1. The two most metal-poor stars both have very unusual abundance patterns, with striking deficiencies of the alpha elements, while the other three stars resemble typical extremely metal-poor Milky Way halo stars. We show that the star-to-star scatter for several elements in Sculptor is larger than that for halo stars in the same metallicity range. This scatter and the uncommon abundance patterns of the lowest metallicity stars indicate that the oldest surviving Sculptor stars were enriched by a small number of earlier supernovae, perhaps weighted toward high-mass progenitors from the first generation of stars the galaxy formed.