The metallicity extremes of the Sagittarius dSph using SALT spectroscopy of PNe

TL;DR

This study presents spectroscopic analysis of planetary nebulae in the Sagittarius dwarf galaxy using SALT, revealing diverse stellar populations and extreme metallicity variations, including a rare neon-rich nebula.

Contribution

First spectroscopic results from SALT on Sagittarius dSph PNe, showing extreme metallicity differences and detailed elemental abundances, including a neon-rich planetary nebula.

Findings

Sagittarius dSph contains young and old stellar populations with distinct metallicities.

One planetary nebula shows exceptionally high neon abundance exceeding oxygen.

The metallicity of BoBn1 is extremely low, about 1/110 of solar, with evidence of stellar dredge-up processes.

Abstract

In this work we present the first spectroscopic results obtained with the Southern African Large Telescope (SALT) telescope during its perfomance-verification phase. We find that the Sagittarius dwarf spheroidal galaxy (Sgr) Sgr contains a youngest stellar population with [O/H] -0.2 and age t>1 Gyr, and an oldest population with [O/H]=-2.0. The values are based on spectra of two planetary nebulae (PNe), using empirical abundance determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He. We confirm the high abundances of PN StWr2-21 with 12+log(O/H) = 8.57+/-0.02 dex. The other PN studied, BoBn1, is an extraordinary object in that the neon abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn1 yield the original stellar metallicity, corresponding to 12+log(O/H) = 6.72+/-0.16 dex which is 1/110 of the solar value. The actual [O/H] is much higher: third dredge-up enriched the material by a factor of ~12 in oxygen, ~240 in nitrogen and ~70 in neon. Neon as well as nitrogen and oxygen content may have been produced in the intershell of low-mass AGB stars. Well defined broad WR lines are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn1. This puts the fraction of [WR]-type central PNe stars to 67% for dSph galaxies.