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

In this work we present the first spectroscopic results obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. We find that the Sagittarius dwarf spheroidal galaxy (Sgr) contains a youngest stellar population with [O/H]≈−0.2 and age t > 1 Gyr, and an...

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Published in:Monthly notices of the Royal Astronomical Society 2008-08, Vol.388 (4), p.1667-1678
Main Authors: Kniazev, Alexei Y., Zijlstra, Albert A., Grebel, Eva K., Pilyugin, Leonid S., Pustilnik, Simon, Väisänen, Petri, Buckley, David, Hashimoto, Yas, Loaring, Nicola, Romero, Encarni, Still, Martin, Burgh, Eric B., Nordsieck, Kenneth
Format: Article
Language:English
Subjects:
Astronomy
Astrophysics
galaxies: individual: Sagittarius dwarf spheroidal
Nebulae
planetary nebulae: general
Radio telescopes
Spectrum analysis
Stars & galaxies
stars: abundances
stars: mass-loss
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Summary:In this work we present the first spectroscopic results obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. We find that the Sagittarius dwarf spheroidal galaxy (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 StWr 2-21 with 12 + log(O/H) = 8.57 ± 0.02 dex. The other PN studied, BoBn 1, is an extraordinary object in that the neon abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn 1 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 asymptotic giant branch (AGB) stars. Well defined broad WR lines are present in the spectrum of StWr 2-21 and absent in the spectrum of BoBn 1. This puts the fraction of [WR]-type central PNe stars to 67 per cent for dSph galaxies.
ISSN:0035-8711
1365-2966
DOI:10.1111/j.1365-2966.2008.13435.x