Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy
Koch, A.; Feltzing, Sofia; Adén, Daniel; Matteucci, F. (2013). Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy. Astronomy & Astrophysics, 554,
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Published
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English
Authors:
Koch, A.
;
Feltzing, Sofia
;
Adén, Daniel
;
Matteucci, F.
Department:
Lund Observatory - Has been reorganised
Abstract:
We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 angstrom absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log epsilon(Ba) less than or similar to 0.7 dex (3 sigma). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 angstrom-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range.
Keywords:
galaxies: evolution ;
abundances ;
nucleosynthesis ;
nuclear reactions ;
stars: abundances ;
stars: Population II ;
galaxies: dwarf ;
galaxies: individual: Hercules
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