The Highly Unusual Chemical Composition of the Hercules Dwarf Spheroidal Galaxy

Abstract

We report on the abundance analysis of two red giants in the faint Hercules dwarf spheroidal (dSph) galaxy. These stars show a remarkable deficiency in the neutron-capture elements, while the hydrostatic a-elements (O, Mg) are strongly enhanced. Our data indicate [Ba/Fe] and [Mg/Fe] abundance ratios of ՇϪ2 and ∼ϩ0.8 dex, respectively, with essentially no detection of other n-capture elements. In contrast to the only other dSph star with similar abundance patterns, Dra 119, which has a very low metallicity at [Fe/H] p Ϫ2.95 dex, our objects, at [Fe/H] ∼ Ϫ2.0 dex, are only moderately metal-poor. The measured ratio of hydrostatic/explosive a-elements indicates that high-mass (∼35 M , ) Type II supernovae progenitors are the main, if not only, contributors to the enrichment of this galaxy. This suggests that star formation and chemical enrichment in the ultrafaint dSphs proceeds stochastically and inhomogeneously on small scales, or that the IMF was strongly skewed to high-mass stars. The neutron capture deficiencies and the [Co/Fe] and [Cr/Fe] abundance ratios in our stars are similar to those in the extremely low metallicity Galactic halo. This suggests that either our stars are composed mainly of the ejecta from the first, massive, Population III stars (but at moderately high [Fe/H]), or that SN ejecta in the Hercules galaxy were diluted with ∼30 times less hydrogen than typical for extreme metal-poor stars.

TOPoS

“…Other local galaxies with at least one such star are Draco (Shetrone et al 2001;Fulbright et al 2004;Cohen & Huang 2009), Sagittarius (Zaggia et al 2004;Bonifacio et al 2006;Sbordone et al 2015;and Monaco et al, in prep. ), Fornax (Tafelmeyer et al 2010), Ursa Minor (Kirby & Cohen 2012;Ural et al 2015), Sextans (Aoki et al 2009b;Tafelmeyer et al 2010), Coma Ber (Frebel et al 2010b), Ursa Maior II (Frebel et al 2010b), Segue 1 (Roederer & Kirby 2014;Frebel et al 2014), Hercules (Koch et al 2008;Adén et al 2011), Boo I (Gilmore et al 2013;Ishigaki et al 2014b), and Boo II (Koch & Rich 2014). Particularly relevant to the present discussion is the discovery of a CEMP-no star in the Sculptor dwarf spheroidal (Skúladóttir et al 2015).…”

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

“…We also inserted the sample of Milky Way halo stars of Barklem et al (2005), similarly excluding carbon-rich and [Ba/Eu] > 0 (s-process enriched stars). As to the UFDs, we use the analysis and upper limits in [Ba/Fe] of Koch et al (2008b) on Hercules (plus sign). In the case of Scl07-49, we could only estimate an upper limit for [Ba/H] in that we indicate with an arrow.…”

Chemical abundances for the outer halo cluster Pal 4 from co-added high-resolution spectroscopy

“…Given its large uncertainty, and because we cannot rule out the possibility that this ratio has been affected by non-LTE effects, we will refrain from drawing any conclusions about the contributions from massive stars yields to these elements' production in Pal 4 (cf. McWilliam et al 1995;Koch et al 2008a). Likewise, the [Mn/Fe] ratio in Pal 4, at −0.18 dex, is marginally higher than the value of ≈−0.4 dex found for halo stars in the same [Fe/H] interval (for which we supplemented the plot with data from Gratton 1989;Feltzing & Gustafsson 1998;Prochaska et al 2000;Nissen et al 2000;Johnson 2002, andCayrel et al 2004; see also McWilliam et al 2003).…”

“…Our comparison therefore focuses on a sample of 13 halo GCs, seven dSph galaxies (Sagittarius, Fornax, Draco, Sextans, Carina, Ursa Minor and Leo II) with abundance data from Shetrone et al (2001Shetrone et al ( , 2003Shetrone et al ( , 2009 (2009); and the five so-called "ultra-faint" dSph galaxies (hereafter UF-dSphs; Hercules, Coma Berenices, Ursa Major II, Bootes I and Leo IV) with published abundance information (Koch et al 2008a;Frebel et al 2010;Feltzing et al 2009;Simon et al 2010).…”