The Chemical Evolution of the Draco Dwarf Spheroidal Galaxy

Abstract

We present an abundance analysis based on high resolution spectra of 8 stars selected to span the full range in metallicity in the Draco dwarf spheroidal galaxy. We find [Fe/H] for the sample stars ranges from −1.5 to −3.0 dex. Combining our sample with previously published work for a total of 14 luminous Draco giants, we show that the abundance ratios [Na/Fe], [Mg/Fe] and [Si/Fe] for the Draco giants overlap those of Galactic halo giants at the lowest [Fe/H] probed, but are significantly lower for the higher Fe-metallicity Draco stars. For the explosive α-elements Ca and Ti, the abundance ratios for Draco giants with [Fe/H] > −2.4 dex are approximately constant and slightly sub-solar, well below values characteristic of Galactic halo stars. The sprocess contribution to the production of heavy elements begins at significantly lower Fe-metallicity than in the Galactic halo.Using a toy model we compare the behavior of the abundance ratios within the sample of Draco giants with those from the literature of Galactic globular clusters, and the Carina and Sgr dSph galaxies. The differences appear to be related to the timescale for buildup of the heavy elements, with Draco having the slowest rate.We note the presence of a Draco giant with [Fe/H] < −3.0 dex in our sample, and reaffirm that the inner Galactic halo could have been formed by early accretion of Galactic satellite galaxies and dissolution of young globular clusters, while the outer halo could have formed from those satellite galaxies accreted later. Stellar Sample and Stellar ParametersOur sample in the Draco dSph galaxy contains 8 stars; details are given in Table 1. It was selected from Table 2.9 of Winnick (2003) to include stars which are known radial velocity members of this satellite to the Galaxy at or near the RGB tip spanning the full range in color and in metallicity and not previously observed at high spectral resolution. Winnick measured the infrared Ca triplet in moderate resolution spectra of Draco stars obtained using the multi-fiber Hydra spectrograph at the WIYN telescope. Her sample was chosen from the photometric survey of Piatek et al. (2001) and previous radial velocity surveys (see, e.g. Armandroff, Olszewski & Pryor 1995). Only confirmed radial velocity members are listed in her table; carbon stars known to be members of Draco were excluded. She developed her own calibration of the relationship between Ca triplet indices and Fe or Ca metallicity based on data for Galactic globular clusters. Details of the calibration and related issues are discussed in the appendix. We adopt the procedures described in Cohen et al. (2002) and used in all subsequent work by the first author published to date to determine the stellar parameters for our sample of luminous Draco giants. Our T ef f determinations are based on the broad band colors V − J and V − K. The optical photometry (we use V only, the V − I color supports the deduced T ef f in most cases, but is not used) is from the SDSS (York et al. 2000) using the transformation equations of Smith e...

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

“…We compare our results with high-resolution spectroscopic analyses of RGBs with [Fe/H] ≤ −2.5 in the Milky Way halo (Cayrel et al 2004;Cohen et al 2008;Aoki et al 2005Aoki et al , 2007Cohen et al 2006;Lai et al 2008;Honda et al 2004) and in faint (Ursa MajorII and Coma Berenices, Boötes I, Leo IV) and classical (Sextans, Draco, Fornax, Carina, Sculptor) dSphs (Letarte et al 2010;Shetrone et al 2001;Fulbright et al 2004;Koch et al 2008a;Cohen & Huang 2009;Aoki et al 2009;Norris et al 2010;Simon et al 2010;Frebel et al 2010b,a). We distinguish between unmixed and mixed RGBs in order to be able to draw a reliable comparison sample for the elements that are sensitive to mixing.…”

“…Large triangles identify the 5 stars analyzed here, while small triangles indicate stars in earlier publications (Letarte et al 2010;Aoki et al 2009;Shetrone et al 2001;Frebel et al 2010a). Open squares: UFDs, Carina and Draco (Fulbright et al 2004;Koch et al 2008a,b;Frebel et al 2010b;Cohen & Huang 2009;Norris et al 2010;Simon et al 2010). The Milky Way halo stars are represented by circles; open circles are unmixed RGBs and filled circles mixed RGBs (Cayrel et al 2004;Cohen et al 2008;Aoki et al 2005;Spite et al 2005;Aoki et al 2007;Cohen et al 2006;Lai et al 2008;Honda et al 2004).…”

“…Aoki et al (2007) argued that given that mixing on the RGB and extra mixing at the tip of the RGB lower the surface abundance of carbon in late-type stars compared to their earlier stage of evolution, the definition of carbon-enriched EMP stars (CEMPS) should depend on the stellar luminosity. (Fulbright et al 2004;Koch et al 2008a;Frebel et al 2010b;Cohen & Huang 2009;Norris et al 2010). Only mixed Milky Way halo stars (black dots) are considered (Cayrel et al 2004;Cohen et al 2008;Honda et al 2004;Aoki et al 2005Aoki et al , 2007Cohen et al 2006;Lai et al 2008).…”

TOPoS

“…To date, the most metal-poor star known in external galaxies is found in Sculptor and has a metallicity near −4.0 (Tafelmeyer et al 2010); two more stars of similar metallicity are known in Sculptor (Frebel et al 2010a;Simon et al 2015) and six more below −3.0 (Starkenburg et al 2013;Simon et al 2015). 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).…”

Galactic evolution of rapid neutron capture process abundances: the inhomogeneous approach

“…Data for the observed stars to compare our simulation results with are taken from the SAGA (Stellar Abundances for Galactic Archaeology) database (e.g., Suda et al 2008, Suda et al 2011, Yamada et al 2013in particular [Eu/Fe] abundance observations are mainly from e.g., Francois et al 2007, Simmerer et al 2004, Barklem et al 2005, Ren et al 2012, Roederer et al 2010, Roederer et al 2014a, Roederer et al 2014b, Roederer et al 2014c, Shetrone, Côté, Stetson 2001, Shetrone et al 2003, Geisler et al 2005, Cohen & Huang 2009, Letarte et al 2010, Starkenburg et al 2013, McWilliam et al 2003. From the raw data, we excluded carbon enriched metal poor stars ("CEMPs") and stars with binary nature, since the surface abundances of such objects are expected to be affected by internal pollution from deeper layers or pollution from the binary companion.…”