Abundance Patterns in the Draco, Sextans, and Ursa Minor Dwarf Spheroidal Galaxies

By M. Shetrone, P. Côté, W. Sargent. 2000

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The Keck I telescope and the High Resolution Echelle Spectrometer (HIRES) have been used to obtain spectra for red giant stars belonging to the Draco, Sextans, and Ursa Minor dwarf spheroidal (dSph) galaxies. An analysis of these spectra is presented, along with abundance ratios for more than 20 elements. The resulting database of element abundances for 17 dSph stars is the most extensive yet assembled for stars in such environments. Our principal findings are summarized as follows. (1) There is unambiguous evidence for a large internal dispersion in metallicity in all three galaxies: our program stars span a range of Δ[Fe/H] = 1.53, 1.40, and 0.73 dex in Draco, Sextans, and Ursa Minor, respectively. (2) The abundance patterns among the dSph stars are remarkably uniform, suggesting that three galaxies have similar nucleosynthetic histories and, presumably, similar initial mass functions. (3) A comparison of the measured element abundance ratios for our sample of dSph stars with published values for Galactic halo and disk field stars suggests that the dSph galaxies have 0.02 ≲ [α/Fe] ≲ 0.13 dex, whereas the halo field star sample has [α/Fe] ~ 0.28 dex over the same range in metallicity. (4) The most metal-rich dSph stars in our sample have [Y/Fe] abundances that are significantly lower than those measured for halo field stars of similar metallicity, while the measured [Ba/Eu] ratios for the dSph stars suggest that the early chemical evolution of these galaxies was dominated by the r-process. Taken together, these results suggest that the Galactic halo is unlikely to have assembled, in its entirety, through the disruption of dwarf galaxies similar to the low-luminosity, ⟨LV⟩ = 3 × 105LV,☉, dSph galaxies studied here. We also note that the measured [Zn/Fe] abundance ratios for the dSph stars are lower than those of damped Lyα systems having low levels of dust depletion by roughly an order of magnitude. The first high-resolution abundance analysis for the distant Galactic globular cluster NGC 2419 is also presented. From a HIRES spectrum of a single red giant, we find a metallicity of [Fe/H] = -2.32 ± 0.11 dex. This is slightly lower than, but still consistent with, published estimates based on low-resolution spectroscopy. With the possible exception of a slight enhancement in the abundances of some heavy elements such as Ce, Nd, Y, and Ba, the observed abundance pattern closely resembles those exhibited by red giants in M92: a nearby, well-studied globular cluster of nearly identical metallicity.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/89c3068b287415b6db9848cac4b0b69f460210d0

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Citations in the corpus

Stellar Chemical Signatures and Hierarchical Galaxy Formation (2004, methodology): “Shetrone et al. (2001, 2003) determined the elemental abundances of 36 red giants in seven dwarf galaxies and compared those with stars in the Galaxy, but they found very little to connect these systems (see also Tolstoy et al. 2003, Fulbright 2002, and Stephens & Boesgaard 2002).”
MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. II. CATALOG OF STARS IN MILKY WAY DWARF SATELLITE GALAXIES (2010, methodology): “Six stars, listed in Table 7, have previously published HRS abundance measurements (Shetrone et al. 2001; Sadakane et al. 2004), and all six were observed in our study.”
The Galaxy and its stellar halo: insights on their formation from a hybrid cosmological approach (2008, result): “A number of recent studies have raised concerns about the Searle & Zinn scenario on the basis of the observation that stars in Local Group dwarf spheroidals (dSph’s) tend to have lower α abundances than stars in the stellar halo (Shetrone et al. 2001; Tolstoy et al. 2003; Venn et al. 2004).”
DETAILED CHEMICAL ABUNDANCES IN THE r-PROCESS-RICH ULTRA-FAINT DWARF GALAXY RETICULUM 2 (2016, background): “Material produced by the main component of the r-process has also been found in Car and UMi, two other dSph galaxies with stellar masses similar to Dra (Shetrone et al. 2001, 2003; Sadakane et al. 2004; Cohen & Huang 2010; Venn et al. 2012).”
A New View of the Dwarf Spheroidal Satellites of the Milky Way from VLT FLAMES: Where Are the Very Metal-poor Stars? (2006, background): “Unfortunately, the currently available high resolution spectra of metal-poor stars in dSph is very sparse with only a few published examples of stars below−2.5 dex (e.g. Shetrone et al. 2001; Sadakane et al. 2004; Fulbright, Rich & Castro 2004) and as such is still inconclusive.”
Properties of stellar generations in globular clusters and relations with global parameters (2010, background): “…of [Al/Mg] with [Mg/H] for metal-poor field stars in the Milky Way (Fulbright et al. 2007; Reddy et al. 2003; Gehren et al. 2006; Jonsell et al. 2005; Fulbright 2000), as well as for stars in dSph galaxies (Koch et al. 2008; Geisler et al. 2004; Shetrone et al. 2001, 2003; Sbordone et al. 2004).”
The Progenitors of Dwarf Spheroidal Galaxies (2002, background): “Moreover, the large metallicity spreads observed in a number of dSphs that are dominated by ancient star formation episodes (Shetrone et al. 2001, 2003; Ikuta & Arimoto 2002; Carigi, Hernandez & Gilmore 2002) as well as radial gradients in old populations (Harbeck et al. 2001) indicate that dSphs…”
Chemical Abundance Distributions of Galactic Halos and Their Satellite Systems in a ΛCDM Universe (2005, background): “Specifically, at metallicities of [Fe/H]≤ −1, stars in the local halo have above solar [α/Fe] ratios ([α/Fe]∼ 0.2 − 0.4), whereas stars in the Local Group dwarf galaxies have nearly solar abundances ([α/Fe]∼ 0.1) (Fuhrmann 1998; Shetrone et al. 2001, 2003; Venn et al. 2004).”
Population Gradients in Local Group Dwarf Spheroidal Galaxies (2001, result): “A recent spectroscopic study by Shetrone et al. (2001) of RGB stars in the dSphÏs Draco, Ursa Minor, and Sextans suggests a decreased a-element abundance (0.02¹ [a/Fe]¹ 0.12) among these LG dSphÏs compared to the Galactic GCs.”
The stellar halo of the Galaxy (2008, methodology): “This in turn has been used to show that dwarf galaxies could not have been significant contributors of the stellar halo if accreted after they formed a significant fraction of their stars (Shetrone et al. 2001; Tolstoy et al. 2003; Venn et al. 2004).”
Theoretical isochrones in several photometric systems. II. The Sloan Digital Sky Survey ugriz system (2004, methodology): “…galaxy Draco is dominated by old populations, it differs from single-age, single-population objects like globular clusters in showing a considerable abundance spread of more than one dex in [Fe/H] (Lehnert et al. 1992; Shetrone, Bolte, & Stetson 1998; Shetrone et al. 2001; and references therein).”
Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo (2004, background): “Like Shetrone et al. (2001), these authors have demonstrated that the chemical signatures of most of the dSph stars are distinct from the stars in each of the components of the Galaxy.”
Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy (2005, background): “If the overall [Fe/H] values for these stars are correct, they would be as metal-poor as the most metalpoor red giants found in other nearby dwarf galaxies (e.g., Shetrone et al. 2001).”
A REVISED PARALLEL-SEQUENCE MORPHOLOGICAL CLASSIFICATION OF GALAXIES: STRUCTURE AND FORMATION OF S0 AND SPHEROIDAL GALAXIES (2011, background): “The α element abundances in dSph galaxies are not much enhanced with respect to solar, also consistent with prolonged star formation histories (Shetrone et al. 1998, 2001, 2003, 2009; Tolstoy et al. 2003; Venn et al. 2004; Geisler et al. 2005).”