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Register a new userHomogeneity in the early chemical evolution of the Sextans dwarf Spheroidal galaxy
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We present the high-resolution spectroscopic analysis of two new extremely metal-poor stars (EMPS) candidates in the dwarf spheroidal galaxy Sextans. These targets were pre-selected from medium resolution spectra centered around the Ca II triplet in the near-infrared and followed-up at higher resolution with VLT/UVES. We confirm their low metallicities with [Fe/H]=-2.95 and [Fe/H]=-3.01, placing them among the most metal-poor stars known in Sextans. The abundances of 18 elements, including C, Na, the alpha-elements, Fe-peak, and neutron capture elements, are determined. In particular, we present the first measurements of Zn in a classical dwarf at extremely low metallicity. There has been previous hints of a large scatter in the abundance ratios of the Sextans stellar population around [Fe/H] -3 when compared to other galaxies. We took the opportunity of this work to re-analyse the full sample of EMPS and find a Milky-Way -like plateau and a normal dispersion at fixed metallicity.
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We present the analysis of the FLAMES dataset targeting the central 25 arcmin region of the Sextans dSph. This dataset is the third major part of the high resolution spectroscopic section of the ESO large program 171.B-0588(A) obtained by the Dwarf galaxy Abundances and Radial-velocities Team (DART). Our sample is composed of red giant branch stars down to the level of the horizontal branch in Sextans. It allows to address questions related to both stellar nucleosynthesis and galaxy evolution. We provide metallicities for 81 stars, which cover the wide [Fe/H]=$-$3.2 to $-$1.5 dex range. The abundances of 10 other elements are derived: Mg, Ca, Ti, Sc, Cr, Mn, Co, Ni, Ba, and Eu. Despite its small mass, Sextans is a chemically evolved system, with evidence for the contribution of core-collapse and Type Ia supernovae as well as low metallicity AGBs. This new FLAMES sample offers a sufficiently large number of stars with chemical abundances derived at high accuracy to firmly establish the existence of a plateau in [$alpha$/Fe] at $sim 0.4$ dex, followed by a decrease above [Fe/H]$sim-2$ dex. This is in stark similarity with the Fornax and Sculptor dSphs despite their very different masses and star formation histories. This suggests that these three galaxies had very similar star formation efficiencies in their early formation phases, probably driven by the early accretion of smaller galactic fragments, until the UV-background heating impacted them in different ways. The parallel between the Sculptor and Sextans dSph is also striking when considering Ba and Eu. Finally, as to the iron-peak elements, the decline of [Co/Fe] and [Ni/Fe] above [Fe/H]$sim -2$ implies that the production yields of Ni and Co in SNeIa is lower than that of Fe. The decrease in [Ni/Fe] favours models of SNeIa based on the explosion of double degenerate sub-Chandrasekhar mass white dwarfs.
We present the star formation history and chemical evolution of the Sextans dSph dwarf galaxy as a function of galactocentric distance. We derive these from the $VI$ photometry of stars in the $42 times 28$ field using the SMART model developed by Yuk & Lee (2007, ApJ, 668, 876) and adopting a closed-box model for chemical evolution. For the adopted age of Sextans 15 Gyr, we find that $>$84% of the stars formed prior to 11 Gyr ago, significant star formation extends from 15 to 11 Gyr ago ($sim$ 65% of the stars formed 13 to 15 Gyr ago while $sim$ 25% formed 11 to 13 Gyr ago), detectable star formation continued to at least 8 Gyr ago, the star formation history is more extended in the central regions than the outskirts, and the difference in star formation rates between the central and outer regions is most marked 11 to 13 Gyr ago. Whether blue straggler stars are interpreted as intermediate age main sequence stars affects conclusions regarding the star formation history for times 4 to 8 Gyr ago, but this is at most only a trace population. We find that the metallicity of the stars increased rapidly up to [Fe/H]=--1.6 in the central region and to [Fe/H]=--1.8 in the outer region within the first Gyr, and has varied slowly since then. The abundance ratios of several elements derived in this study are in good agreement with the observational data based on the high resolution spectroscopy in the literature. We conclude that the primary driver for the radial gradient of the stellar population in this galaxy is the star formation history, which self-consistently drives the chemical enrichment history.
We present the high resolution spectroscopic study of five -3.9<=[Fe/H]<=-2.5 stars in the Local Group dwarf spheroidal, Sculptor, thereby doubling the number of stars with comparable observations in this metallicity range. We carry out a detailed analysis of the chemical abundances of alpha, iron peak, light and heavy elements, and draw comparisons with the Milky Way halo and the ultra faint dwarf stellar populations. We show that the bulk of the Sculptor metal-poor stars follows the same trends in abundance ratios versus metallicity as the Milky Way stars. This suggests similar early conditions of star formation and a high degree of homogeneity of the interstellar medium. We find an outlier to this main regime, which seems to miss the products of the most massive of the TypeII supernovae. In addition to its value to help refining galaxy formation models, this star provides clues to the production of cobalt and zinc. Two of our sample stars have low odd-to-even barium isotope abundance ratios, suggestive of a fair proportion of s-process; we discuss the implication for the nucleosynthetic origin of the neutron capture elements.
We present a homogeneous chemical abundance analysis of five of the most metal-poor stars in the Sculptor dwarf spheroidal galaxy. We analyze new and archival high resolution spectroscopy from Magellan/MIKE and VLT/UVES and determine stellar parameters and abundances in a consistent way for each star. Two of the stars in our sample, at [Fe/H] = -3.5 and [Fe/H] = -3.8, are new discoveries from our Ca K survey of Sculptor, while the other three were known in the literature. We confirm that Scl 07-50 is the lowest metallicity star identified in an external galaxy, at [Fe/H] = -4.1. The two most metal-poor stars both have very unusual abundance patterns, with striking deficiencies of the alpha elements, while the other three stars resemble typical extremely metal-poor Milky Way halo stars. We show that the star-to-star scatter for several elements in Sculptor is larger than that for halo stars in the same metallicity range. This scatter and the uncommon abundance patterns of the lowest metallicity stars indicate that the oldest surviving Sculptor stars were enriched by a small number of earlier supernovae, perhaps weighted toward high-mass progenitors from the first generation of stars the galaxy formed.
We present a large sample of fully self-consistent hydrodynamical Nbody/Tree-SPH simulations of isolated dwarf spheroidal galaxies (dSphs). It has enabled us to identify the key physical parameters and mechanisms at the origin of the observed variety in the Local Group dSph properties. The initial total mass (gas + dark matter) of these galaxies is the main driver of their evolution. Star formation (SF) occurs in series of short bursts. In massive systems, the very short intervals between the SF peaks mimic a continuous star formation rate, while less massive systems exhibit well separated SF bursts, as identified observationally. The delay between the SF events is controlled by the gas cooling time dependence on galaxy mass. The observed global scaling relations, luminosity-mass and luminosity-metallicity, are reproduced with low scatter. We take advantage of the unprecedentedly large sample size and data homogeneity of the ESO Large Programme DART, and add to it a few independent studies, to constrain the star formation history of five Milky Way dSphs, Sextans, LeoII, Carina, Sculptor and Fornax. For the first time, [Mg/Fe] vs [Fe/H] diagrams derived from high-resolution spectroscopy of hundreds of individual stars are confronted with model predictions. We find that the diversity in dSph properties may well result from intrinsic evolution. We note, however, that the presence of gas in the final state of our simulations, of the order of what is observed in dwarf irregulars, calls for removal by external processes.
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