Abundances In Very Metal‐Poor Dwarf Stars

Cohen, Judith G.; Christlieb, N.; McWilliam, Andrew; Shectman, Steve; Thompson, Ian; Wasserburg, G. J.; Ivans, Inese I.; Dehn, M.; Karlsson, Torgny; Meléndez, Jorge

doi:10.1086/422576

Cited by 181 publications

(330 citation statements)

References 90 publications

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“…The difference of 0.2-0.4 dex is due to substantial overionization of Cr i at low metallicity. The LTE abundances determined in this work using Cr i lines are consistent with other LTE studies (Johnson 2002;Cayrel et al 2004;Cohen et al 2004;Lai et al 2008;Bonifacio et al 2009) [Cr/Fe] in the solar neighborhood, computed with the two-infall GCE model of Chiappini et al (1997) with SN Ia yields from Iwamoto et al (1999) and metallicity-dependent SN II yields from , agrees with the NLTE results. The model predicts that ∼60% of the total solar Cr and Fe are produced by SNe Ia and remainder by SNe II, the latter synthesizing both elements in roughly solar proportions.…”

Section: Discussionsupporting

confidence: 88%

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Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Bergemann

Cescutti

2010

A&A

164

124

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Aims. We investigate statistical equilibrium of Cr in the atmospheres of late-type stars to ascertain whether the systematic abundance discrepancy between Cr I and Cr II lines, as often found in previous work, is due to deviations from local thermodynamic equilibrium (LTE). Furthermore, we attempt to interpret the Non-LTE (NLTE) trend of [Cr/Fe] with [Fe/H] using chemical evolution models for the solar neighborhood. Methods. NLTE calculations are performed for the model of the Cr atom, comprising 340 levels and 6806 transitions in total. We use the quantum-mechanical photoionization cross-sections of Nahar (2009) and investigate the sensitivity of the model to uncertain crosssections for H I collisions. NLTE line formation is performed for the MAFAGS-ODF model atmospheres of the Sun and 10 metal-poor stars with −3.2 < [Fe/H] < −0.5, and Cr abundances are derived by comparing the synthetic and observed flux spectra. Results. We achieve good ionization equilibrium of Cr for models with different stellar parameters, if inelastic collisions with H I atoms are neglected. The solar NLTE abundance based on Cr I lines is 5.74 dex with σ = 0.05 dex, which is ∼0.1 dex higher than the LTE abundance. For the metal-poor stars, the NLTE abundance corrections to Cr I lines range from +0.3 to +0.5 dex. The resulting [Cr/Fe] ratio is roughly solar for the range of metallicities analyzed here, which is consistent with current views on the production of these iron peak elements in supernovae. Conclusions. The tendency of Cr to become deficient with respect to Fe in metal-poor stars is an artifact caused by the neglect of NLTE effects in the line formation of Cr i, and has no relation to any peculiar physical conditions in the Galactic ISM or deficiencies of nucleosynthesis theory.

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Section: Discussionsupporting

confidence: 88%

“…On the other hand, most LTE analysis of Cr i lines (McWilliam et al 1995;Cayrel et al 2004;Cohen et al 2004) indicate that the [Cr/Fe] ratio steadily declines towards the lowest metallicity. There is no simple explanation for this trend.…”

Section: Introductionmentioning

confidence: 99%

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Bergemann

Cescutti

2010

A&A

164

124

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“…In all figures, the grey points show the location of RGB stars in the Milky Way halo (Honda et al 2004;Cayrel et al 2004;Spite et al 2005Spite et al , 2006Aoki et al 2005Aoki et al , 2007Cohen et al 2013Cohen et al , 2006Cohen et al , 2004Lai et al 2008;Yong et al 2013;Ishigaki et al 2013). The Sculptor stars are shown in orange.…”

Section: Comparison Samples: References Symbols and Colour Codes Inmentioning

confidence: 99%

The early days of the Sculptor dwarf spheroidal galaxy

Jablonka

North

Mashonkina

et al. 2015

A&A

165

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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 α, iron peak, and 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 follow 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 Type II supernovae. In addition to its help in 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.

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“…Christlieb et al 2002; and characterization (e.g. McWilliam et al 1995;Ryan et al 1996;Carretta et al 2002;Cayrel et al 2004;Cohen et al 2004;François et al 2007;Lai et al 2008;Bonifacio et al 2009) of EMPS in the Galaxy over Tables 6 and 7 are only available in electronic form at http://www.aanda.org the last two decades. But similar studies in extra-galactic systems only became feasible with the advent of the 8−10 m class telescopes.…”

Section: Introductionmentioning

confidence: 99%

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

Tafelmeyer

Jablonka

Hill

et al. 2010

A&A

152

287

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We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor, and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to have [Fe/H] < ∼ −3 and three stars are below [Fe/H] ∼ −3.5. This constitutes the first evidence that the classical dSphs Fornax and Sextans join Sculptor in containing extremely metal-poor stars and suggests that all of the classical dSphs contain extremely metal-poor stars. One giant in Sculptor at [Fe/H] = −3.96 ± 0.06 is the most metal-poor star ever observed in an external galaxy. We carried out a detailed analysis of the chemical abundances of the α, iron peak, and the heavy elements, and we performed a comparison with the Milky Way halo and the ultra faint dwarf stellar populations. Carbon, barium, and strontium show distinct features characterized by the early stages of galaxy formation and can constrain the origin of their nucleosynthesis.

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Abundances In Very Metal‐Poor Dwarf Stars

Cited by 181 publications

References 90 publications

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

The early days of the Sculptor dwarf spheroidal galaxy

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

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