Detailed Abundances for 28 Metal‐poor Stars: Stellar Relics in the Milky Way

Lai, David K.; Bolte, Michael; Johnson, Jennifer A.; Lucatello, S.; Heger, Alexander; Woosley, S. E.

doi:10.1086/588811

Cited by 301 publications

(432 citation statements)

References 101 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%

“…They have revealed severe problems with modelling the excitation and ionisation balance of Cr in the atmospheres of late-type stars. Systematic differences of 0.1-0.5 dex between abundances based on LTE fitting of the Cr i and Cr ii lines were reported for metal-poor giants and dwarfs (Johnson 2002;Gratton et al 2003;Lai et al 2008;Bonifacio et al 2009). The discrepancies are smaller in the atmospheres with a higher metal content, amounting to ∼0.1 dex for Galactic disk stars (Prochaska et al 2000) and the Sun (Sobeck et al 2007;Asplund et al 2009).…”

Section: Introductionmentioning

confidence: 97%

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

Bergemann

Cescutti

2010

A&A

156

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%

Section: Introductionmentioning

confidence: 97%

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

Bergemann

Cescutti

2010

A&A

156

124

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“…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.…”

Section: Comparison Samplesmentioning

confidence: 90%

“…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

151

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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“…For O, we took the results obtained by Cayrel et al (2004) into account with the correction for stellar surface inhomogeneities from Nissen et al (2002); with this correction, the ratio is smaller in 3-D than in the 1-D computations. We also used the chemical abundances of 9 EMP stars from the recent work by Lai et al (2008), who analyzed a total number of 28 stars. We used only the stars for which the abundances for C, N, and O were measured.…”

Section: Observational Datamentioning

confidence: 99%

The effects of stellar winds of fast-rotating massive stars in the earliest phases of the chemical enrichment of the Galaxy

Cescutti

Chiappini

2010

A&A

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Aims. We use the growing data sets of very-metal-poor stars to study the impact of stellar winds of fast rotating massive stars on the chemical enrichment of the early Galaxy. Methods. We use an inhomogeneous chemical evolution model for the Galactic halo to predict both the mean trend and scatter of C/O and N/O. In one set of models, we assume that massive stars enrich the interstellar medium during both the stellar wind and supernovae phases. In the second set, we consider that in the earliest phases (Z < 10 −8 ), stars with masses above 40 M only enrich the interstellar medium via stellar winds, collapsing directly into black holes. Results. We predict a larger scatter in the C/O and N/O ratios at low metallicities when allowing the more massive fast-rotating stars to contribute to the chemical enrichment only via stellar winds. The latter assumption, combined with the stochasticity in the star formation process in the primordial Galactic halo can explain the wide spread observed in the N/O and C/O ratios in normal very-metal-poor stars. Conclusions. For chemical elements with stellar yields that depend strongly on initial mass (and rotation) such as C, N, and neutron capture elements, within the range of massive stars, a large scatter is expected once the stochastic enrichment of the early interstellar medium is taken into account. We also find that stellar winds of fast rotators mixed with interstellar medium gas are not enough to explain the large CNO enhancements found in most of the carbon-enhanced very-metal-poor stars. In particular, this is the case of the most metal-poor star known to date, HE 1327−2326, for which our models predict lower N enhancements than observed when assuming a mixture of stellar winds and interstellar medium. We suggest that these carbon-enhanced very metal-poor stars were formed from almost pure stellar wind material, without dilution with the pristine interstellar medium.

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Detailed Abundances for 28 Metal‐poor Stars: Stellar Relics in the Milky Way

Cited by 301 publications

References 101 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

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

The effects of stellar winds of fast-rotating massive stars in the earliest phases of the chemical enrichment of the Galaxy

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