HE 1327−2326, an Unevolved Star with [Fe/H] &lt; −5.0. I. A Comprehensive Abundance Analysis

Aoki, Wako; Frebel, Anna; Christlieb, N.; Norris, John E.; Beers, Timothy C.; Minezaki, Takeo; Barklem, P. S.; Honda, Satoshi; Takada-Hidai, Masahide; Asplund, Martin; Ryan, Sean G.; Tsangarides, Stelios; Eriksson, K.; Steinhauer, A.; Deliyannis, Constantine P.; Nomoto, Ken’ichi; Fujimoto, Masayuki Y.; Ando, Hiroyasu; Yoshii, Yuzuru; Kajino, Toshitaka

doi:10.1086/497906

Cited by 187 publications

(240 citation statements)

References 88 publications

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“…This specific dilution factor was used to match the C abundance derived from the most metal-poor star discovered to date, the subgiant star HE 1327-2326, with [Fe/H] = −5.96 (Col. 4 of Table 1, with typical error bars ±0.25; Aoki et al 2006;Frebel et al 2008). By applying this dilution factor our model self-consistently reproduces the observed C, N, and O abundances within a factor of 4 for N and 3 for O (observational error bars are ∼a factor of 2), with Fe increasing by only 20%.…”

Section: Surface Composition and Comparison To The Halo Star He 1327-supporting

confidence: 54%

Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars

Campbell

Lugaro

Karakas

2010

A&A

117

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Context. Models of primordial and hyper-metal-poor stars that have masses similar to the Sun are known to experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. Aims. We aim to model the nucleosynthesis occurring during the proton ingestion event to ascertain if any significant neutron-capture nucleosynthesis occurs. Methods. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star with mass 1 M and a metallicity of [Fe/H] = −6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2366 reactions and treats mixing and burning simultaneously. Results. We find that the mixing and burning of protons into the hot convective core leads to the production of 13 C, which then burns via the 13 C(α, n) 16 O reaction, releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14 N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later carried to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr overabundance. Conclusions. Our model provides the possibility of self-consistently explaining the Sr overabundance in HE 1327-2326 together with its C, N, and O overabundances (all within a factor of ∼4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If this is true, it puts constraints on the primordial initial mass function.

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Section: Surface Composition and Comparison To The Halo Star He 1327-supporting

confidence: 54%

Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars

Campbell

Lugaro

Karakas

2010

A&A

117

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“…In addition, this star has a potentially low 12 C/ 13 C (as only a lower limit of 5 is given by Aoki et al 2006) typical of hydrogen-burned material (see Meynet et al 2010). We notice that HE0107−5240 has a 12 C/ 13 C = 60 ± 10 (Christlieb, priv.…”

Section: Resultsmentioning

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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“…Unfortunately, our knowledge of the chemical fingerprints of the enrichment from a few or even single SNe are based on very few stars in dSphs and UFDs (only two stars in Hercules, three in Leo IV and Boo I, and the extreme Dra 119 in the Draco dSph), and a handful of extremely metal-poor halo stars with [Fe/H] around −5 to −3 dex (e.g., Aoki et al 2006;Cohen et al 2008). In the present paper we aim to expand our knowledge on the enrichment patterns and chemical enrichment histories of dSph galaxies by providing more data on the neutron-capture element Ba in the Hercules UFD.…”

Section: Discussionmentioning

confidence: 99%

Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

Koch

Feltzing

Adén

et al. 2013

A&A

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We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 Å absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log ε(Ba) < ∼ 0.7 dex (3σ). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 Å-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range.

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HE 1327−2326, an Unevolved Star with [Fe/H] < −5.0. I. A Comprehensive Abundance Analysis

Cited by 187 publications

References 88 publications

Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars

Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars

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

Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

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