A primordial star in the heart of the Lion

Caffau, E.; Bonifacio, P.; François, P.; Spite, M.; Spite, F.; Zaggia, S.; Ludwig, H.‐G.; Steffen, M.; Mashonkina, L.; Monaco, L.; Sbordone, L.; Molaro, P.; Cayrel, Roger; Plez, B.; Hill, V.; Hammer, F.; Randich, S.

doi:10.1051/0004-6361/201118744

Cited by 117 publications

(147 citation statements)

References 78 publications

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“…Since it is not visible, we are able to place an upper limit [C/Fe]< +0.7. However, our derived metallicity is higher than that determined by Caffau et al (2012), due to the ISM contribution to the Ca ii H and K lines. The metallicity is compatible with our analysis in Aguado et al (2017) with ISIS and UVESsmoothed data.…”

Section: Testing the Methodology With J1019+1729contrasting

confidence: 90%

New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy

Aguado

Prieto

Hernández

et al. 2017

A&A

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Context. The first generation of stars formed in the Galaxy left behind the chemical signatures of their nucleosynthesis in the interstellar medium, visible today in the atmospheres of low-mass stars that formed afterwards. Sampling the chemistry of those low-mass provides insight into the first stars. Aims. We aim to increase the samples of stars with extremely low metal abundances, identifying ultra metal-poor stars from spectra with modest spectral resolution and signal-to-noise ratio (S/N). Achieving this goal involves deriving reliable metallicities and carbon abundances from such spectra. Methods. We carry out follow-up observations of faint, V>19, metal-poor candidates selected from SDSS spectroscopy and observed with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at GTC. The SDSS and follow-up OSIRIS spectra were analyzed using the FERRE code to derive effective temperatures, surface gravities, metallicities and carbon abundances. In addition, a well-known extremely metal-poor star has been included in our sample to calibrate the analysis methodology. Results. We observed and analyzed five metal-poor candidates from modest-quality SDSS spectra. All stars in our sample have been confirmed as extremely metal-poor stars, in the [Fe/H] < −3.3 regime. We report the recognition of J173403+644632, a carbonenhanced ultra metal-poor dwarf star with [Fe/H] = −4.3 and [C/Fe] = +3.1.Conclusions.

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Section: Testing the Methodology With J1019+1729contrasting

confidence: 90%

New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy

Aguado

Prieto

Hernández

et al. 2017

A&A

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“…Our analysis of the SDSS spectrum of this relatively bright star for our sample (g = 17.3) yields estimates of T eff = 5888 K, log g = 4.47, and [Fe/H] = −4.4, which compare well with those from the high-resolution analysis by Caffau et al (2012), namely T eff = 5811 ± 150 K, log g = 4.0 ± 0.5, and [Fe/H] = −4.89 ± 0.10.…”

Section: Additional Validationssupporting

confidence: 77%

Deep SDSS optical spectroscopy of distant halo stars

Prieto

Fernández-Alvar

Schlesinger

et al. 2014

A&A

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Aims. We analyze a sample of tens of thousands of spectra of halo turnoff stars, obtained with the optical spectrographs of the Sloan Digital Sky Survey (SDSS), to characterize the stellar halo population "in situ" out to a distance of a few tens of kpc from the Sun. In this paper we describe the derivation of atmospheric parameters. We also derive the overall stellar metallicity distribution based on F-type stars observed as flux calibrators for the Baryonic Oscillations Spectroscopic Survey (BOSS). Methods. Our analysis is based on an automated method that determines the set of parameters of a model atmosphere that reproduces each observed spectrum best. We used an optimization algorithm and evaluate model fluxes by means of interpolation in a precomputed grid. In our analysis, we account for the spectrograph's varying resolution as a function of fiber and wavelength. Our results for early SDSS (pre-BOSS upgrade) data compare well with those from the SEGUE Stellar Parameter Pipeline (SSPP), except for stars with log g (cgs units) lower than 2.5.Results. An analysis of stars in the globular cluster M 13 reveals a dependence of the inferred metallicity on surface gravity for stars with log g < 2.5, confirming the systematics identified in the comparison with the SSPP. We find that our metallicity estimates are significantly more precise than the SSPP results. We also find excellent agreement with several independent analyses. We show that the SDSS color criteria for selecting F-type halo turnoff stars as flux calibrators efficiently excludes stars with high metallicities, but does not significantly distort the shape of the metallicity distribution at low metallicity. We obtain a halo metallicity distribution that is narrower and more asymmetric than in previous studies. The lowest gravity stars in our sample, at tens of kpc from the Sun, indicate a shift of the metallicity distribution to lower abundances, consistent with what is expected from a dual halo system in the Milky Way.

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“…In addition, the latest findings by Caffau et al (2011Caffau et al ( , 2012) push the lowest metallicity at which low mass stars can form, increasing the need of detailed models for this class of stars. All these results reinforce the necessity of detailed studies of the PIE, both with hydrodynamic simulations and stellar evolutionary models to achieve a more realistic picture of the its properties.…”

Section: Discussionmentioning

confidence: 99%

“…III stars can be formed in multiple systems with a flat protostellar mass function (M ∼ 0.1−10 M ). Moreover, the recent discovery of an EMP star with a metallicity of [Fe/H] = −4.89 and no enhancement in CNO elements (Caffau et al 2011(Caffau et al , 2012, which results in the lowest total metallicity Z ≤ 10 −4 Z observed in a star, also puts into question the claims of a minimum metallicity required for the formation of low-mass stars (Bromm & Loeb 2003;Schneider et al 2003). These results reinforce the necessity of determining the mass range of the stars responsible for the chemical signatures that are observed in EMP stars to constrain the initial mass function in the early Universe.…”

Section: Introductionmentioning

confidence: 99%

S-process in extremely metal-poor, low-mass stars

Cruz¹,

Serenelli²,

Weiß³

2013

A&A

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Context. Extremely metal-poor (EMP), low-mass stars experience an ingestion of protons into the helium-rich layer during the core He-flash, resulting in the production of neutrons through the reactions 12 C(p, γ) 13 N(β) 13 C(α, n) 16 O. This is a potential site for the production of s-process elements in EMP stars, which does not occur in more metal-rich counterparts. The signatures of s-process elements in the two most iron deficient stars observed to date, HE1327-2326 & HE0107-5240, still await for an explanation. Aims. We investigate the possibility that low-mass EMP stars could be the source of s-process elements observed in extremely iron deficient stars, either as a result of self-enrichment or in a binary scenario as the consequence of a mass transfer episode. Methods. We present evolutionary and post-processing s-process calculations of a 1 M stellar model with metallicities of Z = 0, 10 −8 , and 10 −7 . We assess the sensitivity of nucleosynthesis results to uncertainties in the input physics of the stellar models with particular regard to the details of convective mixing during the core He-flash. Results. Our models provide the possibility of explaining the C, O, Sr, and Ba abundance for the star HE0107-5240 as the result of mass-transfer from a low-mass EMP star. The drawback of our model is that nitrogen would be overproduced and the 12 C/ 13 C abundance ratio would be underproduced in comparison to the observed values if mass would be transferred before the primary star enters the asymptotic giant branch phase. Conclusions. Our results show that low-mass EMP stars cannot be ruled out as companion stars that might have polluted HE1327-2326 and HE0107-5240 and produced the observed s-process pattern. However, more detailed studies of the core He-flash and the proton ingestion episode are needed to determine the robustness of our predictions.

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A primordial star in the heart of the Lion

Cited by 117 publications

References 78 publications

New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy

New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy

Deep SDSS optical spectroscopy of distant halo stars

S-process in extremely metal-poor, low-mass stars

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