X-shooter GTO: evidence for a population of extremely metal-poor, alpha-poor stars

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

doi:10.1051/0004-6361/201322213

Cited by 41 publications

(57 citation statements)

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“…In spite of the higher resolution of our UVES spectrum of SDSS J1742+2531 compared to the X-Shooter spectrum presented in Caffau et al (2013a), we can confidently detect only five metallic lines in addition to Ca ii H and K, which were also detected in the X-Shooter spectrum: the two strongest lines of the infrared Ca ii triplet and three Fe i lines. These are listed in Table 10.…”

Section: Sdss J1742+2531mentioning

confidence: 58%

“…This implies that NLTE and granulation effects have opposite directions, and they may tend to cancel; however, for iron it is not possible to simply add the 3D Thompson et al 2008;Aoki et al 2008;Behara et al 2010;Placco et al 2011;Carollo et al 2012;Masseron et al 2010Masseron et al , 2012Yong et al 2013;Cohen et al 2013;Spite et al 2013;Caffau et al 2013a). The bimodal distribution is clear in this plot.…”

Section: Effects Of Granulationmentioning

confidence: 92%

“…For SDSS J1742+2531, in Caffau et al (2013a) we adopted an effective temperature of 6345 K deduced from the (g − z) 0 colour and consistent with the wings of Hα from our X-Shooter spectrum. The wings of Hα from the UVES spectrum are also consistent with this temperature.…”

Section: Sdss J1742+2531mentioning

confidence: 99%

“…As solar abundances we adopted carbon, nitrogen, oxygen and iron from Caffau et al (2011), for the other elements Lodders et al (2009). If too few iron lines were measured to allow the determination of the microturbulence we adopted 1.5 km s −1 as in Caffau et al (2013a).…”

Section: Chemical Abundancesmentioning

confidence: 99%

“…SDSS J1742+2531 was already analysed on the basis of an X-Shooter spectrum by Caffau et al (2013a). In that paper we were able to provide the abundance of C, from the G-band and of Ca, from the Ca ii K line.…”

Section: Introductionmentioning

confidence: 99%

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TOPoS

Bonifacio

Caffau

Spite

et al. 2015

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Context. In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars. Aims. We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns. Methods. We analyse both X-Shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum line formation code. were not able to detect any iron lines, yet we could place a robust (3σ) upper limit of [Fe/H] < −5.0 and measure the Ca abundance, with [Ca/H] = −5.0, and carbon, A(C) = 6.90, suggesting that this star could be even more metal-poor than SDSS J1742+2531. This makes these two stars the seventh and eighth stars known so far with [Fe/H] < −4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a robust upper limit of A(Li) < 1.8 for both stars. Conclusions. Our measured carbon abundances confirm the bimodal distribution of carbon in CEMP stars, identifying a high-carbon band and a low-carbon band. We propose an interpretation of this bimodality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas Article published by EDP Sciences A28, page 1 of 20 A&A 579, A28 (2015) cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements. The abundance pattern of the UIP stars shows a large star-to-star scatter in the [X/Ca] ratios for all elements up to aluminium (up to 1 dex), but this scatter drops for heavier elements and is at most of the order of a factor of two. We propose that this can be explained if these stars are formed from gas that has been chemically enriched by several SNe, that produce the roughly constant [X/Ca] ratios for the heavier elements, and in some cases the gas has also been polluted by the ejecta of a faint SN that contributes the lighter elements in variable amounts. The absence of lithium in four of the five known unevolved UIP stars can be explained by a dominant role of fragmentation in the formation of these stars. This would result either in a destruction of lithium in the pre-main-sequence phase, through rotational mixing or to a lack of late accretion from a reservoir of fresh gas. The phenomenon should have varying degrees of efficiency.

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Section: Sdss J1742+2531mentioning

confidence: 58%

Section: Effects Of Granulationmentioning

confidence: 92%

Section: Sdss J1742+2531mentioning

confidence: 99%

Section: Chemical Abundancesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Caffau

Spite

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Evidence for the accretion origin of halo stars with an extreme r-process enhancement

et al. 2019

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Small stellar systems like dwarf galaxies are suggested to be the main building blocks of our Galaxy by numerical simulations 1 in Lambda CDM models. The existence of star streams like Sagittarius tidal stream 2 indicates that dwarf galaxies play a role in the formation of the Milky Way. However, it is unclear how many and what kind of stars in our Galaxy are originated from satellite dwarf galaxies, which could be constrained by chemical abundances of metal-poor stars. Here we report on the discovery of a metal-poor star with an extreme r-process enhancement and α-element deficiency. In this star, the abundance ratio of the

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TOPoS

Caffau

Bonifacio

Sbordone

et al. 2013

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Context. The metal-weak tail of the metallicity distribution function (MDF) of the Galactic Halo stars contains crucial information on the formation mode of the first generation of stars. To determine this observationally, it is necessary to observe large numbers of extremely metal-poor stars. Aims. We present here the Turn-Off Primordial Stars survey (TOPoS) that is conducted as an ESO Large Programme at the VLT. This project has four main goals: (i) to understand the formation of low-mass stars in a low-metallicity gas: determine the metal-weak tail of the halo MDF below [M/H] = −3.5; in particular, we aim at determining the critical metallicity, that is the lowest metallicity sufficient for the formation of low-mass stars; (ii) to determine in extremely metal-poor stars the relative abundances of the elements that are the signature of the massive first stars; (iii) to determine the trend of the lithium abundance at the time when the Galaxy formed; and (iv) to derive the fraction of C-enhanced extremely metal-poor stars with respect to normal extremely metal-poor stars. The large number of stars observed in the SDSS provides a good sample of candidate stars at extremely low metallicity. Methods. Candidates with turn-off colours down to magnitude g = 20 were selected from the low-resolution spectra of SDSS by means of an automated procedure. X-Shooter has the potential of performing the necessary follow-up spectroscopy, providing accurate metallicities and abundance ratios for several key elements for these stars. Results. We present here the stellar parameters of the first set of stars. The nineteen stars range in iron abundance between -4.1 and −2.9 dex relative to the Sun. Two stars have a high radial velocity and, according to our estimate of their kinematics, appear to be marginally bound to the Galaxy and are possibly accreted from another galaxy.

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X-shooter GTO: evidence for a population of extremely metal-poor, alpha-poor stars

Cited by 41 publications

References 45 publications

TOPoS

TOPoS

Evidence for the accretion origin of halo stars with an extreme r-process enhancement

TOPoS

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