Examinando por Autor "Spite, F."
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Ítem Abundances in a sample of turnoff and subgiant stars in NGC 6121 (M 4)(EDP Sciences, 2016-10) Spite, M.; Spite, F.; Gallagher, A.J.; Monaco, L.; Bonifacio, P.; Caffau, E.; Villanova, S.Context. The stellar abundances observed in globular clusters show complex structures, currently not yet understood. Aims. The aim of this work is to investigate the relations between the abundances of different elements in the globular cluster M 4, selected for its uniform deficiency of iron, to explore the best models explaining the pattern of these observed abundances. Moreover, in turnoff stars, the abundances of the elements are not suspected to be affected by internal mixing. Methods. In M 4, using low and moderate resolution spectra obtained for 91 turnoff (and subgiant) stars with the ESO FLAMES Giraffe spectrograph, we have extended previous measurements of abundances (of Li, C and Na) to other elements (C, Si, Ca, Sr and Ba), using model atmosphere analysis. We have also studied the influence of the choice of the microturbulent velocity. Results. Firstly, the peculiar turnoff star found to be very Li-rich in a previous paper does not show any other abundance anomalies relative to the other turnoff stars in M 4. Secondly, an anti-correlation between C and Na has been detected, the slope being significative at more than 3σ. This relation between C and Na is in perfect agreement with the relation found in giant stars selected below the RGB bump. Thirdly, the strong enrichment of Si and of the neutron-capture elements Sr and Ba, already observed in the giants in M 4, is confirmed. Finally, the relations between Li, C, Na, Sr and Ba constrain the enrichment processes of the observed stars. Conclusions. The abundances of the elements in the turnoff stars appear to be compatible with production processes by massive AGBs, but are also compatible with the production of second generation elements (like Na) and low Li produced by, for example, fast rotating massive stars.Ítem High-speed stars: Galactic hitchhikers(EDP Sciences, 2020-06) Caffau, E.; Monaco, L.; Bonifacio, P.; Sbordone, L.; Haywood, M.; Spite, M.; Di Matteo, P.; Spite, F.; Mucciarelli, A.; François, P.; Matas Pinto, A.M.Context. The search for stars born in the very early stages of the Milky Way star formation history is of paramount importance in the study of the early Universe since their chemistry carries irreplaceable information on the conditions in which early star formation and galaxy buildup took place. The search for these objects has generally taken the form of expensive surveys for faint extremely metal-poor stars, the most obvious but not the only candidates to a very early formation. Aims. Thanks to Gaia DR2 radial velocities and proper motions, we identified 72 bright cool stars displaying heliocentric transverse velocities in excess of 500 km s-1. These objects are most likely members of extreme outer-halo populations, either formed in the early Milky Way build-up or accreted from since-destroyed self-gravitating stellar systems. Methods. We analysed low-resolution FORS spectra of the 72 stars in the sample and derived the abundances of a few elements. Despite the large uncertainties on the radial velocity determination, we derived reliable orbital parameters for these objects. Results. The stars analysed are mainly slightly metal poor, with a few very metal-poor stars. Their chemical composition is much more homogeneous than expected. All the stars have very eccentric halo orbits, some extending well beyond the expected dimension of the Milky Way. Conclusions. These stars can be the result of a disrupted small galaxy or they could have been globular cluster members. Age estimates suggest that some of them are evolved blue stragglers, now on the subgiant or asymptotic giant branches. © E. Caffau et al. 2020.Ítem Investigation of a sample of carbon-enhanced metal-poor stars observed with FORS and GMOS(EDP Sciences, 2018-06) Caffau, E.; Gallagher, A.J.; Bonifacio, P.; Spite, M.; Duffau, S.; Spite, F.; Monaco, L.; Sbordone, L.Aims. Carbon-enhanced metal-poor (CEMP) stars represent a sizeable fraction of all known metal-poor stars in the Galaxy. Their formation and composition remains a significant topic of investigation within the stellar astrophysics community. Methods. We analysed a sample of low-resolution spectra of 30 dwarf stars, obtained using the visual and near UV FOcal Reducer and low dispersion Spectrograph for the Very Large Telescope (FORS/VLT) of the European Southern Observatory (ESO) and the Gemini Multi-Object Spectrographs (GMOS) at the GEMINI telescope, to derive their metallicity and carbon abundance. Results. We derived C and Ca from all spectra, and Fe and Ba from the majority of the stars. Conclusions. We have extended the population statistics of CEMP stars and have confirmed that in general, stars with a high C abundance belonging to the high C band show a high Ba-content (CEMP-s or -r/s), while stars with a normal C abundance or that are C-rich, but belong to the low C band, are normal in Ba (CEMP-no). © ESO 2018.Ítem TOPoS: II. on the bimodality of carbon abundance in CEMP stars Implications on the early chemical evolution of galaxies(EDP Sciences, 2015-07) Bonifacio, P.; Caffau, E.; Spite, M.; Limongi, M.; Chieffi, A.; Klessen, R.S.; François, P.; Molaro, P.; Ludwig, H.-G.; Zaggia, S.; Spite, F.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark, P.C.; Glover, S.C.O.; Hammer, F.; Koch, A.; Monaco, L.; Sbordone, L.; Steffen, M.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. Results. Calcium and carbon are the only elements that can be measured in all six stars. The range is-5.0 ≤ [Ca/H] <-2.1 and 7.12 ≤ A(C) ≤ 8.65. For star SDSS J1742+2531 we were able to detect three Fe i lines from which we deduced [Fe/H] =-4.80, from four Ca ii lines we derived [Ca/H] =-4.56, and from synthesis of the G-band we derived A(C) = 7.26. For SDSS J1035+0641 we 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 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. © 2015 ESO.Ítem TOPoS: III. An ultra iron-poor multiple CEMP system ?(EDP Sciences, 2016-11) Caffau, E.; Bonifacio, P.; Spite, M.; Spite, F.; Monaco, L.; Sbordone, L.; François, P.; Gallagher, A.J.; Plez, B.; Zaggia, S.; Ludwig, H.-G.; Cayrel, R.; Koch, A.; Steffen, M.; Salvadori, S.; Klessen, R.; Glover, S.; Christlieb, N.Aims. One of the primary objectives of the TOPoS survey is to search for the most metal-poor stars. Our search has led to the discovery of one of the most iron-poor objects known, SDSS J092912.32+023817.0. This object is a multiple system, in which two components are clearly detected in the spectrum. Methods. We have analysed 16 high-resolution spectra obtained using the UVES spectrograph at the ESO 8.2 m VLT telescope to measure radial velocities and determine the chemical composition of the system. Results. Cross correlation of the spectra with a synthetic template yields a double-peaked cross-correlation function (CCF) for eight spectra, and in one case there is evidence for the presence of a third peak. Chemical analysis of the spectrum obtained by averaging all the spectra for which the CCF showed a single peak found that the iron abundance is [Fe/H] = −4.97. The system is also carbon enhanced with [C/Fe] = +3.91 (A(C) = 7.44). From the permitted oxygen triplet we determined an upper limit for oxygen of [O/Fe] < +3.52 such that C/O > 1.3. We are also able to provide more stringent upper limits on the Sr and Ba abundances ([Sr/Fe] < +0.70, and [Ba/Fe] < +1.46, respectively).Ítem TOPoS: IV. Chemical abundances from high-resolution observations of seven extremely metal-poor stars(EDP Sciences, 2018-04) Bonifacio, P.; Caffau, E.; Spite, M.; Spite, F.; Sbordone, L.; Monaco, L.; François, P.; Plez, B.; Molaro, P.; Gallagher, A.J.; Cayrel, R.; Christlieb, N.; Klessen, R.S.; Koch, A.; Ludwig, H.-G.; Steffen, M.; Zaggia, S.; Abate, C.Context. Extremely metal-poor (EMP) stars provide us with indirect information on the first generations of massive stars. The TOPoS survey has been designed to increase the census of these stars and to provide a chemical inventory that is as detailed as possible. Aims. Seven of the most iron-poor stars have been observed with the UVES spectrograph at the ESO VLT Kueyen 8.2 m telescope to refine their chemical composition. Methods. We analysed the spectra based on 1D LTE model atmospheres, but also used 3D hydrodynamical simulations of stellar atmospheres. Results. We measured carbon in six of the seven stars: all are carbon-enhanced and belong to the low-carbon band, defined in the TOPoS II paper. We measured lithium (A(Li) = 1.9) in the most iron-poor star (SDSS J1035+0641, [Fe/H] <-5.2). We were also able to measure Li in three stars at [Fe/H] ~-4.0, two of which lie on the Spite plateau. We confirm that SDSS J1349+1407 is extremely rich in Mg, but not in Ca. It is also very rich in Na. Several of our stars are characterised by low α-to-iron ratios. Conclusions. The lack of high-carbon band stars at low metallicity can be understood in terms of evolutionary timescales of binary systems. The detection of Li in SDSS J1035+0641 places a strong constraint on theories that aim at solving the cosmological lithium problem. The Li abundance of the two warmer stars at [Fe/H] ~-4.0 places them on the Spite plateau, while the third, cooler star, lies below. We argue that this suggests that the temperature at which Li depletion begins increases with decreasing [Fe/H]. SDSS J1349+1407 may belong to a class of Mg-rich EMP stars. We cannot assess if there is a scatter in α-to-iron ratios among the EMP stars or if there are several discrete populations. However, the existence of stars with low α-to-iron ratios is supported by our observations. © ESO 2018.Ítem TOPoS: VI. The metal-weak tail of the metallicity distribution functions of the Milky Way and the Gaia -Sausage-Enceladus structure(EDP Sciences, 2021-07) Bonifacio, P.; Monaco, L; Salvadori, S; Caffau, E.; Spite, M; Sbordone, L.; Spite, F.; Ludwig, H.-G; DI Matteo, P.; Haywood, M.; François, P.; Koch-Hansen, A.JContext. The goal of the Turn-Off Primordial Stars survey (TOPoS) project is to find and analyse turn-off (TO) stars of extremely low metallicity. To select the targets for spectroscopic follow-up at high spectral resolution, we relied on low-resolution spectra from the Sloan Digital Sky Survey (SDSS). Aims. In this paper, we use the metallicity estimates we obtained from our analysis of the SDSS spectra to construct the metallicity distribution function (MDF) of the Milky Way, with special emphasis on its metal-weak tail. The goal is to provide the underlying distribution out of which the TOPoS sample was extracted. Methods. We made use of SDSS photometry, Gaia photometry, and distance estimates derived from the Gaia parallaxes to derive a metallicity estimate for a large sample of over 24 million TO stars. This sample was used to derive the metallicity bias of the sample for which SDSS spectra are available. Results. We determined that the spectroscopic sample is strongly biased in favour of metal-poor stars, as intended. A comparison with the unbiased photometric sample allows us to correct for the selection bias. We selected a sub-sample of stars with reliable parallaxes for which we combined the SDSS radial velocities with Gaia proper motions and parallaxes to compute actions and orbital parameters in the Galactic potential. This allowed us to characterise the stars dynamically, and in particular to select a sub-sample that belongs to the Gaia-Sausage-Enceladus (GSE) accretion event. We are thus also able to provide the MDF of GSE. Conclusions. The metal-weak tail derived in our study is very similar to that derived in the H3 survey and in the Hamburg/ESO Survey. This allows us to average the three MDFs and provide an error bar for each metallicity bin. Inasmuch as the GSE structure is representative of the progenitor galaxy that collided with the Milky Way, that galaxy appears to be strongly deficient in metal-poor stars compared to the Milky Way, suggesting that the metal-weak tail of the latter has been largely formed by accretion of low-mass galaxies rather than massive galaxies, such as the GSE progenitor. © P. Bonifacio et al. 2021.