P. Bonifacio

Publications (204)50.96 Total impact

• Source

  Available from: Simone Zaggia

  Article: X-Shooter GTO: Chemical analysis of a sample of EMP candidates

  E. Caffau, P. Bonifacio, P. François, M. Spite, F. Spite, S. Zaggia, H. -G. Ludwig, L. Monaco, L. Sbordone, R. Cayrel, F. Hammer, S. Randich, V. Hill, P. Molaro
  [show abstract] [hide abstract]
  ABSTRACT: Context: Extremely metal-poor stars (EMP) are very rare objects that hold in their atmospheres the fossil record of the chemical composition of the early phases of Galactic evolution. Finding these objects and determining their chemical composition provides important constraints on these early phases. Aims: Using a carefully designed selection method, we chose a sample of candidate EMP stars from the low resolution spectra of the Sloan Digital Sky Survey and observed them with X-Shooter at the VLT to confirm their metallicities and determine abundances for as many elements as possible. Method: The X-Shooter spectra are analysed by means of one-dimensional, plane-parallel, hydrostatic model atmospheres. Corrections for the granulation effects are computed using CO5BOLD hydrodynamical simulations. Results: All the candidates are confirmed to be EMP stars, proving the efficiency of our selection method within about 0.5 dex. The chemical composition of this sample is compatible with those of brighter samples, suggesting that the stars in the Galactic halo are well mixed. Conclusions These observations show that it is feasible to observe, in a limited amount of time, a large sample of about one hundred stars among EMP candidates selected from the SDSS. Such a size of sample will allow us, in particular, to confirm or refute the existence of a vertical drop in the Galactic Halo metallicity distribution function around [Fe/H] ~ -3.5.
  09/2011;
• Source

  Available from: L. Monaco

  Article: Lithium and sodium in the globular cluster M4. Detection of a Li-rich dwarf star: preservation or pollution?

  L. Monaco, S. Villanova, P. Bonifacio, E. Caffau, D. Geisler, G. Marconi, Y. Momany, H. -G. Ludwig
  [show abstract] [hide abstract]
  ABSTRACT: Context. The abundance inhomogeneities of light elements observed in Globular Clusters (GCs), and notably the ubiquitous Na-O anti-correlation, are generally interpreted as evidence that GCs comprise several generations of stars. There is an on-going debate as to the nature of the stars which produce the inhomogeneous elements, and investigating the behavior of several elements is a way to shed new light on this problem. Aims. We aim at investigating the Li and Na content of the GC M 4, that is known to have a well defined Na-O anti-correlation. Methods. We obtained moderate resolution (R=17 000-18 700) spectra for 91 main sequence (MS)/sub-giant branch stars of M 4 with the Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres analysis we measured lithium and sodium abundances. Results. We detect a weak Li-Na anti-correlation among un-evolved MS stars. One star in the sample, # 37934, shows the remarkably high lithium abundance A(Li)=2.87, compatible with current estimates of the primordial lithium abundance. Conclusions. The shallow slope found for the Li-Na anti-correlation suggests that lithium is produced in parallel to sodium. This evidence, coupled with its sodium-rich nature, suggests that the high lithium abundance of star # 37934 may originate by pollution from a previous generations of stars. The recent detection of a Li-rich dwarf of pollution origin in the globular cluster NGC 6397 may also point in this direction. Still, no clear cut evidence is available against a possible preservation of the primordial lithium abundance for star # 37934.
  07/2011;
• Source

  Available from: ArXiv

  Article: The Galactic evolution of phosphorus

  E. Caffau, P. Bonifacio, R. Faraggiana, M. Steffen
  [show abstract] [hide abstract]
  ABSTRACT: As a galaxy evolves, its chemical composition changes and the abundance ratios of different elements are powerful probes of the underlying evolutionary processes. Phosphorous is an element whose evolution has remained quite elusive until now, because it is difficult to detect in cool stars. The infrared weak P I lines of the multiplet 1, at 1050-1082 nm, are the most reliable indicators of the presence of phosphorus. The availability of CRIRES at VLT has permitted access to this wavelength range in stellar spectra.We attempt to measure the phosphorus abundance of twenty cool stars in the Galactic disk. The spectra are analysed with one-dimensional model-atmospheres computed in Local Thermodynamic Equilibrium (LTE). The line formation computations are performed assuming LTE. The ratio of phosphorus to iron behaves similarly to sulphur, increasing towards lower metallicity stars. Its ratio with respect to sulphur is roughly constant and slightly larger than solar, [P/S]=0.10+- 0.10. We succeed in taking an important step towards the understanding of the chemical evolution of phosphorus in the Galaxy. However, the observed rise in the P/Fe abundance ratio is steeper than predicted by Galactic chemical evolution model model developed by Kobayashi and collaborators. Phosphorus appears to evolve differently from the light odd-Z elements sodium and aluminium. The constant value of [P/S] with metallicity implies that P production is insensitive to the neutron excess, thus processes other than neutron captures operate. We suggest that proton captures on 30Si and alpha captures on $27Al are possibilities to investigate. We see no clear distinction between our results for stars with planets and stars without any detected planet.
  07/2011;
• Source

  Available from: ArXiv

  Article: NLTE strontium abundance in a sample of extremely metal poor stars and the Sr/Ba ratio in the early Galaxy

  S. M. Andrievsky, F. Spite, S. A. Korotin, P. Francois, M. Spite, P. Bonifacio, R. Cayrel, V. Hill
  [show abstract] [hide abstract]
  ABSTRACT: Heavy element abundances in extremely metal-poor stars provide strong constraints on the processes of forming these elements in the first stars. We attempt to determine precise abundances of strontium in a homogeneous sample of extremely metal-poor stars. The abundances of strontium in 54 very or extremely metal-poor stars, was redetermined by abandoning the local thermodynamic equilibrium (LTE) hypothesis, and fitting non-LTE (NLTE) profiles to the observed spectral lines. The corrected Sr abundances and previously obtained NLTE Ba abundances are compared to the predictions of several hypothetical formation processes for the lighter neutron-capture elements. Our NLTE abundances confirm the previously determined huge scatter of the strontium abundance in low metallicity stars. This scatter is also found (and is even larger) at very low metallicities (i. e. early in the chemical evolution). The Sr abundance in the extremely metal-poor (EMP) stars is compatible with the main r-process involved in other processes (or by variations of the r-process), as we briefly discuss.
  04/2011;
• Source

  Available from: Manuela Zoccali

  Article: Lithium-rich giants in the Galactic thick disk

  L. Monaco, S. Villanova, C. Moni Bidin, G. Carraro, D. Geisler, P. Bonifacio, O. A. Gonzalez, M. Zoccali, L. Jilkova
  [show abstract] [hide abstract]
  ABSTRACT: Context: Lithium is a fragile element, which is easily destroyed in the stellar interior. The existence of lithium-rich giants still represents a challenge for stellar evolution models. Aims: We have collected a large database of high-resolution stellar spectra of 824 candidate thick-disk giants having 2\,MASS photometry and proper motions measured by the Southern Proper-Motion Program (SPM). In order to investigate the nature of Li-rich giants, we searched this database for giants presenting a strong Li\,I resonance line. Methods: We performed a chemical abundance analysis on the selected stars with the MOOG code along with proper ATLAS-9 model atmospheres. The iron content and atmospheric parameters were fixed by using the equivalent width of a sample of Fe lines. We also derive abundances for C, N, and O and measure or derive lower limits on the $^{12}$C/$^{13}$C isotopic ratios, which is a sensible diagnostic of the stars evolutionary status. Results: We detected five stars with a lithium abundance higher than 1.5, i.e. Li-rich according to the current definition. One of them (SPM-313132) has A(Li)$>$3.3 and, because of this, belongs to the group of the rare super Li-rich giants. Its kinematics makes it a likely thin-disk member and its atmospheric parameters are compatible with it being a 4\,M$_\odot$ star either on the red giant branch (RGB) or the early asymptotic giant branch. This object is the first super Li-rich giant detected at this phase. The other four are likely low-mass thick-disk stars evolved past the RGB luminosity bump, as determined from their metallicities and atmospheric parameters. The most evolved of them lies close to the RGB-tip. It has A(Li)$>$2.7 and a low $^{12}$C/$^{13}$C isotopic ratio, close to the cool bottom processing predictions.
  03/2011;
• Source

  Available from: Simone Zaggia

  Article: Extremely metal‐poor stars in SDSS fields

  P. Bonifacio, E. Caffau, P. François, L. Sbordone, H.-G. Ludwig, M. Spite, P. Molaro, F. Spite, R. Cayrel, F. Hammer, V. Hill, M. Nonino, S. Randich, B. Stelzer, S. Zaggia
  [show abstract] [hide abstract]
  ABSTRACT: Some insight on the first generation of stars can be obtained from the chemical composition of their direct descendants, extremely metal-poor stars (EMP), with metallicity less than or equal to 1/1000 of the solar metallicity. Such stars are exceedingly rare, the most successful surveys, for this purpose, have so far provided only about 100 stars with 1/1000 the solar metallicity and 4 stars with about 1/10000 of the solar metallicity. The Sloan Digital Sky Survey has the potential to provide a large number of candidates of extremely low metallicity. X-shooter has the unique capability of performing the necessary follow-up spectroscopy providing accurate metallicities and abundance ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr,...) for EMP candidates. We here report on the results for the first two stars observed in the course of our Franco-Italian X-shooter GTO. The two stars were targeted to be of metallicity around –3.0, the analysis of the X-shooter spectra showed them to be of metallicity around –2.0, but with a low α to iron ratio, which explains the underestimate of the metallicity from the SDSS spectra. The efficiency of X-shooter allows an in situ study of the outer halo, for the two stars studied here we estimate distances of 3.9 and 9.1 kpc, these are likely the most distant dwarf stars studied in detail to date (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  Astronomische Nachrichten 02/2011; 332(3):251 - 257. · 1.01 Impact Factor
• Source

  Available from: Thirupathi Sivarani

  Article: First stars XIII. Two extremely metal-poor RR Lyrae stars?

  C. J. Hansen, B. Nordstroem, P. Bonifacio, M. Spite, J. Andersen, T. C. Beers, R. Cayrel, F. Spite, P. Molaro, B. Barbuy, E. Depagne, P. Francois V. Hill, B. Plez, T. Sivarani
  [show abstract] [hide abstract]
  ABSTRACT: The chemical composition of extremely metal-poor stars (EMP stars; [Fe/H]<~ -3) is a unique tracer of early nucleosynthesis in the Galaxy. As such stars are rare, we wish to find classes of luminous stars which can be studied at high resolution. We aim to determine the detailed chemical composition of the two EMP stars CS30317-056 and CS22881-039, originally thought to be red horizontal-branch (RHB) stars, and compare it to earlier results for EMP stars as well as to nucleosynthesis yields from various supernova (SN) models. In the analysis, we discovered that our targets are in fact the two most metal-poor RR Lyrae stars known. Our detailed abundance analysis, taking into account the variability of the stars, is based on VLT/UVES spectra (R~ 43000) and 1D LTE OSMARCS model atmospheres and synthetic spectra. For comparison with SN models we also estimate NLTE corrections for a number of elements. We derive LTE abundances for the 16 elements O, Na, Mg, Al, Si, S, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Sr and Ba, in good agreement with earlier values for EMP dwarf, giant and RHB stars. Li and C are not detected in either star. NLTE abundance corrections are newly calculated for O and Mg and taken from the literature for other elements. The resulting abundance pattern is best matched by model yields for supernova explosions with high energy and/or significant asphericity effects. Our results indicate that, except for Li and C, the surface composition of EMP RR Lyr stars is not significantly affected by mass loss, mixing or diffusion processes; hence, EMP RR Lyr stars should also be useful tracers of the chemical evolution of the early Galactic halo. The observed abundance ratios indicate that these stars were born from an ISM polluted by energetic, massive (25 - 40M*) and/or aspherical supernovae, but the NLTE corrections for Sc and certain other elements do play a role in the choice of model.
  01/2011;
• Source

  Available from: Bertrand Plez

  Article: First Stars XIV. Sulfur abundances in extremely metal-poor (EMP) stars

  Monique Spite, E. Caffau, S. M. Andrievsky, S. A. Korotin, E. Depagne, F. Spite, P. Bonifacio, H. -G. Ludwig, R. Cayrel, P. Francois, V. Hill, B. Plez, J. Andersen, B. Barbuy, T. C. Beers, P. Molaro, B. Nordstrom, F. Primas
  [show abstract] [hide abstract]
  ABSTRACT: Sulfur is important: the site of its formation is uncertain, and at very low metallicity the trend of [S/Fe] against [Fe/H] is controversial. Below [Fe/H]=-2.0, [S/Fe] remains constant or it decreases with [Fe/H], depending on the author and the multiplet used in the analysis. Moreover, although sulfur is not significantly bound in dust grains in the ISM, it seems to behave differently in DLAs and in old metal-poor stars. We aim to determine precise S abundance in a sample of extremely metal-poor stars taking into account NLTE and 3D effects. NLTE profiles of the lines of the multiplet 1 of SI have been computed using a new model atom for S. We find sulfur in EMP stars to behave like the other alpha-elements, with [S/Fe] remaining approximately constant for [Fe/H]<-3. However, [S/Mg] seems to decrease slightly as a function of [Mg/H]. The overall abundance patterns of O, Na, Mg, Al, S, and K are best matched by the SN model yields by Heger & Woosley. The [S/Zn] ratio in EMP stars is solar, as found also in DLAs. We obtain an upper limit on the abundance of sulfur, [S/Fe] < +0.5, for the ultra metal-poor star CS 22949-037. This, along with a previous reported measurement of zinc, argues against the conjecture that the light-element abundances pattern in this star, and, by analogy, the hyper metal-poor stars HE 0107-5240 and HE 1327-2326, are due to dust depletion.
  12/2010;
• Source

  Available from: Simone Zaggia

  Article: Li - O anti-correlation in NGC 6752: evidence for Li-enriched polluting gas

  Z. -X. Shen, P. Bonifacio, L. Pasquini, S. Zaggia
  [show abstract] [hide abstract]
  ABSTRACT: Elemental correlations and anti-correlations are known to be present in globular clusters (GCs) owing to pollution by CNO cycled gas. Because of its fragility Li is destroyed at the temperature at which the CNO cycling occurs, and this makes Li a crucial study for the nature of the contaminating stars. We observed 112 un-evolved stars at the Turnoff of the NGC6752 cluster with FLAMES at the VLT to investigate the presence and the extent of a Li-O correlation. This correlation is expected if there is a simple pollution scenario. Li (670.8 nm) and O triplet (771 nm) abundances are derived in NLTE. All stars belong to a very narrow region of the color-magnitude diagram, so they have similar stellar parameters (Teff, log g). We find that O and Li correlate, with a high statistical significance that confirms the early results for this cluster. At first glance this is what is expected if a simple pollution of pristine gas with CNO cycled gas (O-poor, Li-poor) occurred. The slope of the relationship, however, is about 0.4, and differs from unity by over 7 Sigma. A slope of one is the value predicted for a pure contamination model. We confirm an extended Li-O correlation in non evolved stars of NGC 6752. At the same time the characteristic of the correlation shows that a simple pollution scenario is not sufficient to explain the observations. Within this scenario the contaminant gas must have been enriched in Li. This would rule out massive stars as main polluters, and favor the hypothesis that the polluting gas was enriched by intermediate or high-mass AGB stars, unless the former can be shown to be able to produce Li. According to our observations, the fraction of polluting gas contained in the stars observed is a considerable fraction of the stellar mass of the cluster. Comment: 8pages, 2 figures, accepted by A&A Letter
  11/2010;
• Source

  Available from: I. Saviane

  Article: High Resolution HDS/SUBARU chemical abundances of the young stellar cluster Palomar 1

  L. Monaco, I. Saviane, M. Correnti, P. Bonifacio, D. Geisler
  [show abstract] [hide abstract]
  ABSTRACT: Context. Palomar\,1 is a peculiar globular cluster (GC). It is the youngest Galactic GC and it has been tentatively associated to several of the substructures recently discovered in the Milky Way (MW), including the Canis Major (CMa) overdensity and the Galactic Anticenter Stellar Structure (GASS). Aims. In order to provide further insights into its origin, we present the first high resolution chemical abundance analysis for one red giant in Pal\,1. Methods. We obtained high resolution (R=30000) spectra for one red giant star in Pal\,1 using the High Dispersion Spectrograph (HDS) mounted at the SUBARU telescope. We used ATLAS-9 model atmospheres coupled with the SYNTHE and WIDTH calculation codes to derive chemical abundances from the measured line equivalent widths of 18 among $\alpha$, Iron-peak, light and heavy elements. Results. The Palomar~1 chemical pattern is broadly compatible to that of the MW open clusters population and similar to disk stars. It is, instead, remarkably different from that of the Sagittarius (Sgr) dwarf spheroidal galaxy. Conclusions. If Pal\,1 association with either CMa or GASS will be confirmed, this will imply that these systems had a chemical evolution similar to that of the Galactic disk. Comment: 13 pages, 9 figures, 5 tables. Accepted for publication in Astronomy and Astrophysics. Minor changes in the text
  10/2010;
• Source

  Available from: A. C. Robin

  Article: GYES, a multifibre spectrograph for the CFHT

  P. Bonifacio, S. Mignot, J.-L. Dournaux, P. François, E. Caffau, F. Royer, C. Babusiaux, F. Arenou, C. Balkowski, O. Bienaymé, [......], D. Mourard, N. Nardetto, A. Recio-Blanco, N. Robichon, A. C. Robin, M. Rodrigues, C. Soubiran, C. Turon, K. Venn, Y. Viala
  [show abstract] [hide abstract]
  ABSTRACT: We have chosen the name of GYES, one of the mythological giants with one hundred arms, offspring of Gaia and Uranus, for our instrument study of a multifibre spectrograph for the prime focus of the Canada-France-Hawaii Telescope. Such an instrument could provide an excellent ground-based complement for the Gaia mission and a northern complement to the HERMES project on the AAT. The CFHT is well known for providing a stable prime focus environment, with a large field of view, which has hosted several imaging instruments, but has never hosted a multifibre spectrograph. Building upon the experience gained at GEPI with FLAMES-Giraffe and X-Shooter, we are investigating the feasibility of a high multiplex spectrograph (about 500 fibres) over a field of view 1 degree in diameter. We are investigating an instrument with resolution in the range 15000 to 30000, which should provide accurate chemical abundances for stars down to 16th magnitude and radial velocities, accurate to 1 km/s for fainter stars. The study is led by GEPI-Observatoire de Paris with a contribution from Oxford for the study of the positioner. The financing for the study comes from INSU CSAA and Observatoire de Paris. The conceptual study will be delivered to CFHT for review by October 1st 2010. Comment: Contributed talk at the Gaia ELSA conference 2010, S\`evres 7-11 June 2010, to be published on the EAS Series, Editors: C. Turon, F. Arenou & F. Meynadier
  09/2010;
• Source

  Available from: L. Monaco

  Article: The lithium content of omega Centauri. New clues to the cosmological Li problem from old stars in external galaxies

  L. Monaco, P. Bonifacio, L. Sbordone, S. Villanova, E. Pancino
  [show abstract] [hide abstract]
  ABSTRACT: A discrepancy has emerged between the cosmic lithium abundance inferred by the WMAP satellite measurement coupled with the prediction of the standard big-bang nucleosynthesis theory, and the constant Li abundance measured in metal-poor halo dwarf stars (the so-called Spite plateau). Several models are being proposed to explain this discrepancy, involving either new physics, in situ depletion, or the efficient depletion of Li in the pristine Galaxy by a generation of massive first stars. The realm of possibilities may be narrowed considerably by observing stellar populations in different galaxies, which have experienced different evolutionary histories. The WCen stellar system is commonly considered as the remnant of a dwarf galaxy accreted by the Milky Way (MW). We investigate the Li content of a conspicuous sample of unevolved stars in this object. We obtained moderate resolution (R=17000) spectra for 91 main-sequence/early sub-giant branch (MS/SGB) WCen stars using the FLAMES-GIRAFFE/VLT spectrograph. Li abundances were derived by matching the equivalent width of the LiI resonance doublet at 6708A, to the prediction of synthetic spectra computed with different Li abundances. Synthetic spectra were computed using the SYNTHE code along with ATLAS9 model atmospheres. The stars effective temperatures are derived by fitting the wings of the Ha line with synthetic profiles. We obtain a mean content of A(Li)=2.19+-0.14~dex for WCen MS/SGB stars. This is comparable to what is observed in Galactic halo field stars of similar metallicities and temperatures. The Spite plateau seems to be an ubiquitous feature of old, warm metal-poor stars. It exists also in external galaxies, if we accept the current view about the origin of WCen. This implies that the mechanism(s) that causes the "cosmological lithium problem" may be the same in the MW and other galaxies. Comment: 7 pages, 3 figures, 2 tables. Accepted for publication in Astronomy and Astrophysics Letters
  08/2010;
• Conference Proceeding: Codex

  L. Pasquini, S. Cristiani, R. García López, M. Haehnelt, M. Mayor, J. Liske, A. Manescau, G. Avila, H. Dekker, O. Iwert, [......], M.R. Zapatero Osorio, F. Tenegi, B. Carswell, G. Becker, S. Udry, F. Pepe, C. Lovis, D. Naef, M. Dessauges, D. Mégevand
  [show abstract] [hide abstract]
  ABSTRACT: CODEX is the proposed ultra-stable optical high-resolution spectrograph for the E-ELT, which will use novel Laser Comb calibration techniques and an innovative design to open a new era for precision spectroscopy. With its unique combination of light-collecting power and precision, CODEX will make it possible to directly measure the acceleration of the Universe by monitoring the cosmological redshift drift of spectroscopic features at cosmological distances. CODEX will also allow the assembly of the first sizeable sample of earth-like planets in the habitable zones of their stars with the radial velocity technique. CODEX will take this technique to the level of cm/sec radial velocity stability - a factor of about 20 improvement compared to current instruments. These are two of the scientific results anticipated for CODEX, which will be complemented by a wide range of spectacular science in stellar, galactic and extra-galactic Astronomy as well as Fundamental Physics. All the critical technology items are available or (as for the Laser Frequency Comb) are in an advanced state of testing. CODEX is located at the E-ELT coudé focus that will cover the visible range from 370 to 710 nm and provide a resolving power R~120000 with an aperture of 0.8 arcseconds in the sky.
  Ground-based and Airborne Instrumentation for Astronomy III, San Diego; 07/2010
• Conference Proceeding: Project overview of OPTIMOS-EVE: the fibre-fed multi-object spectrograph for the E-ELT

  R Navarro, F Chemla, P Bonifacio, H Flores, I Guinouard, J -M Huet, M Puech, F Royer, J ~H Pragt, G Wulterkens, [......], M ~E Caldwell, I ~A ~J Tosh, M ~S Whalley, G ~F ~W Woodhouse, P Spanò, P Marcantonio, M ~I Andersen, G ~B Dalton, L Kaper, F Hammer
  Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 07/2010
• Source

  Available from: ArXiv

  Article: Sulphur abundances in halo stars from multiplet 3 at 1045 nm

  E. Caffau, L. Sbordone, H.-G. Ludwig, P. Bonifacio, M. Spite
  [show abstract] [hide abstract]
  ABSTRACT: Sulphur is a volatile α -element which is not locked into dust grains in the interstellar medium (ISM). Hence, its abundance does not need to be corrected for dust depletion when comparing the ISM to the stellar atmospheres. The abundance of sulphur in the photosphere of metal-poor stars is a matter of debate: according to some authors, [S/Fe] versus [Fe/H] forms a plateau at low metallicity, while, according to other studies, there is a large scatter or perhaps a bimodal distribution. In metal-poor stars sulphur is detectable by its lines of multiplet 1 at 920 nm, but this range is heavily contaminated by telluric absorptions, and one line of the multiplet is blended by the hydrogen Paschen ζ line. We study the possibility of using multiplet 3 (at 1045 nm) for deriving the sulphur abundance because this range, now observable at the VLT with the infra-red spectrograph CRIRES, is little contaminated by telluric absorption and not affected by blends at least in metal-poor stars. We compare the abundances derived from multiplets 1 and 3, taking into account NLTE corrections and 3D effects. Here we present the results for a sample of four stars, although the scatter is less pronounced than in previous analysis, we cannot find a plateau in [S/Fe], and confirm the scatter of the sulphur abundance at low metallicity (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  Astronomische Nachrichten 06/2010; 331(7):725 - 730. · 1.01 Impact Factor
• Source

  Available from: Jonay I. González Hernández

  Article: Galactic Evolution of Oxygen: OH lines in 3D hydrodynamical model atmospheres

  J. I. Gonzalez Hernandez, P. Bonifacio, H. -G. Ludwig, E. Caffau, N. T. Behara, B. Freytag
  [show abstract] [hide abstract]
  ABSTRACT: The measurement of oxygen lines in metal-poor unevolved stars, in particular near-UV OH lines, can provide invaluable information on the properties of the Early Galaxy. Near-UV OH lines constitute an important tool to derive oxygen abundances in metal-poor dwarf stars. Therefore, it is important to correctly model the line formation of OH lines, especially in metal-poor stars, where 3D hydrodynamical models commonly predict cooler temperatures than plane-parallel hydrostatic models in the upper photosphere. We have made use of a grid of 52 3D hydrodynamical model atmospheres for dwarf stars computed using the code CO5BOLD, extracted from the more extended CIFIST grid. The 52 models cover the effective temperature range 5000-6500K, the surface gravity range 3.5-4.5 and the metallicity range -3<[Fe/H]<0. We determine 3D-LTE abundance corrections in all the 52 3D models for several OH lines and FeI lines of different excitation potentials. These 3D-LTE corrections are generally negative reaching values of roughly -1 dex (for the OH 3167 with excitation potential of approximately 1 eV) for the higher temperatures and surface gravities. We apply these 3D-LTE corrections to the individual O abundances, derived from OH lines, of a sample the metal-poor dwarf stars reported in Israelian et al.(1998, 2001) and Boesgaard et al.(1999), by interpolating the stellar parameters of the dwarfs in the grid of 3D-LTE corrections. The new 3D-LTE [O/Fe] ratio still keeps a similar trend as the 1D-LTE, i.e, increasing towards lower [Fe/H] values. We applied 1D-NLTE corrections to 3D FeI abundances and we still see an increasing [O/Fe] ratio towards lower metallicites. However, the Galactic [O/Fe] ratio must be revisited once 3D-NLTE corrections become available for OH and Fe lines for a grid of 3D hydrodynamical model atmospheres. Comment: Accepted for publication in Astronomy and Astrophysics
  05/2010;
• Source

  Available from: Thirupathi Sivarani

  Article: The metal-poor end of the Spite plateau. 1: Stellar parameters, metallicities and lithium abundances

  L. Sbordone, P. Bonifacio, E. Caffau, H. -G. Ludwig, N. T. Behara, J. I. Gonzalez Hernandez, M. Steffen, R. Cayrel, B. Freytag, C. Van't Veer, P. Molaro, B. Plez, T. Sivarani, M. Spite, F. Spite, T. C. Beers, N. Christlieb, P. Francois, V. Hill
  [show abstract] [hide abstract]
  ABSTRACT: We present VLT-UVES Li abundances for 28 halo dwarf stars between [Fe/H]=-2.5 and -3.5, 10 of which have [Fe/H]<-3. Four different T_eff scales have been used. Direct Infrared Flux Method (IRFM) has been used on the basis of 2MASS infrared photometry. H_alpha wings have been fitted against synthetic grids computed by means of 1D LTE atmosphere models, assuming different self-broadening theories. Finally, a grid of H_alpha profiles has been computed by means of 3D hydrodynamical atmosphere models. The Li I doublet at 670.8 nm has been used to measure A(Li) by means of 3D hydrodynamical NLTE spectral syntheses. An analytical fit of A(Li)(3D, NLTE) as a function of equivalent width, T_eff, log g, and [Fe/H] has been derived and is made available. A(Li) does not exhibit a plateau below [Fe/H]=-3. A strong positive correlation with [Fe/H] appears, not influenced by the choice of the T_eff estimator. From a linear fit, we obtain a strong slope of about 0.30 dex in A(Li) per dex in [Fe/H], significant to 2-3 sigma, and consistent among all the four T_eff estimators. A significant slope is also detected in the A(Li)--T_eff plane, driven mainly by the coolest stars in the sample which appear Li-poor. Removing such stars does not alter the behavior in the A(Li)-[Fe/H] plane. The scatter in A(Li) increases by a factor of 2 towards lower metallicities, while the plateau appears very thin above [Fe/H]=-2.8. The meltdown of the Spite plateau below [Fe/H]\sim-3 is established, but its cause is unclear. If the primordial A(Li) is the one derived from WMAP, it appears difficult to envision a single depletion phenomenon producing a thin, metallicity independent plateau above [Fe/H]=-2.8, and a highly scattered, metallicity dependent distribution below. The fact that no star below [Fe/H]=-3 lies above the plateau suggests that they formed at plateau level and underwent subsequent depletion. Comment: 22 pages, 15 figures, accepted for publication in Astronomy and Astrophysics, replaced with version with final format for table 3, typos corrected in table 4 and in equation B.1
  03/2010;
• Source

  Available from: ArXiv

  Article: The solar photospheric abundance of carbon.Analysis of atomic carbon lines with the CO5BOLD solar model

  E. Caffau, H. -G. Ludwig, P. Bonifacio, R. Faraggiana, M. Steffen, B. Freytag, I. Kamp, T. R. Ayres
  [show abstract] [hide abstract]
  ABSTRACT: The use of hydrodynamical simulations, the selection of atomic data, and the computation of deviations from local thermodynamical equilibrium for the analysis of the solar spectra have implied a downward revision of the solar metallicity. We are in the process of using the latest simulations computed with the CO5BOLD code to reassess the solar chemical composition. We determine the solar photospheric carbon abundance by using a radiation-hydrodynamical CO5BOLD model, and compute the departures from local thermodynamical equilibrium by using the Kiel code. We measure equivalent widths of atomic CI lines on high resolution, high signal-to-noise ratio solar atlases. Deviations from local thermodynamic equilibrium are computed in 1D with the Kiel code. Our recommended value for the solar carbon abundance, relies on 98 independent measurements of observed lines and is A(C)=8.50+-0.06, the quoted error is the sum of statistical and systematic error. Combined with our recent results for the solar oxygen and nitrogen abundances this implies a solar metallicity of Z=0.0154 and Z/X=0.0211. Our analysis implies a solar carbon abundance which is about 0.1 dex higher than what was found in previous analysis based on different 3D hydrodynamical computations. The difference is partly driven by our equivalent width measurements (we measure, on average, larger equivalent widths with respect to the other work based on a 3D model), in part it is likely due to the different properties of the hydrodynamical simulations and the spectrum synthesis code. The solar metallicity we obtain from the CO5BOLD analyses is in slightly better agreement with the constraints of helioseismology than the previous 3D abundance results. (Abridged) Comment: Astronomy and Astrophysics, accepted
  02/2010;
• Source

  Available from: Jonay I. González Hernández

  Article: Three carbon-enhanced metal-poor dwarf stars from the SDSS - Chemical abundances from CO^5BOLD 3D hydrodynamical model atmospheres

  N. T. Behara, P. Bonifacio, H. -G. Ludwig, L. Sbordone, J. I. González Hernández, E. Caffau
  [show abstract] [hide abstract]
  ABSTRACT: The origin of carbon-enhanced metal-poor stars enriched with both s and r elements is highly debated. Detailed abundances of these types of stars are crucial to understand the nature of their progenitors. The aim of this investigation is to study in detail the abundances of SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212, three dwarf CEMP stars, selected from the Sloan Digital Sky Survey. Using high resolution VLT/UVES spectra (R ~ 30 000) we determine abundances for Li, C, N, O, Na, Mg, Al, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni and 21 neutron-capture elements. We made use of CO^5BOLD 3D hydrodynamical model atmospheres in the analysis of the carbon, nitrogen and oxygen abundances. NLTE corrections for C I and O I lines were computed using the Kiel code. We classify SDSS J1349-0229 and SDSS J0912+0216 as CEMP-r+s stars. SDSS J1036+1212 belongs to the class CEMP-no/s, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date. Radial-velocity variations have been observed in SDSS J1349-0229, providing evidence that it is a member of a binary system. The chemical composition of the three stars is generally compatible with mass transfer from an AGB companion. However, many details remain difficult to explain. Most notably of those are the abundance of Li at the level of the Spite plateau in SDSS J1036+1212 and the large over-abundance of the pure r-process element Eu in all three stars. Comment: 12 pages, 15 figures. Accepted for publication in A&A
  02/2010;
• Source

  Available from: ArXiv

  Article: Convection and 6Li in the atmospheres of metal-poor halo stars

  M. Steffen, R. Cayrel, P. Bonifacio, H. -G. Ludwig, E. Caffau
  [show abstract] [hide abstract]
  ABSTRACT: Based on 3D hydrodynamical model atmospheres computed with the CO5BOLD code and 3D non-LTE (NLTE) line formation calculations, we study the effect of the convection-induced line asymmetry on the derived 6Li abundance for a range in effective temperature, gravity, and metallicity covering the stars of the Asplund et al. (2006) sample. When the asymmetry effect is taken into account for this sample of stars, the resulting 6Li/7Li ratios are reduced by about 1.5% on average with respect to the isotopic ratios determined by Asplund et al. (2006). This purely theoretical correction diminishes the number of significant 6Li detections from 9 to 4 (2 sigma criterion), or from 5 to 2 (3 sigma criterion). In view of this result the existence of a 6Li plateau appears questionable. A careful reanalysis of individual objects by fitting the observed lithium 6707 A doublet both with 3D NLTE and 1D LTE synthetic line profiles confirms that the inferred 6Li abundance is systematically lower when using 3D NLTE instead of 1D LTE line fitting. Nevertheless, halo stars with unquestionable 6Li detection do exist even if analyzed in 3D-NLTE, the most prominent example being HD 84937. Comment: To appear in "Light elements in the Universe", Proceedings IAU Symposium No. 268, 2010, C. Charbonnel, M. Tosi, F. Primas & C. Chiappini, eds
  01/2010;