| 1. |
- Caffau, E., et al.
(författare)
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The photospheric solar oxygen project - I. Abundance analysis of atomic lines and influence of atmospheric models
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 488:3, s. 1031-1046
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Tidskriftsartikel (refereegranskat)abstract
- Context. The solar oxygen abundance has undergone a major downward revision in the past decade, the most noticeable one being the update including 3D hydrodynamical simulations to model the solar photosphere. Up to now, such an analysis has only been carried out by one group using one radiation-hydrodynamics code. Aims. We investigate the photospheric oxygen abundance considering lines from atomic transitions. We also consider the relationship between the solar model used and the resulting solar oxygen abundance, to understand whether the downward abundance revision is specifically related to 3D hydrodynamical effects. Methods. We performed a new determination of the solar photospheric oxygen abundance by analysing different high-resolution high signal-to-noise ratio atlases of the solar flux and disc-centre intensity, making use of the latest generation of CO5BOLD 3D solar model atmospheres. Results. We find 8.73 <= log (N-O/N-H) + 12 <= 8.79. The lower and upper values represent extreme assumptions on the role of collisional excitation and ionisation by neutral hydrogen for the NLTE level populations of neutral oxygen. The error of our analysis is +/- (0.04 +/- 0.03) dex, the last being related to NLTE corrections, the first error to any other effect. The 3D "granulation effects" do not play a decisive role in lowering the oxygen abundance. Conclusions. Our recommended value is log (N-O/N-H) = 8.76 +/- 0.07, considering our present ignorance of the role of collisions with hydrogen atoms on the NLTE level populations of oxygen. The reasons for lower O abundances in the past are identified as (1) the lower equivalent widths adopted and (2) the choice of neglecting collisions with hydrogen atoms in the statistical equilibrium calculations for oxygen.
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| 2. |
- Bonifacio, P., et al.
(författare)
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First stars XII. Abundances in extremely metal-poor turnoff stars, and comparison with the giants
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 501:2, s. 519-530
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Tidskriftsartikel (refereegranskat)abstract
- Context. The detailed chemical abundances of extremely metal-poor (EMP) stars are key guides to understanding the early chemical evolution of the Galaxy. Most existing data, however, treat giant stars that may have experienced internal mixing later. Aims. We aim to compare the results for giants with new, accurate abundances for all observable elements in 18 EMP turno. stars. Methods. VLT/UVES spectra at R similar to 45 000 and S/N similar to 130 per pixel (lambda lambda 330-1000 nm) are analysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to derive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba. Results. For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sample of EMP giants, at all metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and Co are similar to 0.2 dex larger than in giants of similar metallicity. Mg and Si abundances are similar to 0.2 dex lower (the giant [Mg/Fe] values are slightly revised), while Zn is again similar to 0.4 dex higher than in giants of similar [Fe/H] (6 stars only). Conclusions. For C, the dwarf/giant discrepancy could possibly have an astrophysical cause, but for the other elements it must arise from shortcomings in the analysis. Approximate computations of granulation (3D) effects yield smaller corrections for giants than for dwarfs, but suggest that this is an unlikely explanation, except perhaps for C, Cr, and Mn. NLTE computations for Na and Al provide consistent abundances between dwarfs and giants, unlike the LTE results, and would be highly desirable for the other discrepant elements as well. Meanwhile, we recommend using the giant abundances as reference data for Galactic chemical evolution models.
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| 3. |
- Bonifacio, P., et al.
(författare)
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Using CO5BOLD models to predict the effects of granulation on colours .
- 2017
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Ingår i: MEMORIE della Società Astronomica Italiana. - 0037-8720 .- 1824-016X. ; 88
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Abstract.In order to investigate the effects of granulation on fluxes and colours, we computedthe emerging fluxes from the models in theCO5BOLDgrid with metallicities [M/H]=0.0,–1.0,–2.0 and –3.0. These fluxes have been used to compute colours in different photometric systems.We explain here how our computations have been performed and provide some results.Key words.Convection – Hydrodynamics - Stars: atmosphere
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| 4. |
- Bonifacio, P., et al.
(författare)
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Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours : I. Grids of 3D corrections in the UBVRI, 2MASS, HIPPARCOS, Gaia, and SDSS systems
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 611:9
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Tidskriftsartikel (refereegranskat)abstract
- Context. The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection. The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities.Aim. We aim to study, in a quantitative way, the impact of granulation on colours.Methods. We use the CIFIST (Cosmological Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2, 3) hydrodynamical models to compute emerging fluxes. These in turn are used to compute theoretical colours in the UBV RI, 2MASS, HIPPARCOS, Gaia and SDSS systems. Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model computed for the same atmospheric parameters, which we used to define a “3D correction” that can be applied to colours computed from fluxes computed from any 1D model atmosphere code. As an example, we illustrate these corrections applied to colours computed from ATLAS models.Results. The 3D corrections on colours are generally small, of the order of a few hundredths of a magnitude, yet they are far from negligible. We find that ignoring granulation effects can lead to underestimation of Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when using colours as diagnostics. We have identified a major shortcoming in how scattering is treated in the current version of the CIFIST grid, which could lead to offsets of the order 0.01 mag, especially for colours involving blue and UV bands. We have investigated the Gaia and HIPPARCOS photometric systems and found that the (G − Hp), (BP − RP) diagram is immune to the effects of granulation. In addition, we point to the potential of the RVS photometry as a metallicity diagnostic.Conclusions. Our investigation shows that the effects of granulation should not be neglected if one wants to use colours as diagnostics of the stellar parameters of F, G, K stars. A limitation is that scattering is treated as true absorption in our current computations, thus our 3D corrections are likely an upper limit to the true effect. We are already computing the next generation of the CIFIST grid, using an approximate treatment of scattering.
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| 5. |
- Cayrel, R., et al.
(författare)
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Determination of [O/Fe] in BD +23 3130 from ESO VLT-UVES observations
- 2001
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Ingår i: New Astronomy Reviews. - 1872-9630. ; 45:8, s. 533-535
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Tidskriftsartikel (refereegranskat)abstract
- We report a new determination of [O/Fe, the relative logarithmicabundance of O/Fe with respect to the Sun, for the very metal-poor starBD+23 3130 ([Fe/H=-2.6). The value was derived from the forbidden line[O I at 630 nm and from six weak Fe II lines, with a S/N ratiosubstantially larger than those obtained before, thanks to theefficiency of the VLT-UVES instrument at Paranal. We obtain[O/Fe=0.71+/-0.25, a value 0.36 dex higher than the value obtained fromthe same lines by Fulbright and Kraft [AJ 118 (1999) 527, but 0.46lower than the one derived by Israelian et al. [ApJ 507 (1998) 805 fromthe UV OH bands.
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| 6. |
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| 7. |
- Cayrel, R., et al.
(författare)
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First stars V - Abundance patterns from C to Zn and supernova yields in the early Galaxy
- 2004
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 416:3, s. 1117-1138
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Tidskriftsartikel (refereegranskat)abstract
- In the framework of the ESO Large Programme ``First Stars'', veryhigh-quality spectra of some 70 very metal-poor dwarfs and giants wereobtained with the ESO VLT and UVES spectrograph. These stars are likelyto have descended from the first generation(s) of stars formed after theBig Bang, and their detailed composition provides constraints on issuessuch as the nature of the first supernovae, the efficiency of mixingprocesses in the early Galaxy, the formation and evolution of the haloof the Galaxy, and the possible sources of reionization of the Universe.This paper presents the abundance analysis of an homogeneous sample of35 giants selected from the HK survey of Beers et al. (cite{BPS92},cite{Be99}), emphasizing stars of extremely low metallicity: 30 of our35 stars are in the range -4.1 <[Fe/H]< -2.7, and 22 stars have[Fe/H] < -3.0. Our new VLT/UVES spectra, at a resolving power ofR∼45 000 and with signal-to-noise ratios of 100-200 per pixel overthe wavelength range 330-1000 nm, are greatly superior to those of theclassic studies of McWilliam et al. (cite{MPS95}) and Ryan et al.(cite{RNB96}).The immediate objective of the work is to determine precise,comprehensive, and homogeneous element abundances for this large sampleof the most metal-poor giants presently known. In the analysis wecombine the spectral line modeling code ``Turbospectrum'' with OSMARCSmodel atmospheres, which treat continuum scattering correctly and thusallow proper interpretation of the blue regions of the spectra, wherescattering becomes important relative to continuous absorption (λ< 400 nm). We obtain detailed information on the trends of elementalabundance ratios and the star-to-star scatter around those trends,enabling us to separate the relative contributions of cosmic scatter andobservational/analysis errors.Abundances of 17 elements from C to Zn have been measured in all stars,including K and Zn, which have not previously been detected in starswith [Fe/H] < -3.0. Among the key results, we discuss the oxygenabundance (from the forbidden [OI] line), the different and sometimescomplex trends of the abundance ratios with metallicity, the very tightrelationship between the abundances of certain elements (e.g., Fe andCr), and the high [Zn/Fe] ratio in the most metal-poor stars. Within theerror bars, the trends of the abundance ratios with metallicity areconsistent with those found in earlier literature, but in many cases thescatter around the average trends is much smaller than found in earlierstudies, which were limited to lower-quality spectra. We find that thecosmic scatter in several element ratios may be as low as 0.05 dex.The evolution of the abundance trends and scatter with decliningmetallicity provides strong constraints on the yields of the firstsupernovae and their mixing into the early ISM. The abundance ratiosfound in our sample do not match the predicted yields frompair-instability hypernovae, but are consistent with element productionby supernovae with progenitor masses up to 100 M⊙.Moreover, the composition of the ejecta that have enriched the matterBased on observations obtained in the frame of the ESO programme ID165.N-0276(A).Full Tables 3 and 8 are available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/1117 This work hasmade use of the SIMBAD database.
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| 8. |
- Cayrel, R., et al.
(författare)
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Measurement of stellar age from uranium decay
- 2001
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 409:6821, s. 691-692
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Tidskriftsartikel (refereegranskat)abstract
- The ages of the oldest stars in the Galaxy indicate when star formationbegan, and provide a minimum age for the Universe. Radioactive dating ofmeteoritic material and stars relies on comparing the present abundanceratios of radioactive and stable nuclear species to the theoreticallypredicted ratios of their production. The radioisotope 232Th(half-life 14Gyr) has been used to date Galactic stars, but it decays byonly a factor of two over the lifetime of the Universe. 238U(half-life 4.5Gyr) is in principle a more precise age indicator, buteven its strongest spectral line, from singly ionized uranium at awavelength of 385.957nm, has previously not been detected in stars. Herewe report a measurement of this line in the very metal-poor starCS31082-0018, a star which is strongly overabundant in itsheavy elements. The derived uranium abundance, log(U/H) = -13.7 +/- 0.14+/- 0.12 yields an age of 12.5 +/- 3Gyr, though this is still modeldependent. The observation of this cosmochronometer gives the mostdirect age determination of the Galaxy. Also, with improved theoreticaland laboratory data, it will provide a highly precise lower limit to theage of the Universe.
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| 9. |
- Depagne, E., et al.
(författare)
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First Stars. II. Elemental abundances in the extremely metal-poor star CS 22949--037. A diagnostic of early massive supernovae
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 390:1, s. 187-198
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Tidskriftsartikel (refereegranskat)abstract
- CS 22949-037 is one of the most metal-poor giants known ([Fe/H]~-4.0),and it exhibits large overabundances of carbon and nitrogen (Norris etal.). Using VLT-UVES spectra of unprecedented quality, regardingresolution and S/N ratio, covering a wide wavelength range (from lambda= 350 to 900 nm), we have determined abundances for 21 elements in thisstar over a wide range of atomic mass. The major new discovery is anexceptionally large oxygen enhancement, [O/Fe] = 1.97+/-0.1, as measuredfrom the [O I] line at 630.0 nm. We find an enhancement of [N/Fe] of2.56+/- 0.2, and a milder one of [C/Fe] = 1.17+/-0.1, similar to thosealready reported in the literature. This implies Zstar =0.01Zsun. We also find carbon isotopic ratios12C/13C =4+/-2.0 and 13C/14N=0.03 +0.035-0.015, close to the equilibrium valueof the CN cycle. Lithium is not detected. Na is strongly enhanced([Na/Fe] = +2.1 +/- 0.2), while S and K are not detected. Thesilicon-burning elements Cr and Mn are underabundant, while Co and Znare overabundant ([Zn/Fe]=+0.7). Zn is measured for the first time insuch an extremely metal-poor star. The abundances of the neutron-captureelements Sr, Y, and Ba are strongly decreasing with the atomic number ofthe element: [Sr/Fe] ~ +0.3, [Y/Fe] ~ -0.1, and [Ba/Fe] ~ -0.6. Amongpossible progenitors of CS 22949-037, we discuss the pair-instabilitysupernovae. Such very massive objects indeed produce large amounts ofoxygen, and have been found to be possible sources of primary nitrogen.However, the predicted odd/even effect is too large, and the predictedZn abundance much too low. Other scenarios are also discussed. Inparticular, the yields of a recent model (Z35Z) from Heger and Woosleyare shown to be in fair agreement with the observations. The onlydiscrepant prediction is the very low abundance of nitrogen, possiblycurable by taking into account other effects such as rotationallyinduced mixing. Alternatively, the absence of lithium in our star, andthe values of the isotopic ratios 12C/13C and13C/14N close to the equilibrium value of the CNcycle, suggest that the CNO abundances now observed might have beenaltered by nuclear processing in the star itself. A 30-40Msun supernova, with fallback, seems the most likelyprogenitor for CS 22949-037. Based on observations made with the ESOVery Large Telescope at Paranal Observatory, Chile (programme ID165.N-0276(A)).
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| 10. |
- François, P., et al.
(författare)
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First Stars. III. A detailed elemental abundance study of four extremely metal-poor giant stars
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 403:3, s. 1105-1114
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports detailed abundance analyses for four extremelymetal-poor (XMP) giant stars with [Fe/H]<-3.8, based onhigh-resolution, high-S/N spectra from the ESO VLT (Kueyen/UVES) and LTEmodel atmosphere calculations. The derived [alpha /Fe] ratios in oursample exhibit a small dispersion, confirming previous findings in theliterature, i.e. a constant overabundance of the alpha -elements with avery small (if any) dependence on [Fe/H]. In particular, the very smallscatter we determine for [Si/Fe] suggests that this element shows aconstant overabundance at very low metallicity, a conclusion which couldnot have been derived from the widely scattered [Si/Fe] values reportedin the literature for less metal-poor stars. For the iron-peak elements,our precise abundances for the four XMP stars in our sample confirm thedecreasing trend of Cr and Mn with decreasing [Fe/H], as well as theincreasing trend for Co and the absence of any trend for Sc and Ni. Incontrast to the significant spread of the ratios [Sr/Fe] and [Ba/Fe], wefind [Sr/Ba] in our sample to be roughly solar, with a much lowerdispersion than previously found for stars in the range -3.5 < [Fe/H]< -2.5.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (Large Programme ID 165.N-0276(A)).The complete version of Table 5 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp:/ /cdsweb.u-strasbg.fr/cgi-bin/qcat?J /A+A/403/1105
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