arXiv subjects

Piercarlo Bonifacio

Publications and source records attributed to Piercarlo Bonifacio.

At least 91 records · Page 5Linked to original sources

arXiv2008

Overview of the lithium problem in metal-poor stars and new results on 6Li

Two problems are discussed here. The first one is the 0.4 dex discrepancy between the 7Li abundance derived from the spectra of metal-poor halo stars on the one hand, and from Big Bang nucleosynthesis, based on the cosmological parameters constrained by the WMAP measurements, on the other hand. Lithium, indeed, can be depleted in the convection zone of unevolved stars. The understanding of the hydrodynamics of the crucial zone near the bottom of the convective envelope in dwarfs or turn-off stars of solar metallicity has recently made enormous progress with the inclusion of internal gravity waves. However, similar work for metal-poor stars is still lacking. Therefore it is not yet clear whether the depletion occurring in the metal-poor stars themselves is adequate to produce a 7Li plateau. The second problem pertains to the large amount of 6Li recently found in metal-poor halo stars. The convection-related asymmetry of the 7Li line could mimic the signal attributed so far to the weak blend of 6Li in the red wing of the 7Li line. Theoretical computations show that the signal generated by the asymmetry of 7Li is 2.0, 2.1, and 3.7 per cent for [Fe/H]= -3.0, -2.0, -1.0, respectively (Teff =6250 K and log g=4.0 [cgs]). In addition we re-investigate the statistical properties of the 6Li plateau and show that previous analyses were biased. Our conclusion is that the 6Li plateau can be reinterpreted in terms of intrinsic line asymmetry, without the need to invoke a contribution of 6Li. (abridged)

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arXiv2008

Spectral analyses of three carbon-enhanced metal-poor stars

We are conducting a high-resolution follow-up of candidate EMP stars extracted from the Sloan Digital Sky Survey (SDSS; York et al. 2000) using UVES at the VLT. Three of the programme stars, SDSS J0912+0216, SDSS J1036+1212 and SDSS J1349-0229, where deliberately targetted as CEMP stars since a strong $G$ band was evident from the SDSS spectra and the weakness of the Ca {\sc ii} K line testified their very low metallicity. The UVES high resolution follow-up confirmed the original findings ([Fe/H] $<-2.50$) and allowed a more detailed investigation of their chemical composition. We determined the carbon abundance from molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by classical 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. 3D carbon abundances were determined for all three stars, using CO$^5$BOLD 3D hydrodynamical model atmospheres. 3D effects on the carbon abundance are found to be quite significant for these stars, with 3D corrections of up to --0.7 dex. Two of the stars, SDSS J0912+0216 and SDSS J1349-0229 exhibit an overabundance of neutron capture elements which classifies them as CEMP-s. Star SDSS J1036+1212, instead belongs to the elusive class of CEMP-no/s stars, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date.

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arXiv2008

The photospheric solar oxygen project: I. Abundance analysis of atomic lines and influence of atmospheric models

The solar oxygen abundance has undergone a major downward revision in the last decade, the most noticeable one being the update including 3D hydrodynamical simulations to model the solar photosphere. Up to now, such an analysis has been carried out only by one group using one radiation-hydrodynamics code. 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. We perform 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. We find 8.73 < logNoxygen/Nhydrogen +12 < 8.79. The lower and upper value 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. 3D ``granulation effects'' do not play a decisive role in lowering the oxygen abundance. Our recommended value, considering our present ignorance of the role of collisions with hydrogen atoms on the NLTE level populations of oxygen, is log(Noxygen/Nhydrogen) = 8.76 +- 0.07. The reasons which have led to 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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arXiv2007

Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination

Context: Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the 6Li/7Li isotopic ratio is prone to suffering from such asymmetries, as the contribution of 6Li is a slight blending reinforcement of the red wing of each component of the corresponding 7Li line, with respect to its blue wing. Aims: The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination. Method: Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as Li I, producing absorption lines of approximately the same equivalent width as individual components of the 7Li I resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the effects of photospheric convection. Results: The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the 7Li absorption feature that mimics the presence of 6Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on 6Li/7Li has thus far been derived. The second method confirms these findings. Conclusions: From this work, it appears that a systematic reappraisal of former determinations of 6Li abundances in halo stars is warranted.

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arXiv2007

The solar photospheric abundance of phosphorus: results from co5bold 3D model atmospheres

aims: We determine the solar abundance of phosphorus using co5bold 3D hydrodynamic model atmospheres. method: High resolution, high signal-to-noise solar spectra of the PI lines of Multiplet 1 at 1051-1068 nm are compared to line formation computations performed on a co5bold solar model atmosphere. results: We find A(P)=5.46+- 0.04, in good agreement with previous analysis based on 1D model atmospheres, due to the fact that the PI lines of Mult. 1 are little affected by 3D effects. We cannot confirm an earlier claim by other authors of a downward revision of the solar P abundance by 0.1 dex employing a 3D model atmosphere. Concerning other stars, we found modest (<0.1 dex) 3D abundance corrections for P among four F dwarf model atmospheres of different metallicity, being largest at lowest metallicity. conclusions: We conclude that 3D abundance corrections are generally rather small for the PI lines studied in this work. They are marginally relevant for metal-poor stars, but may be neglected in the Sun.

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arXiv2007

Sulphur abundances from the SI near-infrared triplet at 1045 nm

context : Unlike silicon and calcium, sulphur is an alpha-element which does not form dust. Some of the available observations of the evolution of sulphur with metallicity indicate an increased scatter of sulphur to iron ratios at low metallicities or even a bimodal distribution, with some stars showing constant S/Fe at all metallicities and others showing an increasing S/Fe ratio with decreasing metallicity. In metal-poor stars SI lines of Multiplet 1 at 920 nm are not yet too weak to permit to measure the sulphur abundance A(S), however in ground-based observations they are severely affected by telluric lines. aims : We investigate the possibility to measure sulphur abundances from SI Mult. 3 at 1045 nm lines, which lie in the near infra-red. These are slightly weaker than those of Mult. 1, but lie in a range not affected by telluric lines. method We investigate the lines of Mult.3 in the Sun (G2V), Procyon (F5V), HD 33256 (F5V), HD 25069 (G9V) and epsilon Eri (HD 22049, K2V). For the Sun and Procyon the analysis has been performed with CO5BOLD 3D hydrodynamical model atmospheres, for the other three stars, for which hydrodynamical simulations are not available, the analysis has been performed using 1D model atmospheres. results For our sample of stars we find a global agreement of A(S) from lines of different multiplets. conclusions : Our results suggest that the infrared lines of Mult. 3 are a viable indicator of the sulphur abundance which, because of the intrinsic strength of this multiplet, should be suitable to study the trend of [S/Fe] at low metallicities.

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arXiv2007

Variations in the lithium abundances of turn off stars in the globular cluster 47 Tuc

aims: Our aim is to determine Li abundances in TO stars of the Globular Cluster 47 Tuc and test theories about Li variations among TO stars. method: We make use of high resolution (R~ 43000), high signal-to-noise ratio (S/N=50--70) spectra of 4 turn off (TO) stars obtained with the UVES spectrograph at the 8.2m VLT Kueyen telescope. results: The four stars observed, span the range 1.6<~A(Li)} <~ 2.14, providing a mean A(Li) = 1.84 with a standard deviation of 0.25 dex. When coupled with data of other two TO stars of the cluster, available in the literature, the full range in Li abundances observed in this cluster is 1.6<~A(Li)<~ 2.3. The variation in A(Li) is at least 0.6 dex (0.7 dex considering also the data available in the literature) and the scatter is six times larger than what expected from the observational error. We claim that these variations are real. A(Li) seems to be anti-correlated with A(Na) exactly as observed in NGC 6752. No systematic error in our analysis could produce such an anti-correlation. conclusions: Na production through p captures on 22Ne at temperatures in excess of 3x10^7 K and the contemporary Li destruction could result in this anti-correlation. However such nuclear processing cannot have taken place in the stars themselves, which do not reach such high temperatures, even at their centre. This points towards the processing in a previous generation of stars. The low N/O ratios in the observed stars and the apparent lack of correlation between N an Li abundances, place a strong constraint on the properties of this previous generation. Our results indicate a different behaviour among the Globular Clusters so far studied as far as the abundance patterns are concerned.

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arXiv2006

Beryllium abundance in turn-off stars of NGC 6752

Aims: To measure the beryllium abundance in two TO stars of the Globular Cluster NGC 6752, one oxygen rich and sodium poor, the other presumably oxygen poor and sodium rich. Be abundances in these stars are used to put on firmer grounds the hypothesis of Be as cosmochronometer and to investigate the formation of Globular Clusters. Method:We present near UV spectra with resolution R$\sim 45000$ obtained with the UVES spectrograph on the 8.2m VLT Kueyen telescope, analysed with spectrum synthesis based on plane parallel LTE model atmospheres. Results:Be is detected in the O rich star with log(Be/H)=-12.04 $\pm$0.15, while Be is not detected in the other star for which we obtain the upper limit log(Be/H)$<$-12.2. A large difference in nitrogen abundance (1.6 dex) is found between the two stars. Conclusions:The Be measurement is compatible with what found in field stars with the same [Fe/H] and [O/H]. The 'Be age' of the cluster is found to be 13.3 Gyrs, in excellent agreement with the results from main sequence fitting and stellar evolution. The presence of Be confirms the results previously obtained for the cluster NGC 6397 and supports the hypothesis that Be can be used as a clock for the early formation of the Galaxy. Since only an upper limit is found for the star with low oxygen abundance, we cannot decide between competing scenarios of Globular Cluster formation, but we can exclude that 'polluted' stars are substantially younger than 'unpolluted' ones. We stress that the Be test might be the only measurement capable of distinguishing between these scenarios.

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arXiv2006

High-Precision Measurements of Delta alpha/alpha from QSO Absorption Spectra

Precise radial velocity measurements (delta v/c ~ 10^{-7}) of FeII lines in damped Ly-alpha systems from very high quality VLT/UVES spectra of quasars HE0515-4414 and Q1101-264 are used to probe cosmological time dependence of the fine structure constant, alpha. It is found that between two redshifts z1 = 1.15 and z2 = 1.84 the value of Delta alpha/alpha changes at the level of a few ppm: (alpha_z2 - alpha_z1)/alpha_0 = 5.43 +/- 2.52 ppm. Variations of alpha can be considered as one of the most reliable method to constrain the dark energy equation of state and improvements on the accuracy of the wavelength calibration of QSO spectra are of great importance.

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arXiv2006

Low metallicity stars in our Galaxy

The advent of m class telescopes has allowed the detailed spectroscopic study of sizeable numbers of extremely metal-poor Galactic stars which are the witnesses of the formation of the early Galaxy. Their chemical composition displays some distinctive trends which should provide a strong constraint on the physical nature of the first generation(s) of stars and on their nucleosynthetic output. I will review recent results in the field following the periodic table, from lithium to uranium and shortly comment on the intriguing classes of Carbon Enhanced Metal Poor (CEMP) stars, for many of which there is no analogue among solar metallicity stars. In spite of these exciting results, the number of known stars of metallicity below [Fe/H]=-3.3 remains quite small and it would be desirable to discover more, both to clearly understand the metal-weak tail of Halo metallicity distribution and to clarify the abundance trends at the lowest metallicities. Most of these extremely rare objects have been discovered by the wide field objective prism surveys, HK survey and Hamburg-ESO survey. In the near future the Sloan Digital Sky Survey and its continuation SEGUE are expected to boost significantly the numbers of known extremely metal poor stars. We are living exciting times but an even more exciting future lies ahead !

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arXiv2005

Chemical Abundances in the Secondary Star of the Neutron Star Binary Centaurus X-4

Using a high resolution spectrum of the secondary star in the neutron star binary {Cen X-4}, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have used a $χ^{2}$ minimization procedure to explore a grid of 1 500 000 LTE synthetic spectra computed for a plausible range of both stellar and veiling parameters. Adopting the best model parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni and Al. These element abundances are super solar ($\mathrm{[Fe/H]}=0.23 \pm 0.10$), but only the abundance of Ti and Ni appear to be moderately enhanced ($\ge1σ$) as compared with the average values of stars of similar iron content. These element abundances can be explained if the secondary star captured a significant amount of matter ejected from a spherically symmetric supernova explosion of a 4 {$M_\odot$} He core progenitor and assuming solar abundances as primordial abundances in the secondary star. The kinematic properties of the system indicate that the neutron star received a natal kick velocity through an aspherical SN and/or an asymmetric neutrino emission. The former scenario might be ruled out since our model computations cannot produce acceptable fits to the observed abundances. We have also examined whether this system could have formed in the Galactic halo, and our simulations show that this possibility seems unlikely. We also report a new determination of the Li abundance consistent with previous studies that is unusually high and close to the cosmic Li abundance in the Galactic disk.

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arXiv2004

Abundances of C, N, O in slightly evolved stars in the globular clusters NGC 6397, NGC 6752 and 47 Tuc

Abundances of C and N are derived from features due to the CH G-band and to the UV CN band measured on high resolution (R ~ 40,000) UVES spectra of more than 40 dwarfs and subgiants in NGC 6397, NGC 6752 and 47 Tuc. Oxygen abundances (or upper limits) are available for all stars in the sample. Isotopic ratios 12C/13C were derived from the CH molecular band. This is the first determination of this ratio in unevolved dwarf stars in globular clusters. By enlarging the sample of subgiants in NGC 6397 studied in Gratton et al. (2001), we uncovered, for the first time, large variations in both Na and O also in this cluster. The origin of the chemical inhomogeneities must be searched for outside the stars under scrutiny. Our data indicate that in unevolved or slightly evolved stars in these clusters C abundances are low but not zero, also in stars with large N-enhancements and O-depletions, and that the isotopic ratios 12C/13C are low, but never reach the equilibrium value of the CN-cycle. When coupled with the run of O and Na abundances, these findings possibly require that, in addition to CNO burning and p-captures, some triple-alpha process is also involved: previously evolved intermediate-mass AGB stars are then the most likely polluters.

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arXiv2004

A new reddening for RR Tel from the HeII Paschen lines

From the available STIS data of RR Tel, that have provided a coverage with absolutely calibrated data in a wide wavelength range, we have obtained a new determination of its reddening (E(B-V)=0.00) from the comparison of the observed HeII Paschen lines decrement relative to HeII lambda 4686 (for 24 HeII Paschen lines down to the region of the head of the series near lambda 2060 A) with the theoretical one as given in Storey and Hummer (1995) for case B, T=10,000 K and log Ne=6. This new E(B-V)=0.0 value has been confirmed from a re-analysis of the IUE low resolution data. We recall that the so far generally adopted value in the literature has been E(B-V)=0.10 as obtained by Penston et al. (1983).

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arXiv2003

Abundance Analysis of Turn-Off and Early Subgiant Stars in the Globular Cluster 47 Tuc (NGC 104)

We used the UVES spectrograph on Kueyen (VLT UT2) to perform the abundance analysis of stars at the base of the giant branch (RGB) and at the main sequence turn-off in the globular cluster 47 Tuc (NGC 104). High dispersion spectra (R ~ 40,000) for 3 dwarfs and 9 subgiants were analyzed. We found an overall [Fe/H] value of -0.67 +/-0.01 +/-0.04, rms=0.05 dex. The mean value obtained from stars at the base of the RGB is virtually the same as that obtained from the dwarfs. Na and O abundances show a star-to-star scatter and are anti-correlated, not unlike previous finding in more metal-poor clusters. The extension of the anticorrelation is not as extreme as found in other clusters, but it is clearly present in unevolved stars. Al abundances do not show a significant spread; this could be indicative that p-captures in high temperature regions are less likely to occur in metal-rich clusters. The [alpha/Fe] ratios suggest a slight excess of Ti with respect to field stars of similar metallicities. Finally, we found an enhanced odd-even effect for Fe-group elements. In particular, the [Mn/Fe] ratio is almost 0.2 dex deficient with respect to the mean value of the dissipative component of field stars in the solar neighborhood. While the theoretical and observational framework for Mn is still poorly understood, the chemistry of 47 Tuc, coupled with its kinematics and its somewhat younger age, might give hints favouring an origin in a formely independent, possibly larger, subsystem of our Galaxy.

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arXiv2003

On the origin of HE0107-5240, the most iron deficient star presently known

We show that the "puzzling" chemical composition observed in the extremely metal poor star HE0107-5240 may be naturally explained by the concurrent pollution of at least two supernovae. In the simplest possible model a supernova of quite low mass (~15 Msun), underwent a "normal" explosion and ejected ~0.06 Msun of 56Ni while a second one was massive enough (~35 Msun) to experience a strong fall back that locked in a compact remnant all the carbon-oxygen core. In a more general scenario, the pristine gas clouds were polluted by one or more supernovae of relatively low mass (less than ~25 Msun). The successive explosion of a quite massive star experiencing an extended fall back would have largely raised the abundances of the light elements in its close neighborhood.

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arXiv2003

How did the metals in a giant star originate?

The chemical composition of stars with extremely low metal contents (taking ``metals'' to mean all elements other than hydrogen and helium) provides us with information on the masses of the stars that produced the first metals. Such a direct connection is not possible, however, if the surface of the star has been polluted by enriched material, either dredged from the star's interior or transferred from a companion star. Here we argue that, in the case of HE0107-5240 (ref. 1), the most iron poor star known, the oxygen abundance could be a discriminant: a ratio of [O/Fe] exceeding +3.5 would favour a pristine origin of metals, whereas an [O/Fe] ratio of less than +3 would favour the pollution hypothesis. Using this criterion, we suggest how the required information on oxygen abundance might be obtained.

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arXiv2002

Automatic abundance analysis of high resolution spectra

We describe an automatic procedure for determining abundances from high resolution spectra. Such procedures are becoming increasingly important as large amounts of data are delivered from 8m telescopes and their high-multiplexing fiber facilities, such as FLAMES on ESO-VLT. The present procedure is specifically targeted for the analysis of spectra of giants in the Sgr dSph; however, the procedure may be, in principle, tailored to analyse stars of any type. Emphasis is placed on the algorithms and on the stability of the method; the external accuracy rests, ultimately, on the reliability of the theoretical models (model-atmospheres, synthetic spectra) used to interpret the data. Comparison of the results of the procedure with the results of a traditional analysis for 12 Sgr giants shows that abundances accurate at the level of 0.2 dex, comparable with that of traditional analysis of the same spectra, may be derived in a fast and efficient way. Such automatic procedures are not meant to replace the traditional abundance analysis, but as an aid to extract rapidly a good deal of the information contained in the spectra.

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arXiv2002

On the lithium content of the globular cluster M92

I use literature data and a new temperature calibration to determine the Li abundances in the globular cluster M 92. Based on the same data, Boesgaard et al. have claimed that there is a dispersion in Li abundances in excess of observational errors. This result has been brought as evidence for Li depletion in metal-poor dwarfs. In the present note I argue that there is no strong evidence for intrinsic dispersion in Li abundances, although a dispersion as large as 0.18 dex is possible. The mean Li abundance, A(Li)=2.36, is in good agreement with recent results for field stars and TO stars in the metal-poor globular cluster NGC 6397. The simplest interpretation is that this constant value represents the primordial Li abundance.

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