arXiv subjects

E. Caffau

Publications and source records attributed to E. Caffau.

At least 109 records · Page 6Linked to original sources

arXiv2014

The Gaia-ESO Survey: Extracting diffuse interstellar bands from cool star spectra, DIB-based interstellar medium line-of-sight structures at the kpc scale

We study how diffuse interstellar bands (DIBs) measured toward distance-distributed target stars can be used to locate dense interstellar (IS) clouds in the Galaxy and probe a line-of-sight (LOS) kinematical structure, a potential useful tool when gaseous absorption lines are saturated or not available in the spectral range. Cool target stars are numerous enough for this purpose. We have devised automated DIB fitting methods appropriate to cool star spectra and multiple IS components. The data is fitted with a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profiles. In parallel, stellar distances and extinctions are estimated self-consistently by means of a 2D Bayesian method, from spectroscopically-derived stellar parameters and photometric data. We have analyzed Gaia-ESO Survey (GES) and previously recorded spectra that probe between $\sim$ 2 and 10 kpc long LOS in five different regions of the Milky Way. Depending on the observed spectral intervals, we extracted one or more of the following DIBs: $λλ$ 6283.8, 6613.6 and 8620.4. For each field, we compared the DIB strengths with the Bayesian distances and extinctions, and the DIB Doppler velocities with the HI emission spectra. For all fields, the DIB strength and the target extinction are well correlated. In case of targets widely distributed in distance, marked steps in DIBs and extinction radial distance profiles match with each other and broadly correspond to the expected locations of spiral arms. For all fields, the DIB velocity structure agrees with HI emission spectra and all detected DIBs correspond to strong NaI lines. This illustrates how DIBs can be used to locate the Galactic interstellar gas and to study its kinematics at the kpc scale.

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arXiv2014

Oxygen in the early Galaxy: OH lines as tracers of oxygen abundance in extremely metal-poor giant stars

Oxygen is a powerful tracer element of Galactic chemical evolution. Unfortunately, only a few oxygen lines are available in the ultraviolet-infrared stellar spectra for the reliable determination of its abundance. Moreover, oxygen abundances obtained using different spectral lines often disagree significantly. In this contribution we therefore investigate whether the inadequate treatment of convection in 1D hydrostatic model atmospheres used in the abundance determinations may be responsible for this disagreement. For this purpose, we used VLT CRIRES spectra of three EMP giants, as well as 3D hydrodynamical CO$^5$BOLD and 1D hydrostatic LHD model atmospheres, to investigate the role of convection in the formation of infrared (IR) OH lines. Our results show that the presence of convection leads to significantly stronger IR OH lines. As a result, the difference in the oxygen abundance determined from IR OH lines with 3D hydrodynamical and classical 1D hydrostatic model atmospheres may reach -0.2 ... -0.3 dex. In case of the three EMP giants studied here, we obtain a good agrement between the 3D LTE oxygen abundances determined by us using vibrational-rotational IR OH lines in the spectral range of 1514-1626 nm, and oxygen abundances determined from forbidden [O I] 630 nm line in previous studies.

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arXiv2014

Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars

The Gaia-ESO Survey is obtaining high-quality spectroscopic data for about 10^5 stars using FLAMES at the VLT. UVES high-resolution spectra are being collected for about 5000 FGK-type stars. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO 2nd internal release and will be part of its 1st public release of advanced data products. The final parameter scale is tied to the one defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. A set of open and globular clusters is used to evaluate the physical soundness of the results. Each methodology is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted-medians of those from the individual methods. The recommended results successfully reproduce the benchmark stars atmospheric parameters and the expected Teff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55 K for Teff, 0.13 dex for log g, and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for Teff, 0.10-0.25 dex for log g, and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex.

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arXiv2014

Clues on the Galactic evolution of sulphur from star clusters

(Abridged) The abundances of alpha-elements are a powerful diagnostic of the star formation history and chemical evolution of a galaxy. Sulphur, being moderately volatile, can be reliably measured in the interstellar medium (ISM) of damped Ly-alpha galaxies and extragalactic HII regions. Measurements in stars of different metallicity in our Galaxy can then be readily compared to the abundances in external galaxies. Such a comparison is not possible for Si or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur is interesting because it probes nucleosynthetic conditions that are very different from those of O or Mg. The measurements in star clusters are a reliable tracers of the Galactic evolution of sulphur. We find NLTE=6.11+/-0.04 for M 4, NLTE=7.17+/-0.02 for NGC 2477, and NLTE=7.13+/-0.06 for NGC 5822. For the only star studied in Trumpler 5 we find A(S)NLTE=6.43+/-0.03 and A(S)LTE=6.94+/-0.05. Our measurements show that, by and large, the S abundances in Galactic clusters trace reliably those in field stars. The only possible exception is Trumpler 5, for which the NLTE sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4 dex than observed in field stars of comparable metallicity, even though its LTE sulphur abundance is in line with abundances of field stars. Moreover the LTE sulphur abundance is consistent only with the abundance of another alpha-element, Mg, in the same star, while the low NLTE value is consistent with Si and Ca. The S abundances in our sample of stars in clusters imply that the clusters are chemically homogeneous for S within 0.05 dex.

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arXiv2014

Science Case and Requirements for the MOSAIC Concept for a Multi-Object Spectrograph for the European Extremely Large Telescope

Over the past 18 months we have revisited the science requirements for a multi-object spectrograph (MOS) for the European Extremely Large Telescope (E-ELT). These efforts span the full range of E-ELT science and include input from a broad cross-section of astronomers across the ESO partner countries. In this contribution we summarise the key cases relating to studies of high-redshift galaxies, galaxy evolution, and stellar populations, with a more expansive presentation of a new case relating to detection of exoplanets in stellar clusters. A general requirement is the need for two observational modes to best exploit the large (>40 sq. arcmin) patrol field of the E-ELT. The first mode ('high multiplex') requires integrated-light (or coarsely resolved) optical/near-IR spectroscopy of >100 objects simultaneously. The second ('high definition'), enabled by wide-field adaptive optics, requires spatially-resolved, near-IR of >10 objects/sub-fields. Within the context of the conceptual study for an ELT-MOS called MOSAIC, we summarise the top-level requirements from each case and introduce the next steps in the design process.

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arXiv2014

Abundances of lithium, oxygen, and sodium in the turn-off stars of Galactic globular cluster 47 Tuc

We aim to determine abundances of Li, O and Na in a sample of of 110 turn-off (TO) stars, in order to study the evolution of light elements in this cluster and to put our results in perspective with observations of other globular and open clusters, as well as with field stars. We use medium resolution spectra obtained with the GIRAFFE spectrograph at the ESO 8.2m Kueyen VLT telescope and use state of the art 1D model atmospheres and NLTE line transfer to determine the abundances. We also employ CO5BOLD hydrodynamical simulations to assess the impact of stellar granulation on the line formation and inferred abundances. Our results confirm the existence of Na-O abundance anti-correlation and hint towards a possible Li-O anti-correlation in the TO stars of 47 Tuc. We find no convincing evidence supporting the existence of Li-Na correlation. The obtained 3D NLTE mean lithium abundance in a sample of 94 TO stars where Li lines were detected reliably, $\langle A({\rm Li})_{\rm 3D~NLTE}\rangle = 1.78 \pm 0.18$ dex, appears to be significantly lower than what is observed in other globular clusters. At the same time, star-to-star spread in Li abundance is also larger than seen in other clusters. The highest Li abundance observed in 47 Tuc is about 0.1 dex lower than the lowest Li abundance observed among the un-depleted stars of the metal-poor open cluster NGC 2243. The lithium abundances in 47 Tuc, when put into context with observations in other clusters and field stars, suggest that stars that are more metal-rich than [FeH] \sim -1.0 experience significant lithium depletion during their lifetime on the main sequence, while the more metal-poor stars do not. Rather strikingly, our results suggest that initial lithium abundance with which the star was created may only depend on its age (the younger the star, the higher its Li content) and not on its metallicity.

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arXiv2014

High-resolution abundance analysis of very metal-poor r-I stars

Moderately r-process-enriched stars (r-I) are at least four times as common as those that are greatly enriched in r-process elements (r-II), and the abundances in their atmospheres are important tools for obtaining a better understanding of the nucleosynthesis processes responsible for the origin of the elements beyond the iron peak. The main aim of this work is to derive abundances for a sample of seven metal-poor stars with classified as r-I stars, to understand the role of these stars for constraining the astrophysical nucleosynthesis event(s) that is(are) responsible for the production of the r-process, and to investigate whether they differ, in any significant way, from the r-II stars. We carried out a detailed abundance analysis based on high-resolution spectra obtained with the VLT/UVES spectrograph. The OSMARCS LTE 1D model atmosphere grid was employed, along with the spectrum synthesis code Turbospectrum. We have derived abundances of light elements Li, C, and N, alpha-elements, odd-Z elements, iron-peak elements, and the trans-iron elements from the first peak, the second peak, the third peak, and the actinides regions. The results are compared with values for these elements for r-II and normal very and extremely metal-poor stars reported in the literature, ages based on radioactive chronometry are explored using different models, and a number of conclusions about the r-process and the r-I stars are presented. Hydrodynamical models were used for some elements, and general behaviors for the 3D corrections were presented.

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arXiv2014

A super lithium-rich red-clump star in the open cluster Trumpler 5

Context. The existence of lithium-rich low-mass red giant stars still represents a challenge for stellar evolution models. Stellar clusters are privileged environments for this kind of investigation. Aims. To investigate the chemical abundance pattern of the old open cluster Trumpler\,5, we observed a sample of four red-clump stars with high-resolution optical spectrographs. One of them (#3416) reveals extremely strong lithium lines in its spectrum. Methods. One-dimensional, local thermodynamic equilibrium analysis was performed on the spectra of the observed stars. A 3D-NLTE analysis was performed to derive the lithium abundance of star #3416. Results. Star #3416 is super Li-rich with A(Li)=3.75\,dex. The lack of $^6$Li enrichment ($^6$Li/$^7$Li$<$2%), the low carbon isotopic ratio ($^{12}$C/$^{13}$C=14$\pm$3), and the lack of evidence for radial velocity variation or enhanced rotational velocity ($v\sin i = 2.8\,$\kms) all suggest that lithium production has occurred in this star through the Cameron & Fowler mechanism. Conclusions. We identified a super Li-rich core helium-burning, red-clump star in an open cluster. Internal production is the most likely cause of the observed enrichment. Given the expected short duration of a star's Li-rich phase, enrichment is likely to have occurred at the red clump or in the immediately preceding phases, namely during the He-flash at the tip of the red giant branch (RGB) or while ascending the brightest portion of the RGB.

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arXiv2013

MyGIsFOS: an automated code for parameter determination and detailed abundance analysis in cool stars

The current and planned high-resolution, high-multiplexity stellar spectroscopic surveys, as well as the swelling amount of under-utilized data present in public archives have led to an increasing number of efforts to automate the crucial but slow process to retrieve stellar parameters and chemical abundances from spectra. We present MyGIsFOS, a code designed to derive atmospheric parameters and detailed stellar abundances from medium - high resolution spectra of cool (FGK) stars. We describe the general structure and workings of the code, present analyses of a number of well studied stars representative of the parameter space MyGIsFOS is designed to cover, and examples of the exploitation of MyGIsFOS very fast analysis to assess uncertainties through Montecarlo tests. MyGIsFOS aims to reproduce a ``traditional'' manual analysis by fitting spectral features for different elements against a precomputed grid of synthetic spectra. Fe I and Fe II lines can be employed to determine temperature, gravity, microturbulence, and metallicity by iteratively minimizing the dependence of Fe I abundance from line lower energy and equivalent width, and imposing Fe I - Fe II ionization equilibrium. Once parameters are retrieved, detailed chemical abundances are measured from lines of other elements. MyGIsFOS replicates closely the results obtained in similar analyses on a set of well known stars. It is also quite fast, performing a full parameter determination and detailed abundance analysis in about two minutes per star on a mainstream desktop computer. Currently, its preferred field of application are high-resolution and/or large spectral coverage data (e.g UVES, X-Shooter, HARPS, Sophie).

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arXiv2013

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. III. Line formation in the atmospheres of giants located close to the base of RGB

We utilize state-of-the-art 3D hydrodynamical and classical 1D stellar model atmospheres to study the influence of convection on the formation properties of various atomic and molecular spectral lines in the atmospheres of four red giant stars, located close to the base of the red giant branch, RGB ($T_{\mathrm eff}\approx5000$ K, $\log g=2.5$), and characterized by four different metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0. The role of convection in the spectral line formation is assessed with the aid of abundance corrections, i.e., the differences in abundances predicted for a given equivalent width of a particular spectral line with the 3D and 1D model atmospheres. We find that for lines of certain neutral atoms the abundance corrections strongly depend both on metallicity of a given model atmosphere and the line excitation potential. While abundance corrections for all lines of both neutral and ionized elements tend to be small at solar metallicity, for lines of neutral elements with low ionization potential and low-to-intermediate $χ$ they quickly increase with decreasing metallicity, reaching in their extremes to -0.6...-0.8 dex. In all such cases the large abundance corrections are due to horizontal temperature fluctuations in the 3D hydrodynamical models. Abundance corrections of molecular lines are very sensitive to metallicity of the underlying model atmosphere and may be larger (in absolute value) than -0.5 dex at [M/H] = -3.0 (-1.5 dex in the case of CO). We also find that an approximate treatment of scattering in the 3D model calculations leads to the abundance corrections that are altered by less than ~0.1 dex, both for atomic and molecular (CO) lines, with respect to the model where scattering is treated as true absorption throughout the entire atmosphere, with the largest differences for the resonance and low-excitation lines.

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arXiv2013

TOPoS: I. Survey design and analysis of the first sample

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

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arXiv2013

Isotope Spectroscopy

The measurement of isotopic ratios provides a privileged insight both into nucleosynthesis and into the mechanisms operating in stellar envelopes, such as gravitational settling. In this article, we give a few examples of how isotopic ratios can be determined from high-resolution, high-quality stellar spectra. We consider examples of the lightest elements, H and He, for which the isotopic shifts are very large and easily measurable, and examples of heavier elements for which the determination of isotopic ratios is more difficult. The presence of 6Li in the stellar atmospheres causes a subtle extra depression in the red wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the highest quality. But even with the best spectra, the derived $^6$Li abundance can only be as good as the synthetic spectra used for their interpretation. It is now known that 3D non-LTE modelling of the lithium spectral line profiles is necessary to account properly for the intrinsic line asymmetry, which is produced by convective flows in the atmospheres of cool stars, and can mimic the presence of 6Li. We also discuss briefly the case of the carbon isotopic ratio in metal-poor stars, and provide a new determination of the nickel isotopic ratios in the solar atmosphere.

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arXiv2013

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

The extremely metal-poor stars are the direct descendants of the first generation stars. They carry the chemical signature of the pristine Universe at the time they formed, shortly after the Big Bang. We aim to derive information about extremely metal-poor stars from their observed spectra. Four extremely metal-poor stars were selected from the Sloan Digital Sky Survey (SDSS) and observed during the guaranteed observing time of X-Shooter. The X-Shooter spectra were analysed using an automatic code, MyGIsFOS, which is based on a traditional analysis method. It makes use of a synthetic grid computed from one-dimensional, plane-parallel, hydrostatic model atmospheres. The low metallicity derived from the SDSS spectra is confirmed here. Two kinds of stars are found. Two stars are confirmed to be extremely metal-poor, with no evidence of any enhancement in carbon. The two other stars are strongly enhanced in carbon. We could not derive iron abundance for one of them, while [Ca/H] is below -4.5. Two of the stars are members of the rare population of extremely metal-poor stars low in alpha elements.

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arXiv2013

Stellar granulation as seen in disk-integrated intensity. II. Theoretical scaling relations compared with observations

A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (tau_eff) and the root-mean-square (rms) brightness fluctuations (sigma) associated with the granulation scale as a function of the peak frequency (nu_max) of the solar-like oscillations. We aim at providing a theoretical background to the observed scaling relations based on a model developed in the companion paper. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-integrated intensity on the basis of the theoretical model. For each PDS we derived tau_eff and sigma and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the Kepler measurements. We derive theoretical scaling relations for tau_eff and sigma, which show the same dependence on nu_max as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (Ma) estimated at the photosphere. The theoretical scaling relations for tau_eff and sigma match the observations well on a global scale. Our modelling provides additional theoretical support for the observed variations of sigma and tau_eff with nu_m max. It also highlights the important role of Ma in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on Ma. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with Ma.

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arXiv2013

Reanalysis of the FEROS observations of HIP 11952

Aims. We reanalyze FEROS observations of the star HIP 11952 to reassess the existence of the proposed planetary system. Methods. The radial velocity of the spectra were measured by cross-correlating the observed spectrum with a synthetic template. We also analyzed a large dataset of FEROS and HARPS archival data of the calibrator HD 10700 spanning over more than five years. We compared the barycentric velocities computed by the FEROS and HARPS pipelines. Results. The barycentric correction of the FEROS-DRS pipeline was found to be inaccurate and to introduce an artificial one-year period with a semi-amplitude of 62 m/s. Thus the reanalysis of the FEROS data does not support the existence of planets around HIP 11952.

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arXiv2013

Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid

3D model atmospheres for giants, dwarfs, and white dwarfs, computed with the CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and asteroseismic studies. Unlike existing plane-parallel 1D structures, these simulations predict the spatially and temporally resolved emergent intensity so that granulation can be analysed, which provides insights on how convective energy transfer operates in stars. The wide range of atmospheric parameters of the CIFIST 3D simulations (3600 < Teff (K) < 13,000 and 1 < log g < 9) allows the comparison of convective processes in significantly different environments. We show that the relative intensity contrast is correlated with both the Mach and Peclet numbers in the photosphere. The horizontal size of granules varies between 3 and 10 times the local pressure scale height, with a tight correlation between the factor and the Mach number of the flow. Given that convective giants, dwarfs, and white dwarfs cover the same range of Mach and Peclet numbers, we conclude that photospheric convection operates in a very similar way in those objects.

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arXiv2013

Micro- and macroturbulence predictions from CO5BOLD 3D stellar atmospheres

We present an overview of the current status of our efforts to derive the microturbulence and macroturbulence parameters (ximic and ximac) from the CIFIST grid of CO5BOLD 3D model atmospheres as a function of the basic stellar parameters Teff, log g, and [M/H]. The latest results for the Sun and Procyon show that the derived microturbulence parameter depends significantly on the numerical resolution of the underlying 3D simulation, confirming that `low-resolution' models tend to underestimate the true value of ximic. Extending the investigation to twelve further simulations with different Teff, log g, and [M/H], we obtain a first impression of the predicted trend of ximic over the Hertzsprung-Russell diagram: in agreement with empirical evidence, microturbulence increases towards higher effective temperature and lower gravity. The metallicity dependence of ximic must be interpreted with care, since it also reflects the deviation between the 1D and 3D photospheric temperature stratifications that increases systematically towards lower metallicity.

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arXiv2013

The influence of convection on the atmospheric structures and observable properties of red giant stars

During the recent years significant progress has been made in the modeling of red giant atmospheres with the aid of 3D hydrodynamical model atmosphere codes. In this contribution we provide an overview of selected results obtained in this context by utilizing 3D hydrodynamical CO5BOLD stellar model atmospheres. Hydrodynamical simulations show that convective motions lead to significant differences in the atmospheric structures of red giants with respect to those predicted by the classical 1D model atmospheres. Results of these simulations also show that in certain cases 1D models fail to reproduce even the average properties of the 3D hydrodynamical models, such as P-T profiles. Large horizontal temperature fluctuations in the 3D model atmospheres, as well as differences between the temperature profiles of the average <3D> and 1D models, lead to large discrepancies in the strengths of spectral lines predicted by the 3D and 1D model atmospheres. This is especially important in models at lowest metallicities ([M/H]<-2.0) where the 3D-1D abundance differences may reach (or even exceed) -0.6 dex for lines of neutral atoms and molecules. We also discuss several simplifications and numerical aspects involved in the present 3D hydrodynamical modeling of red giant atmospheres, and briefly address several issues where urgent progress may be needed.

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