Paula Coelho

Spectral models for solar‐scaled and α‐enhanced stellar populations

We present the first models allowing one to explore in a consistent way the influence of changes in the α-element-to-iron abundance ratio on the high-resolution spectral properties of evolving stellar populations. The models cover the wavelength range from 3000 A to 1.34 μm at a constant resolution of full width at half-maximum (FWHM) = 1 A and a sampling of 0.2 A, for overall metallicities in the range 0.005 � Z � 0.048 and for stellar population ages between 3 and 14 Gyr. These models are based on a recent library of synthetic stellar spectra and a new library of stellar evolutionary tracks, both computed for three different iron abundances ([Fe/H] =− 0.5, 0.0 and 0.2) and two different α-element-to-iron abundance ratios ([α/Fe] = 0.0 and 0.4). We expect our fully synthetic models to be primarily useful for evaluating the differential effect of changes in the α/Fe ratio on spectral properties such as broad-band colours and narrow spectral features. In addition, we assess the accuracy of absolute model predictions in two ways: first, by comparing the predictions of models for scaled-solar metal abundances ([α/Fe] = 0.0) to those of existing models based on libraries of observed stellar spectra; and secondly, by comparing the predictions of models for α-enhanced metal abundances ([α/Fe] = 0.4) to observed spectra of massive early-type galaxies in the Sloan Digital Sky Survey Data Release 4. We find that our models predict accurate strengths for those spectral indices that are strongly sensitive to the abundances of Fe and α elements. The predictions are less reliable for the strengths of other spectral features, such as those dominated by the abundances of C and N, as expected from the fact that the models do not yet allow one to explore the influence of these elements in an independent way. We conclude that our models are a powerful tool for extracting new information about the chemical properties of galaxies for which high-quality spectra have been gathered by modern surveys.

A grid of synthetic spectra and indices Fe5270, Fe5335, Mgb and Mg

We have computed a grid of synthetic spectra in the wavelength range 4600-5600 A using revised model atmo- spheres, for a range of atmospheric parameters and values of (-elements/Fe)= 0: 0a nd+0.4. The Lick indices Fe5270, Fe5335, Mgb and Mg2 are measured on the grid spectra for FWHM= 2t o 8:3 A. Relations between the indices Fe5270, Fe5335 and Mg2 and the stellar parameters eective temperature Te; log g ,( Fe/H) and (/Fe), valid in the range 4000 Te 7000 K, are presented. These fitting functions are given for FWHM= 3:5 and 8.3 A. The indices were also measured for a list of 97 ref- erence stars with well-known stellar parameters observed at ESO and OHP, and these are compared to the computed indices. Finally, a comparison of the indices measured on the observed spectra and those derived from the fitting functions based on synthetic spectra is presented.

_\mathsf{2}

We report on a strictly differential line-by-line analysis of high quality UVES spectra of bright giants in the metal-poor globular cluster NGC 6752. We achieved high precision differential chemical abundance measurements for Fe, Na, Si, Ca, Ti, Cr, Ni, Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu and Dy with uncertainties as low as ~0.01 dex (~2%). We obtained the following main results. (1) The observed abundance dispersions are a factor of ~2 larger than the average measurement uncertainty. (2) There are positive correlations, of high statistical significance, between all elements and Na. (3) For any pair of elements, there are positive correlations of high statistical significance, although the amplitudes of the abundance variations are small. Removing abundance trends with effective temperature and/or using a different pair of reference stars does not alter these results. These abundance variations and correlations may reflect a combination of (a) He abundance variations and (b) inhomogeneous chemical evolution in the pre- or proto-cluster environment. Regarding the former, the current constraints on Delta Y from photometry likely preclude He as being the sole explanation. Regarding the latter, the nucleosynthetic source(s) must have synthesised Na, alpha, Fe-peak and neutron-capture elements and in constant amounts for species heavier than Si; no individual object can achieve such nucleosynthesis. We speculate that other, if not all, globular clusters may exhibit comparable abundance variations and correlations to NGC 6752 if subjected to a similarly precise analysis.

as a function of stellar parameters and [

One of the main ingredients of stellar population synthesis models is a library of stellar spectra. Both empirical and theoretical libraries are used for this purpose, and the question about which one is preferable is still debated in the literature. Empirical and theoretical libraries are being improved significantly over the years, and many libraries have become available lately. However, it is not clear in the literature what are the advantages of using each of these new libraries, and how far behind models are compared to observations. Here we compare in detail some of the major theoretical libraries available in the literature with observations, aiming at detecting weaknesses and strengths from the stellar population modelling point of view. Our test is twofold: we compared model predictions and observations for broad-band colours and for high-resolution spectral features. Concerning the broad-band colours, we measured the stellar colour given by three recent sets of model atmospheres and flux distributions, and compared them with a recent UBVRIJHK calibration which is mostly based on empirical data. We found that the models can reproduce with reasonable accuracy the stellar colours for a fair interval in effective temperatures and gravities. The exceptions are (1) the U - B colour, where the models are typically redder than the observations, and (2) the very cool stars in general (V -K ≥ 3). Castelli & Kurucz is the set of models that best reproduce the bluest colours (U - B, B - V) while Gustafsson et al. and Brott & Hauschildt more accurately predict the visual colours. The three sets of models perform in a similar way for the infrared colours. Concerning the high-resolution spectral features, we measured 35 spectral indices defined in the literature on three high-resolution synthetic libraries, and compared them with the observed measurements given by three empirical libraries. The measured indices cover the wavelength range from ∼3500 to ∼8700 A. We found that the direct comparison between models and observations is not a simple task, given the uncertainties in parameter determinations of empirical libraries. Taking that aside, we found that in general the three libraries present similar behaviours and systematic deviations. For stars with T eff ≤ 7000K, the library by Coelho et al.is the one with best average performance. We detect that lists of atomic and molecular line opacities still need improvement, specially in the blue region of the spectrum, and for the cool stars (T eff ≤ 4500 K).

\mathsf{\alpha}

Aims. Solar colors have been determined on the uvby–β photometric system to test absolute solar fluxes, to examine colors predicted by model atmospheres as a function of stellar parameters (Teff ,l ogg ,[ Fe/H]), and to probe zero-points of Teff and metallicity scales. Methods. New uvby–β photometry is presented for 73 solar-twin candidates. Most stars of our sample have also been observed spectroscopically to obtain accurate stellar parameters. Using the stars that most closely resemble the Sun, and complementing our data with photometry available in the literature, the solar colors on the uvby–β system have been inferred. Our solar colors are compared with synthetic solar colors computed from absolute solar spectra and from the latest Kurucz (ATLAS9) and MARCS model atmospheres. The zero-points of different Teff and metallicity scales are verified and corrections are proposed. Results. Our solar colors are (b − y)� = 0.4105 ± 0.0015, m1,� = 0.2122 ± 0.0018, c1,� = 0.3319 ± 0.0054, and β� = 2.5915 ± 0.0024. The (b − y)� and m1,� colors obtained from absolute spectrophotometry of the Sun agree within 3-σ with the solar colors derived here when the photometric zero-points are determined from either the STIS HST observations of Vega or an ATLAS9 Vega model, but the c1,� and β� synthetic colors inferred from absolute solar spectra agree with our solar colors only when the zero-points based on the ATLAS9 model are adopted. The Kurucz solar model provides a better fit to our observations than the MARCS model. For photometric values computed from the Kurucz models, (b − y)� and m1,� are in excellent agreement with our solar colors independently of the adopted zero-points, but for c1,� and β� agreement is found only when adopting the ATLAS9 zero-points. The c1,� color computed from both the Kurucz and MARCS models is the most discrepant, probably revealing problems either with the models or observations in the u band. The Teff calibration of Alonso and collaborators has the poorest performance (∼140 K off), while the relation of Casagrande and collaborators is the most accurate (within 10 K). We confirm that the Ramirez & Melendez uvby metallicity calibration, recommended by Arnadottir and collaborators to obtain [Fe/H] in F, G, and K dwarfs, needs a small (∼10%) zero-point correction to place the stars and the Sun on the same metallicity scale. Finally, we confirm that the c1 index in solar analogs has a strong metallicity sensitivity.

/Fe]

Low resolution spectra of 23 stars towards the bulge globular clusters NGC 6528 and NGC 6553 are analysed. Radial velocities and atmospheric parameters are derived in order to check their membership in the clusters. Eective temperatures were obtained from photometric data for stars with Te > 3800 K, whereas for cooler stars, they were derived from equivalent widths of TiO bands. Calibrations ofW (TiO) as a function of stellar parameters based on a grid of synthetic spectra are presented. Metallicities were derived from a comparison of the observed spectra to a grid of synthetic spectra. The sample comprises evolutionary stages from the Red Giant Branch to the Horizontal Branch, with parameters in the range 3200 Te 5000 K and 0:5 logg 2:4. The mean metallicities obtained for NGC 6528 and NGC 6553 are (Fe/H) 0: 5a nd 0:7, in both cases with (Mg/Fe) = +0:3; assuming the same overabundance for the elements O, Mg, Si, S, Ca and Ti, this gives (Z=Z )= 0:25 and 0:45. Membership verication by means of low resolution spectra is a crucial step in preparing targets for high resolution spectroscopy with 8 m class telescopes.

High precision differential abundance measurements in globular clusters: chemical inhomogeneities in NGC 6752

We use data from the Multi-Unit Spectroscopic Explorer (MUSE), recently commissioned at the Very Large Telescope (VLT), to study the kinematics and stellar population content of NGC 4371, an early-type massive barred galaxy in the core of the Virgo cluster. We integrate this study with a detailed structural analysis using imaging data from the Hubble and Spitzer space telescopes, which allows us to perform a thorough investigation of the physical properties of the galaxy. We show that the rotationally supported inner components in NGC 4371, an inner disc and a nuclear ring - which, according to the predominant scenario, are built with stars formed from gas brought to the inner region by the bar - are vastly dominated by stars older than 10 Gyr. Our results thus indicate that the formation of the bar occurred at a redshift of about

Testing the accuracy of synthetic stellar libraries

z=1.8^{+0.5}_{-0.4}

uvby–β photometry of solar twins - The solar colors, model atmospheres, and the Teff and metallicity scales

(error bars are derived from 100 Monte Carlo realisations). NGC 4371 thus testifies to the robustness of bars. In addition, the mean stellar age of the fraction of the major disc of the galaxy covered by our MUSE data is above 7 Gyr, with a small contribution from younger stars. This suggests that the quenching of star formation in NGC 4371, likely due to environmental effects, was already effective at a redshift of about

Membership of 23 stars towards the bulge globular clusters NGC 6528 and NGC 6553

z=0.8^{+0.2}_{-0.1}