A. Pietrinferni

A Large Stellar Evolution Database for Population Synthesis Studies. I. Scaled Solar Models and Isochrones

We present a large and updated stellar evolution database for low-, intermediate-, and high-mass stars in a wide metallicity range, suitable for studying Galactic and extragalactic simple and composite stellar populations using population synthesis techniques. The stellar mass range is between � 0.5 and 10 Mwith a fine mass spacing. The metallicity (Fe/H) comprises 10 values ranging from � 2.27 to 0.40, with a scaled solar metal distribution. The initial He mass fraction ranges from Y ¼ 0:245, for the more metal-poor composition, up to 0.303 for the more metal-rich one, with Y =Z � 1:4. For each adopted chemical composition, the evolutionary models have been computed without (canonical models) and with overshooting from the Schwarzschild boundary of the convective cores during the central H-burning phase. Semiconvection is included in the treatment of core convection during the He-burning phase. The whole set of evolutionary models can be used to compute isochrones in a wide age range, from � 30 Myr to � 15 Gyr. Both evolutionary models and isochrones are available in several observational planes, employing an updated set of bolometric corrections and color-TeA relations computed for this project. The number of points along the models and the resulting isochrones is selected in such a way that interpolation for intermediate metallicities not contained in the grid is straightforward; a simple quadratic interpolation produces results of sufficient accuracy for population synthesis applications.We compare our isochrones with results from a series of widely used stellar evolution databases and perform some empirical tests for the reliability of our models. Since this work is devoted to scaled solar chemical compositions, we focus our attention on the Galactic disk stellar populations, employing multicolor photometry of unevolved field main-sequence stars with precise Hipparcos parallaxes, well-studied open clusters, and one eclipsing binary system with precise measurements of masses, radii, and (Fe/H) of both components. We find that the predicted metallicity dependence of the location of the lower, unevolved main sequence in the color magnitude diagram (CMD) appears in satisfactory agreement with empirical data. When comparing our models with CMDs of selected, well-studied, open clusters, once again we were able to properly match the whole observed evolutionary sequences by assuming cluster distance and reddening estimates in satisfactory agreement with empirical evaluations of these quantities. In general, models including overshooting during the H-burning phase provide a better match to the observations, at least for ages below � 4 Gyr. At (Fe/H) around solar and higher ages (i.e., smaller convective cores) before the onset of radiative cores, the selected efficiency of core overshooting may be too high in our model, as well as in various other models in the literature. Since we also provide canonical models, the reader is strongly encouraged to always compare the results from both sets in this critical age range. Subject heading gs: galaxies: stellar content — Galaxy: disk — open clusters and associations: general — stars: evolution — stars: horizontal-branch

A Triple Main Sequence in the Globular Cluster NGC 2808

Accurate photometry with HSTs ACS shows that the main sequence (MS) of the globular cluster NGC 2808 splits into three separate branches. The three MS branches may be associated with the complexities of the clusters horizontal branch and of its abundance distribution. We attribute the MS branches to successive rounds of star formation, with different helium abundances; we discuss possible sources of helium enrichment. Some other massive globulars also appear to have complex populations; we compare them with NGC 2808.

Metallicities on the Double Main Sequence of ω Centauri Imply Large Helium Enhancement

Having shown in a recent paper that the main sequence of ? Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is less metal-poor. We have carefully combined VLTs GIRAFFE spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate signal-to-noise ratio, to show clearly that the stars of the blue main sequence are less metal-poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main-sequence stars, whereas the red main-sequence stars show a 0.2 dex spread in metallicity. We use stellar structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.

Multiple Stellar Populations in 47 Tucanae

We use Hubble Space Telescope (HST) and ground-based imaging to study the multiple populations of 47 Tucanae (47 Tuc), combining high-precision photometry with calculations of synthetic spectra. Using filters covering a wide range of wavelengths, our HST photometry splits the main sequence into two branches, and we find that this duality is repeated in the subgiant and red giant regions, and on the horizontal branch. We calculate theoretical stellar atmospheres for main-sequence stars, assuming different chemical composition mixtures, and we compare their predicted colors through the HST filters with our observed colors. We find that we can match the complex of observed colors with a pair of populations, one with primeval abundance and another with enhanced nitrogen and a small helium enhancement, but with depleted C and O. We confirm that models of red giant and red horizontal branch stars with that pair of compositions also give colors that fit our observations. We suggest that the different strengths of molecular bands of OH, CN, CH, and NH, falling in different photometric bands, are responsible for the color splits of the two populations. Near the cluster center, in each portion of the color-magnitude diagram the population with primeval abundances makes up only ~20% of the stars, a fraction that increases outward, approaching equality in the outskirts of the cluster, with a fraction ~30% averaged over the whole cluster. Thus the second, He/N-enriched population is more concentrated and contributes the majority of the present-day stellar content of the cluster. We present evidence that the color-magnitude diagram of 47 Tuc consists of intertwined sequences of the two populations, whose separate identities can be followed continuously from the main sequence up to the red giant branch, and thence to the horizontal branch. A third population is visible only in the subgiant branch, where it includes ~8% of the stars.

Updated Electron-Conduction Opacities: The Impact on Low-Mass Stellar Models

We review the theory of electron-conduction opacity, a fundamental ingredient in the computation of low-mass stellar models; shortcomings and limitations of the existing calculations used in stellar evolution are discussed. We then present new determinations of the electron-conduction opacity in stellar conditions for an arbitrary chemical composition that improve over previous works and, most importantly, cover the whole parameter space relevant to stellar evolution models (i.e., both the regime of partial and high electron degeneracy). A detailed comparison with the currently used tabulations is also performed. The impact of our new opacities on the evolution of low-mass stars is assessed by computing stellar models along both the H- and He-burning evolutionary phases, as well as main sequence models of very low-mass stars and white dwarf cooling tracks.

The Double Subgiant Branch of NGC 1851: The Role of the CNO Abundance

We explore the possibility that the anomalous split in the subgiant branch (SGB) of the Galactic globular cluster NGC 1851 is due to the presence of two distinct stellar populations with very different initial metal mixtures: a normal α-enhanced component, and one characterized by strong anticorrelations among the CNONa abundances, with a total CNO abundance increased by a factor of 2. We test this hypothesis taking into account various empirical constraints, and conclude that the two populations should be approximately coeval, with the same initial He content. More high-resolution spectroscopic measurements of heavy elements—and in particular of the CNO sum—for this cluster are necessary to prove (or disprove) this scenario.

Transforming observational data and theoretical isochrones into the ACS/WFC Vega-mag system

We propose a zero-point photometric calibration of the data from the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) on board the Hubble Space Telescope, based on a spectrum of Vega and the most up-to-date in-flight transmission curves of the camera. This calibration is accurate at the level of a few hundredths of a magnitude. The main purpose of this effort is to transform the entire set of evolutionary models into a simple observational photometric system for ACS/WFC data, and to make them available to the astronomical community. We provide the zero-points for the most used ACS/WFC bands, and give basic recipes for calibrating both the observed data and the models. We also present the colour-magnitude diagram from ACS data of five Galactic globular clusters, spanning the metallicity range -2.2 <[Fe/H] < -0.04, and we provide fiducial points representing their sequences from several magnitudes below the turn-off to the red giant branch tip. The observed sequences are compared with the models in the newly defined photometric system.

A LARGE STELLAR EVOLUTION DATABASE FOR POPULATION SYNTHESIS STUDIES. V. STELLAR MODELS AND ISOCHRONES WITH CNONa ABUNDANCE ANTICORRELATIONS

We present a new grid of stellar models and isochrones for old stellar populations, covering a large range of [Fe/H] values, for an heavy element mixture characterized by CNONa abundance anticorrelations as observed in Galactic globular cluster stars. The effect of this metal abundance pattern on the evolutionary properties of low-mass stars, from the main sequence to the horizontal branch phase, is analyzed. We perform comparisons between these new models, and our reference α-enhanced calculations, and discuss briefly implications for color-magnitude diagrams showing multiple main sequence or subgiant branches. A brief qualitative discussion of the effect of CN abundances on color-T eff transformations is also presented, highlighting the need to determine theoretical color transformations for the appropriate metal mixture, if one wants to interpret observations in the Stromgren system, or broadband filters blueward of the Johnson V band.

On the metallicity gradient of the Galactic disk

Aims. The iron abundance gradient in the Galactic stellar disk provides fundamental constraints on the chemical evolution of this important Galaxy component, however the spread around the mean slope is, at fixed Galactocentric distance, more than the estimated uncertainties. Methods. To provide quantitative constraints on these trends, we adopted iron abundances for 265 classical Cepheids (more than 50% of the currently known sample) based either on high-resolution spectra or on photometric metallicity indices. Homogeneous distances were estimated using near-infrared period-luminosity relations. The sample covers the four disk quadrants, and their Galactocentric distances range from similar to 5 to similar to 17 kpc. We provided a new theoretical calibration of the metallicity-index-color (MIC) relation based on Walraven and NIR photometric passbands. Results. We estimated the photometric metallicity of 124 Cepheids. Among them 66 Cepheids also have spectroscopic iron abundances and we found that the mean difference is -0.03 +/- 0.15 dex. We also provide new iron abundances, based on high-resolution spectra, for four metal-rich Cepheids located in the inner disk. The remaining iron abundances are based on high-resolution spectra collected by our group (73) or available in the literature (130). A linear regression over the entire sample provides an iron gradient of -0.051 +/- 0.004 dex kpc(-1). The above slope agrees quite well, within the errors, with previous estimates based either on Cepheids or on open clusters covering similar Galactocentric distances. However, Cepheids located in the inner disk systematically appear more metal-rich than the mean metallicity gradient. Once we split the sample into inner (R(G) <8 kpc) and outer disk Cepheids, the slope (-0.130 +/- 0.015 dex kpc(-1)) in the former region is approximate to 3 times steeper than the slope in the latter one (-0.042 +/- 0.004 dex kpc(-1)). In the outer disk the radial distribution of metal-poor (MP, [Fe/H] <-0.02 dex) and metal-rich (MR) Cepheids across the four disk quadrants does not show a clear trend when moving from the innermost to the external disk regions. The relative fractions of MP and MR Cepheids in the 1st and in the 3rd quadrants differ at the 8 sigma (MP) and 15 sigma (MR) levels. Finally, we found that iron abundances in two local overdensities of the 2nd and of the 4th quadrant cover individually a range in iron abundance of approximate to 0.5 dex. Conclusions. Current findings indicate that the recent chemical enrichment across the Galactic disk shows a clumpy distribution.

Optical and near infrared coverage of SN 2004et: physical parameters and comparison with other type IIP supernovae

We present new optical and near-infrared (NIR) photometry and spectroscopy of the Type IIP supernova (SN), SN 2004et. In combination with already published data, this provides one of the most complete studies of optical and NIR data for any Type IIP SN from just after explosion to +500 d. The contribution of the NIR flux to the bolometric light curve is estimated to increase from 15 per cent at explosion to around 50 per cent at the end of the plateau and then declines to 40 per cent at 300 d. SN 2004et is one of the most luminous IIP SNe which has been well studied and characterized, and with a luminosity of log L= 42.3 erg s^(−1) and a ^(56)Ni mass of 0.06 ± 0.04 M_⊙, it is two times brighter than SN 1999em. We provide parametrized bolometric corrections as a function of time since explosion for SN 2004et and three other IIP SNe that have extensive optical and NIR data. These can be used as templates for future events in optical and NIR surveys without full wavelength coverage. We compare the physical parameters of SN 2004et with those of other well-studied IIP SNe and find that the kinetic energies span a range of 10^(50)–10^(51) erg. We compare the ejected masses calculated from hydrodynamic models with the progenitor masses and limits derived from pre-discovery images. Some of the ejected mass estimates are significantly higher than the progenitor mass estimates, with SN 2004et showing perhaps the most serious mass discrepancy. With the current models, it appears difficult to reconcile 100 d plateau lengths and high expansion velocities with the low ejected masses of 5–6 M_⊙ implied from 7–8 M_⊙ progenitors. The nebular phase is studied using very late-time Hubble Space Telescope photometry, along with optical and NIR spectroscopy. The light curve shows a clear flattening at 600 d in the optical and the NIR, which is likely due to the ejecta impacting on circumstellar material. We further show that the [O i] 6300, 6364 A line strengths in the nebular spectra of four Type IIP SNe imply ejected oxygen masses of 0.5–1.5 M_⊙.