S. Degl’Innocenti

STROMGREN PHOTOMETRY OF GALACTIC GLOBULAR CLUSTERS. II. METALLICITY DISTRIBUTION OF RED GIANTS IN ω CENTAURI

We present new intermediate-band Str?mgren photometry based on more than 300 u, v, b, y images of the Galactic globular cluster ? Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J, H, Ks images). The final optical-NIR catalog covers a region of more than 20 ? 20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper-motion data available in the literature to identify candidate cluster red-giant (RG) stars. By adopting different Str?mgren metallicity indices, we estimate the photometric metallicity for 4000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H]phot = ?1.73 ? 0.08, ?1.29 ? 0.03, ?1.05 ? 0.02, ?0.80 ? 0.04, ?0.42 ? 0.12, and ?0.07 ? 0.08 dex) and a sharp cutoff in the metal-poor (MP) tail ([Fe/H]phot ?2 dex) that agree quite well with spectroscopic measurements. We identify four distinct subpopulations, namely, MP ([Fe/H] ? ?1.49), metal-intermediate (MI; ?1.49 < [Fe/H] ? ?0.93), metal-rich (MR; ?0.95 < [Fe/H] ? ?0.15), and solar metallicity ([Fe/H] 0). The last group includes only a small fraction of stars (~8% ? 5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19% ? 1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Str?mgren metallicity indices display a robust correlation with ?-elements ([Ca+Si/H]) when moving from the MI to the MR regime ([Fe/H] ?1.7 dex).

7Li surface abundance in pre-main sequence stars - Testing theory against clusters and binary systems

Context. The disagreement between theoretical predictions and observations for surface lithium abundance in stars is a long-standing problem, which indicates that the adopted physical treatment is still lacking in some points. However, thanks to the recent improvements in both models and observations, it is interesting to analyse the situation to evaluate present uncertainties. Aims. We present a consistent and quantitative analysis of the theoretical uncertainties affecting surface lithium abundance in the current generation of models. Methods. By means of an up-to-date and well tested evolutionary code, FRANEC, theoretical errors on surface 7 Li abundance predictions, during the pre-main sequence (pre-MS) and main sequence (MS) phases, are discussed in detail. Then, the predicted surface 7 Li abundance was tested against observational data for five open clusters, namely Ic 2602, α Per, Blanco1, Pleiades, and NGC 2516, and for four detached double-lined eclipsing binary systems. Stellar models for the aforementioned clusters were computed by adopting suitable chemical composition, age, and mixing length parameter for MS stars determined from the analysis of the colour−magnitude diagram of each cluster. We restricted our analysis to young clusters, to avoid additional uncertainty sources such as diffusion and/or radiative levitation efficiency. Results. We confirm the disagreement, within present uncertainties, between theoretical predictions and 7 Li observations for standard models. However, we notice that a satisfactory agreement with observations for 7 Li abundance in both young open clusters and binary systems can be achieved if a lower convection efficiency is adopted during the pre-MS phase with respect to the MS one.

Star counts in the globular cluster ω centauri. I. Bright stellar components

We present a photometric investigation on HB, RGB, and MSTO stars in ω Cen=NGC 5139. The center of the cluster was covered with a mosaic of F435W, F625W, and F658N band data collected with HST ACS. The outer reaches were covered with a mosaic of U-, B-, V-, and I-band data collected with the 2.2 m ESO/MPI telescope. The final catalog includes ~1.7 million stars. We identified more than 3200 likely HB stars, the largest sample ever collected in a globular cluster. We found that the HB morphology changes with the radial distance from the cluster center. The relative number of extreme HB stars decreases from ~30% to ~21% when moving from the center toward the outer reaches of the cluster, while the fraction of less hot HB stars increases from ~62% to ~72%. The comparison between theory and observations indicates that the empirical star counts of HB stars are on average larger (30%-40%) than predicted by canonical evolutionary models. Moreover, the rate of HB stars is ~43% larger than the MSTO rate. We also compared theory and observations by assuming a mix of stellar populations made with 70% of canonical He (Y = 0.23) stars and 30% of He-enhanced (Y = 0.33, 0.42) stars. We found that the observed RG/MSTO ratio agrees with the predicted lifetimes of He-mixed stellar populations. The discrepancy between theory and observations decreases by a factor of 2 when compared with rates predicted by canonical He content models, but still 15%-25% (Y = 0.42) and 15%-20% (Y = 0.33) higher than observed. Furthermore, the ratios between HB and MSTO star counts are ~24% (Y = 0.42) and 30% (Y = 0.33) larger than predicted lifetime ratios.

Cumulative physical uncertainty in modern stellar models I. The case of low-mass stars

Context. Theoretical stellar evolutionary models are still affected by not negligible uncertainties due to the errors in th e adopted physical inputs. Aims. In this paper, using our updated stellar evolutionary code, we quantitatively evaluate the effects of the uncertainties in the main physical inputs on the evolutionary characteristics of low mass stars, and thus of old stellar clusters, from the main sequence to the zero age horizontal branch (ZAHB). To this aim we calculated more than 3000 stellar tracks and isochrones, with updated solar mixture, by changing the following physical inputs within their current range of uncertainty: 1 H(p,�e + ) 2 H, 14 N(p, ) 15 O, and triple-�reaction rates, radiative and conductive opacities, neutr ino energy losses, and microscopic diffusion velocities. Methods. The analysis was conducted performing a systematic variation on a fixed grid, in a way to obtain a full crossing of the perturbed input values. The effect of the variations of the chosen physical inputs on releva nt stellar evolutionary features, such as the turn-off luminosity, the central hydrogen exhaustion time, the red-giant branch tip luminosity, the helium core mass, and the ZAHB luminosity in the RR Lyrae region are analyzed in a statistical way. Results. We find that, for a 0.9 M⊙ model, the cumulative uncertainty on the turn-off, the red-giant branch tip, and the ZAHB luminosities accounts for±0.02 dex,±0.03 dex, and±0.045 dex respectively, while the central hydrogen exhaustion time varies of about±0.7 Gyr. For all examined features the most relevant effect is due to the radiative opacities uncertainty; for the la ter evolutionary stages the second most important effect is due to the triple-� reaction rate uncertainty. For an isochrone of 12 Gyr, we find that the isochrone turn-off log luminosity varies of±0.013 dex, the mass at the isochrone turn-off varies of±0.015 M⊙, and the difference between ZAHB and turn-off log-luminosity varies of±0.05 dex. The effect of the physical uncertainty affecting the age inferred from turn-off luminosity and from the vertical method are of± 0.375 Gyr and± 1.25 Gyr respectively.

On the Stellar Content of the Carina Dwarf Spheroidal Galaxy

We present deep, accurate, and homogeneous multiband optical (U, B, V, I) photometry of the Carina dwarf spheroidal galaxy, based on more than 4000 individual CCD images from three different ground-based telescopes. Special attention was given to the photometric calibration, and the precision for the B, V, and I bands is generally better than 0.01 mag. We have performed detailed comparisons in the V, B - V, and V, B - I color-magnitude diagrams (CMDs) between Carina and three old, metal-poor Galactic globular clusters (GGCs, M53, M55, M79). We find that only the more metal-poor GCs (M55, [Fe/H] = -1.85; M53, [Fe/H] = -2.02 dex) provide a good match with the Carina giant branch. We have performed a similar comparison in the V, V - I CMD with three intermediate-age clusters (IACs) of the Small Magellanic Cloud (Kron 3, NGC 339, Lindsay 38). We find that the color extent of the subgiant branch (SGB) of the two more metal-rich IACs (Kron 3, [Fe/H] = -1.08; NGC339, [Fe/H] = -1.36 dex) is smaller than the range among Carinas intermediate-age stars. Moreover, the slope of the RGB of these two IACs is shallower than the slope of the Carina RGB. However, the ridge line of the more metal-poor IAC (Lindsay 38, [Fe/H] = -1.59 dex) agrees quite well with the Carina intermediate-age stars. These findings indicate that Carinas old stellar population is metal-poor and appears to have a limited spread in metallicity (Δ[Fe/H] = 0.2-0.3 dex). The Carinas intermediate-age stellar population can hardly be more metal-rich than Lindsay 38, and its spread in metallicity also appears modest. We also find that the synthetic CMD constructed assuming a metallicity spread of 0.5 dex for the intermediate-age stellar component predicts evolutionary features not supported by observations. In particular, red clump stars should attain colors that are redder than red giant stars, but this is not seen. These results are at odds with recent spectroscopic investigations suggesting that Carina stars cover a broad range in metallicity (Δ[Fe/H] ~ 1-2 dex). We also present a new method to estimate the metallicity of complex stellar systems using the difference in color between the red clump and the middle of the RR Lyrae instability strip. The observed colors of Carinas evolved stars indicate a metallicity of [Fe/H] = -1.70 ± 0.19 dex, which agrees quite well with spectroscopic measurements.

Uncertainties in grid-based estimates of stellar mass and radius - SCEPtER: Stellar CharactEristics Pisa Estimation gRid

Context. The availability of high-quality astero-seismological data provided by satellite missions stimulated the development of several grid-based estimation techniques to determine the stellar masses and radii. Some aspects of the systematic and statistical errors a ecting these techniques have still not been investigated well. Aims. We study the impact on mass and radius determination of the uncertainty in the input physics, in the mixing-length value, in the initial helium abundance, and in the microscopic di usion e ciency adopted in stellar model computations. Methods. We consider stars with mass in the range [0.8 - 1.1] M and evolutionary stages from the zero-age main sequence to the central hydrogen exhaustion. To recover the stellar parameters, a maximum-likelihood technique was employed by comparing the observations constraints to a precomputed grid of stellar models. Synthetic grids with perturbed input were adopted to estimate the systematic errors arising from the current uncertainty in model computations. Results. We found that the statistical error components, owing to the current typical uncertainty in the observations, are nearly constant in all cases at about 4.5% and 2.2% on mass and radius determination, respectively. The systematic bias on mass and radius determination due to a variation of 1 in Y= Z is 2:3% and 1:1%; the one due to a change of 0:24 in the value of the mixinglength ml is 2:1% and 1:0%; the one due to a variation of 5% in the radiative opacity is 1:0% and 0:45%. An important bias source is to neglect microscopic di usion, which accounts for errors of about 3.7% and 1.5% on mass and radius. The cumulative e ects of the considered uncertainty sources can produce biased estimates of stellar characteristics. Comparison of the results of our technique with other grid techniques shows that the systematic biases induced by the di erences in the estimation grids are generally greater than the statistical errors involved.

Cumulative physical uncertainty in modern stellar models - II. The dependence on the chemical composition

We extend our work on the effects of the uncertainties on the main input physics for the evolution of low-mass stars. We analyse the dependence of the cumulative physical uncertainty affecting stellar tracks on the chemical composition. We calculated more than 6000 stellar tracks and isochrones, with metallicity ranging from Z = 0.0001 to 0.02, by changing the following physical inputs within their current range of uncertainty: 1H(p,nu e+)2H, 14N(p,gamma)15O and triple-alpha reaction rates, radiative and conductive opacities, neutrino energy losses, and microscopic diffusion velocities. The analysis was performed using a latin hypercube sampling design. We examine in a statistical way the dependence on the variation of the physical inputs of the turn-off (TO) luminosity, the central hydrogen exhaustion time (t_H), the luminosity and the helium core mass at the red-giant branch (RGB) tip, and the zero age horizontal branch (ZAHB) luminosity in the RR Lyrae region. For the stellar tracks, an increase from Z = 0.0001 to Z = 0.02 produces a cumulative physical uncertainty in TO luminosity from 0.028 dex to 0.017 dex, while the global uncertainty on t_H increases from 0.42 Gyr to 1.08 Gyr. For the RGB tip, the cumulative uncertainty on the luminosity is almost constant at 0.03 dex, whereas the one the helium core mass decreases from 0.0055 M_sun to 0.0035 M_sun. The dependence of the ZAHB luminosity error is not monotonic with Z, and it varies from a minimum of 0.036 dex at Z = 0.0005 to a maximum of 0.047 dex at Z = 0.0001. Regarding stellar isochrones of 12 Gyr, the cumulative physical uncertainty on the predicted TO luminosity and mass increases respectively from 0.012 dex to 0.014 dex and from 0.0136 M_sun to 0.0186 M_sun. Consequently, for ages typical of galactic globular clusters, the uncertainty on the age inferred from the TO luminosity increases from 325 Myr to 415 Myr.

On the absolute age of the globular cluster M92

We present precise and deep optical photometry of the globular M92. Data were collected in three different photometric systems: Sloan Digital Sky Survey (g 0 , r 0 , i 0 , and z 0 ; MegaCam at CFHT), Johnson-Kron- Cousins (B, V, and I; various ground-based telescopes), and Advanced Camera for Surveys (ACS) Vegamag (F475W, F555W, and F814W; Hubble Space Telescope). Special attention was given to the photometric calibration, and the precision of the ground-based data is generally better than 0.01 mag. We computed a new set of α-enhanced evolutionary models accounting for the gravitational settling of heavy elements at fixed chemical composition (½α=Fe �¼þ 0:3, ½Fe=H �¼� 2:32 dex, and Y ¼ 0:248). The isochrones—assuming the same true distance mod- ulus (μ ¼ 14:74 mag), the same reddening (EðBV Þ¼ 0:025 � 0:010 mag), and the same reddening law— account for the stellar distribution along the main sequence and the red giant branch in different color-magnitude diagrams (i 0 , g 0 � i 0 ; i 0 , and g 0 � r 0 ; i 0 , g 0 � z 0 ; I, and BI; and F814Wand F475W-F814W). The same outcome applies to the comparison between the predicted zero-age horizontal-branch (ZAHB) and the HB stars. We also found a cluster age of 11 � 1:5 Gyr, in good agreement with previous estimates. The error budget accounts for uncertainties in the input physics and the photometry. To test the possible occurrence of CNO-enhanced stars, we also computed two sets of α- and CNO-enhanced (by a factor of 3) models, both at fixed total metallicity (½M=H �¼� 2:10 dex) and at fixed iron abundance. We found that the isochrones based on the former set give the same cluster age (11 � 1:5 Gyr) as the canonical α-enhanced isochrones. The isochrones based on the latter set also give a similar cluster age (10 � 1:5 Gyr). These findings support previous results concerning the weak sensitivity of cluster isochrones to CNO-enhanced chemical mixtures.

The Carina Project. V. The Impact of NLTE Effects on the Iron Content

We have performed accurate iron abundance measurements for 44 red giants (RGs) in the Carina dwarf spheroidal (dSph) galaxy. We used archival, high-resolution spectra (R ~ 38,000) collected with UVES at ESO/VLT either in slit mode (five RGs) or in fiber mode (39 RGs, FLAMES/GIRAFFE-UVES). The sample is more than a factor of 4 larger than any previous spectroscopic investigation of stars in dSphs based on high-resolution (R≥38000) spectra. We did not impose the ionization equilibrium between neutral and singly ionized iron lines. The effective temperatures and the surface gravities were estimated by fitting stellar isochrones in the V,B - V color-magnitude diagram. To measure the iron abundance of individual lines we applied the LTE spectrum-synthesis fitting method using MARCS model atmospheres of appropriate metallicity. For the 27 stars for which we measured both Fe I and Fe II abundances, we found evidence of NLTE effects between neutral and singly ionized iron abundances. The difference is ~0.1 dex, on average, but steadily increases when moving from the metal-rich to the metal-poor regime. Moreover, the two metallicity distributions differ at the 97% confidence level. Assuming that the Fe II abundances are minimally affected by NLTE effects, we corrected the Fe I stellar abundances using a linear fit between Fe I and Fe II stellar abundance determinations. We found that the Carina metallicity distribution based on the corrected Fe I abundances (44 RGs) has a weighted mean metallicity of [Fe/H] = -1.80 and a weighted standard deviation of σ = 0.24 dex. The Carina metallicity distribution based on the Fe II abundances (27 RGs) gives similar estimates ([Fe/H] = -1.72, σ = 0.24 dex). The current weighted mean metallicities are slightly more metal-poor when compared with similar estimates available in the literature. Furthermore, when we restricted our analysis to stars with the most accurate iron abundances, ~20 Fe I and at least three Fe II measurements (15 stars), we found that the range in iron abundances covered by Carina RGs (~1 dex) agreed quite well with similar estimates based on high-resolution spectra; however, it is a factor of 2-3 smaller than abundance estimates based on the near-infrared calcium triplet. This finding supports previous estimates based on photometric metallicity indicators.

Uncertainties in asteroseismic grid-based estimates of stellar ages - SCEPtER: Stellar CharactEristics Pisa Estimation gRid

We study the impact on stellar age determination by means of grid-based techniques adopting asteroseismic constraints of the uncertainty in the radiative opacity, in the initial helium abundance, in the mixing-length value, in the convective core overshooting, and in the microscopic diffusion efficiency adopted in stellar model computations. We extended our SCEPtER grid (Valle et al. 2014) to include stars with mass in the range [0.8; 1.6] Msun and evolutionary stages from the ZAMS to the central hydrogen depletion. The current typical uncertainty in the observations accounts for 1 sigma statistical relative error in age determination which in mean ranges from about -35% to +42%, depending on the mass. However, due to the strong dependence on the evolutionary phase, the age relative error can be higher than 120% for stars near the ZAMS, while it is typically of the order of 20% or lower in the advanced main-sequence phase. The systematic bias on age determination due to a variation of