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Context.The Galactic globular cluster 47 Tucanae is central to studies of Galaxy formation, and a test-bed for theoretical models, distance determination and extragalactic age-dating techniques. Independent parallax-based distance determinations in the optical spectral range provide discrepant results; also, star counts along the Red Giant Branch from optical data have disclosed a worrying disagreement with theoretical predictions, that impacts not only the theory of red giant stars, but also the calibration of the age scale of extragalactic systems.

Context.HD 23642 is the only known eclipsing binary in the Pleiades, and therefore of importance in determining the distance to this cluster.

Context.Age and metallicity estimates for extragalactic globular clusters, from integrated colour–colour diagrams, are examined.

We present distance modulus and reddening determinations for 72 Galactic globular clusters from the homogeneous photometric database of Piotto et al. ([CITE]), calibrated to the HST flight F439Wand F555Wbands. The distances have been determined by comparison with theoretical absolute magnitudes of the ZAHB. For low and intermediate metallicity clusters, we have estimated the apparent Zero Age Horizontal Branch (ZAHB) magnitude from the RR Lyrae level. For metal rich clusters, the ZAHB magnitude was obtained from the fainter envelope of the red HB. Reddenings have been estimated by comparison of the HST colour-magnitude diagrams (CMD) with ground CMDs of template clusters with low reddening. The homogeneity of both the photometric data and the adopted methodological approach allowed us to obtain highly accurate relative cluster distances and reddenings. Our results are also compared with recent compilations in the literature.

Two alternative scenarios concerning the origin and evolution of extremely metal-poor halo stars are investigated. The first one assumes that the stars have been completely metal-free initially and produced observed carbon and nitrogen overabundances during the peculiar core helium flash typical of low-mass Population III stars. The second scenario assumes that the initial composition resulted from a mixture of primordial material with ejecta from a single primordial supernova. Both scenarios are shown to have problems in reproducing C, N, and O abundances simultaneously, and both disagree with observed $^{12}{\rm C}/^{13}{\rm C}$-ratios, although in different directions. We concentrate on the most iron-poor, carbon-rich object of this class, HE 0107–5240, and conclude that the second scenario presently offers the more promising approach to understand these objects, in particular because evolutionary tracks match observations very well.

In this paper we estimate the initial He content in about 30% of the Galactic globular clusters (GGCs) from new star counts we have performed on the recently published HST snapshot database of Colour Magnitude Diagrams (Piotto et al. [CITE]). More specifically, we use the so-called R-parameter and estimate the He content from a theoretical calibration based on a recently updated set of stellar evolution models. We performed an accurate statistical analysis in order to assess whether GGCs show a statistically significant spread in their initial He abundances, and whether there is a correlation with the cluster metallicity. As in previous works on the subject, we do not find any significant dependence of the He abundance on the cluster metallicity; this provides an important constraint for models of Galaxy formation and evolution. Apart from GGCs with the bluest Horizontal Branch morphology, the observed spread in the individual helium abundances is statistically compatible with the individual errors. This means that either there is no intrinsic abundance spread among the GGCs, or that this is masked by the errors. In the latter case we have estimated a firm 1σupper limit of 0.019 to the possible intrinsic spread. In case of the GGCs with the bluest Horizontal Branch morphology we detect a significant spread towards higher abundances inconsistent with the individual errors; this can be fully explained by additional effects not accounted for in our theoretical calibrations, which do not affect the abundances estimated for the clusters with redder Horizontal Branch morphology. In the hypothesis that the intrinsic dispersion on the individual He abundances is zero, taking into account the errors on the individual R-parameter estimates, as well as the uncertainties on the cluster metallicity scale and theoretical calibration, we have determined an initial He abundance mass fraction $Y_{\rm GGC}=0.250\pm0.006$. This value is in perfect agreement with current estimates based on Cosmic Microwave Background radiation analyses and cosmological nucleosynthesis computations.

We determine ages of 71 old Open Clusters by a two-step method: we use main-squence fitting to 10 selected clusters, in order to obtain their distances, and derive their ages from comparison with our own isochrones used before for Globular Clusters. We then calibrate the morphological age indicator $\delta(V)$, which can be obtained for all remaining clusters, in terms of age and metallicity. Particular care is taken to ensure consistency in the whole procedure. The resulting Open Cluster ages connect well to our previous Globular Cluster results. From the Open Cluster sample, as well as from the combined sample, questions regarding the formation process of Galactic components are addressed. The age of the oldest open clusters (NGC 6791 and Be 17) is of the order of 10 Gyr. We determine a delay by $2.0\pm1.5$Gyr between the start of the halo and thin disk formation, whereas thin and thick disk started to form approximately at the same time. We do not find any significant age–metallicity relationship for the open cluster sample. The cumulative age distribution of the whole open cluster sample shows a moderately significant (${\sim} 2\sigma$level) departure from the predictions for an exponentially declining dissolution rate with timescale of 2.5 Gyr. The cumulative age distribution does not show any trend with galactocentric distance, but the clusters with larger height to the Galactic plane have an excess of objects between 2–4 and 6 Gyr with respect to their counterpart closer to the plane of the Galaxy.

We present observational estimates of the magnitude difference between the luminosity function red giant branch bump and the horizontal branch ($\Delta\mathrm{F555W}^{\mathrm{bump}}_{\mathrm{HB}}$), and of star counts in the bump region ($R_{\mathrm{bump}}$), for a sample of 54 Galactic globular clusters observed by the HST. The large sample of stars resolved in each cluster, and the high photometric accuracy of the data allowed us to detect the bump also in a number of metal poor clusters. To reduce the photometric uncertainties, empirical values are compared with theoretical predictions obtained from a set of updated canonical stellar evolution models which have been transformed directly into the HST flight system. We found an overall qualitative agreement between theory and observations. Quantitative estimates of the confidence level are hampered by current uncertainties on the globular cluster metallicity scale, and by the strong dependence of $\Delta\mathrm{F555W}^{\mathrm{bump}}_{\mathrm{HB}}$on the cluster metallicity. In case of the $R_{\mathrm{bump}}$parameter, which is only weakly affected by the metallicity, we find a very good quantitative agreement between theoretical canonical models and observations. For our full cluster sample the average difference between predicted and observed $R_{\mathrm{bump}}$values is practically negligible, and ranges from -0.002 to -0.028, depending on the employed metallicity scale. The observed dispersion around these values is entirely consistent with the observational errors on $R_{\mathrm{bump}}$. As a comparison, the value of $R_{\mathrm{bump}}$predicted by theory in case of spurious bump detections due to Poisson noise in the stellar counts would be ~0.10 smaller than the observed ones. We have also tested the influence on the predicted $\Delta\mathrm{F555W}^{\mathrm{bump}}_{\mathrm{HB}}$and $R_{\mathrm{bump}}$values of an He-enriched component in the cluster stellar population, as recently suggested by D'Antona et al. ([CITE]). We find that, under reasonable assumptions concerning the size of this He-enriched population and the degree of enrichment, the predicted $\Delta\mathrm{F555W}^{\mathrm{bump}}_{\mathrm{HB}}$and $R_{\mathrm{bump}}$values are only marginally affected.

Recent investigations aimed at estimating the distance to the young LMC cluster NGC 1866 have made use of Red Clump stars in the surrounding LMC field, together with empirical and theoretical Main-Sequence fitting methods, and have found significantly different distances for the field and the cluster, the latter being closer by in distance modulus by Δ(DM) ~ 0.20 mag. In this paper we (re-)consider the Cepheid star population of NGC 1866, to try to shed some light on this discrepancy. By combining various extensive photometric datasets in B, $V, I$and single-epoch 2massJHKphotometry, PLrelationships for the cluster Cepheids are obtained. A comparison between the field LMC and cluster PLrelationships for the reddening free Wesenheit index gives a firm determination of the distance between the cluster and the LMC main body (0.04 mag in distance modulus, the cluster being more distant) which, coupled to a model for the geometry of the LMC disk, provides Δ(DM) ranging between 0.0 and -0.11 mag. The simultaneous comparison of the PLrelationships in B, Vand Ifor the cluster and LMC field gives an estimate of the cluster reddening, which results to be $E(B-V)$= 0.12 ±0.02. This determination is higher than the canonical value of 0.06 mag used in all previous studies, but we show that it is not in contradiction with a re-analysis of independent estimates. The adoption of the LMC extinction law recently presented by Gordon et al. (2003) does not change these results. The cluster Main Sequence fitting distance obtained with this new reddening is DM= 18.58 ±0.08, fully compatible with the Red Clump value of DM= 18.53 $\pm 0.07({\rm random})^{+0.02}_{-0.05}({\rm systematic})$and the Cepheid constraint on Δ(DM). Finally, we determined the distance to the cluster by using a Cepheid Wesenheit PLrelationship with slope coming from LMC observations, and absolute magnitude zero point calibrated on Hipparcosparallaxes of Galactic Cepheids, in the assumption that the relationship is independent of metallicity; the resulting DM= 18.65 ±0.10 is not an accurate estimate of the LMC distance because of possible metallicity effects but, when compared to the revised Main Sequence fitting value, it points out to a possibly weak dependence of the Wesenheit PLrelationship on the Cepheid chemical composition, at least in the period range between 2.5 and 3.5 days.

We investigate whether the observed high number of carbon- and nitrogen-enhanced extremely metal-poor stars could be explained by peculiar evolutionary properties during the core He flash at the tip of the red giant branch. For this purpose we compute a series of detailed stellar models expanding upon our previous work; in particular, we investigate if during the major He flash the penetration of the helium convective zone into the overlying hydrogen-rich layers can produce carbon- and nitrogen-rich abundances in agreement with current spectroscopic observations. The dependence of this phenomenon on selected model input parameters, such as initial metallicity and treatment of convection is examined in detail.

We present homogeneous age determinations for a large sample of 55 Galactic globular clusters, which constitute about 30% of the total Galactic population. A study of their age distribution reveals that all clusters from the most metal poor ones up to intermediate metallicities are coeval, whereas at higher [Fe/H] an age spread exists, together with an age-metallicity relationship. At the same time, all clusters within a certain galactocentric distance appear coeval, whereas an age spread is present further away from the Galactic centre, without any correlation with distance. The precise value of [Fe/H] and galactocentric distance for the onset of the age spread and the slope of the age-metallicity relationship are strongly affected by the as yet uncertain [Fe/H] scale. We discuss how differences in the adopted [Fe/H] scale and cluster sample size may explain discrepant results about the clusters age distribution reached by different authors. Taking advantage of the large number of objects included in our sample, we also tested the possibility that age is the global second parameter which determines the Horizontal Branch morphology, and found indications that age could explain the global behaviour of the second parameter effect.

We present a new method to determine the distance to B-stars in eclipsing binary systems. The method is completely empirical, and it is based on the existence of a very tight linear relationship between the V-band “zero magnitude angular diameter” and the Strömgren colour index c1for B-stars; we have empirically calibrated this relationship using local single B-stars with accurate angular diameters, and B-stars in eclipsing binaries with precise radii and parallax determinations. By studying the differential behaviour of this relationship as predicted by theoretical stellar evolution models, we find that it is independent of the stellar metallicities for a range of [Fe/H] values between the solar one and that of young stars in the Magellanic Clouds. The method, which also provides the value of the reddening to the system, is discussed in detail, together with a thorough estimate of the associated errors. We conclude that accurate Strömgren photometry obtainable with 1.5 m-class telescopes of the LMC eclipsing binaries HV 2274 and HV 982 will allow to obtain an empirical LMC distance with an accuracy of the order of 0.13 mag.

We have computed a grid of up-to-date stellar evolutionary models including atomic diffusion, in order to study the evolution with time of the surface Li abundance in low-mass metal-poor stars. We discuss in detail the dependence of the surface Li evolution on the initial metallicity and stellar mass, and compare the abundances obtained from our models with the available Li measurements in Pop II stars. While it is widely accepted that the existence of the Spite Li-plateau for these stars is a strong evidence that diffusion is inhibited, we show that, when taking into account observational errors, uncertainties in the Li abundance determinations, in the Teffscale, and in particular the size of the observed samples of stars, the Spite plateau and the Li abundances in subgiant branch stars can be reproduced also by models including fully efficient diffusion, provided that the most metal-poor field halo objects are between 13.5 and 14 Gyr old. We provide the value of the minimum number of plateau stars to observe, for discriminating between efficient or inhibited diffusion. From our models with diffusion we derive that the average Li abundance along the Spite plateau is about a factor of 2 lower than the primordial one. As a consequence, the derived primordial Li abundance would be consistent with a high helium and low deuterium Big Bang Nucleosynthesis; this implies a high cosmological baryon density as inferred from the analyses of the cosmic microwave background.

We analyze in detail various possible sources of systematic errors on the distances of globular clusters derived by fitting a local template DA white dwarf sequence to the cluster counterpart (the so-called WD-fitting technique). We find that the unknown thickness of the hydrogen layer of white dwarfs in clusters plays a non negligible role. For reasonable assumptions -supported by the few sparse available observational constraints -about the unknown mass and thickness of the hydrogen layer for the cluster white dwarfs, a realistic estimate of the systematic error on the distance is within ±0.10 mag. However, particular combinations of white dwarf masses and envelope thicknesses -which at present cannot be excluded a priori -could produce larger errors. Contamination of the cluster DA sequence by non-DA white dwarfs introduces a very small systematic error of about -0.03 mag in the $M_{V}/(V-I)$plane, but in the $M_{V}/(B-V)$plane the systematic error amounts to ~+0.20 mag. Contamination by white dwarfs with helium cores should not influence appreciably the WD-fitting distances. Finally, we obtain a derivative $\Delta (m-M)_{V}/\Delta E(B-V)\sim -5.5$for the WD-fitting distances, which is very similar to the dependence found when using the Main Sequence fitting technique.

We reanalyse the UV/optical spectrum and optical broad-band data of the eclipsing binary HV 2274 in the LMC, and derive its distance following the method given by Guinan et al. ([CITE],[CITE]) of fitting theoretical spectra to the stars' UV/optical spectrum plus optical photometry. We describe the method in detail, pointing out the various assumptions that have to be made; moreover, we discuss the systematic effects of using different sets of model atmospheres and different sets of optical photometric data. It turns out that different selections of the photometric data, the set of model atmospheres and the constraints on the value of the ratio of selective to total extinction in the V-band, result in a 25% range in distances (although some of these models have a large ${\chi}^2$). For our best choice of these quantities the derived value for the reddening to HV 2274 is $E(B-V)$= 0.103 ±0.007, and the de-reddened distance modulus is $DM = 18.46$±0.06; the DMto the center of the LMC is found to be 18.42 ±0.07. This is significantly larger than the DMof 18.30 ±0.07 derived by Guinan et al. ([CITE]).

In this paper we investigate the level of agreement between observations and "new" Asymptotic Giant Branch (AGB) models, as produced by updating the physical inputs adopted in previous stellar computations. One finds that the new physics increases the predicted luminosity of Horizontal Branch (HB) and AGB stellar structures by a similar amount, keeping unchanged the predictions about the difference in luminosity between these two evolutionary phases. The best fit of selected globular clusters appears rather satisfactory, disclosing the relevance of the assumption on the mass of the Red Giant Branch (RGB) progenitor in assessing the distance modulus of moderately metal rich clusters. The still existing uncertainties related either to the input physics or to the efficiency of some macroscopic mechanisms, like convection or microscopic diffusion, are critically discussed, ruling out the occurrence of the so called "breathing pulses" during the central He exhaustion, in agreement with earlier suggestions.

We have analyzed ACS/WFC and WFPC2 images from HST, as well as ground-based data to study the radial distribution of the double sub-giant branch (SGB) recently discovered in the Galactic globular cluster NGC 1851. We found that the SGB split can be followed all the way from the cluster center out to at least 8??from the center. Beyond this distance out to the tidal radius at ~11.7?arcmin, there are simply too few SGB stars to identify the sequences. The number ratio of the bright SGB to the faint SGB stars shows no significant radial trend. Furthermore, we have found that the ratio of blue horizontal-branch (HB) stars plus RR Lyrae to the red HB stars also remains constant from the cluster center to the outer envelope.

Context. The mixing length theory (MLT) used to compute the temperature gradient in superadiabatic layers of stellar (interior and atmosphere) models contains in its standard form 4?free parameters. Three parameters are fixed a?priori (and define what we denote as the MLT ?flavour?) whereas one (the so-called mixing length) is calibrated by reproducing observational constraints. The ?classical? B?hm-Vitense flavour is used in all modern MLT-based stellar model computations and, despite its crude approximations, the resulting Teff?scale appears?? perhaps surprisingly?? remarkably realistic, once the mixing length parameter is calibrated with a solar model.Aims. Model atmosphere computations employ parameter choices different from what is used in stellar interior modelling, raising the question of whether a single MLT flavour and mixing length value can be used to compute interiors and atmospheres of stars of all types. As a first step towards addressing this issue, we study whether the MLT flavour (the so-called ML2) and mixing length choice that have been proven adequate to model white dwarf atmospheres, are able to provide, when used in stellar models, results at least comparable to the use of the ?classical? B?hm-Vitense flavour.Methods. We have computed solar models and evolutionary tracks for both low- and intermediate-mass Population?I and?II stars, adopting both solar calibrated B?hm-Vitense and ML2 flavours of the MLT in our stellar evolution code, and state-of-the-art input physics. Results. The two sets of models provide consistent results, with only minor differences. Both calibrations reproduce also the Teffof red giants in a sample of Galactic globular clusters. The ML2?solar model provides a mixing length about half the value of the local pressure scale height, thus alleviating?? but not eliminating?? one of the well known inconsistencies of the MLT employed in stellar models. This mixing length is also consistent with the value used in white dwarf model atmosphere computations.

We calculated synthetic spectra for typical chemical element mixtures (i.e., a standard α-enhanced distribution, and distributions displaying CN and ONa anticorrelations) found in the various subpopulations harboured by individual Galactic globular clusters. From the spectra we determined bolometric corrections to the standard Johnson-Cousins and Strömgren filters and finally predicted colours. These bolometric corrections and colour-transformations, coupled to our theoretical isochrones with the appropriate chemical composition, provided us with a complete and self-consistent set of theoretical predictions for the effect of abundance variations on the observed cluster colour–magnitude diagrams. CNO abundance variations affect mainly wavelengths shorter than  ~400 nm owing to the rise of molecular absorption bands in cooler atmospheres. As a consequence, colour and magnitude changes are largest in the blue filters, independently of using broad or intermediate bandpasses. Colour–magnitude diagrams involving uvyand UB filters (and their various possible colour combinations) are therefore best suited to infer photometrically the presence of multiple stellar generations in individual clusters. They are particularly sensitive to variations in the N abundance, with the largest variations affecting the red giant branch (RGB) and lower main sequence (MS). BVI diagrams are expected to display multiple sequences only if the different populations are characterized by variations of the C+N+O sum and/or helium abundance that lead to changes in luminosity and effective temperature, but leave the flux distribution above 400 nm practically unaffected. A variation of just the helium abundance up to the level we investigate here exclusively affects the interior structure of stars, and is largely irrelevant for the atmospheric structure and the resulting flux distribution in the whole wavelength range spanned by our analysis.

Context.NGC 6791 is a well-studied, metal-rich open cluster that is so close to us that it can be imaged down to luminosities fainter than that of the termination of its white-dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population.

We present new measurements of the magnitude of the main sequence turn off and the red giant branch bump in the luminosity function of a sample of Galactic globular clusters with updated estimates of [Fe/H] and [α/Fe], employing photometric data collected with the Advanced Camera for Survey onboard the HST. We compare measured and predicted values of the magnitude difference between these two features, a rarely employed diagnostic of the internal structure of low-mass stars at the beginning of their red giant evolution. Our analysis discloses a clear discrepancy between theory and observations, because the theoretical red giant branch bump magnitudes are too bright by on average  ~0.2 mag. This corroborates results from the more widely studied magnitude difference between horizontal branch and red giant bump, avoiding the well known problems associated with determining the horizontal branch level from colour-magnitude diagrams and with uncertainties in the luminosity of horizontal branch stellar models. We briefly discuss several potential solutions of this discrepancy.

We investigate the impact of the 14N(p,γ)15O reaction rate as redetermined by the LUNA experiment, on the shell H-burning and core He-burning phases of low-mass, metal-poor stellar models. The new reaction rate has small but noticeable effects, the largest one being a  ~7−8% reduction in the red giant branch lifetimes. To different degrees, the lifetimes and luminosities of horizontal branch models, the mass of the stellar models evolving within the RR Lyrae instability strip, the luminosity of the red giant branch luminosity function bump, the theoretical calibrations of the R-parameter, and the tip of the red giant branch luminosity are also affected. Predictions for the tip of the red giant branch luminosity, in particular, are in very good agreement with the currently available empirical constraints.