Search

We tested the capability of a Bayesian procedure to calibrate both the helium abundance and the mixing length parameter ($\alpha_{ML}$), using precise photometric data for main-sequence (MS) stars in a cluster with negligible reddening and well-determined distance. The method has been applied first to a mock data set generated to mimic Hyades MS stars and then to the real Hyades cluster. We tested the impact on the results of varying the number of stars in the sample, the photometric errors, and the estimated [Fe/H]. The analysis of the synthetic data set shows that $\alpha_{ML}$ is recovered with a very good precision in all the analysed cases (with an error of few percent), while [Fe/H] and the helium-to-metal enrichment ratio $\Delta Y/\Delta Z$ are more problematic. If spectroscopic determinations of [Fe/H] are not available and thus [Fe/H] has to be recovered alongside with $\Delta Y/\Delta Z$ and $\alpha_{ML}$, the well-known degeneracy between [Fe/H]-$\Delta Y/\Delta Z$-$\alpha_{ML}$ could result in a large uncertainty on the recovered parameters, depending on the portion of the MS used for the analysis. On the other hand, the prior knowledge of an accurate [Fe/H] value puts a strong constraint on the models, leading to a more precise parameters recovery. Using the current set of PISA models, the most recent [Fe/H] value and the Gaia photometry and parallaxes for the Hyades cluster, we obtained the average values $=2.01\pm0.05$ and $=2.03\pm0.33$, sensitively reducing the uncertainty in these important parameters., Comment: Accepted for publication in MNRAS

Spectroscopy from the final internal data release of the Gaia-ESO Survey (GES) has been combined with Gaia EDR3 to assign membership probabilities to targets observed towards 63 Galactic open clusters and 7 globular clusters. The membership probabilities are based chiefly on maximum likelihood modelling of the 3D kinematics of the targets, separating them into cluster and field populations. From 43211 observed targets, 13985 are identified as highly probable cluster members ($P>0.9$), with an average membership probability of 0.993. The addition of GES radial velocities successfully drives down the fraction of false positives and we achieve better levels of discrimination in most clusters over the use of astrometric data alone, especially those at larger distances. Since the membership selection is almost purely kinematic, the union of this catalogue with GES and Gaia is ideal for investigating the photometric and chemical properties of clusters as a function of stellar mass, age and Galactic position., Accepted version for publication in MNRAS. 16 pages + 38 pages of Appendices

We aim to constrain the mixing processes in low-mass stars by investigating the behaviour of the Li surface abundance after the main sequence. We take advantage of the data from the sixth internal data release of Gaia-ESO, idr6, and from the Gaia Early Data Release 3, edr3. We select a sample of main sequence, sub-giant, and giant stars in which Li abundance is measured by the Gaia-ESO survey, belonging to 57 open clusters with ages from 120~Myr to about 7 Gyr and to Milky Way fields, covering a range in [Fe/H] between -1.0 and +0.5dex. We study the behaviour of the Li abundances as a function of stellar parameters. We compare the observed Li behaviour in field giant stars and in giant stars belonging to individual clusters with the predictions of a set of classical models and of models with mixing induced by rotation and thermohaline instability. The comparison with stellar evolution models confirms that classical models cannot reproduce the lithium abundances observed in the metallicity and mass regimes covered by the data. The models that include the effects of both rotation-induced mixing and thermohaline instability account for the Li abundance trends observed in our sample, in all metallicity and mass ranges. The differences between the results of the classical models and of the rotation models largely differ (up to ~2 dex), making lithium the best element to constrain stellar mixing processes in low-mass stars. For stars with well-determined masses, we find a better agreement between observed surface abundances and models with rotation-induced and thermohaline mixings, the former dominating during the main sequence and the first phases of the post-main sequence evolution and the latter after the bump in the luminosity function., Accepted for publication in A&A, 19 pages, 16 figures

We collected over 6000 high-resolution spectra of four dozen field RR Lyrae (RRL) variables pulsating either in the fundamental (39 RRab) or in the first overtone (9 RRc) mode. We measured radial velocities (RVs) of four strong metallic and four Balmer lines along the entire pulsational cycle and derived RV amplitudes with accuracies better than 1$-$2~\kmsec. The new amplitudes were combined with literature data for 23~RRab and 3~RRc stars (total sample 74 RRLs) which allowed us to investigate the variation of the Bailey diagram (photometric amplitude versus period) when moving from optical to mid-infrared bands and to re-cast the Bailey diagram in terms of RV amplitudes. We found that RV amplitudes for RRab are minimally affected by nonlinear phenomena (shocks) and multi-periodicity (Blazhko effect). The RV slope ($\log P$--A(V$_r$)) when compared with the visual slope ($\log P$--A($V$)) is shallower and the dispersion, at fixed period, decreases by a factor of two. We constructed homogeneous sets of Horizontal Branch evolutionary models and nonlinear, convective pulsation models of RRLs to constrain the impact of evolutionary effects on their pulsation properties. Evolution causes, on the Bailey diagram based on RV amplitudes, a modest variation in pulsation period and a large dispersion in amplitude. The broad dispersion in period of the Bailey diagram is mainly caused by variation in RRL intrinsic parameters (stellar mass, chemical composition). Empirical evidence indicates that RV amplitudes are an optimal diagnostic for tracing the mean effective temperature across the RRab instability strip., 14 pages, 4 figures, 1 table, Accepted on ApJ Letter

This contribution is meant as a very brief introduction to the principal concepts of stellar physics. First the main physical processes active in stellar structures will be shortly described, then the most important features during the stellar life-cycle up to the central H exhaustion will be summarized with partic-ular attention to the description of solar models.

In the era of large high-resolution spectroscopic surveys, high-quality spectra can contribute to our understanding of the Galactic chemical evolution, providing chemical abundances belonging to the different nucleosynthesis channels, and also providing constraints to stellar age. Some abundance ratios have been proven to be excellent indicators of stellar ages. We aim at providing an empirical relationship between stellar ages and [C/N] using, as calibrators, open star clusters observed by both the Gaia-ESO and APOGEE surveys. We use stellar parameters and abundances from the Gaia-ESO and APOGEE of the Galactic field and open cluster stars. Ages of star clusters are retrieved from the literature sources and validated using a common set of isochrones. We use the same isochrones to determine, for each age and metallicity, the surface gravity at which the first dredge-up and red giant branch bump occur. We study the effect of extra-mixing processes in our sample of giant stars, and we derive the mean [C/N] in evolved stars, including only stars without evidence of extra-mixing. Combining the Gaia-ESO and APOGEE samples of open clusters, we derive a linear relationship between [C/N] and logarithmic cluster ages. We apply our relationship to selected giant field stars in both Gaia-ESO and APOGEE. We find an age separation between thin and thick disc stars and age trends within their populations, with an increasing age towards lower metallicity populations. With such empirical relationship, we are able to provide an age estimate for giant stars in which C and N abundances are measured. Isochrone fitting is less sensitive for giant than dwarf stars at the turn off. The present method can be thus considered as an additional tool to give an independent estimate of the age of giant stars, with uncertainties in their ages comparable to those obtained using isochrone fitting for dwarf stars., Comment: 28 pages, 24 figures. Accepted by A&A

We performed a detailed analysis of the main theoretical uncertainties affecting the age at the lithium depletion boundary (LDB). To do that we computed almost 12 000 pre-main- sequence models with mass in the range (0.06, 0.4) Mby varying input physics (nuclear reaction cross-sections, plasma electron screening, outer boundary conditions, equation of state, and radiative opacity), initial chemical elements abundances (total metallicity, helium and deuterium abundances, and heavy elements mixture), and convection efficiency (mixing length parameter, αML). As a first step, we studied the effect of varying these quantities individually within their extreme values. Then, we analysed the impact of simultaneously perturbing the main input/parameters without an a priori assumption of independence. Such an approach allowed us to build for the first time the cumulative error stripe, which defines the edges of the maximum uncertainty region in the theoretical LDB age. We found that the cumulative error stripe is asymmetric and dependent on the adopted mixing length value. For αML = 1.00, the positive relative age error ranges from 5 to 15percent, while for solar- calibrated mixing length, the uncertainty reduces to 5-10percent. A large fraction of such an error (≈40percent) is due to the uncertainty in the adopted initial chemical elements abundances.

The determination of the age of single stars by means of grid-based techniques is a well established method. We discuss the impact on these estimates of the uncertainties in several ingredients routinely adopted in stellar computations. The systematic bias on age determination caused by varying the assumed initial helium abundance, the mixing-length and convective core overshooting parameters, and the microscopic diffusion are quantified and compared with the statistical error owing to the current uncertainty in the observations. The typical uncertainty in the observations accounts for 1ω statistical relative error in age determination ranging on average from about –35 % to +42 %, depending on the mass. However, the age's relative error strongly depends on the evolutionary phase and can be higher than 120 % for stars near the zero-age main-sequence, while it is typically about 20 % or lower in the advanced main-sequence phase. A variation of ±1 in the helium-to-metal enrichment ratio induces a quite modest systematic bias on age estimates. The maximum bias due to the presence of the convective core overshooting is –7% for β = 0.2 and –13% for β = 0.4. The main sources of bias are the uncertainty in the mixing-length value and the neglect of microscopic diffusion, which account each for a bias comparable to the random error uncertainty.

We analysed the effects of planet ingestion on the characteristics of a pre-MS star similar to the Gamma Velorum cluster member 2MASS J08095427--4721419 (#52). We discussed the effects of changing the age $t_0$ at which the accretion episode occurs, the mass of the ingested planet and its chemical composition. We showed that the mass of the ingested planet required to explain the current [Fe/H]^#52 increases by decreasing the age $t_0$ and/or by decreasing the Iron content of the accreted matter. We compared the predictions of a simplified accretion method -- where only the variation of the surface chemical composition is considered -- with that of a full accretion model that properly accounts for the modification of the stellar structure. We showed that the two approaches result in different convective envelope extension which can vary up to 10 percent. We discussed the impact of the planet ingestion on a stellar model in the colour-magnitude diagram, showing that a maximum shift of about 0.06 dex in the colour and 0.07 dex in magnitude are expected and that such variations persist even much later the accretion episode. We also analysed the systematic bias in the stellar mass and age inferred by using a grid of standard non accreting models to recover the characteristics of an accreting star. We found that standard non accreting models can safely be adopted for mass estimate, as the bias is, Accepted for publication in MNRAS. 13 pages, 3 tables, 9 figures

Context. The cosmological lithium problem, that is, the discrepancy between the lithium abundance predicted by the Big Bang nucleosynthesis and the one observed for the stars of the “Spite plateau”, is one of the long standing problems of modern astrophysics. Recent hints for a possible solution involve lithium burning induced by protostellar mass accretion on Spite plateau stars. However, to date, most of the protostellar and pre-main sequence stellar models that take mass accretion into account have been computed at solar metallicity, and a detailed analysis on the impact of protostellar accretion on the lithium evolution in the metal-poor regime, which is relevant for stars in the Spite plateau, is completely missing. Aims. The purpose of this paper is to fill this gap, analysing, in detail, for the first time the effect of protostellar accretion on low metallicity low-mass stars with a focus on pre-main sequence lithium evolution. Methods. We computed the evolution from the protostar to the main-sequence phase of accreting models with final masses equal to 0.7 and 0.8 M⊙, and three metallicities Z = 0.0001, Z = 0.0010, and Z = 0.0050, corresponding to [Fe/H] ∼ −2.1, −1.1 (typical of Spite plateau stars), and [Fe/H] ∼ −0.42, respectively. We followed the temporal evolution of the chemical composition by considering nuclear burning, convective mixing, and diffusion. The effects of changing some of the main parameters affecting accreting models, that is the accretion energy (i.e. cold versus hot accretion), the initial seed mass Mseed and radius Rseed, and the mass accretion rate ṁ (also considering episodic accretion), have been investigated in detail. Results. As for the main stellar properties and in particular the surface 7Li abundance, hot accretion models converge to standard non-accreting ones within 1 Myr, regardless of the actual value of Mseed, Rseed, and ṁ. Also, cold accretion models with a relatively large Mseed (≳10 MJ) or Rseed (≳1 R⊙) converge to standard non-accreting ones in less than about 10−20 Myr. However, a drastically different evolution occurs whenever a cold protostellar accretion process starts from small values of Mseed and Rseed (Mseed ∼ 1 MJ, Rseed ≲ 1 R⊙). These models almost entirely skip the standard Hayashi track evolution and deplete lithium before the end of the accretion phase. The exact amount of depletion depends on the actual combination of the accretion parameters (ṁ, Mseed, and Rseed), achieving in some cases the complete exhaustion of lithium in the whole star. Finally, the lithium evolution in models accounting for burst accretion episodes or for an initial hot accretion followed by a cold accretion phase closely resemble that of standard non-accreting ones. Conclusions. To significantly deplete lithium in low-mass metal poor stars by means of protostellar accretion, a cold accretion scenario starting from small initial Mseed and Rseed is required. Even in this extreme configuration leading to a non-standard evolution that misses almost entirely the standard Hayashi track, an unsatisfactory fine tuning of the parameters governing the accretion phase is required to deplete lithium in stars of different mass and metallicity – starting from the Big Bang nucleosynthesis abundance – in such a way as to produce the observed Spite plateau.

The high-quality OmegaCAM photometry of the 3x3 deg around the Orion Nebula Cluster (ONC) in r, and i filters by Beccari et al.(2017) revealed three well-separated pre-main sequences in the color-magnitude diagram (CMD). The objects belonging to the individual sequences are concentrated towards the center of the ONC. The authors concluded that there are two competitive scenarios: a population of unresolved binaries and triples with an exotic mass ratio distribution, or three stellar populations with different ages. We use Gaia DR2 in combination with the photometric OmegaCAM catalog to test and confirm the presence of the putative three stellar populations. We also study multiple stellar systems in the ONC for the first time using Gaia DR2. We confirm that the second and third sequence members are more centrally concentrated towards the center of the ONC. In addition we find an indication that the parallax and proper motion distributions are different among the members of the stellar sequences. The age difference among stellar populations is estimated to be 1-2 Myr. We use Gaia measurements to identify and remove as many unresolved multiple system candidates as possible. Nevertheless we are still able to recover two well-separated sequences with evidence for the third one, supporting the existence of the three stellar populations. We were able to identify a substantial number of wide binary objects (separation between 1000-3000 au). This challenges previously inferred values that suggested no wide binary stars exist in the ONC. Our inferred wide-binary fraction is approx 5%. We confirm the three populations correspond to three separated episodes of star formation. Based on this result, we conclude that star formation is not happening in a single burst in this region. (abridged), Comment: Astronomy and Astrophysics (A&A) accepted. 12 pages, 9 figures + appendix. New version with language corrections and new ID values in Tab.A.1

We present the progress in the measurements of the nuclear reaction rates for \(^{6,7}\)Li, \(^9\)Be and \(^{10}\)B (p,\(\alpha \)) burning channels derived using the Trojan Horse Method (THM). The effects on the predicted stellar surface abundances of such elements in pre-main sequence stars are evaluated using the Pisa up-to-date stellar evolutionary code. The present analysis is restricted to the pre-MS evolution of low-mass stars.

We performed an analysis of the main theoretical uncertainties that affect the radius of low- and very-low mass-stars predicted by current stellar models. We focused on stars in the mass range 0.1-1Msun, on both the zero-age main-sequence (ZAMS) and on 1, 2 and 5 Gyr isochrones. First, we quantified the impact on the radius of the uncertainty of several quantities, namely the equation of state, radiative opacity, atmospheric models, convection efficiency and initial chemical composition. Then, we computed the cumulative radius error stripe obtained by adding the radius variation due to all the analysed quantities. As a general trend, the radius uncertainty increases with the stellar mass. For ZAMS structures the cumulative error stripe of very-low mass stars is about $\pm 2$ and $\pm 3$ percent, while at larger masses it increases up to $\pm 4$ and $\pm 5$ percent. The radius uncertainty gets larger and age dependent if isochrones are considered, reaching for $M\sim 1$Msun about $+12(-15)$ percent at an age of 5 Gyr. We also investigated the radius uncertainty at a fixed luminosity. In this case, the cumulative error stripe is the same for both ZAMS and isochrone models and it ranges from about $\pm 4$ percent to $+7$ and $+9$($-5$) percent. We also showed that the sole uncertainty on the chemical composition plays an important role in determining the radius error stripe, producing a radius variation that ranges between about $\pm 1$ and $\pm 2$ percent on ZAMS models with fixed mass and about $\pm 3$ and $\pm 5$ percent at a fixed luminosity., 18 pages, 20 figures, 1 table; accepted for publication in MNRAS

The p(p,e^+ \nu_e)^2H reaction rate is an essential ingredient for theoretical computations of stellar models. In the past several values of the corresponding S-factor have been made available by different authors. Prompted by a recent evaluation of S(E), we analysed the effect of the adoption of different proton-proton reaction rates on stellar models, focusing, in particular, on the age of mid and old stellar clusters (1-12 Gyr) and on standard solar model predictions. By comparing different widely adopted p(p,e^+ \nu_e)^2H reaction rates, we found a maximum difference in the temperature regimes typical of main sequence hydrogen-burning stars (5x10^6 - 3x10^7 K) of about 3%. Such a variation translates into a change of cluster age determination lower than 1%. A slightly larger effect is observed in the predicted solar neutrino fluxes with a maximum difference, in the worst case, of about 8%. Finally we also notice that the uncertainty evaluation of the present proton-proton rate is at the level of few \permil, thus the p(p,e^+ \nu_e)^2H reaction rate does not constitute anymore a significant uncertainty source in stellar models., Comment: accepted

Context. Determination and calibration of the ages of stars, which heavily rely on stellar evolutionary models, are very challenging, while representing a crucial aspect in many astrophysical areas. Aims. We describe the methodologies that, taking advantage of Gaia-DR1 and the Gaia-ESO Survey data, enable the comparison of observed open star cluster sequences with stellar evolutionary models. The final, long-term goal is the exploitation of open clusters as age calibrators. Methods. We perform a homogeneous analysis of eight open clusters using the Gaia-DR1 TGAS catalogue for bright members and information from the Gaia-ESO Survey for fainter stars. Cluster membership probabilities for the Gaia-ESO Survey targets are derived based on several spectroscopic tracers. The Gaia-ESO Survey also provides the cluster chemical composition. We obtain cluster parallaxes using two methods. The first one relies on the astrometric selection of a sample of bona fide members, while the other one fits the parallax distribution of a larger sample of TGAS sources. Ages and reddening values are recovered through a Bayesian analysis using the 2MASS magnitudes and three sets of standard models. Lithium depletion boundary (LDB) ages are also determined using literature observations and the same models employed for the Bayesian analysis. Results. For all but one cluster, parallaxes derived by us agree with those presented in Gaia Collaboration (2017, A&A, 601, A19), while a discrepancy is found for NGC 2516; we provide evidence supporting our own determination. Inferred cluster ages are robust against models and are generally consistent with literature values. Conclusions. The systematic parallax errors inherent in the Gaia DR1 data presently limit the precision of our results. Nevertheless, we have been able to place these eight clusters onto the same age scale for the first time, with good agreement between isochronal and LDB ages where there is overlap. Our approach appears promising and demonstrates the potential of combining Gaia and ground-based spectroscopic datasets.© ESO 2018., Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 188.B-3002, 193.B.0936, and 197.B-1074. These data products have been processed by the Cambridge Astronomy Survey Unit (CASU) at the Institute of Astronomy, University of Cambridge, and by the FLAMES/UVES reduction team at INAF/Osservatorio Astrofisico di Arcetri. These data have been obtained from the Gaia-ESO Survey Data Archive, prepared and hosted by the Wide Field Astronomy Unit, Institute for Astronomy, University of Edinburgh, which is funded by the UK Science and Technology Facilities Council. This work has made use of data from the European Space Agency (ESA) mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research was made possible through the use of the AAVSO Photometric All-Sky Survey (APASS), funded by the Robert Martin Ayers Sciences Fundation. This research has made use of the SIMBAD and VizieR databases, operated at CDS, Strasbourg, France. This research has made use of NASA's Astrophysics Data System. This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell' Istruzione, dell' Universita e della Ricerca (MIUR) in the form of the grant >Premiale VLT 2012> and by PRIN-INAF 2014. The results presented here benefit from discussions held during the Gaia-ESO Survey workshops and conferences supported by the ESF (European Science Foundation) through the GREAT Research Network Programme. E.T., P.G.P.M. and S.D. acknowledge PRA Universita di Pisa 2016 (Stelle di piccola massa: le pietre miliari dell'archeologia galattica, PI: S. Degl'Innocenti) and INFN (Iniziativa specifica TAsP). M.T.C. acknowledge the financial support from the Spanish Ministerio de Economia y Competitividad, through grant AYA2016-75931. U.H. acknowledges support from the Swedish National Space Board (SNSB/Rymdstyrelsen). P.R.F. and A.S. acknowledge useful discussions with Mario Lattanzi. We thank an anonymous referee for the very helpful and constructive comments.

We aim to perform a theoretical evaluation of the impact of the mass loss indetermination on asteroseismic grid based estimates of masses, radii, and ages of stars in the red giant branch phase (RGB). We adopted the SCEPtER pipeline on a grid spanning the mass range [0.8; 1.8] Msun. As observational constraints, we adopted the star effective temperatures, the metallicity [Fe/H], the average large frequency spacing $\Delta \nu,$ and the frequency of maximum oscillation power $\nu_{\rm max}$. The mass loss was modelled following a Reimers parametrization with the two different efficiencies $\eta = 0.4$ and $\eta = 0.8$. In the RGB phase, the average error owing only to observational uncertainty on mass and age estimates is about 8% and 30% respectively. The bias in mass and age estimates caused by the adoption of a wrong mass loss parameter in the recovery is minor for the vast majority of the RGB evolution. The biases get larger only after the RGB bump. In the last 2.5% of the RGB lifetime the error on the mass determination reaches 6.5% becoming larger than the random error component in this evolutionary phase. The error on the age estimate amounts to 9%, that is, equal to the random error uncertainty. These results are independent of the stellar metallicity [Fe/H] in the explored range. Asteroseismic-based estimates of stellar mass, radius, and age in the RGB phase can be considered mass loss independent within the range ($\eta \in [0.0, 0.8]$) as long as the target is in an evolutionary phase preceding the RGB bump., Comment: Accepted for publication in A&A

The direct measurements of nuclear reaction cross sections at astrophysical energies are usually difficult to be performed in terrestrial laboratories, mainly because of the Coulomb barrier. An alternative way of studying these processes is via the indirect methods among which the Trojan Horse (THM) represents a powerful tool for studying charged-particles induced reactions at astrophysically energies without the need of extrapolation procedures. The recent THM studies about the lithium and boron destroying reactions will be here shown and discussed.

We attempt to constrain the initial helium abundance, the age and the efficiency of the convective core overshooting of the binary system TZ Fornacis. Our main aim is in pointing out the biases in the results due to not accounting for some sources of uncertainty. We adopt the SCEPtER pipeline, relying on stellar models computed with two stellar evolutionary codes (FRANEC and MESA). We found multiple independent groups of solutions. The best one suggests a system of age 1.10 $\pm$ 0.07 Gyr (primary star in the central helium burning stage, secondary in the sub-giant branch), with a helium-to-metal enrichment ratio of $\Delta Y/\Delta Z = 1$ and core overshooting parameter $\beta = 0.15 \pm 0.01$ (FRANEC) and $f_{\rm ov}= 0.013 \pm 0.001 $ (MESA). The second class of solutions, characterised by a worse goodness-of-fit, still suggest a primary star in the central helium-burning stage but a secondary in the overall contraction phase, at the end of the main sequence. The FRANEC grid provides an age of $1.16_{-0.02}^{+0.03}$ Gyr and $\beta = 0.25_{-0.01}^{+0.005}$, while the MESA grid gives $1.23 \pm 0.03$ Gyr and $f_{\rm ov} = 0.025 \pm 0.003$. We also show that very precise mass determinations with uncertainty of a few thousandths of solar mass are required to obtain reliable determinations of stellar parameters, as mass errors larger than approximately 1% lead to estimates that are not only less precise but also biased. Moreover, we show that a fit obtained with a grid of models computed at a fixed $\Delta Y/\Delta Z$ can provide biased age and overshooting estimates. The possibility of independent overshooting efficiencies for the two stars of the system is also explored. This work confirms that to constrain the core overshooting parameter by means of binary systems is a very difficult task that requires an observational precision still rarely achieved and robust statistical methods., Comment: A&A accepted. Abstract shortened

Located at the galactic anticenter, Sh 2-284 is a HII region which harbors several young open clusters; Dolidze 25, a rare metal poor (Z~0.004) young cluster, is one of these. Given its association with Sh 2-284, it is reasonable to assume the low metallicity for the whole HII region. Sh~2-284 is expected to host a significant population of Pre-Main Sequence (PMS) stars of both low and intermediate mass stars (Herbig Ae stars). We aim at characterizing these stars by means of a spectroscopic and photometric survey conducted with VIMOS@VLT and complemented with additional optical and infrared observations. In this survey we selected and characterized 23 PMS objects. We derived the effective temperature, the spectral energy distribution and luminosity of these objects; using theoretical PMS evolutionary tracks, with the appropriate metallicity, we estimated the mass and the age of the studied objects. We also estimated a distance of 4 Kpc for Sh 2-284 by using spectroscopic parallax of 3 OB stars. From the age determination we concluded that triggered star formation is in act in this region. Our results show that a significant fraction of the young stellar objects (YSOs) may have preserved their disk/envelopes, in contrast with what is found in other recent studies of low-metallicity star forming regions in the Galaxy. Finally, among the 23 bona fide PMS stars, we identified 8 stars which are good candidates to pulsators of the delta Scuti type.

We present precise and deep optical photometry of the globular M92. Data were collected in three different photometric systems: Sloan Digital Sky Survey (g',r',i',z'; MegaCam@CFHT), Johnson-Kron-Cousins (B, V, I; various ground-based telescopes) and Advanced Camera for Surveys (ACS) Vegamag (F475W, F555W, 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 {\alpha}-enhanced evolutionary models accounting for the gravitational settling of heavy elements at fixed chemical composition ([{\alpha}/Fe]=+0.3, [Fe/H]=-2.32 dex, Y=0.248). The isochrones -- assuming the same true distance modulus ({\mu}=14.74 mag), the same reddening (E(B-V)=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',g'-i' ; i',g'-r' ; i',g'-z' ; I,B-I ; F814W,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 {\alpha}- and CNO-enhanced (by a factor of three) models both at fixed total metallicity ([M/H]=-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 {\alpha}-enhanced isochrones. The isochrones based on the latter set also give a similar cluster age (10 +/- 1.5 Gyr). These indings support previous results concerning the weak sensitivity of cluster isochrones to CNO-enhanced chemical mixtures.

Aims. The capability of grid-based techniques to estimate the age together with the convective core overshooting efficiency of stars in detached eclipsing binary systems for main sequence stars has previously been investigated. We have extended this investigation to later evolutionary stages and have evaluated the bias and variability on the recovered age and convective core overshooting parameter accounting for both observational and internal uncertainties. Methods. We considered synthetic binary systems, whose age and overshooting efficiency should be recovered by applying the SCEPtER pipeline to the same grid of models used to build the mock stars. We focus our attention on a binary system composed of a 2.50 M⊙ primary star coupled with a 2.38 M⊙ secondary. To explore different evolutionary scenarios, we performed the estimation at three different times: when the primary is at the end of the central helium burning, when it is at the bottom of the RGB, and when it is in the helium core burning phase. The Monte Carlo simulations have been carried out for two typical values of accuracy on the mass determination, that is, 1% and 0.1%. Results. Adopting typical observational uncertainties, we found that the recovered age and overshooting efficiency are biased towards low values in all three scenarios. For an uncertainty on the masses of 1%, the underestimation is particularly relevant for a primary in the central helium burning stage, reaching − 8.5% in age and − 0.04 (− 25% relative error) in the overshooting parameter β. In the other scenarios, an undervaluation of the age by about 4% occurs. A large variability in the fitted values between Monte Carlo simulations was found: for an individual system calibration, the value of the overshooting parameter can vary from β = 0.0 to β = 0.26. When adopting a 0.1% error on the masses, the biases remain nearly unchanged but the global variability is suppressed by a factor of about two. We also explored the effect of a systematic discrepancy between the artificial systems and the model grid by accounting for an offset in the effective temperature of the stars by ± 150 K. For a mass error of 1% the overshooting parameter is largely biased towards the edges of the explored range, while for the lower mass uncertainty it is basically unconstrained from 0.0 to 0.2. We also evaluate the possibility of individually recovering the β value for both binary stars. We found that this is impossible for a primary near to central hydrogen exhaustion owing to huge biases for the primary star of + 0.14 (90% relative error), while in the other cases the fitted β are consistent, but always biased by about − 0.04 (− 25% relative error). Finally, the possibility to distinguish between models computed with mild overshooting from models with no overshooting was evaluated, resulting in a reassuring power of distinction greater than 80%. However, the scenario with a primary in the central helium burning was a notable exception, showing a power of distinction lower than 5%.

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 Carina's 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 Carina's old stellar population is metal-poor and appears to have a limited spread in metallicity (Δ[Fe/H] = 0.2-0.3 dex). The Carina's 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 Carina's evolved stars indicate a metallicity of [Fe/H] = -1.70 ± 0.19 dex, which agrees quite well with spectroscopic measurements.

In this contribution I will review the present knowledge about helium and metal diffusion efficiency in the Sun and in old cluster stars and the influence of the uncertainties still present on these mechanisms concerning the stellar characteristics in different evolutionary phases. I will also discuss the relevance of the current uncertainties on the assumed chemical composition for the main evolutionary features of population II stars.

We devised a new method to estimate globular cluster absolute ages by adopting the knee of the bending of the lower main-sequence (MS) in the Near-Infrared (NIR)J,J-Kscolor-magnitude diagram. The color difference between this feature and the Turn-Off point is strongly correlated to the cluster age. This method is marginally affected by distance and reddening uncertainties, and by the possible occurrence of differential reddening. Furthermore, the knee location does not depend on the cluster age and it is a robust theoretical prediction. We adopted accurateJ,Ks-band photometry collected with both MAD/VLT and SOFI/NTT for the Galactic globular cluster NGC 3201 to identify the location of the knee atJ~19.90 ±0.03 andJ-Ks~0.76±0.02 mag. The comparison with different sets of cluster isochrones, transformed adopting different Color–Temperature–Relations (CTRs), shows that the models are slightly redder than the observations forJ> 19 mag. This difference could be due to the presence of a calibration drift or to a problem of the CTRs in this magnitude range.

New accurate and homogeneous optical UBVRI photometry has been obtained for variable stars in the Galactic globular $\omega$ Cen (NGC 5139). We secured 8202 CCD images covering a time interval of 24 years and a sky area of 84x48 arcmin. The current data were complemented with data available in the literature and provided new, homogeneous pulsation parameters (mean magnitudes, luminosity amplitudes,periods) for 187 candidate $\omega$ Cen RR Lyrae (RRLs). Among them we have 101 RRc (first overtone), 85 RRab (fundamental) and a single candidate RRd (double-mode) variables. Candidate Blazhko RRLs show periods and colors that are intermediate between RRc and RRab variables, suggesting that they are transitional objects. The comparison of the period distribution and of the Bailey diagram indicates that RRLs in $\omega$ Cen show a long-period tail not present in typical Oosterhoff II (OoII) globulars. The RRLs in dwarf spheroidals and in ultra faint dwarfs have properties between Oosterhoff intermediate and OoII clusters. Metallicity plays a key role in shaping the above evidence. These findings do not support the hypothesis that $\omega$ Cen is the core remnant of a spoiled dwarf galaxy. Using optical Period-Wesenheit relations that are reddening-free and minimally dependent on metallicity we find a mean distance to $\omega$ Cen of 13.71$\pm$0.08$\pm$0.01 mag (semi-empirical and theoretical calibrations). Finally, we invert the I-band Period-Luminosity-Metallicity relation to estimate individual RRLs metal abundances. The metallicity distribution agrees quite well with spectroscopic and photometric metallicity estimates available in the literature., Comment: AJ, accepted

With the aim to test the present capability of the stellar surface lithium abundance in providing an estimation for the age of PMS stars, we analyze the case of the detached, double-lined, eclipsing binary system PAR 1802. For this system, the lithium age has been compared with the theoretical one, as estimated by applying a Bayesian analysis method on a large grid of stellar evolutionary models. The models have been computed for several values of chemical composition and mixing length, by means of the code FRANEC updated with the Trojan Horse reaction rates involving lithium burning.

STREGA (STRucture and Evolution of the Galaxy) is an ongoing VLT Survey Telescope Guaranteed Time survey, aimed at investigating the mechanisms of formation and evolution of the Galactic Halo on an area of about 150 square degrees. The core programme searches for the signatures of interaction between selected stellar systems and the Galactic Halo and a complementary part is focussed on exploring the southern portion of the Fornax Stream. The adopted stellar tracers are variable stars and main sequence turn-off stars. We present an overview of the survey, the first results, and the perspectives.

Context. A promising technique to derive the physical parameters and the distance of pulsating stars is the fit of the observed light curves by nonlinear pulsation models. Aims. We apply this technique to a subsample of the RR Lyrae belonging to the Galactic globular cluster M3. The application of the method to cluster pulsators has the advantage of dealing with a homogeneous sample at the same distance and with the same chemical composition allowing to be checked the internal consistency of pulsational calculations. Methods. We selected seven pulsators (three RR c and four RR ab ) which cover a significant period range and show detailed light curves in the B, V and in some cases I bands. For four of them, with different periods, pulsation modes and light curve properties, we analyze the dependence of the theoretical light curve variations on the model input parameters. Results. For all selected objects, except the reddest one, we are able to theoretically reproduce the observed light curve morphology for self-consistent ranges of intrinsic stellar parameters, in agreement with the evolutionary predictions for the corresponding metal abundance. It is worth noting that, even if the theoretical reproduction of individual light curves gives for each variable only a range of stellar parameters and distances, the analysis of several variables belonging to the same cluster provides a mean distance modulus, namely μ v = 15.10 ± 0.1 mag, and also checks the self-consistency of the adopted theoretical scenario. Taking also into account the evolutionary constraints, the range of the accepted distance modulus is significantly reduced giving a weighted mean value of 15.05 ± 0.02. Our estimates are in agreement with available results in the literature obtained from independent methods.

We present recent work undertaken by the Evolution and Seismic Tools Activity (ESTA) team of the CoRoT Seismology Working Group. The new ESTA-Task 3 aims at testing, comparing and optimising stellar evolution codes which include microscopic diffusion of the chemical elements resulting from pressure, temperature and concentration gradients. The results already obtained are globally satisfactory, but some differences between the different numerical tools appear that require further investigations., to appear in "Stellar Evolution and Seismic Tools for Asteroseismology", (Eds) C. W. Straka, Y. Lebreton and M. J. P. F. G. Monteiro, EAS Publications Series, 2007

The stellar helium-to-metal enrichment ratio, \Delta Y/\Delta Z, is a widely studied astrophysical quantity. However, its value is still not precisely constrained. This paper is focused on the study of the main sources of uncertainty which affect the \Delta Y/\Delta Z derived from the analysis of the low-main sequence (MS) stars in the solar neighborhood. The possibility to infer the value of \Delta Y/\Delta Z from the study of low-MS stars relies on the dependence of the stellar luminosity and effective temperature on the initial Y and Z. The \Delta Y/\Delta Z ratio is obtained by comparing the magnitude difference between the observed stars and a reference theoretical zero age main sequence (ZAMS) with the related theoretical magnitude differences computed from a new set of stellar models with up-to-date input physics and a fine grid of chemical compositions. A Monte Carlo approach has been used to evaluate the impact on the result of different sources of uncertainty, i.e. observational errors, evolutionary effects, systematic uncertainties of the models. As a check of the procedure, the method has been applied to a different data set, namely the low-MS of the Hyades. Once a set of ZAMS and atmosphere models have been chosen, we found that the inferred value of \Delta Y/\Delta Z is sensitive to the age of the stellar sample, even if we restricted the data set to low luminosity stars. The lack of an accurate age estimate of low mass field stars leads to an underestimate of the inferred \Delta Y/\Delta Z of ~2 units. On the contrary the method firmly recovers the \Delta Y/\Delta Z value for not evolved samples of stars such as the Hyades low-MS. Adopting a solar calibrated mixing-length parameter and the PHOENIX GAIA v2.6.1 atmospheric models, we found \Delta Y/\Delta Z = 5.3 +/- 1.4 once the age correction has been applied. The Hyades sample provided a perfectly consistent value.

We investigated the possibility of producing helium enhanced stars in globular clusters by accreting polluted matter during the pre-main sequence phase. We followed the evolution of two different classes of pre-main sequence accreting models, one which neglects and the other that takes into account the protostellar evolution. We analysed the dependence of the final central helium abundance, of the tracks position in the HR diagram and of the surface lithium abundance evolution on the age at which the accretion of polluted material begins and on the main physical parameters that govern the protostellar evolution. The later is the beginning of the late accretion and the lower are both the central helium and the surface lithium abundances at the end of the accretion phase and in ZAMS (Zero Age Main Sequence). In order to produce a relevant increase of the central helium content the accretion of polluted matter should start at ages lower than 1 Myr. The inclusion of the protostellar evolution has a strong impact on the ZAMS models too. The adoption of a very low seed mass (i.e. 0.001 M$_{\odot}$) results in models with the lowest central helium and surface lithium abundances. The higher is the accretion rate and the lower is the final helium content in the core and the residual surface lithium. In the worst case -- i.e. seed mass 0.001 M$_\odot$ and accretion rate $\ge 10^{-5}$ M$_\odot$ yr$^{-1}$ -- the central helium is not increased at all and the surface lithium is fully depleted in the first few million years., Accepted for pubblication in MNRAS. 19 pages, 15 figures, 2 tables

We supplement current evolutionary computations concerning Magellanic Cloud stars by exploring the evolutionary behavior of canonical stellar models (i.e.,with inefficient core overshooting) with metallicities suitable for stars in the Clouds. After discussing the adequacy of the adopted evolutionary scenario, we present evolutionary sequences as computed following a selected sample of stellar models in the mass range 0.8-8 Mo from the Main Sequence till the C ignition or the onset of thermal pulses in the advanced Asymptotic Giant Branch phase. On this basis, cluster isochrones covering the range of ages from 100 Myr to 15 Gyr are presented and discussed. To allow a comparison with evolutionary investigations appeared in the recent literature, we computed additional sets of models which take into account moderate core overshooting during the H burning phase, discussing the comparison in terms of current uncertainties in the stellar evolutionary models. Selected predictions constraining the cluster ages are finally discussed, presenting a calibration of the difference in magnitude between the luminous MS termination and the He burning giants in terms of cluster age. Both evolutionary tracks and isochrones have been made available at the node http://gipsy.cjb.net as a first step of a planned ``Pisa Evolutionary Library''., 11 pages, 9 eps figures, A&A accepted, evolutionary tracks and isochrones available at http://gipsy.cjb.net at the link ``Pisa Evolutionary Library''

Recent results by Bensby and collaborators on the ages of microlensed stars in the Galactic bulge have challenged the picture of an exclusively old stellar population. However, these age estimates have not been independently confirmed. In this paper we verify these results by means of a grid-based method and quantify the systematic biases that might be induced by some assumptions adopted to compute stellar models. We explore the impact of increasing the initial helium abundance, neglecting the element microscopic diffusion, and changing the mixing-length calibration in theoretical stellar track computations. We adopt the SCEPtER pipeline with a novel stellar model grid for metallicities [Fe/H] from -2.00 to 0.55 dex, and masses in the range [0.60; 1.60] Msun from the ZAMS to the helium flash at the red giant branch tip. We show for the considered evolutionary phases that our technique provides unbiased age estimates. Our age results are in good agreement with Bensby and collaborators findings and show 16 stars younger than 5 Gyr and 28 younger than 9 Gyr over a sample of 58. The effect of a helium enhancement as large as Delta Y/Delta Z = 5 is quite modest, resulting in a mean age increase of metal rich stars of 0.6 Gyr. Even simultaneously adopting a high helium content and the upper values of age estimates, there is evidence of 4 stars younger than 5 Gyr and 15 younger than 9 Gyr. For stars younger than 5 Gyr, the use of stellar models computed by neglecting microscopic diffusion or by assuming a super-solar mixing-length value leads to a mean increase in the age estimates of about 0.4 Gyr and 0.5 Gyr respectively. Even considering the upper values for the age estimates, there are four stars estimated younger than 5 Gyr is in both cases. Thus, the assessment of a sizeable fraction of young stars among the microlensed sample in the Galactic bulge appears robust., Accepted for publication in A&A. Abstract shortened

Burning reaction S(E)-factor determinations are among the key ingredients for stellar models when one has to deal with energy generation evaluation and the genesis of the elements at stellar conditions. To by pass the still present uncertainties in extrapolating low-energies values, S(E)-factor measurements for charged-particle induced reactions involving light elements have been made available by devote Trojan Horse Method (THM) experiments. The recent results are here discussed together with their impact in astrophysics.

We compare theoretical stellar models for Main Sequence (MS) stars with the Hipparcos database for the Hyades cluster to give a warning against the uncritical use of available theoretical scenarios and to show how formal MS fittings can be fortuitous if not fictitious. Moreover, we find that none of the current theoretical scenarios appears able to account for an observed mismatch between theoretical predictions and observations of the coolest Hyades MS stars. Finally, we show that current theoretical models probably give too faint He burning luminosities unlike the case of less massive He burning models, with degenerate progenitors, which have been suggested to suffer the opposite discrepancy., Comment: 5 pages, 8 figures, MNRAS accepted

STREGA (STRucture and Evolution of the GAlaxy) is a Guaranteed Time survey being performed at the VST (the ESO VLT Survey Telescope) to map about 150 square degrees in the Galactic halo, in order to constrain the mechanisms of galactic formation and evolution. The survey is built as a five-year project, organized in two parts: a core program to explore the surrounding regions of selected stellar systems and a second complementary part to map the southern portion of the Fornax orbit and extend the observations of the core program. The adopted stellar tracers are mainly variable stars (RR~Lyraes and Long Period Variables) and Main Sequence Turn-off stars for which observations in the g,r,i bands are obtained. We present an overview of the survey and some preliminary results for three observing runs that have been completed. For the region centered on $\omega$~Cen (37 deg^2), covering about three tidal radii, we also discuss the detected stellar density radial profile and angular distribution, leading to the identification of extratidal cluster stars. We also conclude that the cluster tidal radius is about 1.2 deg, in agreement with values in the literature based on the Wilson model., Comment: Accepted for publication on MNRAS on 2014 August 18

The residual amount of light elements lithium, beryllium and boron (LiBeB) abundances in stellar atmospheres has been largely accepted as one of the most power- ful probes for understanding stellar structure and mixing phenomena. They are in fact gradually destroyed at di erent depths of stellar interior mainly by (p, ), thus their fate in stars is an incomparable tool for studying mixing processes. In order to avoid extrap- olation procedures on the available direct S(E)-factor measurements, the Trojan Horse Method (THM) has been developed, allowing one to measure the bare nucleus S(E)- factor for astrophysically relevant reactions without experiencing Coulomb penetrability e ects. Here, a summary on the recent 6;7 Li and 11 B TH investigations will be given and the corresponding results discussed.

The main characteristics of standard pre-main sequence (PMS) models are described. A discussion of the uncer-tainties affecting the current generation of PMS evolutionary tracks and isochrones is also provided. In particular, the impact of the uncertainties in the adopted equation of state, radiative opacity, nuclear cross sections, and initial chemical abundances are analysed.

We study the temporal evolution of the habitable zone (HZ) of low-mass stars - only due to stellar evolution - and evaluate the related uncertainties. These uncertainties are then compared with those due to the adoption of different climate models. We computed stellar evolutionary tracks from the pre-main sequence phase to the helium flash at the red-giant branch tip for stars with masses in the range [0.70 - 1.10] Msun, metallicity Z in the range [0.005 - 0.04], and various initial helium contents. We evaluated several characteristics of the HZ, such as the distance from the host star at which the habitability is longest, the duration of this habitability, the width of the zone for which the habitability lasts one half of the maximum, and the boundaries of the continuously habitable zone (CHZ) for which the habitability lasts at least 4 Gyr. We developed analytical models, accurate to the percent level or lower, which allowed to obtain these characteristics in dependence on the mass and the chemical composition of the host star. The metallicity of the host star plays a relevant role in determining the HZ. The initial helium content accounts for a variation of the CHZ boundaries as large as 30% and 10% in the inner and outer border. The computed analytical models allow the first systematic study of the variability of the CHZ boundaries that is caused by the uncertainty in the estimated values of mass and metallicity of the host star. An uncertainty range of about 30% in the inner boundary and 15% in the outer one were found. We also verified that these uncertainties are larger than that due to relying on recently revised climatic models, which leads to a CHZ boundaries shift within 5% with respect to those of our reference scenario. We made an on-line tool available that provides both HZ characteristics and interpolated stellar tracks., Accepted for publication in A&A, abstract abridged

Some aspects of the systematic and statistical errors affecting grid-based estimation of stellar masses and radii have still not been investigated well. 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 diffusion efficiency adopted in stellar model computations. We consider stars with mass in the range [0.8 - 1.1] Msun and evolutionary stages from the zero-age main sequence to the central hydrogen exhaustion. Stellar parameters were recovered by a maximum-likelihood technique, comparing the observations constraints to a grid of stellar models. Synthetic grids with perturbed input were adopted to estimate the systematic errors arising from the current uncertainty in model computations. 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 $\pm$ 1 in Delta Y/Delta Z is $\pm$ 2.3% and $\pm$ 1.1%; the one due to a change of $\pm$ 0.24 in the value of the mixing-length is $\pm$ 2.1% and $\pm$ 1.0%; the one due to a variation of $\pm$ 5% in the radiative opacity is $\mp$ 1.0% and $\mp$ 0.45%. An important bias source is to neglect microscopic diffusion, which accounts for errors of about 3.7% and 1.5% on mass and radius. The cumulative effects 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 differences in the estimation grids are generally greater than the statistical errors involved., Accepted for publication in A&A. Abstract shortened

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., Comment: Accepted for publication in A&A

We develop a statistical test on the expected difference in age estimates of two coeval stars in detached double-lined eclipsing binary systems that are only caused by observational uncertainties. We focus on stars in the mass range [0.8; 1.6] Msun, and on stars in the main-sequence phase. The ages were obtained by means of the maximum-likelihood SCEPtER technique. The observational constraints used in the recovery procedure are stellar mass, radius, effective temperature, and metallicity [Fe/H]. We defined the statistic W computed as the ratio of the absolute difference of estimated ages for the two stars over the age of the older one. We determined the critical values of this statistics above which coevality can be rejected. The median expected difference in the reconstructed age between the coeval stars of a binary system -- caused alone by the observational uncertainties -- shows a strong dependence on the evolutionary stage. This ranges from about 20% for an evolved primary star to about 75% for a near ZAMS primary. The median difference also shows an increase with the mass of the primary star from 20% for 0.8 Msun stars to about 50% for 1.6 Msun stars. The reliability of these results was checked by repeating the process with a grid of stellar models computed by a different evolutionary code. We show that the W test is much more sensible to age differences in the binary system components than the alternative approach of comparing the confidence interval of the age of the two stars. We also found that the distribution of W is, for almost all the examined cases, well approximated by beta distributions. The proposed method improves upon the techniques that are commonly adopted for judging the coevality of an observed system. It also provides a result founded on reliable statistics that simultaneously accounts for all the observational uncertainties., Comment: Abstract shortened. Accepted for publication in A&A. One reference fixed

In a robust statistical way, we quantify the uncertainty that affects the calibration of the overshooting efficiency parameter $\beta$ that is owing to the uncertainty on the observational data in double-lined eclipsing binary systems. We also quantify the bias that is caused by the lack of constraints on the initial helium content and on the efficiencies of the superadiabatic convection and microscopic diffusion. We adopted a modified grid-based SCEPtER pipeline using as observational constraints the effective temperatures, [Fe/H], masses, and radii of the two stars. In a reference scenario of mild overshooting $\beta = 0.2$ for the synthetic data, we found both large statistical uncertainties and biases on the estimated $\beta$. For the first 80% of the MS evolution, $\beta$ is biased and practically unconstrained in the whole explored range [0.0; 0.4]. In the last 5% of the MS the bias vanishes and the $1 \sigma$ error is about 0.05. For synthetic data computed with $\beta = 0.0$, the estimated $\beta$ is biased by about 0.12 in the first 80% of the MS evolution, and by 0.05 afterwards. Assuming an uncertainty of $\pm 1$ in the helium-to-metal enrichment ratio $\Delta Y/\Delta Z$, we found that in the terminal part of the MS evolution the error on the estimated $\beta$ values ranges from -0.05 to +0.10, while $\beta$ is basically unconstrained throughout the explored range at earlier evolutionary stages. A uniform variation of $\pm 0.24$ in the mixing-length parameter around the solar-calibrated value causes in last 5% of the MS an uncertainty from -0.09 to +0.15. A complete neglect of diffusion in the stellar evolution computations produces a $1 \sigma$ uncertainty of $\pm 0.08$ in the last 5% of the MS, while $\beta$ is practically unconstrained in the first 80% of the MS. Overall, the calibration appears poorly reliable., Comment: Abstract abridged; accepted for publication in A&A

The Trojan Horse Method has been applied to study the 7 Li ( p , α ) 4 He and 6 ( Li ( d , α ) 4 He reactions through the 7 Li(d, αα)n and 6 Li( 6 Li, αα) 4 He three body processes, respectively. The electron screening potential deduced from these experiments is much larger than the adiabatic approximation prediction for both cases; the systematic discrepancy between data and theoretical predictions is thus confirmed. Astrophysical implications of these measurements are also discussed.

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 (5) or in fiber mode (39, FLAMES/GIRAFFE-UVES). The sample is more than a factor of four larger than any previous spectroscopic investigation of stars in dSphs based on high-resolution (R>38,000) 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. We found evidence of NLTE effects between neutral and singly-ionized iron abundances. Assuming that the FeII abundances are minimally affected by NLTE effects, we corrected the FeI stellar abundances using a linear fit between FeI and FeII stellar abundance determinations. We found that the Carina metallicity distribution based on the corrected FeI abundances (44 RGs) has a weighted mean metallicity of [Fe/H]=-1.80 and a weighted standard deviation of sigma=0.24 dex. The Carina metallicity distribution based on the FeII abundances (27 RGs) gives similar estimates ([Fe/H]=-1.72, sigma=0.24 dex). The current weighted mean metallicities are slightly more metal poor when compared with similar estimates available in the literature. Furthermore, if we restrict our analysis to stars with the most accurate iron abundances, ~20 FeI and at least three FeII measurements (15 stars), we found that the range in iron abundances covered by Carina RGs (~1 dex) agrees quite well with similar estimates based on high-resolution spectra., Accepted for publication in PASP, 16 pages, 7 figures, 3 tables, 1 MR table Note: the electronic version of Table1 is included, but commented, in the tex file

The last decade showed an impressive observational effort from the photometric and spectroscopic point of view for ancient stellar clusters in our Galaxy and beyond. The theoretical interpretation of these new observational results requires updated evolutionary models and isochrones spanning a wide range of chemical composition. With this aim we built the new "Pisa Stellar Evolution Database" of stellar models and isochrones by adopting a well-tested evolutionary code (FRANEC) implemented with updated physical and chemical inputs. In particular, our code adopts realistic atmosphere models and an updated equation of state, nuclear reaction rates and opacities calculated with recent solar elements mixture. A total of 32646 models have been computed in the range of initial masses 0.30 - 1.10 Msun for a grid of 216 chemical compositions with the fractional metal abundance in mass, Z, ranging from 0.0001 to 0.01, and the original helium content, Y, from 0.25 to 0.42. Models were computed for both solar-scaled and alpha-enhanced abundances with different external convection efficiencies. Correspondingly, 9720 isochrones were computed in the age range 8 - 15 Gyr, in time steps of 0.5 Gyr. The whole database is available to the scientific community on the web. Models and isochrones were compared with recent calculations available in the literature and with the color-magnitude diagram of selected Galactic globular clusters. The dependence of relevant evolutionary quantities on the chemical composition and convection efficiency were analyzed in a quantitative statistical way and analytical formulations were made available for reader's convenience., Comment: Accepted for publication in A&A