G. Valle

The Pisa Stellar Evolution Data Base for low-mass stars

INFN, Sezione di Pisa, Largo B. Pontecorvo 3, I-56127, ItalyReceived 12/12/2011; accepted 26/01/2012ABSTRACTContext. The last decade showed an impressive observational effort from the photometric and spectroscopic point of view for ancientstellar clusters in our Galaxy and beyond, leading to important and sometimes surprising results.Aims. The theoretical interpretation of these new observational results requires updated evolutionary models and isochrones spanninga wide range of chemical composition so that the possibility of multipopulations inside a stellar cluster is also taken also into account.Methods. 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 realisticatmosphere models and an updated equation of state, nuclear reaction ratesand opacities calculated withrecent solar elements mixture.Results. A total of 32646 models have been computed in the range of initial masses 0.30 ÷ 1.10 M⊙ for a grid of 216 chemicalcompositions with the fractional metal abundance in mass, Z, ranging from 0.0001 to 0.01, and the original helium content, Y, from0.25 to 0.42. Models were computed for both solar-scaled andα-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 availableto the scientific community on the web. Models and isochrones were compared with recent calculations available in the literature andwith the color-magnitude diagram of selected Galactic globular clusters. The dependence of relevant evolutionary quantities, namelyturn-off and horizontal branch luminosities, on the chemical composition and convection efficiency were analyzed in a quantitativestatistical way and analytical formulations were made available for reader’s convenience. These relations can be useful in severalfields of stellar evolution, e.g. evolutionary properties o f binary systems, synthetic models for simple stellar populations and for starcounts in galaxies, and chemical evolution models of galaxies.Conclusions.Key words. Stars: evolution – Stars: horizontal-branch – Stars: interiors – Stars: low-mass – Hertzsprung-Russell and C-M diagrams– Globular clusters: general

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

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

Evolution of Li, Be, and B in the Galaxy

In this paper we study the production of Li, Be, and B nuclei by Galactic cosmic-ray spallation processes. We include three kinds of processes: (1) spallation by light cosmic rays impinging on interstellar CNO nuclei (direct processes), (2) spallation by CNO cosmic-ray nuclei impinging on interstellar p and 4He (inverse processes), and (3) α-α fusion reactions. The latter dominate the production of 6,7Li. We calculate production rates for a closed box Galactic model, verifying the quadratic dependence of the Be and B abundances for low values of Z. These are quite general results and are known to disagree with observations. We then show that the multizone, multipopulation model that we used previously for other aspects of Galactic evolution produces quite good agreement with the linear trend observed at low metallicities without fine-tuning. We argue that reported discrepancies between theory and observations do not represent a nucleosynthetic problem but instead are the consequences of inaccurate treatments of Galactic evolution.

The AMS-02 electromagnetic calorimeter

Abstract The Electromagnetic Calorimeter (ECAL) of the AMS-02 experiment is a lead-scintillanting fibers sampling calorimeter characterized by high granularity that allows to image the longitudinal and lateral showers development, a key issue to provide high electron/hadron discrimination. The light collection system and the FE electronics are designed to let the calorimeter operate over a wide energy range from few GeV up to 1 TeV. A full-scale prototype of the e.m. calorimeter was tested at Cern in October 2001 using electrons and pions beams with energy ranging from 3 to 100 GeV. Effective sampling thickness, linearity and energy resolution were measured.

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

Context. With their period-luminosity relation, “classical Cepheids” (CC) are the most common primary distance indicators within the Local Group, also providing an absolute calibration of important secondary distance indicators. However, the predicted position of these pulsators in the HR diagram, along the so called blue loop, that is the expected distribution of Cepheids within the instability strip is affected by several model inputs, reflecting upon the predicted PL relation. Aims. The aim of this work is to quantitatively evaluate the effects on the theoretical PL relation of current uncertainties on the chemical abundances of Cepheids in the Large Magellanic Cloud (LMC) and on several physical assumptions adopted in the evolutionary models. We will separately analyse how the different factors influence the evolutionary and pulsational observables and the resulting PL relation. Methods. To achieve this goal we computed new sets of updated evolutionary and pulsational models. Results. As a result, we find that present uncertainties on the most relevant H and He burning reaction rates do not influence in a relevant way the loop extension in temperature. On the contrary, current uncertainties on the LMC chemical composition significantly affect the loop extension and also reflect in the morphology of the instability strip; however their influence on the predicted pulsational parameters is negligible. We also discussed how overshooting and mass loss, sometimes suggested as possible solutions for the long-standing problem of the Cepheid mass discrepancy, influence the ML relation and the pulsational parameters. Conclusions. In summary, the present uncertainties on the physical inputs adopted in the evolutionary codes and in the LMC chemical composition are negligible for the prediction of the main pulsational properties. On the other hand, the inclusion of overshooting in the previous hydrogen burning phase and/or of mass loss is expected to significantly change the resulting theoretical pulsational scenario for Cepheids, as well as the calibration of their distance scale. These systematic effects are expected to influence the theoretical Cepheid calibration of the secondary distance indicators and in turn the resulting evaluation of the Hubble constant.

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

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

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

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

Early star formation in the Galaxy from beryllium and oxygen abundances

We investigate the evolution of the star formation rate in the early Galaxy using beryllium and oxygen abundances in metal poor stars. Specifically, we show that stars belonging to two previously identified kinematical classes (the so-called “accretion” and “dissipative” populations) are neatly separated in the [O/Fe] vs.

Calibrating convective-core overshooting with eclipsing binary systems The case of low-mass main-sequence stars

\log\, ({\rm Be/H})

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

diagram. The dissipative population follows the predictions of our model of Galactic evolution for the thick disk component, suggesting that the formation of this stellar population occurred on a timescale significantly longer (by a factor ∼ 5–10) than the accretion component. The latter shows a large scatter in the [O/Fe] vs.