P. G. Prada Moroni

An eclipsing-binary distance to the Large Magellanic Cloud accurate to two per cent.

In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better. At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale. Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately. The eclipsing-binary method was previously applied to the LMC, but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future.

The Pisa pre-main sequence tracks and isochrones - A database covering a wide range of Z, Y, mass, and age values

Context. In recent years new observations of pre-main sequence stars (pre-MS) with Z≤ Z⊙ have been made available. To take full advantage of the continuously growing amount of data of pre-MS stars in different environments, we need to develop updated pre-MS models for a wide range of metallicity to assign reliable ages and masses to the observed stars. Aims. We present updated evolutionary pre-MS models and isochrones for a fine grid of mass, age, metallicity, and helium values . Methods. We use a standard and well-tested stellar evolutionary code (i.e. FRANEC), that adopts outer boundary conditions from detailed and realistic atmosphere models. In this code, we incorporate additional improvements to the physical inputs related to the equation of state and the low temperature radiative opacities essential to computing low-mass stellar models. Results. We make available via internet a large database of pre-MS tracks and isochrones for a wide range of chemical compositions (Z = 0.0002− 0.03), masses (M = 0.2− 7.0 M⊙), and ages (1− 100 Myr) for a solar-calibrated mixing length parameterα (i.e. 1.68). For each chemical composition, additional models were computed with two different mixing length values, namelyα = 1.2 and 1.9. Moreover, for Z≥ 0.008, we also provided models with two different initial deuterium abundances. The characteristics o f the models have been discussed in detail and compared with other work in the literature. The main uncertainties affecting theoretical predictions have been critically discussed. Comparisons with selected data indicate that there is close agreement between theory and observation.

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

RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system

RR Lyrae pulsating stars have been extensively used as tracers of old stellar populations for the purpose of determining the ages of galaxies, and as tools to measure distances to nearby galaxies. There was accordingly considerable interest when the RR Lyrae star OGLE-BLG-RRLYR-02792 (referred to here as RRLYR-02792) was found to be a member of an eclipsing binary system, because the mass of the pulsator (hitherto constrained only by models) could be unambiguously determined. Here we report that RRLYR-02792 has a mass of 0.26 solar masses () and therefore cannot be a classical RR Lyrae star. Using models, we find that its properties are best explained by the evolution of a close binary system that started with and stars orbiting each other with an initial period of 2.9 days. Mass exchange over 5.4 billion years produced the observed system, which is now in a very short-lived phase where the physical properties of the pulsator happen to place it in the same instability strip of the Hertzsprung–Russell diagram as that occupied by RR Lyrae stars. We estimate that only 0.2 per cent of RR Lyrae stars may be contaminated by systems similar to this one, which implies that distances measured with RR Lyrae stars should not be significantly affected by these binary interlopers.

Recovering the star formation rate in the solar neighborhood

Received; accepted Abstract. This paper develops a method for obtaining the star formation histories of a mixed, resolved population through the use of color-magnitude diagrams (CMDs). The method provides insight into the local star formation rate, analyzing t he observations of the Hipparcos satellite through a comparison with synthetic CMDs computed for different histories with an updated stellar evolution library. Parallax and photometr ic uncertainties are included explicitly and corrected usi ng the Bayesian Richardson-Lucy algorithm. We first describe our verificati on studies using artificial data sets. From this sensitivity study, the critical factors determining the success of a recovery for a known star formation rate are a partial knowledge of the IMF and the age-metallicity relation, and sample contamination by clusters and moving groups (special populations whose histories are different than that of the whole sample). Unresolved binaries are less important impediments. We highlight how these limit the method. For the real field sample, complete to MV < 3.5, we find that the solar neighborhood star formation rate has a characteristic timescale for variation of about 6 Gyr, wit h a maximum activity close to 3 Gyr ago. The similarity of this finding with column integrated star formation rates may indi cate a global origin, possibly a collision with a satellite g alaxy. We also discuss applications of this technique to general phot ometric surveys of other complex systems (e.g. Local Group dwarf galaxies) where the distances are well known.

Calibration of white dwarf cooling sequences: Theoretical uncertainty

White dwarf luminosities are powerful age indicators, whose calibration should be based on reliable models. We discuss the uncertainty of some chemical and physical parameters and their influence on the age estimated by means of white dwarf cooling sequences. Models at the beginning of the white dwarf sequence have been obtained on the basis of progenitor evolutionary tracks computed starting from the zero-age horizontal branch and for a typical halo chemical composition (Z = 0.0001, Y = 0.23). The uncertainties due to nuclear reaction rates, convection, mass loss, and initial chemical composition are discussed. Then, various cooling sequences for a typical white dwarf mass (M = 0.6 M☉) have been calculated under different assumptions on some input physics, namely, conductive opacity, contribution of the ion-electron interaction to the free energy, and microscopic diffusion. Finally, we present the evolution of white dwarfs having mass ranging between 0.5 and 0.9 M☉. Much effort has been spent to extend the equation of state down to the low-temperature and high-density regime. An analysis of the latest improvement in the physics of white dwarf interiors is presented. We conclude that at the faint end of the cooling sequence [log(L/L☉) ~ -5.5] the present overall uncertainty on the age is of the order of 20%, which corresponds to about 3 Gyr. We suggest that this uncertainty could be substantially reduced by improving our knowledge of the conductive opacity (especially in the partially degenerate regime) and by fixing the internal stratification of C and O.

ON A NEW NEAR-INFRARED METHOD TO ESTIMATE THE ABSOLUTE AGES OF STAR CLUSTERS: NGC 3201 AS A FIRST TEST CASE*

We present a new method to estimate the absolute ages of stellar systems. This method is based on the difference in magnitude between the main-sequence turnoff (MSTO) and a well-defined knee located along the lower main sequence (MSK). This feature is caused by the collisionally induced absorption of molecular hydrogen, and it can easily be identified in near-infrared (NIR) and in optical-NIR color-magnitude diagrams of stellar systems. We took advantage of deep and accurate NIR images collected with the Multi-Conjugate Adaptive Optics Demonstrator temporarily available on the Very Large Telescope and of optical images collected with the Advanced Camera for Surveys Wide Field Camera on the Hubble Space Telescope and with ground-based telescopes to estimate the absolute age of the globular NGC 3201 using both the MSTO and the ?(MSTO-MSK). We have adopted a new set of cluster isochrones, and we found that the absolute ages based on the two methods agree to within 1?. However, the errors of the ages based on the ?(MSTO-MSK) method are potentially more than a factor of 2 smaller, since they are not affected by uncertainties in cluster distance or reddening. Current isochrones appear to predict slightly bluer (0.05 mag) NIR and optical-NIR colors than observed for magnitudes fainter than the MSK.

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

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

Physical parameters and the projection factor of the classical Cepheid in the binary system OGLE-LMC-CEP-0227

A novel method of analysis of double-lined eclipsing binaries containing a radially pulsating star is presented. The combined pulsating-eclipsing light curve is built up from a purely eclipsing light curve grid created using an existing modeling tool. For every pulsation phase the instantaneous radius and surface brightness are taken into account, being calculated from the disentangled radial velocity curve of the pulsating star and from its out-of-eclipse pulsational light curve and the light ratio of the components, respectively. The best model is found using the Markov Chain Monte Carlo method. The method is applied to the eclipsing binary Cepheid OGLE-LMC-CEP-0227 (Ppuls = 3.80 d, Porb = 309 d). We analyze a set of new spectroscopic and photometric observations for this binary, simultaneously fitting OGLE V-band, I-band and Spitzer 3.6 µm photometry. We derive a set of fundamental parameters of the system significantly improving the precision comparing to the previous results obtained by our group. The Cepheid mass and radius are M1 = 4.165 ± 0.032M⊙ and R1 = 34.92± 0.34R⊙, respectively. For the first time a direct, geometrical and distance-independent determination of the Cepheid projection factor is presented. The value p =1.21 ± 0.03(stat.) ± 0.04(syst.) is consistent with theoretical expectations for a short period Cepheid and interferometric measurements for δ Cep. We also find a very high value of the optical limb darkening coefficients for the Cepheid component, in strong disagreement with theoretical predictions for static atmospheres at a given surface temperature and gravity.

AN UPDATED 6Li(p, α)3He REACTION RATE AT ASTROPHYSICAL ENERGIES WITH THE TROJAN HORSE METHOD

The lithium problem influencing primordial and stellar nucleosynthesis is one of the most interesting unsolved issues in astrophysics. 6Li is the most fragile of lithiums stable isotopes and is largely destroyed in most stars during the pre-main-sequence (PMS) phase. For these stars, the convective envelope easily reaches, at least at its bottom, the relatively low 6Li ignition temperature. Thus, gaining an understanding of 6Li depletion also gives hints about the extent of convective regions. For this reason, charged-particle-induced reactions in lithium have been the subject of several studies. Low-energy extrapolations of these studies provide information about both the zero-energy astrophysical S(E) factor and the electron screening potential, Ue . Thanks to recent direct measurements, new estimates of the 6Li(p, ?)3He bare-nucleus S(E) factor and the corresponding Ue value have been obtained by applying the Trojan Horse method to the 2H(6Li, ? 3He)n reaction in quasi-free kinematics. The calculated reaction rate covers the temperature window 0.01 to 2T 9 and its impact on the surface lithium depletion in PMS models with different masses and metallicities has been evaluated in detail by adopting an updated version of the FRANEC evolutionary code.