S. Degl'Innocenti

A pulsational approach to near-infrared and visual magnitudes of RR Lyr stars

In this paper, we present an improved theoretical scenario concerning near-infrared and visual magnitudes of RR Lyr variables, as based on up-to-date pulsating models. New relations connecting V and K absolute magnitudeswith periods, mass, luminosity and metal content are discussed separately for fundamental and first-overtone pulsators. We also show that the V -K colours are predicted to supply tight constraints on the pulsator intrinsic luminosity. On this basis, we revisit the case of the prototype variable RR Lyr, showing that the parallax inferred by this new pulsational approach appears in close agreement with Hubble Space Telescope absolute parallax. Moreover, available K and V measurements for field and cluster RR Lyr variables with known reddening and metal content are used to derive a relation connecting the K absolute magnitude to period and metallicity (M K -[Fe/H]-log P) as well as a new calibration of the M V -[Fe/H] relation. The comparison between theoretical prescriptions and observations suggests that RR Lyr stars in the field and in galactic globular clusters (GGCs) should have quite similar evolutionary histories. The comparison between theory and observations also discloses a general agreement that supports the reliability of the current pulsational scenario. On the contrary, current empirical absolute magnitudes based on the Baade-Wesselink (BW) method suggest relations with a zero-point which is fainter than is predicted by pulsation models, together with a milder metallicity dependence. However, preliminary results based on a new calibration of the BW method provided by Cacciari et al. (2000) for RR Cet and SW And appear in a much better agreement with the pulsational predictions.

Galactic globular cluster stars: From theory to observation

We use evolutionary calculations presented in a recent paper (Cassisi et al. 1997a: hereinafter Paper I) to predict B, V , I magnitudes for stars in galactic glob- ulars. The eect of the adopted mixing length on stel- lar magnitudes and colors is discussed, showing that the uncertainty on such a theoretical parameter prevents the use of MS stars as bona de theoretical standard candles. However, comparison with Hipparcos data for eld sub- dwarfs discloses a substantial agreement between theory and observation. Present predictions concerning the mag- nitude of TO and of HB stars are compared with similar results appeared in the recent literature. We nd that our predictions about the dependence on metallicity of ZAHB magnitudes appear in good agreement with observational constraints as recently discussed by Gratton et al. (1997c). We present and discuss a theoretical calibration of the dierence in magnitude between HB and TO as evalu- ated with or without element sedimentation. The eect of a variation of the original helium content on the magni- tude of MS, TO and HB stars is explored and discussed. Finally we use theoretical HB magnitudes to best t the CM diagram of M 68 and M 5, taken as representative of metal poor and intermediate metallicity galactic glob- ulars, deriving an age of 11 1.0 Gyr and 10 1.0 Gyr, respectively, for the adopted chemical compositions, plus an additional uncertainty of1.4 Gyr if the uncertainty on the chemical composition is taken into account. This result is discussed on the basis of current evaluations con- cerning cluster ages and distance moduli.

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

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.

Calibrated stellar models for metal-poor populations

We extend to lower metallicities recent evolutionary computations devoted to Magellanic Clouds stars, presenting and discussing new stellar models with Z = 0.0002, 0.0004, 0.0006, 0.001 and suitable assumptions about the original He con- tent. As in the previous paper, evolutionary results are compared with observational data to properly calibrate the assumptions about the efficiency of the surface convection. On this basis, we follow the evolution of stellar models in the mass range 0.6 to 11 Mfrom the Main Sequence (MS) to the C ignition or the onset of thermal pulses in the advanced Asymptotic Giant Branch (AGB) phase, presenting cluster isochrones covering the range of ages from 20 Myr to 20 Gyr. Selected predictions constraining the cluster ages are discussed, presenting a calibration of the difference in magnitude between the luminous MS ter- mination and the He burning giants in terms of the cluster age. Both evolutionary tracks and isochrones are available at the URL http://astro.df.unipi.it/SAA/PEL/Z0.html � .

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.

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.

\Delta Y/ \Delta Z from the analysis of local K dwarfs

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.

STAR FORMATION HISTORY IN THE SMALL MAGELLANIC CLOUD: THE CASE OF NGC 602*

Deep Hubble Space Telescope/Advanced Camera for Surveys photometry of the young cluster NGC 602, located in the remote low-density wing of the Small Magellanic Cloud (SMC), reveals numerous pre-main-sequence (PMS) stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history (SFH) into recent times and constraining several stellar population properties, such as the present-day mass function (PDMF), the initial mass function, and the binary fraction. To better characterize the PMS population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the SMC (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the SFH in the region by comparing observed and synthetic color-magnitude diagrams. The derived PDMF for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching (0.3-0.7) × 10–3 M ☉ yr–1 in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula.

THE EFFECT OF DIFFUSION ON THE RED GIANT LUMINOSITY FUNCTION 'BUMP'

This paper investigates the effect of microscopic diffusion of helium and heavy elements on the location of the Red Giant Branch Luminosity Function Bump in Population II stellar models. To this aim updated evolutionary models taking into account diffusion from the Main Sequence until the Zero Age Horizontal Branch have been computed. The observational luminosity difference between the RGB bump and the ZAHB, as collected for a sample of galactic globular clusters, has been compared with the corresponding theoretical values obtained by adopting both canonical and diffusive models. We find that the effect of diffusion, even if slightly improving the agreement between observations and theory, is negligible with respect to the observational uncertainties. In any case the theoretical predictions in models with and without diffusion appear in agreement with the observational results within the estimated errors. Thus canonical models can be still safely adopted, at least until much more accurate observational data will be available.