arXiv subjects

V. Castellani

Publications and source records attributed to V. Castellani.

At least 55 records · Page 3Linked to original sources

arXiv2001

Theoretical insights into the RR Lyrae K-band Period-Luminosity relation

Based on updated nonlinear, convective pulsation models computed for several values of stellar mass, luminosity and metallicity, theoretical constraints on the K-band Period-Luminosity (PLK) relation of RR Lyrae stars are presented. We show that for each given metal content the predicted PLK is marginally dependent on uncertainties of the stellar mass and/or luminosity. Then, by considering the RR Lyrae masses suggested by evolutionary computations for the various metallicities, we obtain that the predicted infrared magnitude M_K over the range 0.0001< Z <0.02 is given by the relation MK=0.568-2.071logP+0.087logZ-0.778logL/Lo, with a rms scatter of 0.032 mag. Therefore, by allowing the luminosities of RR Lyrae stars to vary within the range covered by current evolutionary predictions for metal-deficient (0.0001< Z <0.006) horizontal branch models, we eventually find that the infrared Period-Luminosity- Metallicity (PLZK) relation is MK=0.139-2.071(logP+0.30)+0.167logZ, with a total intrinsic dispersion of 0.037 mag. As a consequence, the use of such a PLZK relation should constrain within +-0.04 mag the infrared distance modulus of field and cluster RR Lyrae variables, provided that accurate observations and reliable estimates of the metal content are available. Moreover, we show that the combination of K and V measurements can supply independent information on the average luminosity of RR Lyrae stars, thus yielding tight constraints on the input physics of stellar evolution computations. Finally, for globular clusters with a sizable sample of first overtone variables, the reddening can be estimated by using the PLZK relation together with the predicted MV-logP relation at the blue edge of the instability strip (Caputo et al. 2000).

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arXiv2000

The O-Na and Mg-Al Anticorrelations in Turn-Off and early Subgiants in Globular Clusters

High dispersion spectra (R>40,000) for a quite large number of stars at the main sequence turn-off and at the base of the subgiant branch in NGC6397 and NGC6752 were obtained with the UVES on Kueyen (VLT UT2). The [Fe/H] values we found are -2.03+/-0.02+/-0.04 and -1.42+/-0.02+/-0.04 for NGC6397 and NGC6752 respectively, where the first error bars refer to internal errors and the second ones to systematic errors. In both clusters the [Fe/H] obtained for TO-stars agree perfectly (within a few per cents) with that obtained for stars at the base of the RGB. The [O/Fe]=0.21+/-0.05 value we obtain for NGC6397 is quite low, but it agrees with previous results obtained for giants in this cluster; also, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other side, our results show clearly and for the first time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC6752, a more massive cluster than NGC6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very difficult to explain the observed Na-O, and Mg-Al anticorrelation in NGC6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

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arXiv2000

Stellar models and Hyades: the Hipparcos test

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.

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arXiv2000

Pulsational M_V versus [Fe/H] relation(s) for globular cluster RR Lyrae variables

We use the results from recent computations of updated non-linear convective pulsating models to constrain the distance modulus of Galactic globular clusters through the observed periods of first overtone RRc pulsators. The resulting relation between the mean absolute magnitude of RR Lyrae stars and the heavy element content [Fe/H] appears well in the range of several previous empirical calibrations, but with a non linear dependence on [Fe/H] so that the slope of the relation increases when moving towards larger metallicities. On this ground, our results suggest that metal-poor ([Fe/H]<-1.5) and metal-rich ([Fe/H]>-1.5) variables follow two different linear -[Fe/H] relations. Application to RR Lyrae stars in the metal-poor globular clusters of the Large Magellanic Cloud provides a LMC distance modulus of the order of 18.6 mag, thus supporting the "long" distance scale. The comparison with recent predictions based on updated stellar evolution theory is shortly presented and discussed.

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arXiv2000

Galactic Globular Clusters as a test for Very Low-Mass stellar models

We make use of the Next Generation model atmospheres by Allard et al. (1997) and Hauschildt, Allard & Baron (1999) to compute theoretical models for low and very low-mass stars for selected metallicities in the range Z= 0.0002 to 0.002. On this basis, we present theoretical predictions covering the sequence of H-burning stars as observed in galactic globulars from the faint end of the Main Sequence up to, and beyond, the cluster Turn Off. The role played by the new model atmospheres is discussed, showing that present models appear in excellent agreement with models by Baraffe et al. (1997) as computed on quite similar physical basis. One finds that the theoretical mass-luminosity relations based on this updated set of models, are in good agreement with the empirical data provided by Henry & McCarthy (1993). Comparison with HST observation discloses that the location in the Color-Magnitude diagram of the lower Main Sequence in galactic globular clusters appears again in good agreement with the predicted sensitive dependence of these sequences on the cluster metallicity.

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arXiv2000

The RR Lyrae U Com as a test for nonlinear pulsation models

We use high precision multiband photometric data of the first overtone RR Lyrae U Com to investigate the predictive capability of full amplitude, nonlinear, convective hydrodynamical models. The main outcome of this investigation is that theoretical predictions properly account for the luminosity variations along a full pulsation cycle. Moreover, we find that this approach, due to the strong dependence of this observable and of the pulsation period on stellar parameters, supply tight constraints on stellar mass, effective temperature, and distance modulus. Pulsational estimates of these parameters appear in good agreement with empirical ones. Finally, the occurrence of a well-defined bump just before the luminosity maximum gave the unique opportunity to calibrate the turbulent convection model adopted for handling the coupling between pulsation and convection.

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arXiv2000

Stellar evolution and large extra dimensions

We discuss in detail the information on large extra dimensions which can be derived in the framework of stellar evolution theory and observation. The main effect of large extra dimensions arises from the production of the Kaluza-Klein (KK) excitations of the graviton. The KK-graviton and matter interactions are of gravitational strength, so the KK states never become thermalized and always freely escape. In this paper we first pay attention to the sun. Production of KK gravitons is incompatible with helioseismic constraints unless the 4+n dimensional Planck mass M_s exceeds 300 Gev/c^2. Next we show that stellar structures in their advanced phase of H burning evolution put much more severe constraints, M_s > 3-4 TeV/c^2, improving on current laboratory lower limits.

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arXiv1999

Synthetic Stellar Clusters for Pop III

We present preliminary results of an incoming theoretical work concerning the integrated properties of the Population III clusters of stars. On the basis of synthetic Color-Magnitude Diagrams, we provide a grid of optical and near-IR colors of Simple Stellar Populations with very low metallicity (Z=10$^{-10}$ and Z=10$^{-6}$) and age which spans from 10 Myr to 15 Gyr. A comparison with higher metallicities up to 0.006 is also shown, disclosing sizable differences in the CMD morphology, integrated colors and Spectral Energy Distribution (SED).

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arXiv1999

The puzzling theoretical predictions for the luminosity of clumping He burning stars

This paper deals with theoretical predictions for He burning models in a range of masses covering the so-called Red Giant Branch phase transition. Taking as a guideline the observational constraints given by Hipparcos parallaxes to the predicted luminosity of models originated from Red Giant progenitors with He core undergoing electron degeneracy, we compare models by various authors as recently appeared in the literature, disclosing sensitive differences in the predicted luminosity. The solidity of these theoretical predictions is investigated by exploring the effects of varying the assumptions about the efficiency of core overshooting or the amount of mass loss, giving quantitative estimates of the related uncertainties. However, one finds that theoretical predictions concerning the luminosity of the red giant clump in the Hipparcos sample is scarcely affected by these mechanisms.A comparison among theoretical predictions as recently given by different authors convincingly demonstrates that the different luminosity predictions are the natural results of evolutionary codes with different - but all reasonable- input physics. In this context observations suggests that stellar models based on the most updated input physics are possibly overestimating the luminosity of these structures, raising doubts on several current predictions concerning the luminosity of HB stars in galactic globulars.

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arXiv1999

Theoretical Zero Age Main Sequences revisited

Zero Age Main Sequence (ZAMS) models with updated physical inputs are presented for selected assumptions about the chemical composition, covering the ranges 0.6 < M/Mo < 1.2, 0.0001 < Z < 0.04, 0.23 < Y < 0.34.The HR diagram location of the ZAMS as a function of Y and Z is discussed both in the theoretical and in the observational HR diagrams, showing that the V magnitude presents an increased dependence on Z to be taken into account when discussing observational evidences. Analytical relations quantifying both these dependences are derived. Implications for the galactic helium to heavier elements enrichment are finally discussed.

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arXiv1999

Classical Cepheid Pulsation Models. III. The Predictable Scenario

Within the current uncertainties in the treatment of the coupling between pulsation and convection, limiting amplitude, nonlinear, convective models appear the only viable approach for providing theoretical predictions about the intrinsic properties of radial pulsators. In this paper we present the results of a comprehensive set of Cepheid models computed within such theoretical framework for selected assumptions on their original chemical composition.

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arXiv1999

RR Lyrae variables in M5 as a test of pulsational theory

We present B and V CCD photometry for variables in the cluster central region, adding new data for 32 variables and giving suitable light curves, mean magnitudes and corrected colors for 17 RR Lyrae. Implementing the data given in this paper with similar data already appeared in the literature we discuss a sample of 42 variables, as given by 22 RRab and 20 RRc, to the light of recent predictions from pulsational theories. We find that the observational evidence concerning M5 pulsators appears in marginal disagreement with predictions concerning the color of the First Overtone Blue Edge (FOBE), whereas a clear disagreement appears between the ZAHB luminosities predicted through evolutionary or pulsational theories.

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arXiv1998

Predicted HST FOC and broad band colours for young and intermediate Simple Stellar Populations

This paper presents theoretical HST and broad band colours from population synthesis models based on an homogeneous set of stellar evolutionary tracks as computed under canonical (no overshooting) assumptions, covering the range of cluster ages from t=8 Myr to t=5 Gyr for three different metallicities (Z=0.02, 0.006, and 0.001). Statistical fluctuations in the cluster population have been investigated, assessing the predicted fluctuations of the various colours as a function of the cluster integrated absolute magnitude. We show that the red leak in HST UV filters deeply affects the predicted fluxes and colours. However, we find that for F152M-F307M < 0.5 and for F170M-F278M < 0.5 (which means ages lower than 1 Gyr) the HST UV colours can still be used to infer reliable indications on the age of distant clusters. Moreover, one finds that the age calibration of these colours is scarcely affected by the amount of original helium or by the assumed IMF. On this basis, we present a calibration of the HST UV two-colours (F152M-F307M vs F170M-F278M) in terms of cluster ages for the three above quoted metallicities. We suggest the combined use of HST UV colours and IR colours (V-K in particular) to disentangle the metallicity-age effect in integrated colours of young stellar populations (t< 1 Gyr).

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arXiv1998

Theoretical stellar models for old galactic clusters

We present new evolutionary stellar models suitable for old Population I clusters, discussing both the consequences of the most recent improvements in the input physics and the effect of element diffusion within the stellar structures. Theoretical cluster isochrones are presented, covering the range of ages from 1 to 9 Gyr for the four selected choices on the metallicity Z= 0.007, 0.010, 0.015 and 0.020. Theoretical uncertainties on the efficiency of superadiabatic convection are discussed in some details. Isochrone fitting to the CM diagrams of the two well observed galactic clusters NGC2420 and M67 indicates that a mixing length parameter alpha = 1.9 appears adequate for reproducing the observed color of cool giant stars. The problems in matching theoretical preditions to the observed slope of MS stars are discussed.

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arXiv1998

Galactic Globular Cluster Stars: from Theory to Observation

We use evolutionary calculations presented in a recent paper (Cassisi et al. 1998) to predict B,V,I magnitudes for stars in galactic globulars. The effect of the adopted mixing length on stellar magnitudes and colors is discussed, showing that the uncertainty on such a theoretical parameter prevents the use of MS stars as bona fide theoretical standard candles. However, comparison with Hipparcos data for field subdwarfs discloses a substantial agreement between theory and observation. Present predictions concerning the magnitude of TO and of HB stars are compared with similar results appeared in the recent literature. We present and discuss a theoretical calibration of the difference in magnitude between HB and TO as evaluated with or without element sedimentation. Finally we use theoretical HB magnitudes to best fit the CM diagram of M68 and M5, taken as representative of metal poor and intermediate metallicity galactic globulars, deriving an age of 11$\pm$1.0 Gyr and 10$\pm$1.0 Gyr, respectively, for the adopted chemical compositions, plus an additional uncertainty of $\pm$1.4 Gyr if the uncertainty on the chemical composition is taken into account. This result is discussed on the basis of current evaluations concerning cluster ages and distance moduli.

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arXiv1998

Theoretical Models for Classical Cepheids: II. Period-Luminosity, Period-Color and Period-Luminosity-Color Relations

We present and discuss theoretical pulsational predictions concerning the properties of Classical Cepheids. The computations have been performed for three different chemical compositions taken as representative of Cepheids in the Magellanic Clouds and in the Galaxy. The resulting PL relations are significantly dependent on metallicity, with the amplitude of the metallicity effect decreasing at the longer wavelength. At variance with several empirical suggestions appeared in the literature, we find that at fixed period the metal-rich pulsators should be fainter than the metal-poor ones. Tight Period-Luminosity-Color relations are derived for both visual and near-infrared photometric bands. We present a preliminary use of our relations to evaluate the distance modulus of the Magellanic Clouds.

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arXiv1998

Which radius for the Sun?

The high accuracy reached by solar limb observations, by helioseismic measurements and by Standard Solar Models (SSMs) calculations suggests that general relativity corrections are included when discussing the solar radius. The Allen value (R$_{\odot}$ = 695.99 $\pm$ 0.07 Mm) has to be reduced by 1.5 Km. This correction, which is small as compared with present accuracy, should be kept in mind for future more precise measurements and/or calculations.

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arXiv1998

Nuclear burning rates and population II stellar models

We discuss theoretical predictions concerning the evolution of globular cluster Pop.II stars vis-a-vis current estimates of standard errors in the determination of nuclear burning rates. Numerical evaluations are given for the dependence of TO and HB luminosity from the rate of the relevant nuclear reactions. We conclude that evolutionary predictions appear rather solid in this respect, with a maximum 3$σ$ error of about 1 Gyr in the evaluation of cluster ages derived from the calibration of the difference in luminosity between HB and TO. However, current evaluation of the original He content, as given on the basis of the R parameter, should wait for a much better determination of the C12($α$,$γ$)O16 reaction before reaching a satisfactory accuracy.

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