arXiv subjects

M. Salaris

Publications and source records attributed to M. Salaris.

At least 181 records · Page 10Linked to original sources

arXiv2003

The initial Helium content of Galactic Globular Cluster stars from the R-parameter: comparison with the CMB constraint

Recent precise determinations of the primordial He-abundance (Y_p) from cosmic microwave background (CMB) analyses and cosmological nucleosynthesis computations, provide Y_p=0.248$\pm$0.001. On the other hand, recent works on the initial He-abundance of Galactic globular cluster (GGC) stars, making use of the R parameter as He-indicator, have consistently obtained $Y_{GGC}\sim$0.20. In light of this serious discrepancy that casts doubt on the adequacy of low mass He-burning stellar models, we have rederived the initial He-abundance for stars in two large samples of GGCs, by employing theoretical models computed using new and more accurate determinations of the Equation of State for the stellar matter, and of the uncertain $^{12}$C$(α,γ)^{16}$O reaction rate. Our models include semiconvection during the central convective He-burning phase, while the breathing pulses are suppressed, in agreement with the observational constraints coming from the measurements of the R_2 parameter in a sample of clusters. By taking into account the observational errors on the individual R-parameter values, as well as uncertainties in the GGC [Fe/H] scale, treatment of convection and $^{12}$C$(α,γ)^{16}$O reaction rate, we have obtained, respectively, a mean $Y_{GGC}$=0.243$\pm$0.006 and $Y_{GGC}$=0.244$\pm$0.006 for the two studied GGC samples. These estimates are now fully consistent with Y_p obtained from CMB studies. Moreover, the trend of the individual He-abundances with respect to [Fe/H] is consistent with no appreciable He-enrichment along the GGC metallicity range.

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arXiv2002

First full evolutionary computation of the He-flash induced mixing in Population II stars

The core helium-flash in low-mass stars with extreme mass loss occurs after the tip of the RGB, when the H-rich envelope is very thin. The low efficiency of the H-shell source enables the He-flash driven convective zone to penetrate H-rich layers and trigger a thermonuclear runaway, resulting in a subsequent surface enrichment with He and C. In this work we present the first full computations of Population II low-mass stellar models through this phase. Models experiencing this dredge-up event are significantly hotter than their counterparts with H-rich envelopes, which makes them promising candidates for explaining the existence of stars observed beyond the canonical blue end of the horizontal branch ("blue hook stars"). Moreover, this temperature difference could explain the observed gap in M_V between extreme blue horizontal-branch and blue hook stars. A first comparison with spectroscopic observations of blue hook stars in the globular cluster omega Cen is also presented.

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arXiv2002

The surface carbon and nitrogen abundances in models of ultra metal-poor stars

We investigate whether the observed high number of carbon- and nitrogen-enhanced extremely metal-poor stars could be explained by peculiar evolutionary properties during the core He flash at the tip of the red giant branch. For this purpose we compute a series of detailed stellar models expanding upon our previous work; in particular, we investigate if during the major He flash the penetration of the helium convective zone into the overlying hydrogen-rich layers can produce carbon- and nitrogen-rich abundances in agreement with current spectroscopic observations. The dependence of this phenomenon on selected model input parameters, such as initial metallicity and treatment of convection is examined in detail.

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arXiv2002

Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms

We present homogeneous age determinations for a large sample of 55 Galactic globular clusters, which constitute about 30% of the total Galactic population. A study of their age distribution reveals that all clusters from the most metal poor ones up to intermediate metallicities are coeval, whereas at higher [Fe/H] an age spread exists, together with an age-metallicity relationship. At the same time, all clusters within a certain galactocentric distance appear coeval, whereas an age spread is present further away from the Galactic centre, without any correlation with distance. The precise value of [Fe/H] and galactocentric distance for the onset of the age spread and the slope of the age-metallicity relationship are strongly affected by the as yet uncertain [Fe/H] scale. We discuss how differences in the adopted [Fe/H] scale and cluster sample size may explain discrepant results about the clusters age distribution reached by different authors. Taking advantage of the large number of objects included in our sample, we also tested the possibility that age is the global second parameter which determines the Horizontal Branch morphology, and found indications that age could explain the global behaviour of the second parameter effect.

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arXiv2002

Measuring ^{12}C(&alpha,&gamma)^{16}O from White Dwarf Asteroseismology

During helium burning in the core of a red giant, the relative rates of the 3&alpha and ^{12}C(&alpha,&gamma)^{16}O reactions largely determine the final ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in the 3&alpha reaction at stellar energies due to the extrapolation from high-energy laboratory measurements is relatively small, but this is not the case for the ^{12}C(&alpha,&gamma)^{16}O reaction. Recent advances in the analysis of asteroseismological data on pulsating white dwarf stars now make it possible to obtain precise measurements of the central ratio of carbon to oxygen, providing a more direct way to measure the ^{12}C(&alpha,&gamma)^{16}O reaction rate at stellar energies. We assess the systematic uncertainties of this approach and quantify small shifts in the measured central oxygen abundance originating from the observations and from model settings that are kept fixed during the optimization. Using new calculations of white dwarf internal chemical profiles, we find a rate for the ^{12}C(&alpha,&gamma)^{16}O reaction that is significantly higher than most published values. The accuracy of this method may improve as we modify some of the details of our description of white dwarf interiors that were not accessible through previous model-fitting methods.

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arXiv2002

Red Giant Branch stars: the theoretical framework

Theoretical predictions of Red Giant Branch stars' effective temperatures, colors, luminosities and surface chemical abundances are a necessary tool for the astrophysical interpretation of the visible--near infrared integrated light from unresolved stellar populations, the Color-Magnitude-Diagrams of resolved stellar clusters and galaxies, and spectroscopic determinations of red giant chemical abundances. On the other hand, the comparison with empirical constraints provides a stringent test for the accuracy of present generations of red giant models. We review the current status of red giant stars' modelling, discussing in detail the still existing uncertainties affecting the model input physics (e.g., electron conduction opacity, treatment of the superadiabatic convection), and the adequacy of the physical assumptions built into the model computations. We compare theory with several observational features of the Red Giant Branch in galactic globular clusters, such as the luminosity function bump, the luminosity of the Red Giant Branch tip and the envelope chemical abundance patterns, to show the level of agreement between current stellar models and empirical data concerning the stellar luminosities, star counts, and surface chemical abundances.

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arXiv2001

Asymptotic Giant Branch predictions: theoretical uncertainties

In this paper we investigate the level of agreement between observations and "new" Asymptotic Giant Branch (AGB) models, as produced by updating the physical inputs adopted in previous stellar computations. One finds that the new physics increases the predicted luminosity of Horizontal Branch (HB) and AGB stellar structures by a similar amount, keeping unchanged the predictions about the difference in luminosity between these two evolutionary phases. The best fit of selected globular clusters appears rather satisfactory, disclosing the relevance of the assumption on the mass of the Red Giant Branch (RGB) progenitor in assessing the distance modulus of moderately metal rich clusters. The still existing uncertainties related either to the input physics or to the efficiency of some macroscopic mechanisms, like convection or microscopic diffusion, are critically discussed, ruling out the occurrence of the so called "breathing pulses" during the central He exhaustion, in agreement with earlier suggestions.

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arXiv2001

An empirical method to estimate the LMC distance using B-stars in eclipsing binary systems

We present a new method to determine the distance to B-stars in eclipsing binary systems. The method is completely empirical, and it is based on the existence of a very tight linear relationship between the V-band ``zero magnitude angular diameter'' and the Stroemgren colour index c1 for B-stars; we have empirically calibrated this relationship using local single B-stars with accurate angular diameters, and B-stars in eclipsing binaries with precise radii and parallax determinations. By studying the differential behaviour of this relationship as predicted by theoretical stellar evolution models, we find that it is independent of the stellar metallicities for a range of [Fe/H] values between the solar one and that of young stars in the Magellanic Clouds. The method, which also provides the value of the reddening to the system, is discussed in detail, together with a thorough estimate of the associated errors. We conclude that accurate Stroemgren photometry obtainable with 1.5 m-class telescopes of the LMC eclipsing binaries HV 2274 and HV 982 will allow to obtain an empirical LMC distance with an accuracy of the order of 0.13 mag.

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arXiv2001

The shape of the Red Giant Branch Bump as a diagnostic of partial mixing processes in low-mass stars

We suggest to use the shape of the Red Giant Branch (RGB) Bump in metal-rich globular clusters as a diagnostic of partial mixing processes between the base of the convective envelope and the H-burning shell. The Bump located along the differential luminosity function of cluster RGB stars is a key observable to constrain the H-profile inside these structures. In fact, standard evolutionary models that account for complete mixing in the convective unstable layers and radiative equilibrium in the innermost regions do predict that the first dredge-up lefts over a very sharp H-discontinuity at the bottom of the convective region. Interestingly enough we found that both atomic diffusion and a moderate convective overshooting at the base of the convective region marginally affects the shape of the RGB Bump in the differential Luminosity Function (LF). As a consequence, we performed several numerical experiments to estimate whether plausible assumptions concerning the smoothing of the H-discontinuity, due to the possible occurrence of extra-mixing below the convective boundary, affects the shape of the RGB Bump. We found that the difference between the shape of RGB Bump predicted by standard and by smoothed models can be detected if the H-discontinuity is smoothed over an envelope region whose thickness is equal or larger than 0.5 pressure scale heights. Finally, we briefly discuss the comparison between theoretical predictions and empirical data in metal-rich, reddening free Galactic Globular Clusters (GGCs) to constrain the sharpness of the H-profile inside RGB stars.

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arXiv2001

Constraints on 12C(alpha,gamma)16O from White Dwarf Seismology

For many years, astronomers have promised that the study of pulsating white dwarfs would ultimately lead to useful information about the physics of matter under extreme conditions of temperature and pressure. We can now make good on that promise. Using observational data from the Whole Earth Telescope and a new analysis method employing a genetic algorithm, we empirically determine the central oxygen abundance of the helium-atmosphere variable white dwarf GD 358. We use this value, combined with detailed evolutionary calculations of the internal chemical profiles to place constraints on the 12C(alpha,gamma)16O nuclear reaction cross-section.

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arXiv2001

On the helium flash in low-mass Population III Red Giant stars

We investigate the evolution of initially metal-free, low-mass Red Giant stars through the He core flash at the tip of the Red Giant Branch. The low entropy barrier between the helium- and hydrogen-rich layers enables a penetration of the helium flash driven convective zone into the inner tail of the extinguishing H-burning shell. As a consequence, protons are mixed into high-temperature regions triggering a H-burning runaway. The subsequent dredge-up of matter processed by He and H burning enriches the stellar surface with large amounts of helium, carbon and nitrogen. Extending previous results by Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that the H-burning runaway is an intrinsic property of extremely metal-poor low-mass stars, we found that its occurrence depends on additional parameters like the initial composition and the treatment of various physical processes. We perform some comparisons between predicted surface chemical abundances and observational measurements for extremely metal-deficient stars. As in previous investigations, our results disclose that although the described scenario provides a good qualitative agreement with observations, considerable discrepancies still remain. They may be due to a more complex evolutionary path of `real' stars, and/or some shortcomings in current evolutionary models. In addition, we analyze the evolutionary properties after the He core flash, during both the central and shell He-burning phases, allowing us to deduce some interesting differences between models whose Red Giant Branch progenitor has experienced the H-flash and canonical models. In particular, the Asymptotic Giant Branch evolution of extremely metal-deficient stars and the occurrence of thermal pulses are strongly affected by the previous RGB evolution.

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arXiv2001

On the White Dwarf distances to Galactic Globular Clusters

We analyze in detail various possible sources of systematic errors on the distances of globular clusters derived by fitting a local template DA white dwarf sequence to the cluster counterpart (the so-called WD-fitting technique). We find that the unknown thickness of the hydrogen layer of white dwarfs in clusters plays a non negligible role. For reasonable assumptions - supported by the few sparse available observational constraints - about the unknown mass and thickness of the hydrogen layer for the cluster white dwarfs, a realistic estimate of the systematic error on the distance is within +-0.10 mag. However, particular combinations of white dwarf masses and envelope thicknesses - which at present cannot be excluded a priori - could produce larger errors. Contamination of the cluster DA sequence by non-DA white dwarfs introduces a very small systematic error of about -0.03 mag in the Mv/(V-I) plane, but in the Mv/(B-V) plane the systematic error amounts to ~ +0.20 mag. Contamination by white dwarfs with helium cores should not influence appreciably the WD-fitting distances. Finally, we obtain a derivative D((m-M)v)/D(E(B-V))~ -5.5 for the WD-fitting distances, which is very similar to the dependence found when using the Main Sequence fitting technique.

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arXiv2000

The LMC eclipsing binary HV 2274 revisited

We reanalyse the UV/optical spectrum and optical broad-band data of the eclipsing binary HV 2274 in the LMC, and derive its distance following the method given by Guinan et al. (1998a,b) of fitting theoretical spectra to the stars' UV/optical spectrum plus optical photometry. We describe the method in detail, pointing out the various assumptions that have to be made; moreover, we discuss the systematic effects of using different sets of model atmospheres and different sets of optical photometric data. It turns out that different selections of the photometric data, the set of model atmospheres and the constraints on the value of the ratio of selective to total extinction in the $V$-band, result in a 25% range in distances (although some of these models have a large $χ^2$). For our best choice of these quantities the derived value for the reddening to HV 2274 is E(B-V) = 0.103 $\pm$ 0.007, and the de-reddened distance modulus is DM = 18.46 $\pm$ 0.06; the DM to the center of the LMC is found to be 18.42 $\pm$ 0.07. This is significantly larger than the DM of 18.30 $\pm$ 0.07 derived by Guinan et al. (1998a).

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arXiv2000

The ages of very cool hydrogen-rich white dwarfs

The evolution of white dwarfs is essentially a cooling process that depends primarily on the energy stored in their degenerate cores and on the transparency of their envelopes. In this paper we compute accurate cooling sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres for the full range of masses of interest. For this purpose we use the most accurate available physical inputs for both the equation of state and opacities of the envelope and for the thermodynamic quantities of the degenerate core. We also investigate the role of the latent heat in the computed cooling sequences. We present separately cooling sequences in which the effects of phase separation of the carbon-oxygen binary mixture upon crystallization have been neglected, and the delay introduced in the cooling times when this mechanism is properly taken into account, in order to compare our results with other published cooling sequences which do not include a treatment of this phenomenon. We find that the cooling ages of very cool white dwarfs with pure hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr at log(L/Lo)=-4.5 for an otherwise typical 0.6 Mo white dwarf, when phase separation is neglected. If phase separation of the binary mixture is included then the cooling ages are further increased by roughly 10%. Cooling tracks and cooling isochrones in several color-magnitude diagrams are presented as well.

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arXiv2000

Atomic diffusion in metal poor stars. The influence on the Main Sequence fitting distance scale, subdwarfs ages and the value of Delta Y/DeltaZ

The effect of atomic diffusion on the Main Sequence (MS) of metal-poor low mass stars is investigated. Since diffusion alters the stellar surface chemical abundances with respect to their initial values, one must ensure - by calibrating the initial chemical composition of the theoretical models - that the surface abundances of the models match the observed ones of the stellar population under scrutiny. Since the observed surface abundances of subdwarfs are different from the initial ones due to the effect of diffusion, while the globular clusters stellar abundances are measured in Red Giants, which have practically recovered their initial abundances after the dredge-up, the isochrones to be employed for studying globular clusters and Halo subdwarfs with the same observational value of [Fe/H] are different and do not coincide. We find, however,that the current MS-fitting distances derived from HIPPARCOS subdwarfs using colour corrections from standard isochrones are basically unaltered when diffusion is taken properly into account; on the other hand, the absolute ages, the age dispersion, the age-metallicity relation for Halo subdwarfs, as well as the value of the helium enrichment ratio Delta Y/Delta Z obtained from the width of the empirical Halo subdwarfs MS, are all significantly modified when the properly calibrated isochrones with diffusion are used.

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arXiv1999

Evolution of low-mass metal-free stars including effects of diffusion and external pollution

We investigate the evolution of low-mass metal-free Population III stars. Emphasis is laid upon the question of internal and external sources for CNO-elements, which - if present in sufficient amounts in the hydrogen-burning regions - lead to a strong modification of the stars' evolutionary behavior. For the production of carbon due to nuclear processes inside the stars, we use an extended nuclear network, demonstrating that hot pp-chains do not suffice to produce enough carbon or are less effective than the triple3-alpha-process. As an external source of CNO-elements we test the efficiency of pollution by a nearby massive star combined with particle diffusion. For all cases investigated, the additional metals fail to reach nuclear burning regions before deep convection on the Red Giant Branch obliterates the previous evolution. The surface abundance history of the polluted Pop III stars is presented. The possibilities to discriminate between a Pop II and a polluted Pop III field star are also discussed.

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arXiv1999

The absolute magnitudes of RR Lyrae stars from Hipparcos parallaxes

Using the method of ``reduced parallaxes'' for the Halo RR Lyrae stars in the Hipparcos catalogue we derive a zero point of 0.77 $\pm$ 0.26 mag for an assumed slope of 0.18 in the $M_{\rm V}$-[Fe/H] relation. This is 0.28 magnitude brighter than the value Fernley et al. (1998a) derived by employing the method of statistical parallax for the {\it identical} sample and using the same slope. We point out that a similar difference exists between the ``reduced parallaxes'' method and the statistical parallax method for the Cepheids in the Hipparcos catalogue. We also determine the zero point for the $M_{\rm K}$-$\log P_{0}$ relation, and obtain a value of -1.16 $\pm$ 0.27 mag (for a slope of -2.33). The distance moduli to the Hipparcos RR Lyrae stars derived from the two relations agree well. The derived distance scale is in good agreement with the results from the Main Sequence fitting distances of Galactic globular clusters and with the results of theoretical Horizontal Branch models, and implies a distance modulus to the LMC of 18.61 $\pm$ 0.28 mag.

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arXiv1999

Comparison between predicted and empirical DV(Bump -HB) in Galactic Globular Clusters

We present observational estimates of DV(Bump-HB) in a sample of 28 Galactic Globular Clusters (GGCs) observed by HST. The photometric accuracy and the sizable number of stars measured in each cluster allowed us to single out the RGB Bump both in metal-poor and in metal-rich GGCs. Empirical values are compared with homogeneous theoretical predictions which account for both H and He burning phases over a wide range of metal abundances (0.0001 < Z < 0.02). We found that, within current observational uncertainties on both iron and $α$-element abundances, theory and observations are in very good agreement, provided that the metallicity scale by Carretta & Gratton (1997) as extended by Cohen et al. (1999) is adopted. Interesting enough, we also found that both theoretical and observed values show a change in the slope of the DV(Bump-HB)-[M/H] relation toward higher metal contents.

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