1,721,307 research outputs found
The effect of diffusion on the red giant luminosity function 'bump'
No full textThis paper investigates the effect of microscopic diffusion of helium and heavy elements on the location of the red giant branch (RGB) 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 (ZAHB) are computed. The observational luminosity difference between the RGB bump and the ZAHB (Delta V-hb(bump)), as collected for a sample of Galactic globular clusters, is 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 for Delta V-hb(bump) 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 become available
Confirmation of a metallicity spread amongst first population stars in globular clusters
Full text linkStars in massive star clusters exhibit intrinsic variations in some light elements (the multiple populations phenomenon) that are difficult to explain in a fully coherent formation scenario. In recent years, high quality Hubble Space Telescope (HST) photometry has led to the characterisation of the global properties of these multiple populations in an unparalleled level of detail. In particular, the colour-(pseudo)colour diagrams known as ‘chromosome maps’ have been proven to be very efficient at separating cluster stars with a field-like metal abundance distribution (first population) from an object with distinctive light-element abundance anti-correlations (second population). The unexpected wide colour ranges covered by the first population group – traditionally considered to have a uniform chemical composition – in the chromosome maps of the majority of the investigated Galactic globular clusters have recently been attributed to intrinsic metallicity variations up to ∼0.30 dex from the study of subgiant branch stars in two metal-rich Galactic globular clusters by employing appropriate HST filter combinations. On the other hand, high-resolution spectroscopy of small samples of first populations stars in the globular clusters NGC 3201 and NGC 2808 – both displaying extended sequences of first population stars in their chromosome maps – have provided conflicting results thus far, with a spread of metal abundance detected in NGC 3201 but not in NGC 2808. We present here a new method that employs HST near-UV and optical photometry of red giant branch stars to confirm these recent results independently. Our approach was firstly validated using observational data for M 2, a globular cluster hosting a small group of first population stars with an enhanced (by ≃0.5 dex) metallicity with respect to the main component. We then applied our method to three clusters that cover a much larger metallicity range and that have well populated, extended first population sequences in their chromosome maps, namely M 92, NGC 2808, and NGC 6362. We confirm that metallicity spreads are present among first population stars in these clusters, thus solidifying the case for the existence of unexpected variations up to a factor of two of metal abundances in most globular clusters. We also confirm the complex behaviour of the mean metallicity (and metallicity range) differences between first and second population stars
On the primordial helium content: Cosmic microwave background and stellar constraints
No full textWe present the results of a joint investigation aimed at constraining the primordial He content (Y-P) on the basis of both the cosmic microwave background (CMB) anisotropy and two stellar observables, namely, the tip of the red giant branch (TRGB) and the luminosity of the zero-age horizontal branch (ZAHB). Current baryon density estimates based on CMB measurements cover a wide range of values 0.009less than or similar toOmega(b)h(2)less than or similar to0.045, which according to big bang nucleosynthesis models would imply 0.24less than or similar toY(P)less than or similar to0.26. We constructed several sets of evolutionary tracks and horizontal-branch (HB) models by adopting Y-P = 0.26 and several metal contents. The comparison between theory and observations suggests that ZAHB magnitudes based on He-enhanced models are 1.5 sigma brighter than the empirical ones. The same outcome applies for TRGB bolometric magnitudes. This finding somewhat supports a Y-P abundance close to the canonical 0.23 +/- 0.24 value. More quantitative constraints on this parameter are hampered by the fact that the CMB pattern shows a sizable dependence on both Y-P and the baryon density only at small angular scales, i.e., at high l in the power spectrum (1 greater than or similar to 100). However, this region of the power spectrum could be still affected by deceptive systematic uncertainties. Finally, we suggest using the UV upturn to estimate the He content on Gpc scales. In fact, we find that a strong increase in Y-P causes a decrease in the UV emission in metal-poor, hot HB structures
An updated theoretical scenario for globular cluster stars
No full textIn the first part of this paper we revisit the history of theoretical predictions for HE luminosities in old Population II stellar clusters, starting from the results of "old" evolutionary computations to introduce in various steps all the available "new" physics. We discuss the influence of physical ingredients on selected evolutionary parameters, finally presenting models which incorporate all the most recent updating of the relevant physics. The evolutionary behavior of such models is extensively investigated for selected choices about the cluster metallicity, discussing theoretical predictions concerning both cluster isochrones and the calibration of the parameter R in terms of the original amount of He in stellar matter. One finds that the "new" physics has a relevant influence on both these parameters, moving cluster ages into a much better agreement with current cosmological evaluations. This scenario is implemented by a further set of stellar models where element diffusion is taken into account. The comparison between theoretical scenarios with or without diffusion is presented and discussed. A discussion of current observational constraints in the light of the updated theory closes the paper
Asymptotic giant branch predictions: Theoretical uncertainties
No full textIn 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
Comparison Between Predicted and Empirical DeltaV(bump-HB) in Galactic Globular Clusters
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