The impact of enhanced He and CNONa abundances on globular cluster relative age-dating methods

The impact that unrecognised differences in the chemical patterns of Galactic globular clusters have on their relative age determinations is studied. The two most widely used relative age-dating methods, horizontal and vertical, together with the more recent relative MS-fitting method, were carefully analyzed on a purely theoretical basis. The BaSTI library was adopted to perform the present analysis. We find that relative ages derived using the horizontal and vertical methods are largely dependent on the initial He content and heavy element distribution. Unrecognized cluster-to-cluster chemical abundance differences can lead to an error in the derived relative ages as large as ~0.5 (or ~6 Gyr if an age of 12.8 Gyr is adopted for normalization), and even larger for some extreme cases. It is shown that the relative MS-fitting method is by far the age-dating technique for which undetected cluster-to-cluster differences in the He abundance have less impact. Present results are used in order to pose constraints on the maximum possible spread in the He and CNONa elements abundances on the basis of the estimates - taken from the literature - of the Galactic globular clusters relative age dispersion obtained with the various relative age-dating techniques. Finally, it is shown that the age-metallicity relation found for young Galactic globular clusters by the GC Treasury program is a real age sequence and cannot be produced by variations in the He and/or heavy element distribution.Comment: 26 pages, 8 figures, accepted for publication in ApJ

Stellar models: firm evidence, open questions and future developments

During this last decade our knowledge of the evolutionary properties of stars has significantly improved. This result has been achieved thanks to our improved understanding of the physical behavior of stellar matter in the thermal regimes characteristic of the different stellar mass ranges and/or evolutionary stages. This notwithstanding, the current generation of stellar models is still affected by several, not negligible, uncertainties related to our poor knowledge of some thermodynamical processes and nuclear reaction rates, as well as the efficiency of mixing processes. These drawbacks have to be properly taken into account when comparing theory with observations, to derive evolutionary properties of both resolved and unresolved stellar populations. In this paper we review the major sources of uncertainty along the main evolutionary stages, and emphasize their impact on population synthesis techniques.Comment: 10 pages, 3 figures, Keynote review talk at the IAU Symp. 262 "Stellar Populations - Planning the Next Decade" of the XXVIIth IAU General Assembly held in Rio de Janeiro (Brazil), Proceeding eds. G. Bruzual & S. Charlo

Chemical element transport in stellar evolution models

Stellar evolution computations provide the foundation of several methods applied to study the evolutionary properties of stars and stellar populations, both Galactic and extragalactic. The accuracy of the results obtained with these techniques is linked to the accuracy of the stellar models, and in this context the correct treatment of the transport of chemical elements is crucial. Unfortunately, in many respects calculations of the evolution of the chemical abundance profiles in stars are still affected by sometime sizable uncertainties. Here, we review the various mechanisms of element transport included in the current generation of stellar evolution calculations, how they are implemented, the free parameters and uncertainties involved, the impact on the models, and the observational constraints.Comment: 72 pages, 33 figures, invited review paper to be published in Royal Society Open Science Journa

A critical investigation on the discrepancy between the observational and theoretical Red Giant luminosity function "Bump"

New theoretical evaluations of the RGB luminosity function 'bump' and the ZAHB luminosity covering the range of metallicities typical of galactic globular cluster are presented. The variation of the theoretical RGB bump and ZAHB levels due to the metallicity, original helium content, mixing length value, age, mass loss, bolometric corrections, opacities and equation of state adopted in the evolutionary models is also discussed. These new prescriptions have been taken into account for casting light on a longstanding astrophysical problem connected with the Red Giant Branch evolutionary phase, namely the discrepancy between the observational and the theoretical luminosity of RGB bump. A sample of globular clusters with accurate evaluations of the bump luminosity and spectroscopical metallicity determinations has been selected. The Zero Age Horizontal Branch luminosity at the RR-Lyrae instability strip has been evaluated as accurately as possible, and the observational luminosity difference between the RGB bump and the ZAHB has been compared with the theoretical values. It is shown that there is no significant disagreement between observations and canonical stellar models. The possible applications of this result are also briefly discussed.Comment: 10 pages, 5 figures, Tex file, mnrass.sty style included. To appear in MNRA

Lithium and oxygen in globular cluster dwarfs and the early disc accretion scenario

A new scenario --early disc accretion-- has been recently proposed to explain the discovery of multiple stellar populations in Galactic globular clusters. According to this model, the existence of well defined (anti)-correlations amongst light element abundances (i.e. C, N, O, Na) in the photospheres of stars belonging to the same cluster (and the associated helium enrichment), is caused by accretion of the ejecta of short lived interacting massive binary systems (and single fast rotating massive stars) on fully convective pre-main sequence low- and very low-mass stars, during the early stages of the cluster evolution. We investigated the constraints provided by considering simultaneously the observed spread of lithium and oxygen (and when possible also sodium) abundances for samples of turn-off stars in NGC6752, NGC6121 (M4), and NGC104 (47Tuc), and the helium abundance of their multiple main sequences. These observations provide a very powerful test for the accretion scenario, because the observed O, Li and He abundance distributions at the turn off can be used to constrain the composition (and mass) of the accreted matter, and the timescales of the polluting stars. In case of NGC6752 we could not find a physically consistent solution. In case of M4, spectroscopic errors are too large compared to the intrinsic spread, to constrain the properties of the accreted matter. As for 47Tuc, we could find a physically consistent solution for the abundances of He and O (and Na) in the accreted gas, and predict the abundances of these elements in the accreted matter only if pollution happens with timescales of ~1 Myr, hence polluters are objects with masses of the order of several tens of solar masses (abridged).Comment: 8 pages, 8 figures, accepted for publication in A&