(abridged) The calculation of the thermal stratification in the superadiabatic layers of stellar models with convective envelopes is a long standing problem of stellar astrophysics, and has a major impact on predicted observational properties like ra
dius and effective temperature. The Mixing Length Theory, almost universally used to model the superadiabatic convective layers, contains effectively one free parameter to be calibrated --alpha(ml)-- whose value controls the resulting effective temperature. Here we present the first self-consistent stellar evolution models calculated by employing the atmospheric temperature stratification, Rosseland opacities, and calibrated variable alpha(ml) (dependent on effective temperature and surface gravity) from a large suite of three-dimensional radiation hydrodynamics simulations of stellar convective envelopes and atmospheres for solar stellar composition (Trampedach et al. 2013). From our calculations (with the same composition of the radiation hydrodynamics simulations), we find that the effective temperatures of models with the hydro-calibrated variable alpha(ml) display only minor differences, by at most ~30-50 K, compared to models calculated at constant solar alpha(ml). The depth of the convective regions is essentially the same in both cases. We have also analyzed the role played by the hydro-calibrated T(tau) relationships in determining the evolution of the model effective temperatures, when compared to alternative T(tau) relationships often used in stellar model computations. The choice of the T(tau) can have a larger impact than the use of a variable alpha(ml) compared to a constant solar value. We found that the solar semi-empirical T(tau) by Vernazza et al. (1981) provides stellar model effective temperatures that agree quite well with the results with the hydro-calibrated relationships.
[Abridged] In recent years the view of Galactic globular clusters as simple stellar populations has changed dramatically, as it is now thought that basically all GCs host multiple stellar populations, each with its own chemical abundance pattern and
colour-magnitude diagram sequence. Recent spectroscopic observations of asymptotic giant branch stars in the GC NGC6752 have disclosed a low [Na/Fe] abundance for the whole sample, suggesting that they are all first-generation stars, and that all second-generation stars fail to reach the AGB in this cluster. A scenario proposed to explain these observations invokes strong mass loss in second-generation horizontal branch stars possibly induced by the metal enhancement associated to radiative levitation. This enhanced mass loss would prevent second generation stars from reaching the AGB phase, thus explaining at the same time the low value of the ratio between HB and AGB stars (the R_2 parameter) observed in NGC6752. We have critically discussed this scenario, finding that the required mass-loss rates are of the order of 10^{-9} Mo/yr, significantly higher than current theoretical and empirical constraints. By making use of synthetic HB simulations, we demonstrate that our modelling predicts correctly the R_2 parameter for NGC6752, without the need to invoke very efficient mass loss during the core He-burning stage. Our simulations for NGC6752 HB predict however the presence of a significant fraction - at the level of about 50% - second generation stars along the cluster AGB. We conclude that there is no simple explanation for the lack of second generation stars in the spectroscopically surveyed sample, although the interplay between mass loss (with low rates) and radiative levitation may play a role in explaining this puzzle.
[Abridged] A new scenario --early disc accretion-- has been proposed very recently to explain the origin of the multiple population phenomenon in Galactic globular clusters. It envisages the possibility that a fraction of low- and very low-mass clust
er stars may accrete the ejecta of interacting massive binary (and possibly also fast rotating massive) stars during the fully convective, pre-main sequence stage, to reproduce the CN and ONa anticorrelations observed among stars in individual clusters. This scenario is assumed to be able to explain the presence (and properties) of the multiple populations in the majority of globular clusters in the Milky Way. Here we have considered the well studied cluster NGC 2808, which displays a triple main sequence with well defined and separate He abundances. Knowledge of these abundances allowed us to put strong constraints on the He mass fraction and amount of matter to be accreted by low-mass pre-main sequence stars. We find that the minimum He mass fraction in the accreted gas has to be $sim0.44$ to produce the observed sequences and that at fixed initial mass of the accreting star, different efficiencies for the accretion are required to produce stars placed onto the multiple main sequences. This may be explained by differences in the orbital properties of the progenitors and/or different spatial distribution of intracluster gas with varying He abundances. Both O-Na and C-N anticorrelations appear naturally along the main sequences, once considering the predicted relationship between He and CNONa abundances in the ejecta of the polluters. As a consequence of the accretion, we predict no discontinuity between the abundance ranges covered by intermediate and blue main sequence stars, but we find a sizeable (several 0.1 dex) discontinuity of the N and Na abundances between objects on the intermediate and red main sequences.
During these last decades, our knowledge of evolutionary and structural properties of stars of different mass and chemical composition is significantly improved. This result has been achieved as a consequence of our improved capability in understandi
ng and describing the physical behavior of matter in the different thermal regimes characteristic of the various stellar mass ranges and evolutionary stages. This notwithstanding, current generation of stellar models is still affected by significant and, usually, not negligible uncertainties. These uncertainties are related to our poor knowledge of some physical proceses occurring in the real stars such as, for instance, some thermodynamical processes, nuclear reaction rates, as well as the efficiency of mixing processes. These drawbacks of stellar models have to be properly taken into account when comparing theory with observations in order to derive relevant information about the properties of both resolved and unresolved stellar populations. On the other hand, observations of both field and cluster stars can provide fundamental benchmarks for constraining the reliability and accuracy of the theoretical framework. In the following we review some important evolutionary and structural properties of very-low and low-mass stars, as well as the most important uncertainties affecting the stellar models for such stars. We show what are the main sources of uncertainty along the main evolutionary stages, and discuss the present level of agreement between theory and observations.
We have computed low-mass stellar models and synthetic spectra for an initial chemical composition that includes the full C-N, O-Na, and Mg-Al abundance anticorrelations observed in second generation stars belonging to a number of massive Galactic gl
obular clusters. This investigation extends a previous study that has addressed the effect of only the C-N and O-Na anticorrelations, seen in all globulars observed to date. We find that the impact of Mg-Al abundance variations at fixed [Fe/H] and Helium abundance is negligible on stellar models and isochrones (from the main sequence to the tip of the red giant branch) and bolometric corrections, when compared to the effect of C-N and O-Na variations. We identify a spectral feature at 490-520 nm, for low-mass main sequence stars, caused by MgH molecular bands. This feature has a vanishingly small effect on bolometric corrections for Johnson and Stroemgren filters that cover that spectral range. However, specific narrow-band filters able to target this wavelength region can be powerful tools to investigate the Mg-poor unevolved stars and highlight possible splittings of the MS due to variations of Mg abundances.
We use the pulsational properties of the RR Lyrae variables in the globular cluster NGC 1851 to obtain detailed constraints of the various sub-stellar populations present along its horizontal branch. On the basis of detailed synthetic horizontal bran
ch modeling, we find that minor helium variations (Y~0.248-0.280) are able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae stars. Comparison of number ratios amongst the blue and red horizontal branch components and the two observed subgiant branches also suggest that the RR Lyrae variables originated from the progeny of the bright subgiant branch. The RR Lyrae variables with a slightly enhanced helium (Y~0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables. This correlation does suggest that the pulsational properties of RR Lyrae stars can be very useful for tracing the various sub-populations and can provide suitable constraints on the multiple population phenomenon. It appears of great interest to explore whether this conclusion can be generalized to other globular clusters hosting multiple populations.
We briefly review the main physical and structural properties of Very Low-Mass stars. The most important improvements in the physical inputs required for the stellar models computations are also discussed. We show some comparisons with observational
measurements concerning both the Color-Magnitude diagrams, mass-luminosity relations and mass-radius one, in order to disclose the level of agreement between the present theoretical framework and observations.
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 characteris
tic 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.
This paper is the 4th in a series describing the latest additions to the BaSTI stellar evolution database, which consists of a large set of homogeneous models and tools for population synthesis studies. Here we present a new set of low and high resol
ution synthetic spectra based on the BaSTI stellar models, covering a large range of simple stellar populations (SSPs) for both scaled solar and alpha-enhanced metal mixtures. This enables a completely consistent study of the photometric and spectroscopic properties of both resolved and unresolved stellar populations, and allows us to make detailed tests on their integrated properties. Our low resolution spectra are suitable for deriving broadband magnitudes and colors in any photometric system. These spectra cover the full wavelength range (9-160000nm) and include all evolutionary stages up to the end of AGB evolution. Our high resolution spectra are suitable for studying the behaviour of line indices and we have tested them against a large sample of Galactic globular clusters. We find that the range of ages, iron abundances [Fe/H], and degree of alpha-enhancement predicted by the models matches observed values very well. We have also tested the global consistency of the BaSTI models by making detailed comparisons between ages and metallicities derived from isochrone fitting to observed CMDs, and from line index strengths, for the Galactic globular cluster 47Tuc and the open cluster M67. For 47Tuc we find reasonable agreement between the 2 methods, within the estimated errors. From the comparison with M67 we find non-negligible effects on derived line indices caused by statistical fluctuations, which are a result of the specific method used to populate an isochrone and assign appropriate spectra to individual stars. (abridged)
