Mass‐loss of red giant branch (RGB) stars is still poorly determined, despite its crucial role in the chemical enrichment of galaxies. Thanks to the recent detection of solar‐like oscillations in G–K giants in open clusters with Kepler, we can now directly determine stellar masses for a statistically significant sample of stars in the old open clusters NGC 6791 and 6819. The aim of this work is to constrain the integrated RGB mass‐loss by comparing the average mass of stars in the red clump (RC) with that of stars in the low‐luminosity portion of the RGB [i.e. stars with L≲L(RC)]. Stellar masses were determined by combining the available seismic parameters νmax and Δν with additional photometric constraints and with independent distance estimates. We measured the masses of 40 stars on the RGB and 19 in the RC of the old metal‐rich cluster NGC 6791. We find that the difference between the average mass of RGB and RC stars is small, but significant [ (random) ±0.04 (systematic) M⊙]. Interestingly, such a small does not support scenarios of an extreme mass‐loss for this metal‐rich cluster. If we describe the mass‐loss rate with Reimers prescription, a first comparison with isochrones suggests that the observed is compatible with a mass‐loss efficiency parameter in the range 0.1 ≲η≲ 0.3. Less stringent constraints on the RGB mass‐loss rate are set by the analysis of the ∼2 Gyr old NGC 6819, largely due to the lower mass‐loss expected for this cluster, and to the lack of an independent and accurate distance determination. In the near future, additional constraints from frequencies of individual pulsation modes and spectroscopic effective temperatures will allow further stringent tests of the Δν and νmax scaling relations, which provide a novel, and potentially very accurate, means of determining stellar radii and masses.

The majority of globular clusters show chemical inhomogeneities in the composition of their stars, apparently due to a second stellar generation in which the forming gas is enriched by hot‐CNO cycled material processed in stars belonging to a first stellar generation. Clearly this evidence prompts questions on the modalities of formation of globular clusters. An important preliminary input to any model for the formation of multiple generations is to determine which is today the relative number fraction of ‘normal’ and anomalous stars in each cluster. As it is very difficult to gather very large spectroscopic samples of globular cluster stars to achieve this result with good statistical significance, we propose to use the horizontal branch (HB). We assume that, whichever the progenitors of the second generation, the anomalies also include enhanced helium abundance. In fact, helium variations have been recently recognized to be able to explain several puzzling peculiarities (gaps, RR Lyr periods and period distribution, ratio of blue to red stars, blue tails) in HBs. We summarize previous results and extend the analysis in order to infer the percentage in number of the first and second generation in as many clusters as possible. We show that, with few exceptions, approximately 50 per cent or more of the stars belong to the second generation. In other cases, in which at first sight one would think of a simple stellar population, we give arguments and suggest that the stars might all belong to the second generation. We provide in the appendix a detailed discussion and new fits of the optical and ultraviolet data of NGC 2808, the classic example of a multiple helium populations cluster, consistently including a reproduction of the main‐sequence splittings and an examination of the problem of ‘blue hook’ stars. We also show a detailed fit of the totally blue HB of M13, one among the clusters that are possibly fully made up by second generation stars. We conclude that the formation of the second generation is a crucial event in the life of globular clusters. The problem of the initial mass function required to achieve the observed high fraction of second generation stars can be solved only if the initial cluster was much more massive than the present one and most of the first generation low‐mass stars have been preferentially lost. As shown by D'Ercole et al., by modelling the formation and dynamical evolution of the second generation, the mass loss due to the explosions of the Type II supernovae of the first generation may be the process responsible for triggering the expansion of the cluster, the stripping of its outer layers and the loss of most of the first generation low‐mass stars.

Abstract. In the course of a systematic exploration of the uncertainties associated with the input micro-and macro-physics in the modeling of the evolution of intermediate mass stars during their Asymptotic Giant Branch (AGB) phase, we focus on the role of the nuclear reactions rates and mass loss. We consider masses 3 ≤ M/M ≤ 6.5 for a metallicity typical for globular clusters, Z = 0.001, and compare the results obtained by computing the full nucleosynthesis with hot bottom burning (HBB), for a network of 30 elements, using either the NACRE or the Cameron & Fowler (CF88) cross-sections. The results differ in particular with respect to the 23 Na nucleosynthesis (which is more efficient in the NACRE case) and the magnesium isotopes ratios. For both choices, however, the CNO nucleosynthesis shows that the C+N+O is constant within a factor of two, in our models employing a very efficient convection treatment. Different mass loss rates alter the physical conditions for HBB and the length of the AGB phase, indirectly changing the chemical yields. These computations show that the predictive power of our AGB models is undermined by these uncertainties. In particular, it appears at the moment very difficult to strongly accept or dismiss that these sources play a key-role in the pollution of Globular Clusters (GCs), and that they have been the main stellar site responsible for the chemical anomalies that are observed at the surface of giant and turn-off stars of GCs, in the self-enrichment scenarios.

We explain the multiple populations recently found in the 'prototype' Globular Cluster (GC) NGC 2808 in the framework of the asymptotic giant branch (AGB) scenario. The chemistry of the five -or more-populations is approximately consistent with a sequence of star formation events, starting after the supernovae type II epoch, lasting approximately until the time when the third dredge up affects the AGB evolution (age ∼ 90-120Myr), and ending when the type Ia supernovae begin exploding in the cluster, eventually clearing it from the gas. The formation of the different populations requires episodes of star formation in AGB gas diluted with different amounts of pristine gas. In the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV Legacy Survey of GCs, the nitrogen increase is due to the third dredge up in the smallest mass AGB ejecta involved in the star formation of this population. The possibly-iron-rich small population in NGC 2808 may be a result of contamination by a single type Ia supernova. The NGC 2808 case is used to build a general framework to understand the variety of 'second generation' stars observed in GCs. Cluster-to-cluster variations are ascribed to differences in the effects of the many processes and gas sources which may be involved in the formation of the second generation. We discuss an evolutionary scheme, based on pollution by delayed type II supernovae, which accounts for the properties of s-Feanomalous clusters.

The stars in globular clusters are known to differ in their surface chemistry: spectroscopic investigations in recent decades outlined the presence of star‐to‐star differences in the abundances of the light elements, up to aluminium (and possibly silicon), suggesting that some stars were contaminated by an advanced proton‐capture nucleosynthesis. The asymptotic giant branch (AGB) stars are one of the most promising candidates in producing the pollution of the intracluster medium, via the ejection of gas processed by hot bottom burning, from which new stellar generations are formed. This work is focused on the degree of nucleosynthesis involving magnesium, aluminium and silicon that these sources may experience. The key ingredient in determining the degree of magnesium depletion, and the amount of aluminium that can be produced, is the rate of proton capture on 25Mg, forming 26Al; an increase in this cross‐section by a factor of 2 with respect to the highest value allowed by the NACRE compilation allows the reproduction of the extent of the Mg depletion observed, and is in qualitative agreement with the positive Al–Si correlation observed in a few clusters. The main uncertainties associated with the macro‐ and microphysics input are discussed and commented upon, and a comparison with recent spectroscopic results for globular clusters showing some degree of Mg–Al anticorrelation and Al–Si correlation is presented.