We examine the photometric data for Fornax clusters, focussing our attention on their horizontal branch color distribution and, when available, on the RR Lyr variables fraction and period distribution. Based on our understanding of the HB morphology
in terms of varying helium content in the context of multiple stellar generations, we show that clusters F2, F3 and F5 must contain substantial fractions of second generation stars (~54-65%). On the basis of a simple chemical evolution model we show that the helium distribution in these clusters can be reproduced by models with cluster initial masses ranging from values equal to ~4 to ~10 times larger than the current masses. Models with a very short second generation star formation episode can also reproduce the observed helium distribution but require larger initial masses up to about twenty times the current mass. While the lower limit of this range of possible initial GC masses is consistent with those suggested by the observations of the low metallicity field stars, we also discuss the possibility that the metallicity scale of field stars (based on CaII triplet spectroscopy) and the metallicities derived for the clusters in Fornax may not be consistent with each other. The reproduction of the HB morphology in F2,F3,F5 requires two interesting hypotheses: 1) the first generation HB stars lie all at red colours. According to this interpretation, the low metallicity stars in the field of Fornax, populating the HB at colours bluer than the blue side ((V-I)o<=0.3 or (B-V)o<=0.2) of the RR Lyrs, should be second generation stars born in the clusters;a preliminary analysis of available colour surveys of Fornax field provides a fraction ~20% of blue HB stars, in the low metallicity range; 2) the mass loss from individual second generation red giants is a few percent of a solar mass larger than the mass loss from first generation stars.
Some globular clusters in our Galaxy are noticeably rich in low-mass X-ray binaries. Terzan 5 has the richest population among globular clusters of X- and radio-pulsars and low-mass X-ray binaries. The detection and study of optical/IR counterparts o
f low-mass X-ray binaries is fundamental to characterizing both the low-mass donor in the binary system and investigating the mechanisms of the formation and evolution of this class of objects. We aim at identifying the near-IR counterpart of the 11 Hz pulsar IGRJ17480-2446 discovered in Terzan 5. Adaptive optics (AO) systems represent the only possibility for studying the very dense environment of GC cores from the ground. We carried out observations of the core of Terzan 5 in the near-IR bands with the ESO-VLT NAOS-CONICA instrument. We present the discovery of the likely counterpart in the Ks band and discuss its properties both in outburst and in quiescence. Archival HST observations are used to extend our discussion to the optical bands. The source is located at the blue edge of the turn-off area in the color-magnitude diagram of the cluster. Its luminosity increase from quiescence to outburst, by a factor 2.5, allows us to discuss the nature of the donor star in the context of the double stellar generation population of Terzan 5 by using recent stellar evolution models.
Globular cluster stars show chemical abundance patterns typical of hot-CNO processing. Lithium is easily destroyed by proton capture in stellar environments, so its abundance may be crucial to discriminate among different models proposed to account f
or multiple populations. In order to reproduce the observed O-Na anticorrelation and other patterns typical of multiple populations, the formation of second generation stars must occur from the nuclearly processed stellar ejecta, responsible of the chemical anomalies, diluted with pristine gas having the composition of first generation stars. The lithium abundance in the unprocessed gas -which is very likely to be equal to the lithium abundance emerging from the Big Bang- affects the lithium chemical patterns among the cluster stars. This paper focuses on a scenario in which processed gas is provided by asymptotic giant branch (AGB) stars. We examine the predictions of this scenario for the lithium abundances of multiple populations. We study the role of the non-negligible lithium abundance in the ejecta of massive AGB (A(Li)~2), and, at the same time, we explore how our models can constrain the extremely large ---and very model dependent--- lithium yields predicted by recent super--AGB models. We show that the super--AGB yields may be tested by examining the lithium abundances in a large set of blue main sequence stars in wCen and/or NGC2808. In addition, we examine the different model results obtained by assuming for the pristine gas either the Big Bang abundance predicted by the standard models (A(Li)=2.6-2.7), or the abundance detected at the surface of population II stars (A(Li)=2.2-2.3). Once a chemical model is well constrained, the O--Li distribution could perhaps be used to shed light on the primordial lithium abundance.
The color magnitude diagram (CMD) of NGC 1851 presents two subgiant branches (SGB), probably due the presence of two populations differing in total CNO content. We test the idea that a difference in total CNO may simulate an age difference when compa
ring the CMD of clusters to derive relative ages. We compare NGC 1851 with NGC 6121 (M4), a cluster of very similar [Fe/H]. We find that, with a suitable shift of the CMDs that brings the two red horizontal branches at the same magnitude level, the unevolved main sequence and red giant branch match, but the SGB of NGC 6121 and its red giant branch bump are fainter than in NGC 1851. In particular, the SGB of NGC 6121 is even slightly fainter than the the faint SGB in NGC 1851. Both these features can be explained if the total CNO in NGC 6121 is larger than that in NGC 1851, even if the two clusters are coeval. We conclude by warning that different initial C+N+O abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the turnoff morphology that can be easily attributed to an age difference.
Among the newly discovered features of multiple stellar populations in Globular Clusters, the cluster NGC 1851 harbours a double subgiant branch, that can be explained in terms of two stellar generations, only slightly differing in age, the younger o
ne having an increased total C+N+O abundance. Thanks to this difference in the chemistry, a fit can be made to the subgiant branches, roughly consistent with the C+N+O abundance variations already discovered two decades ago, and confirmed by recent spectroscopic data. We compute theoretical isochrones for the main sequence turnoff, by adopting four chemical mixtures for the opacities and nuclear reaction rates. The standard mixture has Z=10$^{-3}$ and [$alpha$/Fe]=0.4, the others have C+N+O respectively equal to 2, 3 and 5 times the standard mixture, according to the element abundance distribution described in the text. We compare tracks and isochrones, and show how the results depend on the total CNO abundance. We notice that different initial CNO abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the turnoff morphology that can be easily attributed to an age difference. We simulate the main sequence and subgiant branch data for NGC 1851 and show that an increase of C+N+O by a factor $sim$3 best reproduces the shift between the subgiant branches. We compare the main sequence width in the color m$_{F336W}$-m$_{F814W}$ with models, and find that the maximum helium abundance compatible with the data is Y$simeq$0.29. We consider the result in the framework of the formation of the second stellar generation in globular clusters, for the bulk of which we estimate a helium abundance of Y$simlt 0.26$.
