Is mass loss along the red giant branch of globular clusters sharply peaked? The case of M3

There is a growing evidence that several globular clusters must contain multiple stellar generations, differing in helium content. This hypothesis has helped to interpret peculiar unexplained features in their horizontal branches. In this framework we model the peaked distribution of the RR Lyr periods in M3, that has defied explanation until now. At the same time, we try to reproduce the colour distribution of M3 horizontal branch stars. We find that only a very small dispersion in mass loss along the red giant branch reproduces with good accuracy the observational data. The enhanced and variable helium content among cluster stars is at the origin of the extension in colour of the horizontal branch, while the sharply peaked mass loss is necessary to reproduce the sharply peaked period distribution of RR Lyr variables. The dispersion in mass loss has to be <~ 0.003 Msun, to be compared with the usually assumed values of ~0.02 Msun. This requirement represents a substantial change in the interpretation of the physical mechanisms regulating the evolution of globular cluster stars.Comment: Accepted for publication in The Astrophysical Journa

The early evolution of Globular Clusters: the case of NGC 2808

Enhancement and spread of helium among globular cluster stars have been recently suggested as a way to explain the horizontal branch blue tails, in those clusters which show a primordial spread in the abundances of CNO and other elements involved in advanced CNO burning (D'Antona et al. 2002). In this paper we examine the implications of the hypothesis that, in many globular clusters, stars were born in two separate events: an initial burst (first generation), which gives origin to probably all high and intermediate mass stars and to a fraction of the cluster stars observed today, and a second, prolonged star formation phase (second generation) in which stars form directly from the ejecta of the intermediate mass stars of the first generation. In particular, we consider in detail the morphology of the horizontal branch in NGC 2808 and argue that it unveils the early cluster evolution, from the birth of the first star generation to the end of the second phase of star formation. This framework provides a feasible interpretation for the still unexplained dichotomy of NGC 2808 horizontal branch, attributing the lack of stars in the RR Lyr region to the gap in the helium content between the red clump, whose stars are considered to belong to the first stellar generation and have primordial helium, and the blue side of the horizontal branch, whose minimum helium content reflects the helium abundance in the smallest mass (~4Msun)contributing to the second stellar generation. This scenario provides constraints on the required Initial Mass Function, in a way that a great deal of remnant neutron stars and stellar mass black holes might have been produced.Comment: 23 pages, 7 figures, in press on The Astrophysical Journa

Towards a working model for the abundance variations within Globular Clusters stars

A popular self--enrichment scenario for the formation of globular clusters assumes that the abundance anomalies shown by the stars in many clusters are due to a second stage of star formation occurring from the matter lost by the winds of massive asymptotic giant branch (AGB) stars. Until today, the modellizations of the AGB evolution by several different groups failed, for different reasons, to account for the patterns of chemical anomalies. Here we show that our own modelling can provide a consistent picture if we constrain the three main parameters which regulate AGB evolution: 1) adopting a high efficiency convection model; 2) adopting rates of mass loss with a high dependence on the stellar luminosity; 3) assuming a very small overshooting below the formal convective regions during the thermal pulse (TP) phase. The first assumption is needed to obtain an efficient oxygen depletion in the AGB envelopes, and the second one is needed to lose the whole stellar envelope within few thermal pulses, so that the sum of CNO elements does not increase too much, consistently with the observations. The third assumption is needed to fully understand the sodium production. We also show that the Mg - Al anticorrelation is explained adopting the higher limit of the NACRE rates for proton captures by Mg25 and Mg26, and the models are consistent with the recently discovered F-Al correlation. Problems remain to fully explain the observed Mg isotopes ratios

PSR J1740-5340: accretion inhibited by radio-ejection in a binary millisecond pulsar in the Globular Cluster NGC 6397

We present an evolutionary scenario for the spin-up and evolution of binary millisecond pulsars, according to which the companion of the pulsar PSR J 1740-5340, recently discovered as a binary with orbital period of 32.5 hr in the Globular Cluster NGC 6397, is presently in a phase of ``radio-ejection'' mass loss from the system. At present, Roche lobe overflow due to the nuclear evolution of the pulsar companion and to systemic angular momentum losses by magnetic braking is still going on, but accretion is inhibited by the momentum exerted by the radiation of the pulsar on the matter at the inner Lagrangian point. The presence of this matter around the system is consistent with the long lasting irregular radio eclipses seen in the system. Roche lobe deformation of the mass losing component is also necessary to be compatible with the optical light curve. The "radio-ejection" phase had been recently postulated by us to deal with the problem of the lack of submillisecond pulsars (Burderi et al. 2001, ApJ, 560, L71).Comment: Accepted for publication in The Astrophysical Journa

Predictions for self-pollution in globular cluster stars

Full evolutionary models have been built to follow the phases of asymptotic giant branch evolution with mass loss for metal mass fractions from Z=0.0002 to Z=0.004. For the first time, we find that temperatures close to or even larger than 10^8 K are achieved at low Z; the full CNO cycle operates at the base of the envelope, the Oxygen abundance for the most metal-poor models of 4 and 5 solar masses is drastically reduced, and sodium and aluminum production by by proton capture on neon and magnesium can occur. These results may be relevant for the evolution of primordial massive globular clusters: we suggest that the low-mass stars may have been polluted at the surface by accretion from the gas that was lost from the evolving intermediate-mass stars at early ages.Comment: 14 pages, 6 figure

Rapidly rotating second-generation progenitors for the blue hook stars of {\omega} Cen

Horizontal Branch stars belong to an advanced stage in the evolution of the oldest stellar galactic population, occurring either as field halo stars or grouped in globular clusters. The discovery of multiple populations in these clusters, that were previously believed to have single populations gave rise to the currently accepted theory that the hottest horizontal branch members (the blue hook stars, which had late helium-core flash ignition, followed by deep mixing) are the progeny of a helium-rich "second generation" of stars. It is not known why such a supposedly rare event (a late flash followed by mixing) is so common that the blue hook of {\omega} Cen contains \sim 30% of horizontal branch stars 10 , or why the blue hook luminosity range in this massive cluster cannot be reproduced by models. Here we report that the presence of helium core masses up to \sim 0.04 solar masses larger than the core mass resulting from evolution is required to solve the luminosity range problem. We model this by taking into account the dispersion in rotation rates achieved by the progenitors, whose premain sequence accretion disc suffered an early disruption in the dense environment of the cluster's central regions where second-generation stars form. Rotation may also account for frequent late-flash-mixing events in massive globular clusters.Comment: 44 pages, 8 figures, 2 tables in Nature, online june 22, 201

Where May Ultra-Fast Rotating Neutron Stars Be Hidden?

The existence of ultra-fast rotating neutron stars (spin period P < 1 ms) is expected on the basis of current models for the secular evolution of interacting binaries, though they have not been detected yet. Their formation depends on the quantity of matter accreted by the neutron star which, in turn, is limited by the mechanism of mass ejection from the binary. An efficient mass ejection can avoid the formation of ultra-fast pulsars or their accretion induced collapse to a black hole. We propose that significant reductions of the mass-transfer rate may cause the switch-on of a radio pulsar phase, whose radiation pressure may be capable of ejecting out of the system most of the matter transferred by the companion. This can prevent, for long orbital periods and if a sufficiently fast spin has been reached, any further accretion, even if the original transfer rate is restored, thus limiting the minimum spin period attainable by the neutron star. We show that close systems (orbital periods P_orb \sim 1 hr are the only possible hosts for ultra-fast spinning neutron stars. This could explain why ultra-fast radio pulsars have not been detected so far, as the detection of pulsars with very short spin periods in close systems is hampered, in current radio surveys, by strong Doppler modulation and computational limitations.Comment: 6 pages, including 1 figure. To appear in ApJ

A peculiar formula of essential amino acids prevents rosuvastatin myopathy in mice

Aims: Myopathy, characterized by mitochondrial oxidative stress, occurs in ∼10% of statin-treated patients, and a major risk exists with potent statins such as rosuvastatin (Rvs). We sought to determine whether a peculiar branched-chain amino acid-enriched mixture (BCAAem), found to improve mitochondrial function and reduce oxidative stress in muscle of middle-aged mice, was able to prevent Rvs myopathy. Results: Dietary supplementation of BCAAem was able to prevent the structural and functional alterations of muscle induced by Rvs in young mice. Rvs-increased plasma 3-methylhistidine (a marker of muscular protein degradation) was prevented by BCAAem. This was obtained without changes of Rvs ability to reduce cholesterol and triglyceride levels in blood. Rather, BCAAem promotes de novo protein synthesis and reduces proteolysis in cultured myotubes. Morphological alterations of C2C12 cells induced by statin were counteracted by amino acids, as were the Rvs-increased atrogin-1 mRNA and protein levels. Moreover, BCAAem maintained mitochondrial mass and density and citrate synthase activity in skeletal muscle of Rvs-treated mice beside oxygen consumption and ATP levels in C2C12 cells exposed to statin. Notably, BCAAem assisted Rvs to reduce oxidative stress and to increase the anti-reactive oxygen species (ROS) defense system in skeletal muscle. Innovation and Conclusions: The complex interplay between proteostasis and antioxidant properties may underlie the mechanism by which a specific amino acid formula preserves mitochondrial efficiency and muscle health in Rvs-treated mice. Strategies aimed at promoting protein balance and controlling mitochondrial ROS level may be used as therapeutics for the treatment of muscular diseases involving mitochondrial dysfunction, such as statin myopathy

The effects of Population III stars and variable IMF on the chemical evolution of the Galaxy

We studied the effects of a hypothetical initial stellar generation (PopIII) of only massive and very massive stars (VMS) on the chemical evolution of the Galaxy. We adopted the two-infall chemical evolution model of Chiappini et al. and tested several sets of yields for primordial VMS (Pair-Creation SNe), which produce different amounts of heavy elements than lower mass stars. We focused on the evolution of alpha-elements, C, N, Fe. The effects of PopIII stars on the Galactic evolution of these elements is negligible if a few generations of such stars occurred, whereas they produce different results from the standard models if they formed for a longer period. Also the effects of a more strongly variable IMF were discussed, making use of suggestions appeared in the literature to explain the lack of metal-poor stars in the Galactic halo with respect to model predictions. The predicted variations in abundances, SN rates, G-dwarf [Fe/H] distribution are here more dramatic and in contrast with observations; we concluded that a constant or slightly varying IMF is the best solution. Our main conclusion is that if VMS existed they must have formed only for a very short period of time (until the halo gas reached the threshold metallicity for the formation of very massive objects); in this case, their effects on the evolution of the studied elements was negligible also in the earliest phases. We thus cannot prove or disprove the existence of such stars on the basis of the available data. Due to their large metal production and short lives, primordial VMS should have enriched the halo gas beyond the metallicity of the most metal poor stars known in a few Myrs. This constrains the number of Pair-Creation SNe: we find that a number of 2-20 of such SNe occurred in our Galaxy depending on the stellar yields.Comment: 30 pages, 10 figures, accepted for publication in New Astronom