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ABSTRACT: We present new models for the evolution of stars with mass in the range 1Msun
< M < 7.5Msun, followed from the pre-main-sequence through the asymptotic giant
branch phase. The metallicity adopted is $Z=3*10^{-4} (which, with an
alpha-enhancement of +0.4, corresponds to [Fe/H]=-2). Dust formation is
described by following the growth of dust grains of various types as the wind
expands from the stellar surface.
Models with mass M>3Msun experience Hot Bottom Burning, thus maintaining the
surface C/O below unity. Unlike higher Z models, the scarcity of silicon
available in the envelope prevents the formation of silicates in meaningful
quantities, sufficient to trigger the acceleration of the wind via radiation
pressure on the dust grains formed. No silicate formation occurs below a
threshold metallicity of Z=10^{-3}.
Low--mass stars, with M< 2.5Msun become carbon stars, forming solid carbon
dust in their surroundings. The total dust mass formed depends on the uncertain
extent of the inwards penetration of the convective envelope during the Third
Dredge--Up episodes following the Thermal Pulses. Carbon grains have sizes 0.08
micron < a_C < 0.12 micron and the total amount of dust formed (increasing with
the mass of the star) is M_C=(2-6)*10^{-4}Msun.
Our results imply that AGB stars with Z=3*10^{-4} can only contribute to
carbon dust enrichment of the interstellar medium on relatively long
timescales, > 300 Myr, comparable to the evolutionary time of a 3Msun star. At
lower metallicities the scarcity of silicon available and the presence of Hot
Bottom Burning even in M< 2Msun, prevents the formation of silicate and carbon
grains. We extrapolate our conclusion to more metal--poor environments, and
deduce that at Z < 10^{-4} dust enrichment is mostly due to metal condensation
in supernova ejecta.
04/2013;
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ABSTRACT: We follow the scenario of formation of second generation stars in globular
clusters by matter processed by hot bottom burning (HBB) in massive asymptotic
giant branch (AGB) stars and Super AGB stars (SAGB). In the cluster NGC 2419 we
assume the presence of an extreme population directly formed from the AGB and
SAGB ejecta, so we can directly compare the yields for a metallicity Z=0.0003
with the chemical inventory of the cluster NGC 2419. At such a low metallicity,
the HBB temperatures (well above 108K) allow a very advanced nucleosynthesis.
Masses of about 6Mo deplete Mg and synthesize Si, going beyond Al, so this
latter element results only moderately enhanced; sodium can not be enhanced.
The models are consistent with the observations, although the predicted Mg
depletion is not as strong as in the observed stars. We predict that the oxygen
abundance must be depleted by a huge factor in the Mg poor stars. The HBB
temperatures are close to the region where other p capture reactions on heavier
nuclei become possible. We show that high potassium abundance found in Mg poor
stars can be achieved during HBB, by p captures on the argon nuclei, if the
relevant cross section(s) are larger than listed in the literature or if the
HBB temperature is higher. Finally, we speculate that some calcium production
is occurring owing to proton capture on potassium. We emphasize the importance
of a strong effort to measure a larger sample of abundances in this cluster.
11/2012;
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ABSTRACT: The internal structure of low-mass stars turns out to depend critically on the thermodynamic description of the partial ionization
regime. In this contribution we show the differences among the results obtained with some of the various equations of state
(EOS) available in the literature.
Stars-Equation of state
Astrophysics and Space Science 04/2012; 328(1):167-169. · 1.69 Impact Factor
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ABSTRACT: We have used new, deep, visible and near infrared observations of the compact
starburst cluster in the giant HII region NGC 3603 and its surroundings with
the WFC3 on HST and HAWK-I on the VLT to study in detail the physical
properties of its intermediate mass (~ 1 - 3 M_sun) stellar population. We show
that after correction for differential extinction and actively accreting stars,
and the study of field star contamination, strong evidence remains for a
continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~
30 Myr within the temporal resolution available. Existing differences among
presently available theoretical models account for the largest possible
variation in shape of the measured age histograms within these limits. We also
find that this isochronal age spread in the near infrared and visible
Colour-Magnitude Diagrams cannot be reproduced by any other presently known
source of astrophysical or instrumental scatter that could mimic the luminosity
spread seen in our observations except, possibly, episodic accretion. The
measured age spread and the stellar spatial distribution in the cluster are
consistent with the hypothesis that star formation started at least 20-30 Myrs
ago progressing slowly but continuously up to at least a few million years ago.
All the stars in the considered mass range are distributed in a flattened
oblate spheroidal pattern with the major axis oriented in an approximate
South-East - North-West direction, and with the length of the equatorial axis
decreasing with increasing age. This asymmetry is most likely due to the fact
that star formation occurred along a filament of gas and dust in the natal
molecular cloud oriented locally in this direction.
03/2012;
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ABSTRACT: In order to account for the chemical composition of a stellar second
generation (SG), Globular Clusters (GCs) evolution models based on the
asymptotic giant branch (AGB) scenario so far included only the yields
available for the massive AGB stars, while the possible role of super-AGB
ejecta was either extrapolated or not considered. In this work, we explore the
role of super-AGB ejecta using yields recently calculated by Ventura and
D'Antona. Models of clusters showing extended Na-O anticorrelations, like NGC
2808, indicate that a SG formation history similar to that outlined in our
previous work is required: formation of an Extreme population with very large
helium content from the pure ejecta of super-AGB stars, followed by formation
of an Intermediate population by dilution of stellar ejecta with pristine gas.
The very O-poor Na-rich Extreme stars can be accounted for once deep-mixing is
assumed in SG giants forming in a gas with helium abundance Y> 0.34, which
significantly reduces the atmospheric oxygen content, while preserving the
sodium abundance. On the other hand, for clusters showing a mild O-Na
anticorrelation, like M 4, the use of the new yields broadens the range of SG
formation routes leading to abundance patterns consistent with observations. It
is shown that models in which SG stars form only from super-AGB ejecta promptly
diluted with pristine gas can reproduce the observations. We discuss the
variety of small helium variations occurring in this model and its relevance
for the horizontal branch morphology. In some of these models the duration of
the SG formation episode can be as short as \sim10 Myr; the formation time of
the SG is thus compatible with the survival of a cooling flow in the GC,
previous to the explosion of the SG core collapse supernovae. We also explore
models with formation of multiple populations in individual bursts, each
lasting no longer than \sim10 Myr.
03/2012;
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ABSTRACT: The stars in globular clusters are known to differ in their surface
chemistry: the spectroscopic investigations in the last 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 AGB stars are
one of the most promising candidates in producing the pollution of the
intra-cluster 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 to determine the degree of magnesium
depletion, and the amount of aluminium that can be produced, is the rate of
proton capture on Mg25, forming Al26; an increase in this cross-section by a
factor 2 with respect to the highest value allowed by the NACRE compilation
allows to reproduce 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 micro-physics
input, are discussed and commented, and the comparison with recent
spectroscopic results for the globular cluster showing some degree of Mg-Al
anticorrelation and Al-Si correlation is presented.
05/2011;
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ABSTRACT: A new paradigm is emerging for the formation of Globular Clusters, supported by recent surprising discoveries concerning the presence of more than one, numerically significant, stellar population in most of the clusters so far examined spectroscopically and∕or photometrically.
AIP Conference Proceedings. 12/2010; 1314(1):151-156.
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ABSTRACT: We investigate the physical and chemical evolution of population II stars with initial masses in the range 6.5-8 Msun, which undergo an off centre carbon ignition under partially degenerate conditions, followed by a series of thermal pulses, and supported energetically by a CNO burning shell, above a O-Ne degenerate core. In agreement with the results by other research groups, we find that the O-Ne core is formed via the formation of a convective flame that proceeds to the centre of the star. The evolution which follows is strongly determined by the description of the mass loss mechanism. Use of the traditional formalism with the super-wind phase favours a long evolution with many thermal pulses, and the achievement of an advanced nucleosynthesis, due the large temperatures reached by the bottom of the external mantle. Use of a mass loss recipe with a strong dependence on the luminosity favours an early consumption of the stellar envelope, so that the extent of the nucleosynthesis, and thus the chemical composition of the ejecta, is less extreme. The implications for the multiple populations in globular clusters are discussed. If the "extreme" populations present in the most massive clusters are a result of direct formation from the super-AGB ejecta, their abundances may constitute a powerful way of calibrating the mass loss rate of this phase. This calibration will also provide informations on the fraction of super-AGBs exploding as single e-capture supernova, leaving a neutron star remnant in the cluster. Comment: In press on MNRAS
09/2010;
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ABSTRACT: A large number of spectroscopic studies have provided evidence of the presence of multiple populations in globular clusters by revealing patterns in the stellar chemical abundances. This paper is aimed at studying the origin of these abundance patterns. We explore a model in which second generation (SG) stars form out of a mix of pristine gas and ejecta of the first generation of asymptotic giant branch stars. We first study the constraints imposed by the spectroscopic data of SG stars in globular clusters on the chemical properties of the asymptotic and super asymptotic giant branch ejecta. With a simple one-zone chemical model, we then explore the formation of the SG population abundance patterns focussing our attention on the Na-O, Al-Mg anticorrelations and on the helium distribution function. We carry out a survey of models and explore the dependence of the final SG chemical properties on the key parameters affecting the gas dynamics and the SG formation process. Finally, we use our chemical evolution framework to build specific models for NGC 2808 and M4, two Galactic globular clusters which show different patterns in the Na-O and Mg-Al anticorrelation and have different helium distributions. We find that the amount of pristine gas involved in the formation of SG stars is a key parameter to fit the observed O-Na and Mg-Al patterns. The helium distribution function for these models is in general good agreement with the observed one. Our models, by shedding light on the role of different parameters and their interplay in determining the final SG chemical properties, illustrate the basic ingredients, constraints and problems encountered in this self-enrichment scenario which must be addressed by more sophisticated chemical and hydrodynamic simulations. Comment: 19 pages, 10 figures, MNRAS accepted
05/2010;
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ABSTRACT: Core helium burning is the dominant source of energy of extreme horizontal branch stars, as the hydrogen envelope is too small to contribute to the nuclear energy output. The evolution of each mass in the HR diagram occurs along vertical tracks that, when the core helium is consumed, evolve to higher Teff and then to the white dwarf stage. The larger is the mass, the smaller is the Teff of the models, so that the zero age horizontal branch (ZAHB) is "horizontal". In this paper we show that, if the helium mass fraction (Y) of the envelope is larger than Y~0.5, the shape of the tracks changes completely: the hydrogen burning becomes efficient again also for very small envelope masses, thanks to the higher molecular weight and to the higher temperatures of the hydrogen shell. The larger is Y, the smaller is the envelope mass that provides strong H-shell burning. These tracks have a curled shape, are located at a Teff following the approximate relation Teff=8090+ 32900xY, and become more luminous for larger envelope masses. Consequently, the ZAHB of the very high helium models is "vertical" in the HR diagram. Synthetic models based on these tracks nicely reproduce the location and shape of the "blue hook" in the globular cluster wCen, best fit by a very high Teff (bluer) sequence with Y=0.80 and a cooler (redder) one with Y=0.65. Although these precise values of Y may depend on the color-Teff conversions, we know that the helium content of the progenitors of the blue hook stars can not be larger than Y~0.38-0.40, if they are descendants of the cluster blue main sequence. Consequently, this interpretation implies that all these objects must in fact be progeny of the blue main sequence, but they have all suffered further deep mixing, that has largely and uniformly increased their surface helium abundance, during the red giant branch evolution. A late helium flash can not be the cause of this deep mixing, as the models we propose have hydrogen rich envelopes much more massive than those required for a late flash. We discuss different models of deep mixing proposed in the literature, and conclude that our interpretation of the blue hook can not be ruled out, but requires a much deeper investigation before it can be accepted. Comment: in press on MNRAS
03/2010;
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ABSTRACT: The possible sites of hot CNO processing for the gas which allowed the formation of “second generation”, chemically anomalous, stars in Globular Clusters are either the envelopes of massive rotating stars or the envelopes of massive asymptotic giant branch (AGB) stars. As the massive stars destroy Lithium, while the massive AGBs can also produce it, the Lithium abundance of unevolved stars of GCs could be a powerful discriminant between the two scenarios. We show the status of art of the computation of Lithium production in massive AGBs for different population II metallicities, provide explanations for the trends with metallicity and mass loss rate of the models, and compare the results with observations. Unfortunately, today's determinations of Lithium abundance in GC stars are not sufficient, and not enough precise, to allow a choice between self–enrichment by AGB or massive star models.
AIP Conference Proceedings. 04/2008; 1001(1):41-48.
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ABSTRACT: We present this paper in conjunction with a companion paper as the first results in the Canada-France-Hawaii Telescope Open Star Cluster Survey. This survey is a large BVR imaging data set of 19 open star clusters in our Galaxy. This data set was taken with the CFH12K mosaic CCD (42' × 28'), and the majority of the clusters were imaged under excellent photometric, subarcsecond seeing, conditions. The combination of multiple exposures extending to deep (V ~ 25) magnitudes with short (≤10 s) frames allows for studies ranging from faint white dwarf stars to bright turnoff, variable, and red giant stars. The primary aim of this survey is to catalog the white dwarf stars in these clusters and establish observational constraints on the initial-final mass relationship for these stars and the upper mass limit to white dwarf production. Additionally, we hope to better determine the properties of the clusters, such as age and distance, and also test evolution and dynamical theories by analyzing luminosity and mass functions. In order to more easily incorporate these data in further studies, we have produced a catalog of positions, magnitudes, colors, and stellarity confidence for all stars in each cluster of the survey. This reduction, along with the computed calibration parameters for all three nights of the observing run will encourage others to use these data in different astrophysical studies outside of our goals. Additionally, the data set is reduced using the new TERAPIX photometric reduction package, PSFex, which is found to compare well with other packages. This paper is intended both as a source for the astronomical community to obtain information on the clusters in the survey and as a detailed reference of reduction procedures for further publications of individual clusters. We discuss the methods employed to reduce the data and compute the photometric catalog. We reserve both the scientific results for each individual cluster and global results from the study of the entire survey for future publications. The first of these further publications is devoted to the old rich open star cluster, NGC 6819, and appears as a companion paper in the same issue of the Journal.
The Astronomical Journal 12/2007; 122(1):257. · 4.03 Impact Factor
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ABSTRACT: We present analysis of deep CCD photometry for the very rich, old open star cluster NGC 6819. The science goals are to catalog the white dwarfs in the cluster and measure the cluster luminosity and mass functions. These CFH12K data results represent the first of nineteen open star clusters which were imaged as a part of the CFHT Open Star Cluster Survey. We find a tight, very rich, main-sequence and turnoff consisting of over 2900 cluster stars in the V, B-V color-magnitude diagram (CMD). Main-sequence fitting of the unevolved cluster stars with the Hyades star cluster yields a distance modulus of (m - M)V = 12.30 ± 0.12, for a reddening of E(B-V) = 0.10. These values are consistent with a newly calculated theoretical stellar isochrone of age 2.5 Gyr, which we take to be the age of the cluster. Both the depth gained in the photometry and the increased projected area of the CFH12K Mosaic CCD allow for detailed star counts in concentric annuli out to large angular radii. These indicate a much larger cluster extent (R = 95 ± 10), by a factor of ~2 over some previous estimates. Incompleteness tests confirm a slightly negatively sloped luminosity function extending to faint (V ~ 23) magnitudes which is indicative of a dynamically evolved cluster. Further luminosity function and mass segregation tests indicate that low-mass objects (M ≤ 0.65 M) predominate in the outer regions of the cluster, 35 ≤ R ≤ 95. The estimation of the number of white dwarfs in NGC 6819, based on stellar evolution models, white dwarf cooling timescales, and conservation of star number arguments applied to the red giant stars of the cluster are in good agreement with the observed number. For those white dwarf candidates which pass both a statistical subtraction that removes background galaxies and field stars and a high star/galaxy confidence by using image classification, we show comparisons with white dwarf isochrones and cooling models which suggest the need for spectroscopy to confirm the white dwarf nature of the brighter objects. This is entirely feasible for all objects, before a statistical subtraction cut, with the current generation of 8 m–class telescopes and multiobject spectrometers.
The Astronomical Journal 12/2007; 122(1):266. · 4.03 Impact Factor
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ABSTRACT: We present stellar yields computed from detailed models of intermediate mass asymptotic giant branch stars of low metallicity. In this work, the whole main microphysics inputs have been updated, and in particular alpha-enhancement is explicitly taken into account both in the opacities and equation of state. The target of this work is to provide a basis to test the reliability of the AGB self-enrichment scenario for Globular Clusters of intermediate metallicity. These Globular Clusters exhibit well defined abundance patterns, which have often been interpreted as a consequence of the pollution of the interstellar medium by the ejecta of massive AGBs. We calculated a grid of intermediate mass models with metallicity Z=0.001; the evolutionary sequences are followed from the pre-Main sequence along the whole AGB phase. We focus our attention on those elements largely studied in the spectroscopic investigations of Globular Clusters stars, i.e. oxygen, sodium, aluminum, magnesium and fluorine.} The predictions of our models show an encouraging agreement with the demand of the self-enrichment scenario for what concerns the abundances of oxygen, aluminum, fluorine and magnesium. The question of sodium is more tricky, due to the large uncertainties of the cross-sections of the Ne-Na cycle. The present results show that only a relatively small range of initial masses (M=5,6 solar masses) can be responsible for the self enrichment.
12/2007;
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ABSTRACT: We summarize the status of art of the secular evolution of low mass X-ray binaries (LMXBs) and take a close look at the orbital period distribution of LMXBs and of binary millisecond pulsars (MSP), in the hypothesis that this latter results from the LMXB evolution. The deficiency of systems below the period gap, which in cataclysmic binaries occurs between ~ 2 and 3 hr, points to a very different secular evolution of LMXBs with respect to their counterparts containing a white dwarf compact object. The presence of several ultrashort period LMXBs (some of which are also X-ray millisecond pulsars), the important fraction of binary MSPs at periods between 0.1 and 1 day, the periods (26 and 32hr) of two ``interacting'' MSPs in Globular Clusters are other pieces of the puzzle in the period distribution. We consider the possible explanations for these peculiarities, and point out that Grindlay's old proposal that all (most of) LMXBs in the field were originally born in globular clusters must be carefully reconsidered.
01/2007;
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ABSTRACT: The majority of the inhomogeneities in the chemical composition of Globular Cluster (GC) stars appear due to primordial enrichment. The most studied model today claims that the ejecta of Asymptotic Giant Branch (AGB) stars of high mass -those evolving during the first ~100Myr of the Clusters life- directly form a second generation of stars with abundance anomalies. In this talk, we review the status of the art with regard to this model, whose major problems are i) the modelling of the chemical anomalies is still not fully complete, and ii) it requires an IMF peculiarly enhanced in the intermediate mass stars. The model predicts enhanced helium abundance in the stars showing chemical anomalies, and the helium abundance distribution can be roughly derived from the morphology of the horizontal branch. Such distribution may possibly help to falsify the model for the first phases of evolution of GCs. As an illustration, we compare the results of the analysis of the HB morphology of some clusters.
01/2007;
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ABSTRACT: Rotational evolution in the pre-main sequence (PMS) is described with new sets of PMS evolutionary tracks including rotation, non-gray boundary conditions (BCs) and either low (LCE) or high convection efficiency (HCE). Using observational data and our theoretical predictions, we aim at constraining 1) the differences obtained for the rotational evolution of stars within the ONC by means of these different sets of models; 2) the initial angular momentum of low mass stars, by means of their templates in the ONC. We discuss the reliability of current stellar models for the PMS. While the 2D radiation hydrodynamic simulations predict HCE in PMS, semi-empirical calibrations either seem to require that convection is less efficient in PMS than in the following MS phase or are still contradictory. We derive stellar masses and ages for the ONC by using both LCE and HCE. The resulting mass distribution for the bulk of the ONC population is in the range 0.2$-$0.3 {\msun} for our non-gray models and in the range 0.1$-$0.3{\msun} for models having gray BCs. In agreement with Herbst et al. (2002) we find that a large percentage ($\sim$70%) of low-mass stars (M\simlt 0.5{\msun} for LCE; M\simlt0.35{\msun} for HCE) in the ONC appears to be fast rotators (P$<$4days). Three possibilities are open: 1) $\sim$70% of the ONC low mass stars lose their disk at early evolutionary phases; 2)their locking period is shorter; 3) the period evolution is linked to a different morphology of the magnetic fields of the two groups of stars. We also estimate the range of initial angular momentum consistent with the observed periods. The comparisons made indicate that a second parameter is needed to describe convection in the PMS, possibly related to the structural effect of a dynamo magnetic field. Comment: 17 pages, 11 figures
09/2006;
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ABSTRACT: 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.
11/2005;
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ABSTRACT: Most inhomogeneities in the chemical composition of GC stars are due to primordial enrichment. The model today most credited is that the winds lost by high mass Asymptotic Giant Branch (AGB) stars, evolving during the first $\simlt$200Myr of the Clusters life, directly form a second generation of stars with abundance anomalies. The best indirect hint towards this suggestion is the recognition that some peculiarities in the Horizontal Branch (HB) stars distribution (blue tails, gaps, anomalous luminosity slope of the flat part of the HB) can be attributed to the larger helium abundance in the matter, processed through Hot Bottom Burning, from which these stars are born. The model has been reinforced by finding a peculiar main sequence distribution in the cluster NGC 2808, which also has a bimodal HB distribution and an extended blue tail: the excess of blue objects on the main sequence has been interpreted as stars with very high helium. We remark that the RR Lyr distribution may be affected by the helium spread, and this can be at the basis of the very long periods of the RRab variables of the metal rich clusters NGC 6388 and NGC 6441, longer than for the very metal poor Oosterhoff II clusters. These periods imply that the RR Lyr are brighter than expected for their metallicities, consistent with a larger helium abundance.
11/2005;
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ABSTRACT: In the course of a systematic exploration of the uncertainties associated to 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/Msun<6.5 for a metallicity typical for Globular Cluster, 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 (1988) cross-sections. The results differ in particular with respect to the Na23 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, changing indirectly 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) Comment: 27 pages (referee format) + 19 figures
05/2005;
