arXiv subjects

Francesca D'Antona

Publications and source records attributed to Francesca D'Antona.

53 records · Page 3Linked to original sources

arXiv2006

Were most Low Mass X ray Binaries born in Globular Clusters?

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.

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arXiv2006

Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster

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.

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arXiv2006

New light on the old problem of lithium pre--MS depletion: models with 2D RHD convection

The Teff location of Pre-Main Sequence (PMS) evolutionary tracks depends on the treatment of over-adiabaticity. We present here the PMS evolutionary tracks computed by using the mixing length theory of convection (MLT) in which the alpha_MLT=l/H_p parameter calibration is based on 2D--hydrodynamical models (Ludwig et al. 1999). These MLT--α^2D stellar models and tracks are very similar to those computed with non--grey ATLAS9 atmospheric boundary conditions and Full Spectrum of Turbulence (FST) convection model both in the atmosphere and in the interior. The comparison of the new tracks with the location on the HR diagram of pre--MS binariesis not completely satisfactory, as some binary components are located at too low \teff. Besides, the pre--MS lithium depletion in the MLT--α^2D tracks is still much larger than that expected from the observations of lithium in young open clusters. This result is similar to that of FST models. Thus, in spite of the fact that 2D RHD models should provide a better convection description than any local model, their introduction is not sufficient to reconcile theory and observations. Lithium depletion in young clusters points towards a convection efficiency which, in pre--MS, should be smaller than in the MS. The pre--MS lithium depletion decreases significantly in FST models if we reduce the solar metallicity down to the value suggested by Asplund et al.(2004), but the corresponding solar model does not reproduce the depth of the convective zone as determined by helioseismology.

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arXiv2005

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.

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arXiv2005

Multiple helium abundances in Globular Clusters stars: Consequences for the Horizontal Branch and RR Lyrae

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.

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arXiv2005

Full computation of massive AGB evolution. II. The role of mass loss and cross-sections

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)

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arXiv2005

The role of General Relativity in the evolution of Low Mass X-ray Binaries

We study the evolution of Low Mass X-ray Binaries (LMXBs) and of millisecond binary radio pulsars (MSPs), with numerical simulations that keep into account the evolution of the companion, of the binary system and of the neutron star. According to general relativity, when energy is released, the system loses gravitational mass. Moreover, the neutron star can collapse to a black hole if its mass exceeds a critical limit, that depends on the equation of state. These facts have some interesting consequences: 1) In a MSP the mass-energy is lost with a specific angular momentum that is smaller than the one of the system, resulting in a positive contribution to the orbital period derivative. If this contribution is dominant and can be measured, we can extract information about the moment of inertia of the neutron star, since the energy loss rate depends on it. Such a measurement can therefore help to put constraints on the equation of state of ultradense matter. 2) In LMXBs below the bifurcation period (\sim 18 h), the neutron star survives the period gap only if its mass is smaller than the maximum non-rotating mass when the companion becomes fully convective and accretion pauses. Therefore short period (P < 2h) millisecond X-ray pulsar like SAX J1808.4-3658 can be formed only if either a large part of the accreting matter has been ejected from the system, or the equation of state of ultradense matter is very stiff. 3) In Low Mass X-ray binaries above the bifurcation period, the mass-energy loss lowers the mass transfer rate. As side effect, the inner core of the companion star becomes 1% bigger than in a system with a non-collapsed primary. Due to this difference, the final orbital period of the system becomes 20% larger than what is obtained if the mass-energy loss effect is not taken into account.

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arXiv2004

Full computation of massive AGB evolution. I. The large impact of convection on nucleosynthesis

It is well appreciated that the description of overadiabatic convection affects the structure of the envelopes of luminous asymptotic giant branch (AGB) stars in the phase of ``hot bottom burning '' (HBB). We stress that this important uncertainty in the modeling plays a role which is much more dramatic than the role which can be ascribed, e.g., to the uncertainty in the nuclear cross-sections. Due to the role tentatively attributed today to the HBB nucleosynthesis as the site of self-enrichment of Globular Clusters stars, it is necessary to explore the difference in nucleosynthesis obtained by different prescriptions for convection. We present results of detailed evolutionary calculations of the evolution of stars of intermediate mass during the AGB phase. We follow carefully the nucleosynthesis at the base of the external convective region, showing that very different results can be obtained according to the presciption adopted to find out the temperature gradient within the instability regions. We discuss the uncertainties in the yields of the various chemical species and the role which these sources can play as polluters of the interstellar medium.

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arXiv2004

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.

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arXiv2002

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).

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arXiv2002

Radio ejection in the evolution of X-ray binaries: the bridge between low mass X-ray binaries and millisecond pulsars

We present a scenario for the spin-up and evolution of binary millisecond pulsars. This can explain the observational properties of the recently discovered binary millisecond pulsar PSR J1740-5340, with orbital period 32.5 hrs, in the Globular Cluster NGC 6397. The optical counterpart of this system is a star as luminous as the cluster turnoff stars, but with a lower Teff (a larger radius) which we model with a star of initial mass compatible with the masses evolving in the cluster (~0.85 Msun). This star has suffered Roche lobe overflow while evolving off the main sequence, spinning up the neutron star to the present period of 3.65 ms. There are evidences that at present, Roche lobe overflow is still going on. Indeed Roche lobe deformation of the mass losing component is necessary to be compatible with the optical light curve. The presence of matter around the system is also consistent with the long lasting irregular radio eclipses seen in the system. We propose that this system is presently in a phase of `radio-ejection' mass loss. The radio-ejection phase can be initiated only if the system is subject to intermittency in the mass transfer during the spin-up phase. In fact, when the system is detached the pulsar radio emission is not quenched, and may be able to prevent further mass accretion due to the action of the pulsar pressure at the inner Lagrangian point.

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arXiv2001

The CFHT Open Star Cluster Survey. III. The White Dwarf Cooling Age of the Rich Open Star Cluster NGC 2099 (M37)

We present deep CCD photometry of the very rich, intermediate aged (similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-V color-magnitude diagram (CMD) for the cluster shows an extremely well populated and very tightly constrained main-sequence extending over 12 magnitudes from the turn-off. The CFH12K photometry for this cluster is faint enough (V ~ 24.5) to detect the remnants of the most massive progenitor cluster stars under the Type I SNe limit. Therefore, the CMD of the cluster also exhibits a well defined white dwarf `clump' caused by the decreased rate of cooling of these stars as they age, and a subsequent gap with very few objects. The termination point (Mv = 11.95 +/- 0.30) of the white dwarf luminosity function gives a white dwarf cooling age of 566 +/- 154/176 Myrs which is in excellent agreement with the main-sequence turn-off isochrone age (520 Myrs). By carefully accounting for errors, we show that the cooling age confirms that models including convective core overshooting are preferred for young-intermediate aged clusters. We also derive the reddening (E(B-V) = 0.21 +/- 0.03) and distance ((m-M)v = 11.55 +/- 0.13) to NGC 2099 by matching main-sequence features in the cluster to a new fiducial main-sequence for the Hyades. As a continuing part of the goals of the CFHT Open Star Cluster Survey to better understand dynamical processes of open clusters, we also fit a King model to the cluster density distribution and investigate the cluster main-sequence luminosity and mass functions in increasing concentric annuli. We find some evidence for mass segregation within the boundary of NGC 2099 as expected given the cluster's age relative to the dynamical age. The present global mass function for the cluster is found to be shallower than a Salpeter IMF.

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arXiv2001

The CFHT Open Star Cluster Survey I -- Cluster Selection and Data Reduction

We present this paper in conjuction with the following as the first results in the CFHT 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' X 28') and the majority of the clusters were imaged under excellent photometric, sub-arcsecond seeing conditions. The combination of multiple exposures extending to deep (V ~ 25) magnitudes with short (< 10 second) frames allows for many different studies. The primary aim of this survey is to catalogue 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 this data in further studies, we have produced a catalogue of positions, magnitudes, colors, and stellarity confidence for all stars in each cluster of the survey. 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 catalogue. The scientific results for each individual cluster and global results from the study of the entire survey will be presented in further publications (results for the rich old open cluster NGC 6819 immediately follow this paper).

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arXiv2001

The CFHT Open Star Cluster Survey II -- Deep CCD Photometry of the Old Open Star Cluster NGC 6819

We present analysis of deep CCD photometry for the very rich, old open star cluster NGC 6819. 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 turn-off consisting of over 2900 cluster stars in the V, B-V color-magnitude diagram (CMD). Main-sequence fitting of the un-evolved 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 Gyrs, which we take to be the age of the cluster. Detailed star counts indicate a much larger cluster extent (R = 9.5' +/- 1.0'), 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 < 0.65Mo) predominate in the outer regions of the cluster, 3.5 < R < 9.5. The estimation of the number of white dwarfs in NGC 6819 are in good agreement with the observed number. For those white dwarf candidates which pass both a statistical and image classification tests, we show comparisons to white dwarf isochrones and cooling models which suggest the need for spectroscopy to confirm the white dwarf nature of the brighter objects.

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arXiv2001

The stellar origin of 7Li - Do AGB stars contribute a substantial fraction of the local Galactic lithium abundance?

We adopt up-to-date 7Li yields from asymptotic giant branch stars in order to study the temporal evolution of 7Li in the solar neighbourhood in the context of a revised version of the two-infall model for the chemical evolution of our galaxy. We consider several lithium stellar sources besides the asymptotic giant branch stars such as Type II supernovae, novae, low-mass giants as well as Galactic cosmic rays and low-mass X-ray binaries. We conclude that asymptotic giant branch stars cannot be considered as important 7Li producers as believed in so far and that the contribution of low-mass giants and novae is necessary to reproduce the steep rise of the 7Li abundance in disk stars as well as the meteoritic 7Li abundance. Lithium production in low-mass X-ray binaries hardly affects the temporal evolution of 7Li in the solar neighbourhood.

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arXiv2001

Evolution of Lithium in the Milky Way

We adopt up-to-date 7Li yields from asymptotic giant branch stars in order to study the temporal evolution of this element in the solar neighbourhood. Several lithium stellar sources are considered besides the AGBs: Type II supernovae, novae, low-mass giants. Galactic cosmic ray nucleosynthesis is taken into account as well. We conclude that AGB stars do not substantially contribute to 7Li enrichment on a Galactic scale. Therefore, a significant 7Li production from novae and low-mass stars is needed to explain the late, steep rise of the 7Li abundance in disk stars and the meteoritic 7Li abundance.

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arXiv1999

The HR diagram of Globular Clusters: Theorist' view(s)

I list the characteristic features of Globular Cluster (GC) HR diagrams which provide a complete test of the stellar evolution of low mass stars: morphologies describing the different evolutionary phases, number ratios and luminosity functions, which add quantitative information. Then I explore the stage of todays' understanding of the classical distance scale indicators (providing warnings against model construction which, underneath, already is based on a choice of distance), and compare them to the `new' indicators, such as the white dwarfs and the first and second kink of the low main sequence. The classical and new distance indicators are still subject to uncertainties due to tiny details of the theory, which are all of the same order of magnitude, \~0.25mag and the absolute ages of GCs can not be constrained to better than 10-16Gyr. However, most of recent theoretical and observational results (including both classical and new indicators) point more towards the lower range of ages (10-12) than to the upper range (15-16).

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