arXiv subjects

O. G. Benvenuto

Publications and source records attributed to O. G. Benvenuto.

At least 37 records · Page 2Linked to original sources

arXiv2013

An evolutionary model for the gamma-ray system PSR J1311-3430 and its companion

The most recent member of the millisecond pulsar with very low-mass companions and short orbital periods class, PSR J1311-3430 (Pletsch et al. 2012) is a remarkable object in various senses. Besides being the first discovered in gamma-rays, its measured features include the very low or absent hydrogen content. We show in this Letter that this important piece of information leads to a very restricted range of initial periods for a given donor mass. For that purpose, we calculate in detail the evolution of the binary system self-consistently, including mass transfer and evaporation, finding the features of the new evolutionary path leading to the observed configuration. It is also important to remark that the detailed evolutionary history of the system naturally leads to a high final pulsar mass, as it seems to be demanded by observations.

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arXiv2012

Evolutionary trajectories of ultra-compact "black widow" pulsars with very low mass companions

The existence of millisecond pulsars with planet-mass companions (Bailes et al., 2011) in close orbits is challenging from the stellar evolution point of view. We calculate in detail the evolution of binary systems self-consistently, including mass transfer, evaporation and irradiation of the donor by X-rays feedback, demonstrating the existence of a new evolutionary path leading to short periods and compact donors as required by the observations of PSR J1719-1438. We also point out the alternative of an exotic nature of the companion planet-mass star.

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arXiv2012

Evolution of low mass close binary systems with a neutron star: its dependence with the initial neutron star mass

We construct a set of binary evolutionary sequences for systems composed by a normal, solar composition, donor star together with a neutron star. We consider a variety of masses for each star as well as for the initial orbital period corresponding to systems that evolve to ultra-compact or millisecond pulsar-helium white dwarf pairs. Specifically, we select a set of donor star masses of 0.50, 0.65, 0.80, 1.00, 1.25, 1.50, 1.75, 2.00, 2.25, 2.50, 3.00, and 3.50 solar masses, whereas for the accreting neutron star we consider initial masses values of 0.8, 1.0, 1.2, and 1.4 solar masses. The considered initial orbital period interval ranges from 0.5 to 12 days. It is found that the evolution of systems, with fixed initial values for the orbital period and the mass of the normal donor star, heavily depends upon the mass of the neutron star. In some cases, varying the initial value of the neutron star mass, we obtain evolved configurations ranging from ultra-compact to widely separated objects. We also analyse the dependence of the final orbital period with the mass of the white dwarf. In agreement with previous expectations, our calculations show that the final orbital period-white dwarf mass relation is fairly insensitive to the initial neutron star mass value. A new period-mass relation based on our own calculations is proposed, which is in good agreement with period-mass relations available in the literature. As consequence of considering a set of values for the initial neutron star mass, these models allow finding different plausible initial configurations (donor and neutron star masses and orbital period interval) for some of the best observed binary systems of the kind we are interested in here. We apply our calculations to analyse the case of PSR J0437-4715.

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arXiv2012

The evolution of low mass, close binary systems with a neutron star component: a detailed grid

In close binary systems composed of a normal, donor star and an accreting neutron star, the amount of material received by the accreting component is, so far, a real intrigue. In the literature there are available models that link the accretion disk surrounding the neutron star with the amount of material it receives, but there is no model linking the amount of matter lost by the donor star to that falling onto the neutron star. In this paper we explore the evolutionary response of these close binary systems when we vary the amount of material accreted by the neutron star. We consider a parameter β, which represents the fraction of material lost by the normal star that can be accreted by the neutron star. βis considered as constant throughout evolution. We have computed the evolution of a set of models considering initial donor star masses (in solar units) between 0.5 and 3.50, initial orbital periods (in days) between 0.175 and 12, initial masses of neutron stars (in solar units) of 0.80, 1.00, 1.20 and 1.40 and several values of beta. We assumed solar abundances. These systems evolve to ultracompact or to open binary systems, many of which form low mass helium white dwarfs. We present a grid of calculations and analyze how these results are affected upon changes in the value of β. We find a weak dependence of the final donor star mass with respect to β. In most cases this is also true for the final orbital period. The most sensitive quantity is the final mass of the accreting neutron star. As we do not know the initial mass and rotation rate of the neutron star of any system, we find that performing evolutionary studies is not helpful for determining β.

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arXiv2011

Simultaneous formation of Solar System giant planets

In the last few years, the so-called "Nice model" has got a significant importance in the study of the formation and evolution of the solar system. According to this model, the initial orbital configuration of the giant planets was much more compact than the one we observe today. We study the formation of the giant planets in connection with some parameters that describe the protoplanetary disk. The aim of this study is to establish the conditions that favor their simultaneous formation in line with the initial configuration proposed by the Nice model. We focus in the conditions that lead to the simultaneous formation of two massive cores, corresponding to Jupiter and Saturn, able to achieve the cross-over mass (where the mass of the envelope of the giant planet equals the mass of the core, and gaseous runway starts) while Uranus and Neptune have to be able to grow to their current masses. We compute the in situ planetary formation, employing the numerical code introduced in our previous work, for different density profiles of the protoplanetary disk. Planetesimal migration is taken into account and planetesimals are considered to follow a size distribution between $r_p^{min}$ (free parameter) and $r_p^{max}= 100$ km. The core's growth is computed according to the oligarchic growth regime. The simultaneous formation of the giant planets was successfully completed for several initial conditions of the disk. We find that for protoplanetary disks characterized by a power law ($Σ\propto r^{-p}$), smooth surface density profiles ($p \leq 1.5$) favor the simultaneous formation. However, for steep slopes ($p\sim 2$, as previously proposed by other authors) the simultaneous formation of the solar system giant planets is unlikely ...

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arXiv2011

On the occurrence and detectability of Bose-Einstein condensation in helium white dwarfs

It has been recently proposed that helium white dwarfs may provide promising conditions for the occurrence of the Bose-Einstein condensation. The argument supporting this expectation is that in some conditions attained in the core of these objects, the typical De Broglie wavelength associated with helium nuclei is of the order of the mean distance between neighboring nuclei. In these conditions the system should depart from classical behavior showing quantum effects. As helium nuclei are bosons, they are expected to condense. In order to explore the possibility of detecting the Bose-Einstein condensation in the evolution of helium white dwarfs we have computed a set of models for a variety of stellar masses and values of the condensation temperature. We do not perform a detailed treatment of the condensation process but mimic it by suppressing the nuclei contribution to the equation of state by applying an adequate function. As the cooling of white dwarfs depends on average properties of the whole stellar interior, this procedure should be suitable for exploring the departure of the cooling process from that predicted by the standard treatment. We find that the Bose-Einstein condensation has noticeable, but not dramatic effects on the cooling process only for the most massive white dwarfs compatible with a helium dominated interior (\approx 0.50 M_\odot) and very low luminosities (say, Log(L/L_\odot) < -4.0). These facts lead us to conclude that it seems extremely difficult to find observable signals of the Bose-Einstein condensation. Recently, it has been suggested that the population of helium white dwarfs detected in the globular cluster NGC 6397 is a good candidate for detecting signals of the Bose-Einstein condensation. We find that these stars have masses too low and are too bright to have an already condensed interior.

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arXiv2010

Consequences of the simultaneous formation of giant planets by the core accretion mechanism

The core accretion mechanism is presently the most widely accepted cause of the formation of giant planets. For simplicity, most models presently assume that the growth of planetary embryos occurs in isolation. We explore how the simultaneous growth of two embryos at the present locations of Jupiter and Saturn affects the outcome of planetary formation. We model planet formation on the basis of the core accretion scenario and include several key physical ingredients. We consider a protoplanetary gas disk that exponentially decays with time. For planetesimals, we allow for a distribution of sizes from 100~m to 100~km with most of the mass in the smaller objects. We include planetesimal migration as well as different profiles for the surface density $Σ$ of the disk. The core growth is computed in the framework of the oligarchic growth regime and includes the viscous enhancement of the planetesimal capture cross-section. Planet migration is ignored. By comparing calculations assuming formation of embryos in isolation to calculations with simultaneous embryo growth, we find that the growth of one embryo generally significantly affects the other. This occurs in spite of the feeding zones of each planet never overlapping. The results may be classified as a function of the gas surface density profile $Σ$: if $Σ\propto r^{-3/2}$ and the protoplanetary disk is rather massive, Jupiter's formation inhibits the growth of Saturn. If $Σ\propto r^{-1}$ isolated and simultaneous formation lead to very similar outcomes; in the the case of $Σ\propto r^{-1/2}$ Saturn grows faster and induces a density wave that later acclerates the formation of Jupiter. Our results indicate that the simultaneous growth of several embryos impacts the final outcome and should be taken into account by planet formation models.

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arXiv2009

Oligarchic planetesimal accretion and giant planet formation II

The equation of state calculated by Saumon and collaborators has been adopted in most core-accretion simulations of giant-planet formation performed to date. Since some minor errors have been found in their original paper, we present revised simulations of giant-planet formation that considers a corrected equation of state. We employ the same code as Fortier and collaborators in repeating our previous simulations of the formation of Jupiter. Although the general conclusions of Fortier and collaborators remain valid, we obtain significantly lower core masses and shorter formation times in all cases considered. The minor errors in the previously published equation of state have been shown to affect directly the adiabatic gradient and the specific heat, causing an overestimation of both the core masses and formation times.

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arXiv2007

Oligarchic planetesimal accretion and giant planet formation

Aims. In the context of the core instability model, we present calculations of in situ giant planet formation. The oligarchic growth regime of solid protoplanets is the model adopted for the growth of the core. Methods. The full differential equations of giant planet formation were numerically solved with an adaptation of a Henyey-type code. The planetesimals accretion rate was coupled in a self-consistent way to the envelope's evolution. Results. We performed several simulations for the formation of a Jupiter-like object by assuming various surface densities for the protoplanetary disc and two different sizes for the accreted planetesimals. We find that the atmospheric gas drag gives rise to a major enhancement on the effective capture radius of the protoplanet, thus leading to an average timescale reduction of 30% -- 55% and ultimately to an increase by a factor of 2 of the final mass of solids accreted as compared to the situation in which drag effects are neglected. With regard to the size of accreted planetesimals, we find that for a swarm of planetesimals having a radius of 10 km, the formation time is a factor 2 to 3 shorter than that of planetesimals of 100 km, the factor depending on the surface density of the nebula. Moreover, planetesimal size does not seem to have a significant impact on the final mass of the core.

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arXiv2006

The evolutionary status of the white dwarf companion of the binary pulsar PSR J1713+0747

Splaver and coworkers have measured the masses of the white dwarf and the neutron star components of the PSR J1713+0747 binary system pair by Shapiro Delay. We attempt to find the original configuration of this system performing a set of binary evolution calculations to simultaneously account for the masses of both stars and the orbital period. We considered initial masses of 1.5 and 1.4 \msun for the normal (donor) and the neutron star, respectively. We assumed two metallicity values (Z = 0.010 and 0.020), and an initial orbital period near 3 days. We assume that the neutron star is only able to retain \lesssim 0.10 of the matter transferred by the donor star. Calculations were performed employing our binary hydro code that handles the mass transfer rate in a fully implicit way together with state-of-the-art physical ingredients, diffusion and a non-grey atmospheres. We compare the structure of the resulting white dwarfs with the characteristic age of PSR J1713+0747 finding a nice agreement with observations by Lundgren et al. especially for the case of a donor star with Z= 0.010. This result indicates that the evolution of this kind of binary system is well understood. The models predict that, due to diffusion, the atmosphere of the white dwarf is an almost hydrogen-pure one. We find that such structures are unable to account for the colours measured by Lundgren et al. within their error bars. Thus, some discrepances in the white dwarf emergent radiation remain to be explained.

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arXiv2004

The Formation of a Helium White Dwarf in Close Binary System with Diffusion

We study the evolution of a system composed by a 1.4 \msun neutron star and a normal, solar composition star of 2 \msun in orbit with a period of 1 day. Calculations were performed employing the binary hydro code presented in Benvenuto & De Vito (2003) that handle the mass transfer rate in a fully implicit way. Now we included the main standard physical ingredients together with diffusion processes and a proper outer boundary condition. We have assumed fully non - conservative mass transfer episodes. In order to study the interplay of mass loss episodes and diffusion we considered evolutionary sequences with and without diffusion in which all Roche lobe overflows (RLOFs) produce mass transfer. Another two sequences in which thermonuclearly-driven RLOFs are not allowed to drive mass transfer have been computed with and without diffusion. To our notice, this study represents the first binary evolution calculations in which diffusion is considered. The system produces a helium white dwarf of \sim 0.21 \msun in an orbit with a period of \sim 4.3 days for the four cases. We find that mass transfer episodes induced by hydrogen thermonuclear flashes drive a tiny amount of mass transfer. As diffusion produces stronger flashes, the amount of hydrogen - rich matter transferred is slightly higher than in models without diffusion. We find that diffusion is the main agent in determining the evolutionary timescale of low mass white dwarfs even in presence of mass transfer episodes.

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arXiv2003

A Code for Stellar Binary Evolution and its Application to the Formation of Helium White Dwarfs

We present a numerical code intended for calculating stellar evolution in close binary systems. In doing so, we consider that mass transfer episodes occur when the stellar size overflows the corresponding Roche lobe. In such situation we equate the radius of the star with the equivalent radius of the Roche lobe. This equation is handled implicitly together with those corresponding to the whole structure of the star. We describe in detail the necessary modifications to the standard Henyey technique for treating the mass loss rate implicitly together with thin outer layers integrations. We have applied this code to the calculation of the formation of low mass, helium white dwarfs in low mass close binary systems. We found that the global numerical convergence properties are fairly good. In particular, the onset and end of mass transfer episodes is computed automatically.

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arXiv2002

Evolution and colours of helium-core white dwarf stars: the case of low metallicity progenitors

The present work is designed to explore the evolution of helium-core white dwarf (HeWD) stars for the case of metallicities much lower than the solar one (Z=0.001 and Z=0.0002). Evolution is followed in a self-consistent way with the predictions of detalied and new non-grey atmospheres, time-dependent element diffusion and the history of the white dwarf progenitor. Reliable initial models for low mass HeWDs are obtained by applying mass loss rates to a 1msun stellar model. The loss of angular momentum caused by gravitational wave emission and magnetic stellar wind braking are considered. Model atmospheres, based on a detailed treatment of the microphysics entering the WD atmosphere enable us to provide accurate colours and magnitudes at both early and advanced evolutionary stages. We find that most of our evolutionary sequences experience several episodes of hydrogen thermonuclear flashes. In particular, the lower the metallicity, the larger the minimum stellar mass for the occurrence fo flashes induced by CNO cycle reactions. The existence of a mass-threshold for the occurrence of diffusion-induced CNO flashes leadss to a marked dichotomy in the age of our models. Another finding of this study is that our HeWD models experience unstable hydrogen burning via PP nuclear reactions at late cooling stages as a result of hydrogen chemically diffusing inwards. Such PP flashes take place in models with very low metal content. We also find that models experiencing CNO flashes exhibit a pronouncede turn-off in most of their colours at M_V=16 approximately. Finally, colour-magnitude diagrams for our models are presented and compared with recent observational data of HeWD candidates in the globular clusters NGC 6397 and 47 Tucanae.

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arXiv2002

Improved synthetic spectra of helium-core white dwarf stars

We examine the emergent fluxes from helium-core white dwarfs following their evolution from the end of pre-white dwarf stages down to advanced cooling stages. For this purpose, we include a detailed treatment of the physics of the atmosphere, particularly an improved representation of the state of the gas by taking into account non-ideal effects according to the so-called occupation probability formalism. The present calculations also incorporate hydrogen line opacity from Lyman, Balmer and Paschen series, pseudo-continuum absorptions and new updated induced-dipole absorption from H$_2$-H$_2$, H$_2$-He and H-He pairs. We find that the non-ideal effects and line absorption alter the appearance of the stellar spectrum and have a significant influence upon the photometric colours in the UBVRI-JHKL system. This occurs specially for hot models $T_{\rm eff}\ga 8000$ due to line and pseudo-continuum opacities, and for cool models $T_{\rm eff}\la 4000$ where the perturbation of atoms and molecules by neighbour particles affects the chemical equilibrium of the gas. In the present study, we also include new cooling sequences for helium-core white dwarfs of very low mass (0.160 and 0.148 M$_\odot$) with metallicity $Z=0.02$. These computations provide theoretical support to search for and identify white dwarfs of very low mass, specially useful for recent and future observational studies of globular cluster, where these objects have began to be detected.

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arXiv2002

On mode trapping in pulsating DA white dwarf stars

The present work is designed to explore the effects of the time-dependent element diffusion on the mode trapping properties of DA white dwarf models with various thickness of the hydrogen envelope. Our predictions are compared with the standard assumption of diffusive equilibrium in the trace element approximation. We find that element diffusion markedly weakens the presence of mode trapping originated in the outer layers of the models, even for the case of thin hydrogen envelopes.

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arXiv2002

The mode trapping properties of full DA white dwarf evolutionary models

An adiabatic, non-radial pulsation study of a 0.563 M_sun DA white dwarf model is presented on the basis of new evolutionary calculations performed in a self-consistent way with the predictions of time dependent element diffusion, nuclear burning and the history of the white dwarf progenitor. Emphasis is placed on the role played by the internal chemical stratification of these new models in the behaviour of the eigenmodes, and the expectations for the full g-spectrum of periods. The implications for the mode trapping properties are discussed at length. In this regard, we find that, for high periods, the viability of mode trapping as a mode selection mechanism is markedly weaker for our models, as compared with the situation in which the hydrogen-helium transition region is treated assuming equilibrium diffusion in the trace element approximation.

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arXiv2002

The Effects of Element Diffusion on the Pulsational Properties of Variable DA White Dwarf Stars

We explore the effects of element diffusion due to gravitational settling and thermal and chemical diffusion on the pulsational properties of DA white dwarfs. To this end, we employ an updated evolutionary code coupled with a pulsational, finite difference code for computing the linear, non-radial g-modes in the adiabatic approximation. We follow the evolution of a 0.55 \msun white dwarf model in a self-consistent way with the evolution of chemical abundance distribution as given by time dependent diffusion processes. Results are compared with the standard treatment of diffusive equilibrium in the trace element approximation. Appreciable differences are found between the two employed treatments. We conclude that time dependent element diffusion plays an important role in determining the whole oscillation pattern and the temporal derivative of the periods in DAV white dwarfs. In addition, we discuss the plausibility of the standard description employed in accounting for diffusion in most of white dwarf asteroseismological studies.

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arXiv2002

Time dependent diffusion in pulsating white dwarf stars: Asteroseismology of G117-B15A

We study the structural characteristic of the variable DA white dwarf G117B-15A by applying the methods of asteroseismology. For such a purpose, we construct white dwarf evolutionary models considering a detailed and up-to-date physical description as well as several processes responsible for the occurrence of element diffusion. We have considered several thickness for the outermost hydrogen layer, whereas for the inner helium-, carbon- and oxygen-rich layers we considered realistic profiles predicted by calculations of the white dwarf progenitor evolution. The evolution of each of the considered model sequences were followed down to very low effective temperature; in particular, from 12500K on we computed the dipolar, linear, adiabatic oscillations with low radial order. We find that asteroseismological results are not univocal regarding mode identification for the case of G117B-15A. However, our asteroseismological results are compatible with spectroscopical data only if the observed periods of 215.2, 271.0 and 304.4 s are due to dipolar modes with $k=2, 3, 4$ respectively. Our calculations indicate that the best fit to the observed period pattern of G117B-15A corresponds to a DA white dwarf structure with a stellar mass of 0.525 \msun, with a hydrogen mass fraction \lmh$\gtrsim$-3.83 at an effective temperature \teff$\approx$11800K.

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