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ABSTRACT: We present radial velocity observations of four extremely low-mass (0.2 Msol) white dwarfs. All four stars show peak-to-peak radial velocity variations of 540 - 710 km/s with 1.0 - 5.9 hr periods. The optical photometry rules out main-sequence companions. In addition, no milli-second pulsar companions are detected in radio observations. Thus the invisible companions are most likely white dwarfs. Two of the systems are the shortest period binary white dwarfs yet discovered. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.46 Msol; thus those are carbon/oxygen core white dwarfs. The unknown inclination angles prohibit a definitive conclusion about the future of these systems. However, the chance of a supernova Ia event is only 1% to 5%. These systems are likely to form single R Coronae Borealis stars, providing evidence for a white dwarf + white dwarf merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70% chance of having a low-mass white dwarf companion. This system will probably form a single helium-enriched subdwarf O star. All four white dwarf systems have unusal mass ratios of < 0.2-0.8 that may also lead to the formation of AM CVn systems. Comment: ApJ, accepted version
11/2009;
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ABSTRACT: We present full evolutionary calculations appropriate for the study of hot hydrogen-deficent DO white dwarfs, PG 1159 stars, and DB white dwarfs. White dwarf sequences are computed for a wide range of stellar masses and helium envelopes on the basis of a complete treatment of the evolutionary history of progenitors stars, including the core hydrogen and helium burning phases, the thermally-pulsing AGB phase, and the born-again episode that is responsible for the hydrogen deficiency. We also provide colors and magnitudes for the new sequences for $T_{\rm eff} < 40 000$ K, where the NLTE effects are not dominant. These new calculations provide an homogeneous set of evolutionary tracks appropriate for mass and age determinations for both PG 1159 stars and DO white dwarfs. The calculations are extended down to an effective temperature of 7 000 K. We applied these new tracks to redetermine stellar masses and ages of all known DO white dwarfs with spectroscopically-determined effective temperatures and gravities, and compare them with previous results. We also compare for the first time consistent mass determinations for both DO and PG 1159 stars, and find a considerably higher mean mass for the DO white dwarfs. We discuss as well the chemical profile expected in the envelope of variable DB white dwarfs from the consideration of the evolutionary history of progenitor stars. Finally, we present tentative evidence for a different evolutionary channel, other than that involving the PG 1159 stars, for the formation of hot, hydrogen-deficient white dwarfs. Comment: To be published in The Astrophysical Journal
09/2009;
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ABSTRACT: Motivated by the recent detection of single and binary He-core white dwarfs in metal-rich clusters, we present a full set of evolutionary calculations and colors appropriate for the study of such white dwarfs. The paper is also aimed at investigating whether stable hydrogen burning may constitute a main source of energy for massive He-core white dwarfs resulting from high-metallicity progenitors. White dwarf sequences are derived by taking into account the evolutionary history of progenitor stars with supersolar metallicities. We also incorporate a self-consistent, time-dependent treatment of gravitational settling and chemical diffusion, as well as of the residual nuclear burning. We find that the influence of residual nuclear burning during the late stages of white dwarf evolution is strongly dependent on the occurrence of chemical diffusion at the base of the hydrogen-rich envelope. When no diffusion is considered, residual hydrogen burning strongly influences the advanced stages of white dwarf cooling, introducing evolutionary delays of several Gyr. By contrast, when diffusion is taken into account the role of residual nuclear burning is strongly mitigated, and the evolution is dictated only by the thermal content stored in the ions. In addition, for all of our sequences, we provide accurate color and magnitudes on the basis of new and improved non gray model atmospheres which explicitly include Ly$\alpha$ quasi-molecular opacity. Comment: 10 pages, 10 figures, 1 table. To be published in Astronomy & Astrophysics
05/2009;
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ABSTRACT: We study the full evolution of low-mass white dwarfs with helium and oxygen cores. We revisit the age dichotomy observed in many white dwarf companions to millisecond pulsar on the basis of white dwarf configurations derived from binary evolution computations. We evolve 11 dwarf sequences for helium cores with final masses of 0.1604, 0.1869, 0.2026, 0.2495, 0.3056, 0.3333, 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M⊙. In addition, we compute the evolution of five sequences for oxygen cores with final masses of 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M⊙. A metallicity of Z= 0.02 is assumed. Gravitational settling, chemical and thermal diffusion are accounted for during the white dwarf regime. Our study reinforces the result that diffusion processes are a key ingredient in explaining the observed age and envelope dichotomy in low-mass helium-core white dwarfs, a conclusion we arrived at earlier on the basis of a simplified treatment for the binary evolution of progenitor stars. We determine the mass threshold where the age dichotomy occurs. For the oxygen white dwarf sequences, we report the occurrence of diffusion-induced, hydrogen-shell flashes, which, as in the case of their helium counterparts, strongly influence the late stages of white dwarf cooling. Finally, we present our results as a set of white dwarf mass–radius relations for helium and oxygen cores.
Monthly Notices of the Royal Astronomical Society 11/2007; 382(2):779 - 792. · 4.90 Impact Factor
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ABSTRACT: We present evolutionary calculations aimed at describing the born-again scenario for post-AGB remnant stars of 0.5842 and 0.5885 \msun. Results are based on a detailed treatment of the physical processes responsible for the chemical abundance changes. We considered two theories of convection: the standard mixing length theory (MLT) and the double-diffusive GNA convection developed by Grossman et al. The latter accounts for the effect of the chemical gradient ($\nabla\mu$) in the mixing processes and in the transport of energy. We also explore the dependence of the born-again evolution on some physical hypothesis, such as the effect of the existence of non-zero chemical gradients, the prescription for the velocity of the convective elements and the size of the overshooting zones. Attention is given to the behavior of the born-again times and to the chemical evolution during the ingestion of protons. We find that in our calculations born again times are dependent on time resolution. In particular when the minimum allowed time step is below $5 \times 10^{-5}$ yr we obtain, with the standard mixing length theory, born again times of 5-10 yr. This is true without altering the prescription for the efficiency of convective mixing during the proton ingestion. On the other hand we find that the inclusion of the chemical gradients in the calculation of the mixing velocity tend to increase the born again times by about a factor of two. In addition we find that proton ingestion can be seriously altered if the occurrence of overshooting is modified by the $\nabla\mu$-barrier at the H-He interface, strongly altering born again times. Comment: 15 pages including 13 figures and 2 tables
11/2005;
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ABSTRACT: This paper is designed to explore the formation and evolution of hydrogen-deficient post-AGB white dwarfs. To this end, we compute the complete evolution of an initially 2.7 M_sun star from the zero-age main sequence through the thermally pulsing and mass-loss phases to the white dwarf stage. Particular attention is given to the chemical abundance changes during the whole evolution. A time-dependent scheme for the simultaneous treatment of abundance changes caused by nuclear reactions, diffusive overshooting, salt fingers and convection is considered. We employed the double-diffusive mixing-length theory of convection for fluids with composition gradients (Grossman & Taam 1996). The study can therefore be considered as a test of its performance in low-mass stars. Also, time-dependent element diffusion for multicomponent gases is taken into account during the white dwarf evolution. The evolutionary stages corresponding to the last helium thermal pulse on the early white-dwarf cooling branch and the following born-again episode are carefully explored. Relevant aspects for PG1159 stars and DB white dwarf evolution are studied in the frame of these new evolutionary models that take into account the history of the white dwarf progenitor. The scope of the calculations is extended to the domain of the helium-rich, carbon-contaminated DQ white dwarfs with the aim of exploring the plausibility of the evolutionary connection PG1159-DB-DQ. In this regard, the implications for the double-layered chemical structure in pulsating DB white dwarfs is investigated. Another aspect of the investigation concerned the consequences of mass-loss episodes during the PG1159 stage for the chemical stratification of the outer layer of DB and DQ white dwarfs. Comment: 20 pages, 15 figures. Accepted by Astronomy & Astrophysics
01/2005;
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ABSTRACT: The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we follow the evolution of a 0.33 Msun white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. The evolutionary stages prior to the white dwarf formation are also fully accounted for by computing the conservative binary evolution of an initially 2.5-Msun Pop. I star with a 1.25 Msun companion, and period P_i= 3 days. Evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. We find that chemical diffusion induces the occurrence of an additional hydrogen thermonuclear flash which leads to stellar models with thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced stages of evolution. In addition, we explore the adiabatic pulsational properties of the resulting white dwarf models. As compared with their helium-core counterparts, low-mass oxygen-core white dwarfs are characterized by a pulsational spectrum much more featured, an aspect which could eventually be used for distinguishing both types of stars if low-mass white dwarfs were in fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass white dwarf models. Comment: 13 Pages, including 16 Postscript figures. Accepted for publication in MNRAS
09/2003;
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Revista Mexicana de Astronomía y Astrofísica. 01/2002;
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ABSTRACT: Recent measurements by Hipparcos provide strong observational evidence supporting the existence of white dwarf stars with iron-rich core composition. Here we examine the evolution of iron-rich white dwarfs, for which the cooling is substancially accelerated as compared with the standard carbon-oxigen white dwarfs.
Revista Mexicana de Astronomía y Astrofísica. 01/2001;
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ABSTRACT: Recent measurements made by Hipparcos (Provencal et al. 1998) present observational evidence supporting the existence of some white dwarf (WD) stars with iron - rich, core composition. In this connection, the present paper is aimed at exploring the structure and evolution of iron - core WDs by means of a detailed and updated evolutionary code. In particular, we examine the evolution of the central conditions, neutrino luminosity, surface gravity, crystallization, internal luminosity profiles and ages. We find that the evolution of iron - rich WDs is markedly different from that of their carbon - oxygen counterparts. In particular, cooling is strongly accelerated as compared with the standard case. Thus, if iron WDs were very numerous, some of them would have had time enough to evolve at lower luminosities than that corresponding to the fall - off in the observed WD luminosity function. Comment: 8 pages, 21 figures. Accepted for publication in MNRAS
11/1999;
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ABSTRACT: The purpose of this work is to present accurate and detailed mass-radius relations for white dwarf (WD) models with helium, carbon, oxygen, silicon and iron cores, by using a fully updated stellar evolutionary code. We considered masses from 0.15 to 0.5 Mo for the case of helium core, from 0.45 to 1.2 Mo for carbon, oxygen and silicon cores and from 0.45 to 1.0 Mo for the case of an iron core. In view of recent measurements made by Hipparcos that strongly suggest the existence of WDs with an iron-dominated core, we focus our attention mainly on the finite-temperature, mass-radius relations for WD models with iron interiors. Furthermore, we explore the effects of gravitational, chemical and thermal diffusion on low-mass helium WD models with hydrogen and helium envelopes.
10/1999;
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ABSTRACT: We explore the formation and evolution of hydrogen-deficient post-AGB white dwarfs. To this end, we compute the complete evolution of an initially $2.7 ~M_{\odot}$ star from the zero-age main sequence through the thermally pulsing and mass-loss phases to the white dwarf stage. Particular attention is given to the chemical abundance changes during the whole evolution. A time-dependent scheme for the simultaneous treatment of abundance changes caused by nuclear reactions, diffusive overshooting, salt fingers and convection is considered. We employed the double-diffusive mixing-length theory of convection for fluids with composition gradients. The study can therefore be considered as a test of its performance in low-mass stars. Also, time-dependent element diffusion for multicomponent gases is taken into account during the white dwarf evolution. The evolutionary stages corresponding to the last helium thermal pulse on the early white-dwarf cooling branch and the following born-again episode are carefully explored. Relevant aspects for PG 1159 stars and DB white dwarf evolution are studied in the framework of these new evolutionary models that take into account the history of the white dwarf progenitor. The scope of the calculations is extended to the domain of the helium-rich, carbon-contaminated DQ white dwarfs with the aim of exploring the plausibility of the evolutionary connection PG 1159-DB-DQ. In this regard, the implications for the double-layered chemical structure in pulsating DB white dwarfs is investigated. We examine the consequences of mass-loss episodes during the PG 1159 stage for the chemical stratification of the outer layer of DB and DQ white dwarfs.
http://dx.doi.org/10.1051/0004-6361:20041965.
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ABSTRACT: We present evolutive results for stars where we have employed the
chemistry diffusion scheme; for masses from 5 to 30 Msolar
starting from Main Sequence. In particular, we show in detail the
stellar evolution for 5 Msolar to thermal pulses and we
compute the chemistry diffusion in the convective zones. At the present,
we have computed the chemistry mixing in the convective zones, such as
it being instantly, it is equivalent to a fully effective diffusion,
that is with huge diffusion coeficients. Nevertheless, no occur like it,
therefore is necesary the study of the diffusion of the different
isotopes in the convective zones. The solution of this problem, has
inside the inversion the huge matrixes, if we call with N to number of
layers of the any convective zone, and M to number of isotopes have in
account in whole star, then the systems to resolve would be of N*M
× N*M, and if the typical numbers are about of 2000 to 4000
layers, or more for huge convective regions, and if we consider that the
isotope number would be bigger than 10, we have at moment of the
computes that the time employed is very important. Resolve the diffusion
will let us, for example, have right results of Lithium abundances in
the stars, since with an instantaneus mixing is not possible. The
Lithium has being ever object of investigation because to cosmologic
importance.
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina. 44:45.
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ABSTRACT: The present work is designed to explore the evolutionary and
pulsational properties of low-mass white dwarfs with carbon/oxygen
cores. In particular, we follow the evolution of a 0.33 solar masses
white dwarf remnant in a self-consistent way with the predictions of
nuclear burning, element diffusion and the history of the white dwarf
progenitor. The evolutionary stages prior to the white dwarf formation
are also fully accounted for by computing the conservative binary
evolution of an initially 2.5 solar masses Pop. I star with a 1.25
solar masses companion, and orbital period of 3 days. Evolution is
followed down to the domain of the ZZ Ceti stars on the white dwarf
cooling branch.We find that chemical diffusion induces the occurrence
of an additional hydrogen thermonuclear flash which leads to stellar
models with thin hydrogen envelopes. As a result, a fast cooling is
encountered at advanced stages of evolution. In addition, we
explore the adiabatic pulsational properties of the resulting white
dwarf models. As compared with their helium-core counterparts,
low-mass oxygen-core white dwarfs are characterized by a pulsational
spectrum much more featured, an aspect which could eventually be
used for distinguishing both types of stars if low-mass white dwarfs
were in fact found to pulsate as ZZ Ceti - type variables.
Finally, we perform a non-adiabatic pulsational analysis on the
resulting carbon/oxygen low-mass white dwarf models.
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina. 46:32-33.
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ABSTRACT: Recientes mediciones de la masa y el radio hechas por Hipparcos de las
estrellas enanas blancas 40 Eri B y Procyon B (Shipman, H. &
Provencal, J. - ApJ. 1998, 494, 759), sugieren un núcleo
compuesto de hierro para dichas estrellas, en lugar de carbono y
oxígeno como predice la teoría standard de
evolución estelar. Para interpretar estas observaciones,
presentamos aquí, relaciones masa-radio para configuraciones
degeneradas a temperatura finita para distintas composiciones
químicas centrales. Para tal fin hemos calculado secuencias
evolutivas de enanas blancas utilizando el código de
evolución estelar, desarrollado en el Observatorio de La Plata.
Dicho código resuelve las ecuaciones de estructura y
evolución estelar mediante la técnica de relajación
de Henyey, y esta basado en una descripción física muy
detallada y actualizada.
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina. 42:22-22.
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ABSTRACT: Podríamos identificar a las estrellas de helio con estrellas
Wolf-Rayet (WR) que han perdido su envoltura rica en hidrógeno ya
sea porque las mismas pertenecen a sistemas binarios o a través
de fuertes vientos estelares. Las WR representan una etapa evolucionaria
normal de las estrellas masivas, cuya pérdida de masa es >= 3
× 10-5Msolar/yr y la cual es sufrida por la
estrella en un tiempo de escala mucho menor que el tiempo en que se
produce la quema del He. Esto garantiza la ``homogeneidad'' de las
estrellas de helio para nuestros modelos. Este tipo de estrellas
serían posibles progenitores de SN tipo Ib y Ic. Aquí
presentamos un estudio sobre la evolución de estrellas de helio a
partir de la secuencia principal de helio, pasando por el flash de
carbono, hasta agotarlo en la región central; como así
también la dependencia con la variable masa y con la
pérdida de la misma para distintos tipos de masas. Para tal fin
hemos utilizado un código de evolución estelar completo
que realiza todas las reacciones de Fowler en forma simultánea.
También se han tenido en cuenta los procesos de mezcla
convectiva, los principales mecanismos de emisión de neutrinos y
los efectos de la pérdida de masa. Las opacidades utilizadas
fueron las de Rogers & Iglesias (1992). Debido a la pérdida
de masa en este tipo de estrellas, hemos encontrado que los perfiles
convectivos, la composición química, las condiciones
centrales de temperatura y presión, luminosidad y temperatura
efectiva dependen en forma esencial de la velocidad de pérdida de
masa adoptada, lo que tendría profundas implicaciones en la
evolución posterior de estos objetos.
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina. 40:18-19.
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ABSTRACT: We present stellar evolutionary calculations for models with stellar
masses ranging from 1.2 to 20 Msolar. We follow the
calculations from the Main Sequence up to the phase of thermal pulses.
The emphasis is placed mainly on the analysis of the behaviour of a 5
Msolar model. The evolutionary code is based on the
Kippenhahn, Weigert, & Hofmeister (1967) method to compute stellar
evolution. The structure and stellar evolution equations for the stellar
interior are integrated using the standard Henyey method. The degree of
superadiabaticity is computed from the mixing length theory of
convection (Böhm - Vitense 1958). The equation of state we employed
takes into account partial ionization, radiation pressure and
relativistic degeneracy for electrons at finite temperature. Radiative
opacities with metallicity Z=0.02 are taken from Rogers & Iglesias
(1996). Conductive opacities for the low - density regime are from the
fits of Iben (1975) to the calculations of Hubbard & Lampe (1969).
For higher densities we use the results of Itoh et. al (1983). The
molecular opacities are those of Alexander & Ferguson (1994). The
different mechanisms of neutrino emission are also taken account. In
particular, photo and pair neutrinos are from Itoh et al. (1989); plasma
neutrinos from Itoh et al. (1989) and Bremsstrahlung from Itoh et al.
(1992). Because the aim in this work has been to calculate the stages
corresponding to the thermal pulses, particular attention has been
devoted to the treatment of the numerical difficulties appearing in this
kind of calculation. To this end, we solve the equations describing the
structure and evolution of a star in terms of differences with respect
to time, instead of iterating the value of the physical variables
directly. This change has allowed us to calculate advanced evolutionary
stages such as the thermal pulses. In this regard, we find that our
models experiencies up to 10 thermal flashes.
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina. 43:34-35.
