Context. The OWN Survey has detected several O-type stars with composite spectra whose individual components show very different line broadening. Some of these stars have been revealed as binary ...systems whose components are asynchronous. This fact may be related to the processes acting in these systems (e.g., angular-momentum transfer, tidal forces, etc.) or to the origin of the binaries themselves. Aims. We aim to determine the orbital and physical parameters of the massive star HD 96264A in order to confirm its binary nature and to constrain the evolutionary status of its stellar components. Methods. We computed the spectroscopic orbit of the system based on the radial velocity analysis of 37 high-resolution, high-S/N, multi-epoch optical spectra. We disentangled the composite spectrum and determined the physical properties of the individual stellar components using FASTWIND models incorporated to the IACOB-GBAT tool. We also computed a set of evolutionary models to estimate the age of the system and explore its tidal evolution. Results. HD 96264A is a binary system composed of an O9.2 IV primary and a B0 V(n) secondary, with minimum masses of 15.0 ± 0.5 M⊙ and 9.9 ± 0.4 M⊙, respectively, in a wide and eccentric orbit (P = 124.336 ± 0.008 d; e = 0.265 ± 0.005). The primary and secondary components have different projected rotational velocities (∼40 and ∼215 km s−1 respectively), and the physical properties derived through quantitative spectroscopic analyses include masses of ∼20.5 M⊙ and 16.8 M⊙, respectively. The evolutionary models indicate an approximate age of 4.5 Myr for both stars in the pair, corresponding to current masses and radii of 26.0 M⊙ and 10.8 R⊙ for the primary, and 17.9 M⊙ and 7.0 R⊙ for the secondary. Conclusions. The youth and wide orbit of the system indicate that the non-synchronous rotational nature of its components is a consequence of the stellar formation process rather than tidal evolution. This circumstance should be accounted for in theories of binary star formation.
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 a situation we equate the radius of the star to 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-layer 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 find that the global numerical convergence properties are fairly good. In particular, the onset and end of mass transfer episodes are computed automatically.
We study the properties of the non-radial pulsations of strange dwarf stars. These objects are white dwarfs (WDs) with a compact core made up of strange quark matter (SQM). We show that the SQM core ...compresses the surrounding normal matter strongly enough to give rise to the occurrence of a sharp peak in the Brunt–Väisälä frequency. This, in turn, allows for the existence of a completely new resonant cavity for gravity (g-) modes, which is absent in standard WDs. We study the cases in which the mass of the SQM core is 10−2, 10−3, 10−4 and 10−5 of the total stellar mass, which have been added to a 0.525 M⊙ WD model adequate to account for the period structure of the DAV G117B15A, showing that this new resonant cavity is present for such a large range of core mass fractions. Due to the extremely short wavelength of g-modes in the new resonant cavity, we treat oscillations there with an asymptotic analysis up to an intermediate, evanescent zone (located at ≈10 per cent of the stellar radius). At such a point, we consider the asymptotic treatment as a boundary condition for a self-consistent numerical calculation of the g-mode spectrum of oscillations. In particular, we consider dipolar oscillations, which are currently identified with the observed oscillations in standard WDs. We find a very distinctive signal for the presence of a SQM core inside a WD: the difference of periods between two consecutive modes is far shorter than it is in standard WDs due to the oscillations in the new resonant cavity, being even shorter than a second. This confirms previous expectations based on very simplified calculations. Our calculations indicate that, while the period spacing between consecutive modes is a smooth function of the period, the square of the amplitude of the modes near the SQM core is a strongly varying function. While some modes will have large amplitude there, and thus large kinetic energy, others will have far lower energy. Then, if (as usual) we assume that the excited modes are those with low kinetic energy, we expect a very particular spectrum of dipolar oscillations of WDs with SQM cores. The spectrum should be characterized by several well-detached sets of a very large number of evenly (in period) spaced modes. This should be considered as a clearly distinctive, observable signature of the presence of SQM inside WDs.
Context. We study the evolution of close binary systems composed of a normal, intermediate mass star and a neutron star considering a chemical composition typical of that present in globular clusters ...(Z = 0.001). Aims. We look for similarities and differences with respect to solar composition donor stars, which we have extensively studied in the past. As a definite example, we perform an application on one of the redbacks located in a globular cluster. Methods. We performed a detailed grid of models in order to find systems that represent the so-called redback binary radio pulsar systems with donor star masses between 0.6 and 2.0 solar masses and orbital periods in the range 0.2–0.9 d. Results. We find that the evolution of these binary systems is rather similar to those corresponding to solar composition objects, allowing us to account for the occurrence of redbacks in globular clusters, as the main physical ingredient is the irradiation feedback. Redback systems are in the quasi-RLOF state, that is, almost filling their corresponding Roche lobe. During the irradiation cycle the system alternates between semi-detached and detached states. While detached the system appears as a binary millisecond pulsar, called a redback. Circumstellar material, as seen in redbacks, is left behind after the previous semi-detached phase. Conclusions. The evolution of binary radio pulsar systems considering irradiation successfully accounts for, and provides a way for, the occurrence of redback pulsars in low-metallicity environments such as globular clusters. This is the case despite possible effects of the low metal content of the donor star that could drive systems away from redback configuration.
We continue the study of the properties of non-radial pulsations of strange dwarfs. These stars are essentially white dwarfs with a strange quark matter (SQM) core. We have previously shown that the ...spectrum of oscillations should be formed by several, well-detached clusters of modes inside which the modes are almost evenly spaced. Here, we study the relation between the characteristics of these clusters and the size of the SQM core. We do so assuming that, for a given cluster, the kinetic energy of the modes is constant. For a constant amplitude of the oscillation at the stellar surface, we find that the kinetic energy of the modes is very similar for the cases of models with LogQ
SQM=−2, −3 and −4, while it is somewhat lower for LogQ
SQM=−5 (here Q
SQM≡M
SQM/M; M
SQM and M are the masses of the SQM core and the star, respectively). Remarkably, the shape (amplitude of the modes versus period of oscillation) of the clusters of periods is very similar. However, the number of modes inside each cluster is strongly (and non-monotonously) dependent upon the size of the SQM core.
The characteristics of the spectrum of oscillations of strange dwarf stars are very different from the ones corresponding to normal white dwarfs and should be, in principle, observable. Consequently, the stars usually considered as white dwarfs may indeed provide an interesting and affordable way to detect SQM in an astrophysical environment.
The purpose of this work is to explore the evolution of helium-core white dwarf stars in a self-consistent way with the predictions of detailed non-grey model atmospheres and element diffusion. To ...this end, we consider helium-core white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 M⊙ and follow their evolution from the end of mass-loss episodes, during their pre-white dwarf evolution, down to very low surface luminosities. We find that when the effective temperature decreases below 4000 K, the emergent spectrum of these stars becomes bluer within time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour–colour and in the colour–magnitude diagrams and find that helium-core white dwarfs with masses ranging from ∼0.18 to 0.3 M⊙ can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than MV≈16.5. In view of these results, many low-mass helium white dwarfs could have had enough time to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.
The present work is designed to explore the evolution of helium-core white dwarf (He WD) stars for the case of metallicities much lower than the solar metallicity (Z= 0.001 and 0.0002). Evolution is ...followed in a self-consistent way with the predictions of detailed and new non-grey model atmospheres, time-dependent element diffusion and the history of the white dwarf progenitor. Reliable initial models for low-mass He WDs are obtained by applying mass-loss rates to a 1-M⊙ stellar model in such a way that the stellar radius remains close to the Roche lobe radius. 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 (such as the formalism of Hummer—Mihalas to deal with non-ideal effects) and hydrogen line and pseudo-continuum opacities, 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 of flashes induced by CNO cycle reactions. The existence of a mass threshold for the occurrence of diffusion-induced CNO flashes leads to a marked dichotomy in the age of our models. Another finding of this study is that our He WD 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 pronounced turn-off in most of their colours at MV≈ 16. Finally, colour—magnitude diagrams for our models are presented and compared with recent observational data of He WD candidates in the globular clusters NGC 6397 and 47 Tucanae.
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 H2–H2, H2–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 Teff≳ 8000 owing to line and pseudo-continuum opacities, and for cool models Teff≲ 4000 where the perturbation of atoms and molecules by neighbouring 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⊙) 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 clusters, where these objects have began to be detected.
Context.
The OWN Survey has detected several O-type stars with composite spectra whose individual components show very different line broadening. Some of these stars have been revealed as binary ...systems whose components are asynchronous. This fact may be related to the processes acting in these systems (e.g., angular-momentum transfer, tidal forces, etc.) or to the origin of the binaries themselves.
Aims.
We aim to determine the orbital and physical parameters of the massive star HD 96264A in order to confirm its binary nature and to constrain the evolutionary status of its stellar components.
Methods.
We computed the spectroscopic orbit of the system based on the radial velocity analysis of 37 high-resolution, high-S/N, multi-epoch optical spectra. We disentangled the composite spectrum and determined the physical properties of the individual stellar components using
FASTWIND
models incorporated to the
IACOB-GBAT
tool. We also computed a set of evolutionary models to estimate the age of the system and explore its tidal evolution.
Results.
HD 96264A is a binary system composed of an O9.2 IV primary and a B0 V(n) secondary, with minimum masses of 15.0 ± 0.5
M
⊙
and 9.9 ± 0.4
M
⊙
, respectively, in a wide and eccentric orbit (
P
= 124.336 ± 0.008 d;
e
= 0.265 ± 0.005). The primary and secondary components have different projected rotational velocities (∼40 and ∼215 km s
−1
respectively), and the physical properties derived through quantitative spectroscopic analyses include masses of ∼20.5
M
⊙
and 16.8
M
⊙
, respectively. The evolutionary models indicate an approximate age of 4.5 Myr for both stars in the pair, corresponding to current masses and radii of 26.0
M
⊙
and 10.8
R
⊙
for the primary, and 17.9
M
⊙
and 7.0
R
⊙
for the secondary.
Conclusions.
The youth and wide orbit of the system indicate that the non-synchronous rotational nature of its components is a consequence of the stellar formation process rather than tidal evolution. This circumstance should be accounted for in theories of binary star formation.
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