Hydrogen burning on a white dwarf accreting hydrogen in a binary system
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Deflagration
Intermediate polar
Roche lobe
Accretion in white-dwarf binary systems can occur through discs, accretion columns or a combination of these, depending on the magnetic field of the white dwarf. Recent high-quality X-ray observations with the XMM-Newton and Chandra observatories have significantly advanced our understanding of the physics of the accretion process, and place severe tests on our existing models. There have been some surprises, such as the strong dependence of atmospheric heating on accretion rate. However, we believe that we are now confident that we understand in general the physical processes in the accretion region, although some complicating factors, such as absorption, remain. We also discuss new developments in ultra-short-period white-dwarf binary systems.
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Abstract The growth rate of a white dwarf which accretes hydrogen-rich or helium matter is studied. If the accretion rate is relatively small, unstable shell flash occurs and during which the envelope mass is lost. We have followed the evolutions of shell flashes by steady state approach with wind mass loss solutions to determined the mass lost from the system for wide range of binary parameters. The time-dependent models are also calculated in some cases. The mass loss due to the Roche lobe overflow are taken into account. This results seriously affects the existing scenarios on the origin of the type I supernova or on the neutron star formation induced by accretion.
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Cataclysmic variable star
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We present a detailed calculation of the evolution of low-mass helium white dwarfs. These white dwarfs are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with masses between 0.001 to 0.06 M⊙. We found that the majority of our computed models experience one or two hydrogen shell flashes. The duration of the flashes is between a few ×106 y to a few ×107 y. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. The presence of low-mass helium white dwarf secondaries in millisecond pulsar binaries allows to determine the age of the systems independently of the rotational history of the pulsars. The same method may be applied to double degenerate systems. We discuss the cooling history of the low-mass, helium core white dwarfs in short orbital period millisecond pulsars.
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Roche lobe
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An analysis of X-ray and optical light curves of the magnetic cataclysmic variable (MCV) BY Cam is presented. This system is one of three MCVs in which the spin period of the white dwarf and the binary orbital period differ by ∼1 per cent. As such these 'BY Cam' stars are important objects with which to probe the field structure of the magnetic white dwarf and ultimately the nature of synchronization of AM Her binaries. We confirm asynchronous rotation of the magnetic white dwarf with respect to the binary. We find evidence that the accretion stream accretes directly on to the white dwarf as in AM Her systems, but further, the stream impacts on to different magnetic poles over the course of the beat period. We present evidence that the optical and hard X-ray light curves modulate in phase, but together they are out of phase with the soft X-ray light curve. We confirm the spin down of the white dwarf which is expected to lead to the synchronization of the spin and orbital periods of BY Cam.
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Cataclysmic variable star
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Rotation period
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Magnetic cataclysmic variables are close binary systems containing a strongly magnetized white dwarf that accretes matter coming from an M-dwarf companion. High-energy radiation coming from those objects is emitted from the accretion column close to the white dwarf photosphere at the impact region. Its properties depend on the characteristics of the white dwarf and an accurate accretion column model allows the properties of the binary system to be inferred, such as the white dwarf mass, its magnetic field, and the accretion rate. We study the temporal and spectral behaviour of the accretion region and use the tools we developed to accurately connect the simulation results to the X-ray and optical astronomical observations. The radiation hydrodynamics code Hades was adapted to simulate this specific accretion phenomena. Classical approaches were used to model the radiative losses of the two main radiative processes: bremsstrahlung and cyclotron. The oscillation frequencies and amplitudes in the X-ray and optical domains are studied to compare those numerical results to observational ones. Different dimensional formulae were developed to complete the numerical evaluations. The complete characterization of the emitting region is described for the two main radiative regimes: when only the bremsstrahlung losses and when both cyclotron and bremsstrahlung losses are considered. The effect of the non-linear cooling in- stability regime on the accretion column behaviour is analysed. Variation in luminosity on short timescales (~ 1 s quasi-periodic oscillations) is an expected consequence of this specific dynamic. The importance of secondary shock instability on the quasi-periodic oscillation phenomenon is discussed. The stabilization effect of the cyclotron process is confirmed by our numerical simulations, as well as the power distribution in the various modes of oscillation.
Intermediate polar
Oscillation (cell signaling)
Photosphere
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Roche lobe
Roche limit
Intermediate polar
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We present a detailed calculation of the evolution of low-mass (<0.25 M⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M⊙. As a result of mixing between the core and outer envelope, the surface hydrogen content (Xsurf) is 0.5−0.35, depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M⊙ (Z=0.003), 0.198 M⊙ (Z=0.01), 0.192 M⊙ (Z=0.02) or 0.183 M⊙ (Z=0.03). The duration of the flashes (independent of chemical composition) is between a few ×106 and a few ×107 yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M⊙ (depending on the heavy element value).
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Chandrasekhar limit
Deflagration
Thermonuclear Fusion
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One of the challenges to increasing the mass of a white dwarf through accretion is the tendency for the accumulating hydrogen to ignite unstably and potentially trigger mass loss. It has been known for many years that there is a narrow range of accretion rates for which the hydrogen can burn stably, allowing for the white dwarf mass to increase as a pure helium layer accumulates. We first review the physics of stable burning, providing a clear explanation for why radiation pressure stabilization leads to a narrow range of accretion rates for stable burning near the Eddington limit, confirming the recent work of Nomoto and collaborators. We also explore the possibility of stabilization due to a high luminosity from beneath the burning layer. We then examine the impact of the β-decay-limited "hot" CNO cycle on the stability of burning. Although this plays a significant role for accreting neutron stars, we find that for accreting white dwarfs, it can only increase the range of stably burning accretion rates for metallicities <0.01 Z☉.
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Carbon-oxygen white dwarf (CO WD) and helium star binary is one of the ways that can lead to accretion-induced collapse (AIC). The continuous accretion may increase the mass of the white dwarf until at a certain condition, carbon burning off-center might be initiated and alter CO into ONe WD. This paper is intended to analyze the long-term evolution of the CO WD accreting helium material. The stellar evolution code used in this research is MESA (Modules for Experiments in Stellar Astrophysics). MESA creates CO WD by evolving ZAMS star with an initial mass of 6 M ⊙ to produce 0.9 M ⊙ CO WD. The accretion rates are 4×10 −6 and 4×10 −7 M ⊙ /year which consist of mostly helium. It shows that for the high accretion rate, the helium burning on the surface of the WD is stable. It is predicted that it will continue to be stable for a very long time because of the high supply of matter. For the lower one, the burning experiences a fluctuation from the beginning of the accretion. For both cases, if carbon burning off-center has occurred, it may lead to the creation of ONe WD and eventually AIC.
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