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Register a new userJets from Accreting White Dwarfs
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Added by
Jennifer L. Sokoloski
Publication date
2004
fields
Physics
and research's language is
English
Authors
J. L. Sokoloski
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Collimated outflows from accreting white dwarfs have an important role to play in the study of astrophysical jets. Observationally, collimated outflows are associated with systems in which material is accreted though a disk. Theoretically, accretion disks provide the foundation for many jet models. Perhaps the best-understood of all accretion disks are those in cataclysmic variable stars (CVs). Since the disks in other accreting white-dwarf (WD) binaries are probably similar to CV disks (at least to the extent that one does not expect complications such as, for example, advection-dominated flows), with WD accretors one has the advantage of a relatively good grasp of the region from which the outflows are likely to originate. We briefly compare the properties of the three main classes of WD accretors, two of which have members that produce jets, and review the cases of three specific jet-producing WD systems.
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Thermonuclear (type Ia) supernovae are explosions in accreting white dwarfs, but the exact scenario leading to these explosions is still unclear. An important step to clarify this point is to understand the behaviour of accreting white dwarfs in close binary systems. The characteristics of the white dwarf (mass, chemical composition, luminosity), the accreted material (chemical composition) and those related with the properties of the binary system (mass accretion rate), are crucial for the further evolution towards the explosion. An analysis of the outcome of accretion and the implications for the growth of the white dwarf towards the Chandrasekhar mass and its thermonuclear explosion is presented.
Interacting binaries in which a white dwarf accretes material from a companion - cataclysmic variables (CVs) in which the mass donor is a Roche-lobe filling star on or near the main sequence, and symbiotic stars in which the mass donor is a late type giant - are relatively commonplace. They display a wide range of behaviors in the optical, X-rays, and other wavelengths, which still often baffle observers and theorists alike. Here I review the existing body of research on X-ray emissions from these objects for the benefits of both experts and newcomers to the field. I provide introductions to the past and current X-ray observatories, the types of known X-ray emissions from these objects, and the data analysis techniques relevant to this field. I then summarize of our knowledge regarding the X-ray emissions from magnetic CVs, non-magnetic CVs and symbiotic stars, and novae in eruption. I also discuss space density and the X-ray luminosity functions of these binaries and their contribution to the integrated X-ray emission from the Galaxy. I then discuss open questions and future prospects.
Over 1500 DBZ or DZ white dwarfs (WDs) have been observed so far, and polluted atmospheres with metal elements have been found among these WDs. The surface heavy element abundances of known DBZ or DZ WDs show an evolutionary sequence. By using Modules for Experiments in Stellar Evolution, we create DB WDs, and simulate the element diffusion due to high gravitational fields and the metal-rich material accretion coming from the planet disrupted by the WD. In our models, the input parameters ($alpha_{rm MLT}$, $alpha_{rm th}$ and $Z$) have very weak effect on DB WD structures including interior temperatures, chemical profiles and convective zones.The mass-accretion rate and the effective temperature of DB WDs determine the abundances of heavy elements. The evolutionary sequence of Ca element for about 1500 observed DB or DBZ WDs cannot be explained by the model with a constant mass-accretion rate, but is consistent well with the model in which the mass-accretion rate decreases by one power law when $T_{rm eff}>10$ kK and slightly increases by another power law when $T_{rm eff}<10$ kK. The observed DB WD evolutionary sequence of heavy element abundances originates from WD cooling and the change of mass-accretion rate.
The double-degenerate model, involving the merger of double carbon-oxygen white dwarfs (CO WDs), is one of the two classic models for the progenitors of type Ia supernovae (SNe Ia). Previous studies suggested that off-centre carbon burning would occur if the mass-accretion rate (Macc) is relatively high during the merging process, leading to the formation of oxygen-neon (ONe) cores that may collapse into neutron stars. However, the off-centre carbon burning is still incompletely understood, especially when the inwardly propagating burning wave reaches the centre. In this paper, we aim to investigate the propagating characteristics of burning waves and the subsequently evolutionary outcomes of these CO cores. We simulated the long-term evolution of CO WDs that accrete CO-rich material by employing the stellar evolution code MESA on the basis of the thick-disc assumption. We found that the final outcomes of CO WDs strongly depend on Macc (Msun/yr) based on the thick-disc assumption, which can be divided into four regions: (1) explosive carbon ignition in the centre, then SNe Ia (Macc < 2.45*10^-6); (2) OSi cores, then neutron stars (2.45*10^-6 < Macc < 4.5*10^-6); (3) ONe cores, then e-capture SNe (4.5*10^-6 < Macc < 1.05*10^-5); (4) off-centre oxygen and neon ignition, then off-centre explosion or Si-Fe cores (Macc > 1.05*10^-5). Our results indicate that the final fates of double CO WD mergers are strongly dependent on the merging processes (e.g. slow merger, fast merger, composite merger, violent merger, etc.).
The carbon-oxygen white dwarf (CO WD) + He star channel is one of the promising ways for producing type Ia supernovae (SNe Ia) with short delay times. Recent studies found that carbon under the He-shell can be ignited if the mass-accretion rate of CO WD is higher than a critical rate (about 2 * 10^-6 Msun/yr), triggering an inwardly propagating carbon flame. Previous studies usually supposed that the off-centre carbon flame would reach the centre, resulting in the formation of an oxygen-neon (ONe) WD that will collapse into a neutron star. However, the process of off-centre carbon burning is not well studied. This may result in some uncertainties on the final fates of CO WDs. By employing MESA, we simulated the long-term evolution of off-centre carbon burning in He-accreting CO WDs. We found that the inwardly propagating carbon flame transforms the CO WDs into OSi cores directly but not ONe cores owing to the high temperature of the burning front. We suggest that the final fates of the CO WDs may be OSi WDs under the conditions of off-centre carbon burning, or explode as iron-core-collapse SNe if the mass-accretion continues. We also found that the mass-fractions of silicon in the OSi cores are sensitive to the mass-accretion rates.
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