scholarly journals Winds from Disks

1990 ◽  
Vol 122 ◽  
pp. 228-235
Author(s):  
J.E. Drew
Keyword(s):  
Magnetic Field ◽  
Numerical Modelling ◽  
Mass Loss ◽  
Weak Magnetic Field ◽  
Cataclysmic Variables ◽  
Dwarf Novae ◽  
Loss Mechanism ◽  
Fundamental Cause ◽  
Magnetic Cataclysmic Variables ◽  
Thermonuclear Runaway

The disks referred to in the title of this paper are specifically those present in cataclysmic variables in which the accreting white dwarf has a relatively weak magnetic field (≪ 1 MG). Such systems are classified either as nova-like variables or as dwarf novae, and are of interest here because they are believed to be novae in quiescence (Ritter and Livio discuss this point elsewhere in this volume).This review aims to do two things: i) to summarise what has been learned about the winds associated with non-magnetic cataclysmic variables both from observation and from numerical modelling, and ii) to outline ideas about the nature of the mass loss mechanism. By contrast with the certainty that nova outflows are the consequence of thermonuclear runaway, it shall be seen that the fundamental cause of mass loss from cataclysmic variables remains obscure. An earlier review of this subject is by Cordova and Howarth (1986). Also of interest are some sections of the monograph on dwarf novae and nova-like variables by LaDous (1989).

scholarly journals The EUVE Observations of Dwarf Novae

1996 ◽  
Vol 152 ◽  
pp. 301-308
Author(s):  
Knox S. Long
Keyword(s):  
Boundary Layer ◽  
Effective Temperature ◽  
White Dwarf ◽  
Extreme Ultraviolet ◽  
Cataclysmic Variables ◽  
Minimum Size ◽  
Standard Theory ◽  
Dwarf Novae ◽  
Extended Source ◽  
Magnetic Cataclysmic Variables

In the standard theory of dwarf novae in outburst, the boundary layer region between the inner edge of the accretion disk and the white dwarf surface radiates primarily in the extreme ultraviolet. Using EUVE, observers have been able to obtain spectra with sufficient spectral resolution to characterize accurately the emission from several dwarf novae in outburst, including U Geminorum and SS Cygni. I present an overview of the observations and early analyses of the dwarf nova observations. The spectra obtained of dwarf novae are complex compared to the EUV spectra of magnetic cataclysmic variables and single white dwarfs. Detailed spectral modeling of an expanding atmosphere will most like be required to fully understand the spectra. Nevertheless, we already know there were significant differences in the effective temperatures and other properties of the EUV emissions. If we assume the EUV emission arises primarily from the boundary layer and parameterize the EUV spectrum in terms of a blackbody, then for U Gem the derived boundary luminosity is comparable to the disk luminosity, consistent with the standard theory, and the minimum size of the emitting region is about that of the white dwarf surface. The count rates from U Gem were modulated strongly with the orbital period; differences in the shape of the spectrum in eclipse and out of eclipse suggest that while the bulk of the emission arose from the vicinity of the white dwarf, there was an extended source of emission as well. For SS Cyg, however, the derived boundary layer luminosity was a small fraction of the disk luminosity. In U Gem, the effective temperature dropped during the decline from outburst. In contrast, in SS Cyg, the effective temperature remained constant as the count rate rose by a factor of 100 and the effective size increased. Thus while the observations of U Gem seem broadly consistent with the standard theory for the boundary layer emission from dwarf novae, SS Cyg appears to present fundamental challenges to that theory.

scholarly journals The Role of Magnetic Field for Quiescence-Outburst Models in CVs

2015 ◽  
Vol 2 (1) ◽  
pp. 192-196
Author(s):  
S. De Bianchi ◽  
V. F. Braga ◽  
S. Gaudenzi
Keyword(s):  
Magnetic Field ◽  
Complex Systems ◽  
Cataclysmic Variables ◽  
The Other ◽  
Suitable Candidate ◽  
Global Behaviour ◽  
The One ◽  
Thermonuclear Runaway ◽  
The Magnetic Field

In this paper we present the elementary assumptions of our research on the role of the magnetic field in modelling the quiescence-outbursts cycle in Cataclysmic Variables (CVs). The behaviour of the magnetic field is crucial not only to integrate the disk instability model (Osaki 1974), but also to determine the cause and effect nexus among parameters affecting the behavior of complex systems. On the ground of our interpretation of the results emerging from the literature, we suggest that in models describing DNe outbursts, such as the disk instability model, the secondary instability model (Bath 1973) and the thermonuclear runaway model (Mitrofanov 1978), the role of the magnetic field is at least twofold. On the one hand, it activates a specific dynamic pathway for the accreting matter by channelling it. On the other hand, it could be indirectly responsible for switching a particular outburst modality. In order to represent these two roles of the magnetic field, we need to integrate the disk instability model by looking at the global behaviour of the system under analysis. Stochastic resonance in dynamo models, we believe, is a suitable candidate for accomplishing this task. We shall present the MHD model including this mechanism elsewhere.

scholarly journals Magnetic Field Reconnection in Differentially Rotating Accretion Disks

1985 ◽  
Vol 107 ◽  
pp. 477-482
Author(s):  
David Gilden ◽  
T. Tajima
Keyword(s):  
Magnetic Field ◽  
Accretion Disks ◽  
Hot Spots ◽  
Small Amplitude ◽  
Uniform Magnetic Field ◽  
Cataclysmic Variables ◽  
Dwarf Novae ◽  
The Magnetic Field ◽  
Viscous Stresses ◽  
Fast Reconnection

Differentially rotating accretion disks threaded by a uniform magnetic field have been numerically simulated. Fast reconnection followed by coalescence allows the magnetic field to drive small amplitude radial oscillations in the disk. These oscillations may be observable as the viscous stresses cause the disk to brighten and fade as the disk expands and contracts. Episodes of reconnection may also be observable as hot spots produced locally at the sites of coalescence. Cataclysmic variables, and in particular dwarf novae, provide a natural interpretation for these calculations.

scholarly journals Instabilities in Accretion Disks of Cataclysmic Variables: A Unification Model for Dwarf Nova Outburst

1997 ◽  
Vol 163 ◽  
pp. 269-278
Author(s):  
Yoji Osaki
Keyword(s):  
Accretion Disks ◽  
Thermal Instability ◽  
Mass Transfer Rate ◽  
Cataclysmic Variables ◽  
Variable Stars ◽  
Limit Cycle Oscillation ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Magnetic Cataclysmic Variables ◽  
Nova Outbursts

AbstractInstabilities of accretion disks in cataclysmic variable stars are reviewed in relation to dwarf nova outbursts. Two different kinds of instabilities of accretion disks are now known: the thermal instability and the tidal instability. The thermal instability is produced by hydrogen ionization-recombination transition, which gives rises to a thermal limit-cycle oscillation in accretion disks and it is thought to be responsible for outbursts of U Gem-type dwarf novae. The tidal instability is produced by the tidal effects of the secondary star on accretion disks, by which the disk is deformed to eccentric form and it slowly precesses in the inertial frame of reference. The tidal instability is thought to be responsible for the superhump phenomenon observed during superoutbursts of SU UMa-type dwarf novae. There is a rich variety in outburst behaviors of non-magnetic cataclysmic variables, starting from non-outbursting nova-like stars to various sub-classes of dwarf novae. A unification model for dwarf nova outbursts is then proposed based on these two instabilities. In this model, the non-magnetic cataclysmic variables are classified in the orbital-period versus mass-transfer-rate diagram into four regions depending on different combination to these two instabilities, and their observed outburst behaviors are basically understood on this diagram.

scholarly journals Magnetic Field Topology of Accreting White Dwarfs

2004 ◽  
Vol 190 ◽  
pp. 71-77 ◽  
Author(s):  
Klaus Reinsch ◽  
Fabian Euchner ◽  
Klaus Beuermann ◽  
Stefan Jordan
Keyword(s):  
Magnetic Field ◽  
White Dwarfs ◽  
Cataclysmic Variables ◽  
Systematic Investigation ◽  
Magnetic Field Topology ◽  
First Results ◽  
Global Magnetic Field ◽  
Magnetic Cataclysmic Variables ◽  
Polarization Spectra ◽  
The Magnetic Field

AbstractWe report first results of our systematic investigation of the magnetic field structure of rotating single magnetic white dwarfs and of white dwarfs in magnetic cataclysmic variables. The global magnetic field distributions on the isolated white dwarf HE 1045-0908 and the accreting white dwarfs in EF Eri and CP Tuc have been derived from phase-resolved flux and polarization spectra obtained with FORS1 at the ESO VLT using the systematic method of Zeeman tomography.

Non-Magnetic Cataclysmic Variables in the ROSAT WFC Survey

1996 ◽  
Vol 152 ◽  
pp. 337-341
Author(s):  
Peter J. Wheatley
Keyword(s):  
Wavelength Range ◽  
Cataclysmic Variables ◽  
Dwarf Novae ◽  
X Ray ◽  
Dwarf Nova ◽  
Euv Emission ◽  
Magnetic Cataclysmic Variables ◽  
Nova Outbursts

Six non-magnetic cataclysmic variables were detected during the ROSAT WFC survey; four dwarf novae and two nova-like variables. In two dwarf novae (VW Hyi & SS Cyg) the flux evolution through outburst was followed across a broad wavelength range. The two other detections (Z Cam & RX J0640−24) also suggest the presence of distinct luminous EUV emission components; supporting the view that such components are a ubiquitous feature of dwarf nova outbursts. Two bright nova-like variables were detected, but these detections are found to be consistent with the soft tail of the X-ray emission.

scholarly journals The EUV Excess in Magnetic Cataclysmic Variables

1996 ◽  
Vol 152 ◽  
pp. 325-329
Author(s):  
John K. Warren ◽  
Koji Mukai
Keyword(s):  
Magnetic Field ◽  
Orbital Period ◽  
White Dwarf ◽  
Preliminary Analysis ◽  
Cataclysmic Variables ◽  
X Rays ◽  
System Parameters ◽  
Magnetic Capture ◽  
Magnetic Cataclysmic Variables ◽  
Capture Radius

We present preliminary analysis of EUVE pointed data of 8 magnetic cataclysmic variables. Blackbody temperatures, luminosities, and interstellar columns have been better constrained. Using these luminosities we look for correlations between the EUV excess (over optical and hard X-rays) and various system parameters. While it appears there is no correlation between the EUV excess and the system inclination and orbital period, correlations are suggested between the EUV excess and the longitude of the accretion spot, the colatitude of the accretion spot, the white dwarf magnetic field, and the magnetic capture radius.

Accretion Geometry of Synchronously Rotating Magnetic Cataclysmic Variables: Theories versus Observations

1994 ◽  
Vol 11 (2) ◽  
pp. 198-200 ◽  
Author(s):  
Kinwah Wu ◽  
D. T. Wickramasinghe ◽  
J. Bailey ◽  
A. F. Tennant
Keyword(s):  
Magnetic Field ◽  
White Dwarf ◽  
Cataclysmic Variables ◽  
Quadrupole Field ◽  
Companion Star ◽  
Magnetic Cataclysmic Variables ◽  
Theoretical Results

AbstractWe review the magnetic locking of the white dwarf and the companion star in AM Herculis-type binaries for various white dwarf magnetic field structures, and compare the theoretical results with observations. The model in which the white dwarf has a dipole plus a quadrupole field is found to be in closest agreement with the recent observations.

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