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 A Unified Model of Dwarf Nova Outbursts Based on the Disk Instability

1993 ◽  
Vol 134 ◽  
pp. 359-360
Author(s):  
Y. Osaki ◽  
M. Hirose ◽  
S. Ichikawa
Keyword(s):  
Accretion Disks ◽  
Transfer Rate ◽  
Thermal Instability ◽  
Mass Transfer Rate ◽  
Variable Stars ◽  
Unified Model ◽  
Model Parameters ◽  
Cataclysmic Variable ◽  
Dwarf Novae ◽  
Nova Outbursts

AbstractA unified model for outbursts of dwarf novae is proposed based on the disk instability model in cataclysmic variable stars. In this model, two different intrinsic instabilities (i.e., the thermal instability and the tidal instability) within accretion disks are considered in non-magnetic cataclysmic variable stars. It is suggested that all of three sub-classes of dwarf novae (i.e., U Gem-type, Z Cam-type and SU UMa-type dwarf novae) may be explained in terms of two model parameters of the orbital period of the binary and of the mass transfer rate within the framework of the disk instability model.

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 Accretion Effects on Disks Around Non-Magnetic Compact Objects

2012 ◽  
Vol 8 (S290) ◽  
pp. 279-280
Author(s):  
Michele M. Montgomery
Keyword(s):  
Accretion Disks ◽  
Mass Transfer Rate ◽  
Cataclysmic Variables ◽  
Compact Objects ◽  
Additional Time ◽  
Compact Binaries ◽  
Particle Hydrodynamics ◽  
Smoothed Particle ◽  
Low Mass ◽  
Magnetic Cataclysmic Variables

AbstractAccretion disks in compact binaries are thought to sometimes tilt and precess in the retrograde direction as indicated by modulations in light curves and/or signals. Using 3D Smoothed Particle Hydrodynamics and a low mass transfer rate, Montgomery (2012) shows the disk in non-magnetic Cataclysmic Variables tilts naturally after enough time has passed. In that work, twice the fundamental negative superhump signal 2ν_ is associated with disk tilt around the line of nodes, gas stream overflow approximately twice per orbital period, and retrograde precession. In this work, we show that after enough additional time has passed in the same simulation, the 4ν_ harmonic appears. The decrease in the 2ν_ amplitude approximately equals the amplitude of the 4ν_ harmonic. We discuss the implications.

scholarly journals Accretion Disks with Convective Viscosity

1997 ◽  
Vol 163 ◽  
pp. 173-176
Author(s):  
J. Smak
Keyword(s):  
Accretion Disks ◽  
Critical Points ◽  
Effective Temperature ◽  
Surface Density ◽  
Vertical Structure ◽  
Turbulent Viscosity ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Nova Outbursts

AbstractModels of vertical structure are calculated for a range of parameters applicable to dwarf novae with two types of viscosity being included: the standard α-disk viscosity and the additional turbulent viscosity in the convective regions. The resulting surface density (∑) υs. effective temperature (Te) relations, compared to those without convective viscosity, show larger separation between the two critical points of the ∑ – Te relation, ∑min and ∑max. This could suggest that with such a modification the dwarf nova outbursts could be reproduced with a single value of α. (Note that in the standard α approach two different α’s are needed on the hot and cool branch of the ∑ – Te relation, with αcool ≈ αhot/4). It turns out, however, that this is not the case. This is due to the fact that additional convective viscosity makes also ∑max smaller than in the pure α case.

scholarly journals Theoretical Spectra of Cataclysmic Variables

1987 ◽  
Vol 93 ◽  
pp. 341-349
Author(s):  
S. Křiž ◽  
I. Hybený
Keyword(s):  
Unique Solution ◽  
Radiation Field ◽  
Accretion Disks ◽  
Mass Flux ◽  
Main Sequence ◽  
Cataclysmic Variables ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Moderate Mass

AbstractWe present two methods of modelling accretion disks in dwarf novae. The first one determines self-consistently the structure of a disk together with the radiation field. The computed theoretical spectra agree well with the observations of the dwarf nova WX Hyi in quiescence as well as in superoutburst. The second method is a modification of Tylenda’s (1981) approach. We have used it to calculate models of disks around main-sequence accretors. The most interesting result is that no unique solution exists for a moderate mass flux around 10−6 M⊙yr−1. Such disks are thus probably unstable.

scholarly journals The disappearance and reformation of the accretion disc during a low state of FO Aquarii

2017 ◽  
Vol 606 ◽  
pp. A7 ◽  
Author(s):  
J.-M. Hameury ◽  
J.-P. Lasota
Keyword(s):  
Mass Transfer ◽  
Transfer Rate ◽  
Hot Spot ◽  
Mass Transfer Rate ◽  
Accretion Disc ◽  
Numerical Code ◽  
Light Sources ◽  
Instability Strip ◽  
Dwarf Nova ◽  
Nova Outbursts

Context. FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate polar) went into a low state in 2016, from which it slowly and steadily recovered without showing dwarf nova outbursts. This requires explanation since in a low state, the mass-transfer rate is in principle too low for the disc to be fully ionised and the disc should be subject to the standard thermal and viscous instability observed in dwarf novae. Aims. We investigate the conditions under which an accretion disc in an intermediate polar could exhibit a luminosity drop of two magnitudes in the optical band without showing outbursts. Methods. We use our numerical code for the time evolution of accretion discs, including other light sources from the system (primary, secondary, hot spot). Results. We show that although it is marginally possible for the accretion disc in the low state to stay on the hot stable branch, the required mass-transfer rate in the normal state would then have to be extremely high, of the order of 1019 g s-1 or even larger. This would make the system so intrinsically bright that its distance should be much larger than allowed by all estimates. We show that observations of FO Aqr are well accounted for by the same mechanism that we have suggested as explaining the absence of outbursts during low states of VY Scl stars: during the decay, the magnetospheric radius exceeds the circularisation radius, so that the disc disappears before it enters the instability strip for dwarf nova outbursts. Conclusions. Our results are unaffected, and even reinforced, if accretion proceeds both via the accretion disc and directly via the stream during some intermediate stages; the detailed process through which the disc disappears still requires investigation.

scholarly journals Pulsating white dwarfs in cataclysmic variables: The marriage of ZZ Cet and dwarf nova

2004 ◽  
Vol 193 ◽  
pp. 382-386 ◽  
Author(s):  
Brian Warner ◽  
Patrick A. Woudt
Keyword(s):  
White Dwarf ◽  
Large Amplitude ◽  
White Dwarfs ◽  
Cataclysmic Variables ◽  
Sloan Digital Sky Survey ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Sky Survey

AbstractThere are now four dwarf novae known with white dwarf primaries that show large amplitude non-radial oscillations of the kind seen in ZZ Cet stars. We compare the properties of these stars and point out that by the end of the Sloan Digital Sky Survey more than 30 should be known.

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 Thermal-viscous instability in tilted accretion disks: A possible application to IW Andromeda-type dwarf novae

2020 ◽  
Vol 72 (2) ◽  
Author(s):  
Mariko Kimura ◽  
Yoji Osaki ◽  
Taichi Kato ◽  
Shin Mineshige
Keyword(s):  
Accretion Disks ◽  
Outer Edge ◽  
Light Curves ◽  
Dwarf Novae ◽  
Transfer Rates ◽  
Secondary Star ◽  
Viscous Instability ◽  
Simplifying Assumptions ◽  
Nova Outbursts

Abstract IW And stars are a subgroup of dwarf novae characterized by repetition of the intermediate brightness state with oscillatory variations terminated by brightening. This group of dwarf novae is also known to exhibit a wide variety even within one system in long-term light curves, including the usual dwarf-nova outbursts, Z Cam-type standstills, and so on, besides the typical IW And-type variations mentioned above. Following recent observations suggesting that some IW And stars seem to have tilted disks, we have investigated how the thermal-viscous instability works in tilted accretion disks in dwarf novae and whether it could reproduce the essential features of the light curves in IW And stars. By adopting various simplifying assumptions for tilted disks, we have performed time-dependent one-dimensional numerical simulations of a viscous disk by taking into account various mass supply patterns to the disk; that is, the gas stream from the secondary star flows not only to the outer edge of the disk but also to the inner portions of the disk. We find that tilted disks can achieve a new kind of accretion cycle, in which the inner disk almost always stays in the hot state while the outer disk repeats outbursts, thereby reproducing alternating mid-brightness intervals with dips and brightening, which are quite reminiscent of the most characteristic observational light variations of IW And stars. Further, we have found that our simulations produce diverse light variations, depending on different mass supply patterns even without time variations in mass transfer rates. This could explain the wide variety in long-term light curves of IW And stars.

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.

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