Composite Accretion Disk and White Dwarf Model Analyses of the Quiescence of Dwarf Novae: EM Cygni, CZ Orionis, and WW Ceti

2003 ◽  
Vol 582 (1) ◽  
pp. 352-357 ◽  
Author(s):  
Lisa Winter ◽  
Edward M. Sion
Keyword(s):  
Accretion Disk ◽  
White Dwarf ◽  
Dwarf Novae

scholarly journals Superoutbursts and Superhumps of SU UMa Stars

1988 ◽  
Vol 108 ◽  
pp. 238-239
Author(s):  
Yoji Osaki ◽  
Masahito Hirose
Keyword(s):  
Accretion Disk ◽  
Orbital Period ◽  
White Dwarf ◽  
Binary Systems ◽  
Roche Lobe ◽  
Light Curves ◽  
Close Binary ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Close Binary Systems

SU UMa stars are one of subclasses of dwarf novae. Dwarf novae are semi-detached close binary systems in which a Roche-lobe filling red dwarf secondary loses matter and the white dwarf primary accretes it through the accretion disk. The main characteristics of SU UMa subclass is that they show two kinds of outbursts: normal outbursts and superoutbursts. In addition to the more frequent narrow outbursts of normal dwarf nova, SU UMa stars exhibit “superoutbursts”, in which stars reach about 1 magnitude brighter and stay longer than in normal outburst. Careful photometric studies during superoutburst have almost always revealed the “superhumps”: periodic humps in light curves with a period very close to the orbital period of the system. However, the most curious of all is that this superhump period is not exactly equal to the orbital period, but it is always longer by a few percent than the orbital period.

scholarly journals Impact of convection and resistivity on angular momentum transport in dwarf novae

2018 ◽  
Vol 609 ◽  
pp. A77 ◽  
Author(s):  
N. Scepi ◽  
G. Lesur ◽  
G. Dubus ◽  
M. Flock
Keyword(s):  
Angular Momentum ◽  
Accretion Disk ◽  
White Dwarf ◽  
Momentum Transport ◽  
Simple Relationship ◽  
Dwarf Novae ◽  
Angular Momentum Transport ◽  
Viscous Instability ◽  
S Curve ◽  
The Impact

The eruptive cycles of dwarf novae are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to imply enhanced angular momentum transport in the accretion disk during outburst. This is measured by the stress to pressure ratio α, with α ≈ 0.1 required in outburst compared to α ≈ 0.01 in quiescence. Such an enhancement in α has recently been observed in simulations of turbulent transport driven by the magneto-rotational instability (MRI) when convection is present, without requiring a net magnetic flux. We independently recover this result by carrying out PLUTO magnetohydrodynamic (MHD) simulations of vertically stratified, radiative, shearing boxes with the thermodynamics and opacities appropriate to dwarf novae. The results are robust against the choice of vertical boundary conditions. The thermal equilibrium solutions found by the simulations trace the well-known S-curve in the density-temperature plane that constitutes the core of the disk thermal-viscous instability model. We confirm that the high values of α ≈ 0.1 occur near the tip of the hot branch of the S-curve, where convection is active. However, we also present thermally stable simulations at lower temperatures that have standard values of α ≈ 0.03 despite the presence of vigorous convection. We find no simple relationship between α and the strength of the convection, as measured by the ratio of convective to radiative flux. The cold branch is only very weakly ionized so, in the second part of this work, we studied the impact of non-ideal MHD effects on transport. Ohmic dissipation is the dominant effect in the conditions of quiescent dwarf novae. We include resistivity in the simulations and find that the MRI-driven transport is quenched (α ≈ 0) below the critical density at which the magnetic Reynolds number Rm ≤ 104. This is problematic because the X-ray emission observed in quiescent systems requires ongoing accretion onto the white dwarf. We verify that these X-rays cannot self-sustain MRI-driven turbulence by photo-ionizing the disk and discuss possible solutions to the issue of accretion in quiescence.

scholarly journals Do We See Magnetic Effects in Dwarf Nova Outbursts?

1995 ◽  
Vol 151 ◽  
pp. 285-285
Author(s):  
N. Vogt ◽  
E. Meyer-Hofmeister ◽  
F. Meyer
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Accretion Disk ◽  
White Dwarf ◽  
Magnetic Activity ◽  
Light Curves ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Cyclic Behaviour ◽  
Nova Outbursts

Many observations indicate that fast rotating late type stars show magnetic activity. We therefore argue that some of the secondary stars in cataclysmic binaries might also have magnetic fields. Such magnetic fields would reach over the accretion disk around the white dwarf primary. We investigate their effect on dwarf nova outbursts. The magnetic field lines will penetrate the disk and remove angular momentum. This shifts the accumulation of mass towards the inner disk, closer to the white dwarf, and therefore leads to a different outburst behaviour, which can be recognized in observed light curves of dwarf novae. If a magnetic field of the order 50 - 100 gauss is acting on the accretion disk, we expect narrower and more frequent outbursts as compared to the non-magnetic case. Outburst records of three dwarf novae above the period gap (P > 3h), whose long-term light curves are well covered, were used to search for traces of magnetic activity. All three cases display a pronounced bimodality in the distribution of their outburst width, i.e. either narrow or wide outbursts occur. We found evidence for a cyclic behaviour in one case, SS Aur: possible “magnetic episodes” repeating every 18 years reveal epochs with abnormally frequent narrow outbursts and nearly or totally missing wide ones. There are also indications for a similar behaviour of SS Cyg with a 7-years cycle, but with less pronounced periodicity. The third case, U Gem, does not show clear evidence of magnetic activity although we found some indications for a transitory oscillation of the width of wide outbursts after 1926, with a period of 13.6 years. The behaviour of SS Aur and SS Cyg resembles the theoretical predictions, there is, however, an important difference: Throughout the entire cycle, marked by the “magnetic episodes”, neither wide nor narrow outbursts alter their mean light curves: magnetic fields seem to affect only the observed proportion in the frequency of both outburst types without altering the light curves of individual outbursts. Remarkably, most of the “anomalous outbursts” (which are characterized by an abnormally slow rising branch to an outburst) occur also at or near the “magnetic episodes”.

scholarly journals Accretion Disk Instabilities

1986 ◽  
Vol 89 ◽  
pp. 249-267
Author(s):  
F. Meyer
Keyword(s):  
Light Curve ◽  
Accretion Disk ◽  
Accretion Disks ◽  
White Dwarf ◽  
Cataclysmic Variables ◽  
Outer Region ◽  
Stationary Flow ◽  
Close Binaries ◽  
Dwarf Novae ◽  
Low Mass

In this article we discuss two instabilities of stationary accretion disks which lead to an understanding of observed light variations in accretion disk systems, the dwarf novae and the rapid burster MXB 17030-335. The accretion disks in these systems avoid instability at the cost of stationarity and perform stable cycles in which sudden changes of the accretion flow lead to corresponding, often dramatic, variations of their accretion luminosity.Figure 1 shows a light curve of U Geminorum. It was discovered In 1855 by J.R. Hind and has become a prototype of the dwarf novae. In these systems an extended time of quiescence of up to several weeks Is followed by a short outburst of a few days during which the luminosity rises by a factor of 30 to 100. The dwarf novae belong to the cataclysmic variables. They are all close binaries In which a white dwarf primary is orbited by a Roche lobe-filling low mass secondary. Through the inner Lagrangian point mass flows over from the secondary and forms a luminious accretion disk around the white dwarf. In the case of the dwarf novae this disk has temperatures below about 10000K in Its outer region. It will be discussed how partial lonizatlon and convection then affect the vertical structure of the disk such that the stationary flow becomes unstable.Fig. 1. Light curve of the dwarf nova U Geminorum. Abszissa in days С [2])

scholarly journals The Benefit of Archival Research in Investigating Dwarf Novae

1995 ◽  
Vol 151 ◽  
pp. 282-284
Author(s):  
C. la Dous
Keyword(s):  
Accretion Disk ◽  
White Dwarf ◽  
Cataclysmic Variables ◽  
Physical Parameters ◽  
Late Type ◽  
Dwarf Novae ◽  
Interested Reader ◽  
Entire Class ◽  
Short Period ◽  
Individual Observation

Dwarf novae and nova-like stars are a sub-group of the class of cataclysmic variables. Most of these stars show high and low brightness states (maxima or outbursts and minima or quiescent states, respectively). They all are short-period interacting binaries consisting of a white dwarf primary, a late-type Roche-lobe filling secondary, and an accretion disk around the white dwarf. For details on observations of cataclysmic variables and their theoretical explanations and modelling, the interested reader is referred to recent reviews.The main problem when trying to understand these systems is the wealth of patterns presented by the whole group: no two objects are nearly identical and it is hard to say which the ‘typical’ features are. As a result of this, from an individual observation, or from observations of a few systems only, it is not possible to decide which of the observed features are characteristic of the entire class, and which are specific of individual members. The only way is to investigate many different members of the same class, arrange them according to their different physical parameters, and try to determine what the general characteristics are.

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.

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