EUVE photometry of SS Cygni: Dwarf nova outbursts and oscillations Page: 6 of 13
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C. W. Mauche: EUVE Photometry of SS Cygni
3. Dwarf Nova Outbursts
The first application of the EUVE photometry of SS Cyg deals with the relative shapes
of the optical and EUV light curves shown in Figure 1 and the implications for the
processes responsible for dwarf nova outbursts. For an excellent review of the physics
of dwarf nova outbursts, see Cannizzo (1993), from which the following synopsis was
prepared.
The vertical structure (the run of pressure, temperature, and flux) of an accretion
disk is determined by the equations of hydrostatic equilibrium, energy transport by
radiation and convection, and, unlike a stellar atmosphere, energy generation via viscous
dissipation of orbital shear. Using standard assumptions, it is found that the effective
temperature Teff of the disk as a function of surface density E is double-valued: in the
E-Teff plane, the thermal stability of the disk is described by an "S-shaped" curve. That
the form of this relationship leads to an instability in the thermal state of the disk can
be understood as follows. Imagine that the surface density of a given annulus increases
because mass is added to the annulus from "above" (from higher in the gravitational well)
faster than it is removed from "below." Initially, the temperature of the annulus will
increase monotonically. However, when the surface density reaches some value Emax when
the temperature reaches ~ 6000 K, hydrogen begins to become ionized, causing a sharp
increase in both the opacity and the specific heat of the gas. These microscopic changes
in the gas drive macroscopic changes in the vertical structure of the disk as radiation
replaces convection as the main source of the transport of energy out of the disk. To
cope with these changes, the temperature of the annulus must increase discontinuously
to remain in thermal equilibrium. On the upper branch of the S-shaped curve, the
viscous dissipation is higher because the temperature and hence the pressure is higher.
Therefore, it is possible for the surface density of the annulus to decrease as mass drains
out of the annulus faster than it is added from "above." Initially, the temperature of the
annulus will decrease monotonically. However, when surface density reaches some value
Emin when the temperature reaches - 104 K, hydrogen begins to recombine, causing a
sharp decrease in both the opacity and the specific heat of the gas. The gas again falls
out of thermal equilibrium, this time cooling significantly at fixed surface density.
To describe the response of the full disk to the above instability, it is necessary to
specify the dependence of the critical surface densities Emax and Emin on radius, and to
solve the time-dependent hydrodynamic equations of the conservation of mass, energy,
and angular momentum. These calculations demonstrate that when a given annulus
reaches the value of Emax appropriate to its radius and makes the transition to the high
state, it generates a heating wave which transforms the entire disk to the high state. This
disturbance can begin in the inner disk and sweep outward, or begin in the outer disk
and sweep inward, generating, respectively, inside-out and outside-in outbursts. Since
mass is predominantly transferred inward through the disk by viscous dissipation, the
minimum surface density Emin is reached first in the outer disk. When that annulus
makes the transition to the low state, it generates a cooling wave which transforms the
entire disk to the low state. The cooling wave shuts down the disk by moving gas to
larger radii, raising it out of the gravitational well and transforming its thermal energy
into gravitational potential energy.
The differences between the inside-out and outside-in outbursts qualitatively describe
the differences in the optical and EUV light curves shown in Figure 1. The 1993 outburst
was of the inside-out variety. As the heating wave swept outward though the disk, it
transformed successively larger annuli to the high state, and in the process set this
material in motion toward the white dwarf, constantly increasing the rate at which3
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Mauche, C.W. EUVE photometry of SS Cygni: Dwarf nova outbursts and oscillations, report, May 15, 1995; California. (https://digital.library.unt.edu/ark:/67531/metadc793210/m1/6/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.