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Cataclysmic Variable Stars
Viki Joergens
[Joergens et al. 2000a, A&A Letter
]
[Joergens et al. 2000b, A&A ]
Cataclysmic variables (CVs) are close
interacting binary systems compromised of an accreting
white dwarf star and a Roche lobe filling
secondary, typically a main sequence star, which
loses mass via the inner Langrangian point into the Roche lobe of
the white dwarf.
Conservation of angular momentum causes the stream material in non-magnetic
CVs to form
a disk around the massive star, which is the
dominating light source of the system.
Dwarf novae
are a subclass of CVs. They frequently show outbursts, which
are episodes of enhanced accretion
through the disk and onto the
central object.
The described basic model for CVs is well
established, however,
the processes leading to mass
transfer through the disk and the transport
of angular momentum within the disk
are still poorly
constrained.
The system EX Dra:
The CV EX Dra is a double-eclipsing dwarf nova of the U Gem class with a
quiescence magnitude
of about 14mag, a relatively small outburst
amplitude of 1-2.3mag and an orbital period of about
5 hours. The reconstruction of two-dimensional images of EX Dra
during quiescence as well as
during outburst by means of Doppler
tomography (see below for details to this image reconstruction
technique) allowed us to locate
line-emitting sites in the system
(Joergens, Mantel, Barwig et al. 2000).
They are based on time-resolved optical spectra taken at the Calar Alto 3.5m
telescope with the
TWIN spectrograph.
The Doppler maps of EX Dra showed, that the main emission sources are the
accretion disk itself,
the hot spot, where the gas stream hits the disk,
as well as the irradiated side of
the secondary. And it can be seen that the
heating of the secondary becomes the dominant emission
source during outburst,
which is attributed to an enhanced accretion rate.
Spiral structures in the disk of EX Dra:
Observations at higher spectral and temporal resolution of the dwarf nova EX Dra during outburst
were carried out at the 4.2m William
Herschel telescope. The disk seen in the He I line reconstructed
by Doppler tomography shows a clear
two-armed spiral pattern pointing to spiral shocks in the
accretion disk of EX Dra
(Joergens, Spruit, Rutten 2000,
A&A Letter). Click here for a
plot of the
He I map. The Balmer and He II maps
of EX Dra also give evidence for the presence of spirals.
EX Dra is the second CV after IP Peg (Steeghs et al. 1997),
for which spiral structures have been
found in the accretion disk.
See the astronomical picture of the day
August 22, 1997
for an image
of the spirals in IP Peg's disk.
Doppler tomography - an image reconstruction technique:
Line profiles of spectral lines
broadened by Doppler shifting retain an imprint of the
line emission
region from which it originated.
Doppler tomography is an image reconstruction technique
developed by Marsh & Horne (1988), which allows to
gain information about
the distribution of
line emission over
the surface of the disk in velocity space.
Under the assumption that Doppler
shifting is the only broadening
mechanism of significance, it is possible to locate the line forming
regions of the binary system by
means of Doppler tomography.
The Doppler broadened line profiles represent a projection of the
velocity distribution in the
direction of the observer's line of sight, while
rotation of the binary gives the observer a continuously
varying sequence of velocity projections. This combination of Doppler
shifting and binary rotation
provides sufficient information for the
assembly of two-dimensional maps in velocity space.
Viki Joergens /
viki @ mpia.de
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