Cataclysmic Variable Stars

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Cataclysmic Variable Stars

• Nataliya Ostrova
• Astronomical observatory of the Odessa National
  University,
• natali_ostrova_at_ukr.net
• T.G. Shevchenko Park, Odessa, Ukraine
• astro_at_paco.odessa.ua
• Cracow, 2005

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Odessa Astronomical Observatory

• Director Prof. Dr. Valentin G. Karetnikov
• Departments
• Physics of Stars and Galaxies (head Dr. T.V.
  Mishenina)
• Chemical Composition of Cool Giants
  (supervisor Dr. T.V. Mishenina)
  
  
• Chemical Composition of Galaxy (supervisor
  Dr. S.M. Andrievsky)
• Periodic and Aperiodic Processes in
  Variable Stars (supervisor Prof. I.L. Andronov)
• Asteroids and Artificial Satellites (head Dr.
  N.I. Koshkin)
• Physics of Minor Bodies of the Solar System
  (supervisor Prof. V.G. Karetnikov)

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• The Astronomical Observatory in Odessa as the
  scientific institution was founded in 1870. Now
  it has two mounteneous and suburban observational
  stations. The observatory is equipped by two
  80-cm, a 60-cm telescopes, a seven-camera
  Astrograph.
• Significant part of observations is obtained at
  the other observatories (6m-telescope of the
  Special Astrophysical Observatory, Russian
  Academy of Scienses, 2.6-m Shain Telescope of the
  Crimean astrophysical Observatory etc.) In 1993
  we renewed edition of the journal with a title
  Odessa Astronomical Publications

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What are cataclysmic variables?

• non-magnetic cataclysmic binary stars (ex-Nova,
  dwarf Nova, Nova-like)
• semi-magnetic cataclysmic binary stars
  (intermediate polars)
• magnetic cataclysmic binary stars
• (polars)

• Of the 6000 stars visible to the naked eye from
  the Earth, well over half of two ore more bodies
  locked in gravitational bound orbits. About half
  of them consist of interacting binary systems
  where the two component stars are unable to
  complete there normal without being influenced by
  the presence of the other. On of the classes of
  interacting binary are the cataclysmic variables,
  or CVs, whose members include the novae, dwarf
  novae and the novalikes.

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The CVs consist of a white dwarf (the primary
star), and a red dwarf (secondary), which is
typically a main-sequence star cooler than the
Sun. These variables are characterized by their
cataclysmic (i.e. violent but non-destructive)
eruptions, which are associated with the presence
of an accretion disc around the primary star.
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• The image depits the five principal components
  of typical CV the primary star, the secondary
  star, the gas stream (formed by the transfer of
  material from the secondary to the primary), the
  bright spot (formed by the collision between the
  gas stream and the edge of the accretion disc),
  and the accretion disc.
• The distance between the stellar components is
  approximately a Solar radius (700000km) and the
  orbital period is typically a few hours. The
  orbital periods of CVs typically range from
  approximately 0.6 day (14 hr) to 0.06 day (90
  min). These binaries are quite small by
  astronomical standards the binary separation is
  1.1 (Porb/3 hr)2/3 (M1M2)1/3 times the Sun's
  radius of 0.7 x 106 km (where Porb is the binary
  orbital period in hours and M1M2 is the total
  mass of the binary in solar masses).

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Why study CVs

• CVs provide a unique laboratory for the study of
  two fundamental astrophysical processes
  accretion and binary star evolution.
• Accretion is the process by which matter is able
  to overcome the angular momentum barrier which
  would normally prevent material from spiralling
  inwards to form compact objects like the Sun, the
  Earth and black holes.
• Cataclysmic variable stars have been central to
  many developments in the thory of accretion
  disks. This is because the disk in these systems
  are nearby (and hence bright), they evolve on
  very short timescales (hour to weeks).
• Binary star evolution describes how to widely
  separated stellar companions may come together
  and interact, leading to some of the most exotic
  inhabtants of our Galaxy (black hole binaries,
  supernovae).
• CVs are vital link in the evolutionary chain of
  binary stars, comming immediately after a
  common-envelop phase and evolving via magnetic
  braking and gravitational radiation
  observations of CVs have play the key role in the
  development of these theories.

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Inter-Longitude Astronomy (ILA)

• many observations of our group have been
  obtained in an international collaboration
  according to the program ILA in Greece, Japan,
  Korea, Slovakia , Spain, Hungary, Germany.

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My Interests
My research interests centre on the study of
cataclysmic variables, and in particular, their
evolution and the study of instabilities of
accretion processies on them.
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Cataclysmic Variable Types

• CVs are classified into various subgroups based
  primarily on the strength of the white dwarf's
  magnetic field
• 1) Nominally non-magnetic systems (dwarf novae
  and novalike variables), Blt0.1-1 MG
• 2) Magnetic systems with field strengths in
  excess of about 106 gauss. Magnetic CVs are
  further subdivided into
• Intermediate Polars or DQ Her stars with magnetic
  field strengths 1-10 MG
• Polars or AM Her stars with magnetic field
  strengths 10-100 MG.

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• Non-Magnetic Cataclysmic Variables
• There are two important structures in a
  non-magnetic CV
• 1) The accretion disk, where about half of the
  gravitational potential energy of the accreting
  material is released, and
•  2) The boundary layer between the accretion disk
  and the surface of the white dwarf, where the
  kinetic energy of the flow is thermalized and
  radiated.
•  Because the effective temperature of the
  accretion disk ranges from 5000 K at its outer
  edge to few x 104 K at its inner edge, it
  radiates over a broad energy range from the
  optical through the far ultraviolet.
• Because of the small size and high luminosity of
  the boundary layer, its temperature is
  significantly higher than that of the accretion
  disk. When the mass-accretion rate is high (Mdot
  10-8 Msun/yr e.g., novalike variables and
  dwarf novae in outburst), the boundary layer is
  optically thick and its temperature 105 K (10
  eV), so it radiates primarily in the extreme
  ultraviolet and soft X-ray bandpasses. When the
  mass-accretion rate is low (Mdot 10-11
  Msun/yr e.g., dwarf novae in quiescence), the
  boundary layer is optically thin and its
  temperature 108 K (10 keV), so it radiates
  primarily in the X-ray bandpass.

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New dwarf nova subtype SU Uma star V368 Peg.
In the figure, the overall light curve is shown,
representing 4 nights during the superoutburst
and 3 nights after. Here D R - is the average
difference between the brightness of the variable
star and of the comparison star.
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• The analysis of the brightness variations during
  separate nights has confirmed that this star
  belongs to the SU UMa - subtype because of the
  presence of superhumps. They may originate from
  the precessing accretion disk because of tidal
  resonance with the secondary component.

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Intermediate Polars (DQ Her stars)

• In intermediate polars, the accretion disk is
  disrupted at small radii by the white dwarf
  magnetosphere the accreting material then leaves
  the disk and follows the magnetic field lines
  down to the white dwarf surface in the vicinity
  of the magnetic poles.
• As the accreting material rains down onto the
  white dwarf surface, it passes through a strong
  shock where its free-fall kinetic energy is
  converted into thermal energy. The shock
  temperature is 108 K (10 keV), so the
  post-shock plasma is a strong source of hard
  X-rays.
• The X-ray, ultraviolet, and optical radiation is
  pulsed at the spin period Pspin of the white
  dwarf and the beat period between spin and
  orbital periods Pbeat (1/Pspin 1/ Porb)-1.

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The spin period variations (Pspin 20.9min) of
FO Aqr. From 1981 to 1987, the white dwarf showed
spin-down, which was then changed to a spin-up.
Hellier (2001) discusses period variations as
fluctuations near the equilibrium value (cf.
Warner 1990) with a characteristic time of tens
years.

• From top to bottom the phase folded V and R mean
  light curves of FO Aqr and the V-R color index
  for the ephemeris by Patterson et al. (1998) and
  our ephemeris (bottom).
• The vertical line marks the position of maximum.

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The historical change in 1987 from spin-down to
a spin-up does not reflect accretion rate
variations, as the mean magnitude remains
constant within 0.1 mag, and a fast acceleration
of the spin-up may be caused by changes of the
magnetosphere e.g. owed to the precession of the
white dwarf. Our data support the fit 3" model
of Williams (2003) for the cycle counting.
The O-C diagram for spin-period variations of FO
Aqr. Pspin 20.9min Patterson et al. (1998).
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Polars (AM Her stars)
In polars, the white dwarf magnetic field is so
strong that 1) The white dwarf is
spin-synchronized with the binary (Pspin Porb),
and 2) No disk forms - accretion takes places
directly into the white dwarf magnetosphere.
Like intermediate polars, polars are strong hard
X-ray sources, but the X-ray, extreme
ultraviolet, ultraviolet, and optical radiation
is pulsed at the binary orbital period.
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The EndThank you for attention