Cataclysmic Variable Stars. Nataliya Ostrova Astronomical observatory of the Odessa National University, email@example.com T.G. Shevchenko Park, Odessa, Ukraine firstname.lastname@example.org Cracow, 2005. Odessa Astronomical Observatory. Director: Prof. Dr. Valentin G. Karetnikov Departments:
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Cataclysmic Variable Stars
Astronomical observatory of the Odessa National University,
T.G. Shevchenko Park, Odessa, Ukraine
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”
non-magnetic cataclysmic binary stars (ex-Nova, dwarf Nova, Nova-like)
“semi-magnetic” cataclysmic binary stars (intermediate polars)
magnetic cataclysmic binary stars
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.
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.
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 (M1+M2)1/3 times the Sun's radius of 0.7 x 106 km (where Porb is the binary orbital period in hours and M1+M2 is the total mass of the binary in solar masses).
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.
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.
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.
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), B<0.1-1 MG
2) Magnetic systems with field strengths in excess of about 10^6 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.
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 isthermalized and radiated.
Because the effective temperature of the accretion disk ranges from ~ 5000 K at its outer edge to ~ few x 10^4 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 ~ 10^5 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 ~ 10^8 K (10 keV), so it radiates primarily in the X-ray bandpass.
In the figure, the overall light curve is shown, representing 4 nights during the superoutburst and 3 nights after. Here DR - is the average difference between the brightness of the variable star and of the comparison star.
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.
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 ~ 10^8 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.
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 indexfor the ephemeris by Patterson et al. (1998) and our ephemeris (bottom).
The vertical line marks the position of maximum.
From Williams G., 2003, PASP, 115, 618
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
Pattersonet al. (1998).
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.