Cataclysmic Variable StarsCataclysmic Variable Stars

Nataliya OstrovaNataliya Ostrova

Astronomical observatory of the Odessa National University,natali_ostrova@ukr.net

T.G. Shevchenko Park, Odessa, Ukraineastro@paco.odessa.ua

Cracow, 2005

Odessa Astronomical ObservatoryOdessa Astronomical Observatory► Director: Prof. Dr. Valentin G. KaretnikovDirector: Prof. Dr. Valentin G. Karetnikov► Departments:Departments:► Physics of Stars and Galaxies (head Dr. Physics of Stars and Galaxies (head Dr.

T.V. Mishenina)T.V. Mishenina)► * Chemical Composition of Cool * Chemical Composition of Cool

Giants (supervisor Dr. T.V. Mishenina) Giants (supervisor Dr. T.V. Mishenina)

► * Chemical Composition of Galaxy * Chemical Composition of Galaxy (supervisor Dr. S.M. Andrievsky)(supervisor Dr. S.M. Andrievsky)

► * Periodic and Aperiodic Processes in * Periodic and Aperiodic Processes in Variable Stars (supervisor Prof. I.L. Variable Stars (supervisor Prof. I.L. Andronov)Andronov)

► Asteroids and Artificial Satellites (head Asteroids and Artificial Satellites (head Dr. N.I. Koshkin)Dr. N.I. Koshkin)

► Physics of Minor Bodies of the Solar Physics of Minor Bodies of the Solar System (supervisor Prof. V.G. System (supervisor Prof. V.G. Karetnikov)Karetnikov)

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 non-magnetic cataclysmic binary stars (ex-Nova, dwarf Nova, Nova-like)Nova, Nova-like)

► ““semi-magnetic” cataclysmic binary stars (intermediate semi-magnetic” cataclysmic binary stars (intermediate polars)polars)

► magnetic cataclysmic binary stars magnetic cataclysmic binary stars (polars)(polars)

What are What are cataclysmic cataclysmic variables?variables?

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

The CVs consist of a white dwarf (the The CVs consist of a white dwarf (the primary star), and a red dwarf (secondary), primary star), and a red dwarf (secondary), which is typically a main-sequence star which is typically a main-sequence star cooler than the Sun. These variables are cooler than the Sun. These variables are characterized by their „cataclysmic” (i.e. characterized by their „cataclysmic” (i.e. violent but non-destructive) eruptions, violent but non-destructive) eruptions, which are associated with the presence of which are associated with the presence of an accretion disc around the primary star. 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 CVs provide a unique laboratory for the study of two fundamental astrophysical processes: accretion and binary star evolution. astrophysical processes: accretion and binary star evolution.

Accretion is the process by which matter is able to overcome the angular Accretion is the process by which matter is able to overcome the angular momentum barrier which would normally prevent material from momentum barrier which would normally prevent material from spiralling inwards to form compact objects like the Sun, the Earth and spiralling inwards to form compact objects like the Sun, the Earth and black holes.black holes.

Cataclysmic variable stars have been central to many developments in Cataclysmic variable stars have been central to many developments in the thory of accretion disks. This is because the disk in these systems the thory of accretion disks. This is because the disk in these systems are nearby (and hence bright), they evolve on very short timescales are nearby (and hence bright), they evolve on very short timescales (hour to weeks).(hour to weeks).

Binary star evolution describes how to widely separated stellar Binary star evolution describes how to widely separated stellar companions may come together and interact, leading to some of the companions may come together and interact, leading to some of the most exotic inhabtants of our Galaxy (black hole binaries, supernovae).most exotic inhabtants of our Galaxy (black hole binaries, supernovae).

CVs are vital link in the evolutionary chain of binary stars, comming CVs are vital link in the evolutionary chain of binary stars, comming immediately after a common-envelop phase and evolving via magnetic immediately after a common-envelop phase and evolving via magnetic braking and gravitational radiation – observations of CVs have play the braking and gravitational radiation – observations of CVs have play the key role in the development of these theories.key role in the development of these theories.

Why study CVs

Inter-Longitude Astronomy (ILA):Inter-Longitude Astronomy (ILA):

many observations of our many observations of our group have been obtained in group have been obtained in

an international an international collaboration according to collaboration according to

the program „ILA” in Greece, the program „ILA” in Greece, Japan, Korea, Slovakia , Japan, Korea, Slovakia ,

Spain, Hungary, Germany.Spain, Hungary, Germany.

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.

Cataclysmic Cataclysmic Variable TypesVariable TypesCVs 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:

a) Intermediate Polars or DQ Her stars with magnetic field strengths ~ 1-10 MG

b) Polars or AM Her stars with magnetic field strengths ~ 10-100 MG.

Non-MagneticNon-Magnetic Cataclysmic VariablesCataclysmic 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 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.

New New dwarf dwarf novanova subtype SU Uma subtype SU Uma star V368 star V368 Peg.Peg.

In the figIn the figureure, the overall , the overall light curve is shown, light curve is shown, representing 4 nights during representing 4 nights during the superoutburst and 3 the superoutburst and 3 nights after. Here nights after. Here RR - is - is the average difference the average difference between the brightness of between the brightness of the variable star and of the the variable star and of the comparison star.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.

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 ~ 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.

From top to bottom the From top to bottom the phase folded V and R phase folded V and R

mean light curves mean light curves of FO of FO Aqr Aqr and the V-R color and the V-R color

indexindex for the ephemeris by for the ephemeris by Patterson et al. (1998) and Patterson et al. (1998) and

our ephemeris (bottom).our ephemeris (bottom).The vertical line marks the The vertical line marks the

position of maximum.position of maximum.

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

From Williams G., 2003, PASP, 115, 618

The O-C diagram for spin-The O-C diagram for spin-period period variations of FO variations of FO AqrAqr. . Pspin =Pspin = 20.9 20.9minmin PattersonPatterson et alet al. (. (1991998)8). .

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.

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.

The End

Thank you for attention