Transcript Cataclysmic Variable Stars

Cataclysmic Variable Stars
Nataliya Ostrova
Astronomical observatory of the Odessa National University,
[email protected]
T.G. Shevchenko Park, Odessa, Ukraine
[email protected]
Cracow, 2005
Odessa Astronomical Observatory
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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)
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 80cm, a 60-cm telescopes, a sevencamera 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”
What are cataclysmic
variables?
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non-magnetic cataclysmic binary stars
(ex-Nova, dwarf Nova, Nova-like)
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“semi-magnetic” cataclysmic binary stars
(intermediate polars)
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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.
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).
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.
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.
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
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), 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-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 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 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.
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.
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.
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 spinup 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).
From Williams G., 2003, PASP, 115, 618
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