Transcript Accretion Processes of Binaries of White Dwarfs

Accretion Processes in
Binaries of White Dwarfs
Hee-Won Lee
Dept. of Astronomy
Astrophysical Research Center for the
Structure and Evolution of the Cosmos
Sejong University
Contents
• Accretion in Astrophysics
• Binary Systems of White Dwarfs
• Cataclysmic Variables
Non-magnetic CV
Polars : AM Her Stars
Intermediate Polars : DQ Her
• Symbiotic Stars
• Raman Scattering in Symbiotic Stars
• Discussion and Summary
Energy Production in the Universe
• Nuclear interaction –
Stellar Radiation
0.007 mp c2 per
proton
• Gravitational
interactionAccretion onto
compact objects
Accretion in the Universe I
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Gravitational Potential
Energy
GMm/R = 0.5mc2 (RSch/R)
Eddington Luminosity =
Maximally allowed energy
output for steady spherical
accretion
Importance of Accretion
1. Formation of Planetary
Rings, Solar System and
Stars
2. Mysteries of Quasars
IGM, galaxy evolution
3. Various high energy
processes around
compact objects
Accretion in the Universe II
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Super Massive Black Holes (Quasars)
Protostars
Stellar Black Holes
Neutron Stars
White Dwarfs
Roche Geometry
• Effective Potential in the
Co-rotating Frame
• 5 Lagrangian Points where
the gradient of the effective
potential vanishes.
L2
• Through L1, matter may
overflow from one onto the
other
• Stellar matter may reach L1
by filling the lobe by the star
or by stellar wind – Roche
Lobe Overflow or Wind
Accretion
L4
L3
L5
Formation of Accretion Disk
1 .Through the L1 point, the stream has an angular momentum that
is hard to eliminate.
2. Energy may be dissipated via stream collision.
3. Eventually the mass stream will settle into the orbit with minimum
energy for given angular momentum  Circularization of the orbit
and the accretion ring is formed.
Angular Momentum Transport
• Viscosity is responsible for the
angular momentum transport
from inner region to outer region.
• High accretion rate is possible
in the presence of high viscosity.
• Accretion flow is stable, when
mass is transferred from the
lighter star to the heavier star.
• Magnetic field for slowly rotating
systems and gravitational wave
radiation may operate for fast
rotating systems
Binaries of White Dwarfs
• Non-interacting binaries
– e.g. Sirius A, B
• Roche Lobe Overflowing
Systems -Cataclysmic
Variables
• Stellar Wind Accretion
Systems – Symbiotic
Stars
Sirius B
In optical
Sirius B
In X-ray
Cataclysmic Variables
• Ideal systems for investigation
of accretion processes
• Quasars are too far away.
• Protostars are severely
obscured by dust.
• Neutron star and black hole
systems emit most energy in
X-rays.
Introduction of CV
• Consists of an accreting
white dwarf (primary)
and a low-mass main
sequence star
(secondary) filling its
Roche lobe
• Orbital periods : 90 min
– 14 hr
• Accretion Luminosity –
several times of the
solar luminosity (mainly
in UV through X-ray)
Classification of CVs
• Non-magnetic CV
(1) Dwarf Novae (U Gem stars)
(2) Z Cam Stars
(3) Nova-like Variable
• Magnetic CV
(1) Intermediate Polar (DQ
Her)-partially disrupted
accretion disk by magnetic
field
(2) Polar (AM Her) – totally
disrupted accretion disk
U Gem (or SS Cyg) Stars
• Dwarf Nova Outbursts
are semi-regular
(repeating
quiescence and
outburst state)
• Brightening by 2-5
Mag
• Orbital period > 3h
• Angular momentum
loss via magnetic
field
Disk Thermal Instability Model
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Y. Osaki proposed that Dwarf
nova outbursts occur due to
Thermal Instability
Viscosity may change abruptly.
There are two branches of
equation of state between the
surface mass density of disk and
temperature (or accretion rate).
High temperature state
corresponds to high viscosity
state.
In cold disk state, material piles
up in the disk, because accretion
is inefficient and secondary keeps
dumping mass.
In hot disk state, accretion rate
exceeds the mass transfer rate to
become cooler.
Instability due to B
Hot disk
High viscosity
Cold disk
Low
viscosity
Z Cam Stars
1. Light curves are
characterized by standstills.
2. Brightness remains at a
constant level before
outbursts resume.
3. Mass transfer rate is high,
close to the lower limit of
the upper branch of Osaki’s
model
Novalike Variables
• Classical Novae : historical
record
• Novalike variables: without a
historical record but similar
observational characters
• Quasi-steady light curves
with high mass transfer rate
SU UMa Stars
• Superoutbursts
• Longer duration
• Small mass ratio
q=M2/M1
• Superhumps
• A few percent
longer than the
orbital period
• Eccentric Disk :
precession and
orbital (3:1)
resonance
Superhumps of SU UMa
• The potential is not exactly
Keplerian (due to the presence
of the secondary), and the orbit
is not closed.
• Prograde precession of an
eccentric accretion disk
• Beating phenomena between
the precession period and the
binary orbital period
Polars : AM Her Stars
• Strong magnetic fields (B ~ 107-8 Gauss) completely disrupt the
accretion flow, preventing the formation of an accretion disk.
• Accretion column forms at the polar regions, where charged
particles falls almost freely.
• X-ray pulses with the half-period that of the primary spin.
Polars : Accretion Column
• Accretion column
forms at the polar
regions, where
charged particles fall
almost freely.
• Hard X-rays are
emitted at the shock
front.
• Soft X-rays and UV
photons are emitted at
the photosphere.
X-ray Observation of AM Her Star
• Quasi-sinusoidal hard
X-ray light curve
• Square wavelike soft
X-ray light curve out of
phase with hard X-ray
• Existence of two
accretion poles with
one pole dominating in
soft X-rays and the
other in hard X-rays
Intermediate Polar : DQ Her
• With intermediate
strength of B field (1067 Gauss), accretion
flow is partially
disrupted inside the
accretion disk.
• Asynchronous rotation :
Weaker magnetic field
is not sufficient to lock
the primary spin to the
binary orbital rotation
X-ray Observations of IP
• Low Energy Dip before
Eclipse
• No or Little Dip is seen
in hard X-ray light
curve
• Dip is characterized
by high hardness ratio
• Absorption by
accretion disk for soft
X-ray
AE Aqr : Propeller System
Intermediate polar
Known as a magnetic propeller
Very rapidly rotating WD P_spin =
33.08s,
P_orbit = 9.88 hr
Spin-down power = 1034 erg s-1
Luminosity several orders less
than spin-down power
-Propeller system : most material
is ejected by the primary
Doppler Tomography
• Keplerian disk : symmetric
double peak profile
• Isovelocity contour: regions
giving rise to the same velocity
component w.r.t. the
observer’s line of sight
• Bright spot contributes
significantly to the line profile,
destroying the symmetry and
exhibits variations as the
binary phase.
Doppler Tomography
• Useful tool to investigate the accretion flow and
the bright spot
Polarimetry for Magnetic Stars
• Cyclotron emission is generally circularly
polarized.
• = eB/(2 mc)=3x1014 B8 Hz
• Higher harmonics are observed in the
optical region
• Zeeman effect can be used to derive the
strength of B field.
Evolution of CV
• Period minimum at 78 min and period
gap between 2 and 3 h
• Reaching the period minimum, the
secondary red dwarf becomes
degenerate with the size increasing as
mass decreases.
• Binary expands and angular momentum
transport is less efficient leading to faint
accretion luminosity.
Symbiotic Stars
• Wide binary systems of a
giant and a hot white
dwarf
• TiO absorption band,
Prominent Emission
Lines
• Detached system
(underfills the Roche
lobe)
• S(stellar) type and
D(dusty) type
• D type symbiotics have
Mira and OH/IR sources
as the giant component.
Symbiotic Stars
• Often accompany bipolar
nebulae
• 10 percent of planetary
nebulae are bipolar 
Binarity of the central
star system?
• Severe mass loss rate
through slow stellar wind
from the giant.
• Fast stellar wind from the
white dwarf component
• Collision of slow stellar
wind and fast wind
Wind Accretion in Symbiotics
1. Mass losing giant may
shed a large amount of
mass in the form of the
slow stellar wind
2. SPH simulations show
that steady accretion
disk can be formed via
gravitational capture of
slow stellar wind
Raman Scattering in Symbiotic Stars I
• Very broad emission
features at 6830 and
7088 are only known
to exist in about a
half of symbiotic
stars.
• They were identified
in 1989 by Schmid
as Raman scattered
features of O VI 1032
and 1038
Raman Scattering in Symbiotics II
• O VI 1032 and 1038 are less
energetic than Ly beta 1025.
• If Ly beta 1025 is absorbed by
a hydrogen atom, Balmer alpha
can be generated with the deexcitation into 2s state.
• The hydrogen atom is excited
by an incident O VI photon and
may de-excite into 2s state
with an re-emission of an
optical photon redward of
Balmer alpha.
Raman Scattering in Symbiotics III
• The scattering cross
section can be computed
from the 2nd order
perturbation theory in
quantum mechanics.
σ=10-22 cm2 for O VI
σ=10-20 cm2 for He II
Raman Scattering in Symbiotics IV
• O VI 1032 and 1038 are
prominently emitted near
white dwarf.
• Hydrogen atoms are
Giant
abundantly found around
the giant, forming an
extended atmosphere or a
part of slow stellar wind
• The condition of operation
of Raman scattering is
ideally met only in
symbiotic stars.
H I Scatering
• So far 6830 and 7088
Region
features have never been
observed other than in
symbiotic stars
Stellar
Wind
Stream
OVI 1032
Dis
k
White Dwarf
Hot
spot
O VI Emission
Region
Spectroscopic and
Polarimetric Properties
• Double or Triply Peaked
Profile
• Strongly polarized
• Polarization flip is shown
in the red wing.
• 6830 and 7088 exhibits
different profiles
 Characteristic of wind
accretion disk emission
in symbiotic stars
Wind Accretion Disk Model
• Lee & Park (1999)
proposed to
interpret Raman
O VI adopting the
wind accretion
disk emission
model.
• Refer to the next
Talk by Suna
Kang
Giant
Stellar
Wind
Stream
OVI 1032
Dis
k
White Dwarf
Hot
spot
H I Scatering
O VI Emission
Region
Region
BOES Observation of Symbiotic Stars
BOES Spectrograph
H
HeII4860
Raman
4850
Mass Loss Rate and He II Raman Spectroscopy
• He II Raman scattered
feature has flux
proportional to the
extent of H I region.
• He II optical emission
lines allow one to
estimate exactly the
Raman scattering
efficiency.
• 3.6X10-7 M/yr for
V1016 Cyg (determined
by Yang Chan Jung)
Summary
• Accretion processes take
place from a small scale of
planetary systems to a huge
galactic scale involving spiral
galaxies.
• Photometric and
spectroscopic, polarimetric
observations from radio to Xray can all contribute to
understanding accretion
processes.
• White dwarf system is
particularly interesting to
investigate various aspect of
accretion processes.
Sciences for the 2.4m Telescope
• High Resolution
Spectroscopy of CV,
Symbiotic Stars will be very
interesting.
• Doppler Tomography is a
useful tool to investigate the
accretion processes.
• Raman Spectroscopy
provides unique
opportunities to look into the
wind accretion processes in
symbiotic stars.
Thank you