Transcript Cataclysmic Variable Stars A Brief Overview

Cataclysmic Variable Stars
A Brief Overview
What is a Cataclysmic
Variable?
• Close binary system
– Primary = White Dwarf
– Secondary = (usually) Red Dwarf
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Mass transfer from secondary
X-Ray emission
Roche Lobe Geometry
Accretion Disks (not always though)
Characterized by strong, somewhat irregular
variations
Light Curve of SS Cyg
How do CVs form?
• Close binary system
• Primary star (more massive) evolves to
WD
• Go through a common envelope period
• Orbital distance decreases
• Binary system acts as propeller pushing
gas away
• Left with naked binary system
How do CVs form?
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Red Dwarf fills Roche Lobe
Material begins to accrete onto the WD
Inner Lagrangian Point
Effect of filled Roche lobe is tidal locking of Red Dwarf
Secondary (Red Dwarf) star’s outer layers are distorted
by the WD (remember it’s a close binary) - Ellipsoidal
variations
Mass Transfer
• Red Dwarf fills Roche Lobe and
accretes matter onto the WD through
Lagrangian point
• Turbulence and friction cause the
stream of matter to spread into a disc
(sometimes)
• How does the system maintain this
mass transfer?
Mass Transfer
it’s all about conserving angular momentum
• Gravitational Radiation
– Radiation of energy in
gravity waves
– Usually only significant
with VERY massive
objects
– Becomes significant in
extremely close systems
with very short periods
• Magnetic Braking
– Corotating mag fields
accelerate stellar wind
particles to high speeds
carrying away angular
momentum
– Usually this would cause
the star to slow down its
rotation
– Can’t happen because of
tidal locking, so instead
the orbital distance
decreases
Non-Magnetic CVs: The
Accretion Disc
• Mass transferred in stream through the
Lagrangian point is slowed down and spread
out into a disk by turbulence and friction
• Creation of the ‘Bright Spot’
– Stream from secondary strikes edge of disc
– Turbulence - KE of stream converted the heat and
radiated away
– Outcome - a very hot bright spot (radiates in x-ray)
The Bright Spot
• Characteristic
light curves
• Orbital Humps
• Bright Spot
Eclipses
• Grazing
Eclipses
Light curve of Z Cha
Spectra of accretion disc
• Sometimes emission,
sometimes absorption
• Indicates changes
between optically
thick and optically thin
• Expect double peaked
profile from disc
material, not always
seen
• Not fully understood
Distribution of orbital periods
Period min
Period gap
Long
Period
Cut-off
Dwarf Novae
• Changes of several Mag in short time
– Stay bright for ~week, then decline. Cycle repeats
months later
– U Gem and SS Cyg
• Disc instability
– Viscosity of disc causes pile up
– Disc becomes unstable and heats up and expands
both inward and outward
• Increased mass transfer from secondary
• Opinion leans toward disc instability
– Observations of increased disc radius
– Uniformity of bright spot L
– Theoretical success with instability
Dwarf Novae - accretion disc
physics
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Viscosity
Magnetic Turbulence
Thermal Instabilities
Heating and cooling waves
Lead to different shapes of outbursts
and different durations of the outburst
Novalikes (UX Uma)
• Disc remains hot (higher mass flow) and
viscous
• Stays in a state of constant outburst
• Can be brought back to quiescence when
something like a star spot crosses
Lagrangian point
• Z Cam stars - intermediate between Dwarf
Novae and Novalike (standstills)
– Behaves like regular dwarf novae until an outburst
brings it back to novalike state
Other causes for variation
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Elliptical Discs
Tidal torques and resonances
Spiral shocks
Flared discs
– SW Sex Stars
• Winds
• Disc-stream spill over
• Superoutbursts - SU UMa stars
– Similar to DN but last much longer, more regular
– Also display superhumps
– Relation to period gap
• Infrahumps?
• … the lesson - this is complicated
Novae
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Accretion builds up
Runaway thermonuclear reactions
System re-enter ‘common envelope’ phase
Blow off outer shell (P Cygni profile)
Recurring Novae
– Amount of accretion necessary depends on mass of WD
– Short time scale (~100yrs) could occur for stars near the
Chandrasekhar limit
– Also possible in systems with evolving secondary (possibly
not actually novae)
– There are a few systems known that could possibly be
recurring novae
• Can CVs Supernova?
– Super soft x-ray sources
Magnetic CVs
• Strong Mag fields
– Feedback from charged particles and mag
fields
– End result - particles frozen in to field
• Inner zone dominated by B field
(Magnetosphere)
• Outer zone acts like non-magnetic CV
• Boundary layer, poorly understood
AM Her Stars (Polars)
• STRONG B fields (~10-100 MG or more)
• Synchronous rotation (WD and orbital period)
• B field from WD can still dominates at Lagrangian point
– Even when it doesn’t, magnetosphere is close enough that disc
never forms
• Stream diverted along field lines
• Form very concentrated accretion regions at the poles
AM Her Stars
• Accretion hot spots at
poles
• Accretion shocks (xrays)
• Orientation changes s.t.
one pole of WD aligns
with stream
• Makes for very strong
obvious eclipse of the
accretion hot spot
Accretion regions
• Smaller particles form hot accretion column
• Other particles collide with accretion column
and slow down - accretion shock
• Accretion columns are sources of hard x-rays
• Denser blobs not affected by accretion
column, go directly to surface of WD
• Denser material landing on WD leads to more
soft x-rays
Polarization in AM Her Stars
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Cyclotron radiation
Extremely polarized
Known as ‘Polars’
Measurement of polarization can give
you orientation of magnetic axis of WD
• Can also give you knowledge about
binary inclination
• Can tell you about field strength
Asynchronous Rotation
• Observation of light curves of accretion
regions can show asynchronous rotation
• Usually only off by ~1%
• Possibly knocked out of synchronous rotation
– V1500Cyg Nova
• System might have B field just a little too
weak to cause synchronous rotation
Intermediate Polars - DQ Her
• Non synchronous rotation (WD spin period and xray flux periods don’t match)
• Discless
– Magnetosphere rotation adjusts so it is at the same
speed as the Keplerian orbit
• Can they form discs?
– If r of the magnetosphere < rmin of material - YES
– If r is magnetosphere > rmin - No
– What about in between?
• Diamagnetic blobs with induced current
• Polarization not observed in most intermediate
polars (smaller B field, and presence of disc)
• Mag field presence deduced from pulsed x-rays
Intermediate Polars (Cont’d)
• Disc fed accretion
– Pulsations due to accretion curtain
– These can be observed in the optical also,
because the ‘curtain’ material is optically bright
• DQ Her
– Not a real DQ Her star
– No x-ray flux
– Effect of x-ray flux detectable in disc
• XY Ari
– DN behavior from disc - but pulsed x-rays like AM
Her
Propellers
• Magnetosphere radius > corotation
• Extremely rapid rotations
• Diamagnetic blobs accelerated by field lines
and expelled from the system
• AE Aqr
– 33 sec period of WD with 9.9 hr orbital period
– Mass tranfer estimated 1000x > amount acreting
onto WD
• WZ Sge
– Long periods of quiescence followed by super
outbursts
– Could be due to build up prevented by propeller
behavior
Flickering and Quasi-Periodic
Oscillations
• Mass transfer is turbulent
• U Gem (grazing eclipse) implies flickering
occurs in the bright spot
• HT Cas (full eclipse) indicates it occurs at the
inner disc
• QPOs observed in dwarf novae and AM Her
• Not fully understood
• White dwarf pulsations
– ZZ Ceti stars
Secondary star variations
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Pulsation in secondary star
Star spots
Evolution of secondary star
These would all have strong effect on mass
transfer rates
• VY Scl
– Novalike stars that appear to go through periods
of no mass transfer
• Another factor to consider
Summary
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CVs do share common characteristics
Many causes of variation
Lots of physics (complicated)
Two major classes of CVs
– Non Magnetic
• Accretion Disc, Bright Spot
– Magnetic
• Magetosphere
• Polar accretion hot spots
• Mass Transfer - not a simple process
– Turbulence, friction, variation in secondary
• CVs can tell us a lot!
Questions
• Not much that isn’t a question
• Mechanisms for processes such as turbulence poorly
understood
• Accretion disc physics
– Magneto-hydrodynamics
• A few major questions
– What happens at the boundary layer of
magnetosphere?
– How does the disc behave with different levels of mass
transfer?
– Are there more long term behaviors?
– How long do classes of CVs last?
– How many distinct classes are there?
– How important are variations in the secondary?
References
• Hellier, C. 2001, Cataclysmic Variable Stars: How
and Why They Vary
• Wood, J., et al. 1986, MNRAS, 219, 629
• Hessman, F. V., et al. 1984, ApJ, 286, 747
• Ritter and Kolb, 2005, Catalogue of Cataclysmic
Binaries, LMXBs, and related objects
• Honeycutt, R. K., et al. 1998, PASP, 110, 676
• Kube, J. G., et al. 2000, AAP, 356, 490
• Martin Wood - astro.fit.edu (for the CV tree
diagram)