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Cataclysmic variables
Sander Bus
Kapteyn Astronomical Institute – Groningen
October 6, 2011
Overview
I Types of cataclysmic stars
I How to form a cataclysmic variable
I X-ray production
I Variation in outburst lightcurve, length and time
Important source:Cataclysmic variable starshow and why they vary by Coel Hellier
Types of cataclysmic stars
I Supernovae Ia
I Novae
I Recurent Novae
I Dwarf Novae
Supernovae Ia
Very violent event involving the destruction of astar, 20 magnitudes or more increase in luminosity.
Novae
Sudden nuclear ignition of accreted matter from asolar type star onto a white dwarf, 7 to 16magnitudes increase.
Recurrent novae
Similar to the nova, only these events happenmultiple times during the observation history,magnitudes are a bit lower.
Dwarf novae
This system involves a WD and a red dwarf in closeorbit, 2 to 6 magnitudes increase in luminosity.
Properties of a cataclysmic variable dwarf novae
I Binary system of a white dwarf and a red dwarf.
I They are in close orbit, within the radius of thesun.
I Novae are fed by accretion onto the white dwarf.
Robert Kraft, 60’s
WD-RD binary system
Problems
I How to get the stars in close orbit?
I How to accrete matter?
I How to have periodicity?
I How to have different lightcurve shapes?
How to get the stars in close orbit?
Roche geometry
Roche lobe overflow
Mass flows from the primary to the secondary:M2 > 0
a
a=
2J
J+−2M2
M2
(1− M2
M1
)
Common envelope
Cataclysmic configuration
How to loose angular momentum?
I gravitational radiation
I magnetic braking
Gravitional radiation
Angular momentum loss due to gravitationalradiation
J
J∝ M1M2M
a4
Magnetic braking
Ingredients
I Stellar wind
I Stellar magnetic field
Magnetic braking
Orbital period distribution
Kepler’s law
P2 ∝ a3
M1 + M2
Roche lobe geometry
Feature: long-period cutoff
Feature: period gap
Feature: short-period cutoff
Time evolution of orbital period
Dwarf novae
What is the outburst mechanism?
Two outburst mechanism theories
Osaki Instability in the disk
Bath Instability in the secondary
Solution: The intensity of the brightspot doesn’tchange significantly during the outburst, so it can’tbe an extra flow through the Lagrange point.
Two outburst mechanism theories
Osaki Instability in the disk
Bath Instability in the secondary
Solution: The intensity of the brightspot doesn’tchange significantly during the outburst, so it can’tbe an extra flow through the Lagrange point.
Instability in the disk
I Msec > Mdisk due to too low viscous interactions.
I Pile up of material
I This makes the disk unstable: increase inviscosity
I Great increase in mass transport
I Increased accretion on the WD: higherluminosity and drain of disk
I Back to quiescent, low viscous state
What is this viscosity & where does it come from?
Instability in the disk
I Msec > Mdisk due to too low viscous interactions.
I Pile up of material
I This makes the disk unstable: increase inviscosity
I Great increase in mass transport
I Increased accretion on the WD: higherluminosity and drain of disk
I Back to quiescent, low viscous state
What is this viscosity & where does it come from?
Viscosity
Viscosity causes mass to flow inward and angularmomentum to flow outward.
ν = αcsH
for a turbulent α-disk from the theory of Shakura& Sunyaev.
I Quiescent state: α ≈ 0.01− 0.05
I Outburst state: α ≈ 0.1− 0.5
Magnetic turbulence: Balbus-Hawley instability
Thermal instability
We need a way to flip between the hot (highlyviscous) state and the cold (low viscous) state.
I If the density rises, so does the temperature.
I Until the temperature is so high that H is beingionized (7000k)
I Opacity kicks in, trapping of energy
I The opacity goes as T 10 in partial ionized gas
I very unstable
Thermal instability
Lightcurve of a Dwarf Nova
Summary: How to form an outburst system
I A heavy and a light star → WD & RD
I Loss of angular momentum due to magneticbreaking and gravitational radiation
I Balbus-Hawley instability in the disk when itshot
I Thermal instability due to opacity
High accretion rate
The outburst will emit in the extreme UV
Low accretion rate
Siphon effect: The corona handles accretion ontothe WD, the corona will emit in X- and γ-rays
Measurements
Outburst: 2.8± 0.2 10−3 counts/sQuiescent: 8.1± 0.7 10−3 counts/s
for OY carinae by ROSAT
Different lightcurve shapes
Mass distribution after an outburst
Burst lightcurves
Long burst lightcurve: fast rise
I Σ > Σmax at an outerannulus
I viscosity works morein than out
I Σmax is higher athigher r
I The inner annuli areflooded with materialfrom outside
Long burst lightcurve: plateau
I Entire disk issustained in theoutburst
Long burst lightcurve: cooling wave
I Cooling wave movesinward
I Outer annuli willbecome quiescent first
I They don’t radiateanymore
Summary of high energy processes
I The corona is the principle place of emission ofX-rays
I Present when the binary is optically quiescent
I Becomes dim when binary is in optical outburst
Thank you for your attention!Do you have questions?
I chandrasekhar ’31,http://adsabs.harvard.edu/abs/1931ApJ....74...81C
I Sn1a collissions/accretion ’10, http://arxiv.org/abs/1002.3359
I life on a HR, http://astro.wsu.edu/worthey/astro/html/lec-hr.html
I samenvatting WD characteristics, http://www.astronomy.ohio-state.edu/ jaj/Ast162/lectures/notesWL22.pdf
I measures of OY carinae,http://onlinelibrary.wiley.com/doi/10.1046/j.1365-8711.1999.02900.x/full
I Sirius A & B optical,http://upload.wikimedia.org/wikipedia/commons/f/f3/Sirius A and B Hubble photo.jpg
I Sirius A & B X-ray,http://upload.wikimedia.org/wikipedia/commons/d/d6/Sirius A %26 B X-ray.jpg
I VW Hyi,http://www.aavso.org/sites/default/files/images/vwhyilc2.gif
I lcSN,http://www.aavso.org/sites/default/files/images/lightcurves/sn1987a.jpg
I lcN,http://www.aavso.org/sites/default/files/images/lightcurves/v2467cyg.jpg
I lcRN,http://www.aavso.org/sites/default/files/images/lightcurves/rsoph.jpg
I lcDN,http://www.aavso.org/sites/default/files/images/lightcurves/ugem.jpg
I sirart, http://www.sciencephoto.com/media/331231/enlarge
I Waves, http://space.mit.edu/ kcooksey/special/images/Vaulin.jpg
I fieldlines,http://scienceblogs.com/startswithabang/upload/2010/10/some matter is strange but tha/070820 neutron star 02-thumb-500x471-57492.jpeg
I period CV diagram,http://www.caha.es/newsletter/news03b/gaensicke/index.html
I MRI,http://rsta.royalsocietypublishing.org/content/366/1884/4453/F5.large.jpg
I accr,http://universe-beauty.com/albums/astronomy photo/Accretion-Disk-Binary-System.gif
I DN, http://1.bp.blogspot.com/ KBRiQuRcYAQ/TTxTNb-udTI/AAAAAAAAAzQ/JkBbKN07Xyk/s1600/Dwarf+Nova.jpg
I Sdiag, Hellier
I roche, http://www.physics.unc.edu/ evans/pub/A31/Lecture18-Stellar-Evolution/roche-lobe.jpg
From Hellier on cataclysmic variables:
I Vary irregulary
I Robert Kraft: They are made up out of two stars, where materialflows from one to the other. One a compact object: a white dwarfand the other a red dwarf.
I WD was about a solar mass star, which due to Hydrogen shellburning became a red giant. The radiation pressure will overcomethe gravitational force: Layers are expelled: Planetary nebula(LOOK UP!!).
I Primary: The compact object M1. Secondary: the companion M2
I The flow goes through the Lagrangian point (show point figures:equipotential en the potential fig 2.4 of white en red dwarves.
I The material can’t flow immediatly to the WD, due to the spinningmotion of the system. (Vlagrangian ≈ 10km/s and vrot ≈ 100km/s.So the material will rotate, untill it hits its own stream: turbelence:heating of the medium: energy is radiated away: smaller orbits arepossible, however Angular momentum must be conserved. So massmoves inward & outward while interacting. The outward movingmaterial will at some point come into the tidal influence sphere ofthe red dwarf, transfering its excess angular momentum to the reddwarf.
I Disk is fueled by the flow through L1 both in material and inangular momentum. The angular momentum is taken from the diskby tidal interactions with the secondary, while material is accretedby the primary.
I The accretion disk dominates the cataclysmic story, however thereare many more disk than cataclysmic stars. This is partly due to notbeing abble to see the effect in detail, like in quasars (whole starsare torn appart by BH’s) and star formation (planetary disk whichshrouds the new star). We hope to be able to extrapolate theknowledge of accretion from cataclysmic variables to theseprocesses.
I Eclipsing binaries give us a lot of information about period,distance, size and masses.
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