Cataclysmic variables Sander Bus Kapteyn Astronomical Institute Groningen October 6, 2011
Overview Types of cataclysmic stars How to form a cataclysmic variable X-ray production Variation in outburst lightcurve, length and time Important source: Cataclysmic variable stars how and why they vary by Coel Hellier
Types of cataclysmic stars Supernovae Ia Novae Recurent Novae Dwarf Novae
Supernovae Ia Very violent event involving the destruction of a star, 20 magnitudes or more increase in luminosity.
Novae Sudden nuclear ignition of accreted matter from a solar type star onto a white dwarf, 7 to 16 magnitudes increase.
Recurrent novae Similar to the nova, only these events happen multiple times during the observation history, magnitudes are a bit lower.
Dwarf novae This system involves a WD and a red dwarf in close orbit, 2 to 6 magnitudes increase in luminosity.
Properties of a cataclysmic variable dwarf novae Binary system of a white dwarf and a red dwarf. They are in close orbit, within the radius of the sun. Novae are fed by accretion onto the white dwarf. Robert Kraft, 60 s
WD-RD binary system
Problems How to get the stars in close orbit? How to accrete matter? How to have periodicity? 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: Ṁ 2 > 0 ȧ a = 2J J + 2Ṁ ( 2 1 M ) 2 M 2 M 1
Common envelope
Cataclysmic configuration
How to loose angular momentum? gravitational radiation magnetic braking
Gravitional radiation Angular momentum loss due to gravitational radiation J J M 1M 2 M a 4
Magnetic braking Ingredients Stellar wind Stellar magnetic field
Magnetic braking
Orbital period distribution
Kepler s law P 2 a 3 M 1 + M 2 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
Two outburst mechanism theories Osaki Instability in the disk Bath Instability in the secondary Solution: The intensity of the brightspot doesn t change significantly during the outburst, so it can t be an extra flow through the Lagrange point.
Instability in the disk Ṁ sec > Ṁdisk due to too low viscous interactions. Pile up of material This makes the disk unstable: increase in viscosity Great increase in mass transport Increased accretion on the WD: higher luminosity and drain of disk Back to quiescent, low viscous state
Instability in the disk Ṁ sec > Ṁdisk due to too low viscous interactions. Pile up of material This makes the disk unstable: increase in viscosity Great increase in mass transport Increased accretion on the WD: higher luminosity and drain of disk Back to quiescent, low viscous state What is this viscosity & where does it come from?
Viscosity Viscosity causes mass to flow inward and angular momentum to flow outward. ν = αc s H for a turbulent α-disk from the theory of Shakura & Sunyaev. Quiescent state: α 0.01 0.05 Outburst state: α 0.1 0.5
Magnetic turbulence: Balbus-Hawley instability
Thermal instability We need a way to flip between the hot (highly viscous) state and the cold (low viscous) state. If the density rises, so does the temperature. Until the temperature is so high that H is being ionized (7000k) Opacity kicks in, trapping of energy The opacity goes as T 10 in partial ionized gas very unstable
Thermal instability
Lightcurve of a Dwarf Nova
Summary: How to form an outburst system A heavy and a light star WD & RD Loss of angular momentum due to magnetic breaking and gravitational radiation Balbus-Hawley instability in the disk when its hot 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 onto the WD, the corona will emit in X- and γ-rays
Measurements Outburst: 2.8 ± 0.2 10 3 counts/s Quiescent: 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 Σ > Σ max at an outer annulus viscosity works more in than out Σ max is higher at higher r The inner annuli are flooded with material from outside
Long burst lightcurve: plateau Entire disk is sustained in the outburst
Long burst lightcurve: cooling wave Cooling wave moves inward Outer annuli will become quiescent first They don t radiate anymore
Summary of high energy processes The corona is the principle place of emission of X-rays Present when the binary is optically quiescent Becomes dim when binary is in optical outburst
Thank you for your attention! Do you have questions?
chandrasekhar 31, http://adsabs.harvard.edu/abs/1931apj...74...81c Sn1a collissions/accretion 10, http://arxiv.org/abs/1002.3359 life on a HR, http://astro.wsu.edu/worthey/astro/html/lec-hr.html samenvatting WD characteristics, http://www.astronomy.ohiostate.edu/ jaj/ast162/lectures/noteswl22.pdf measures of OY carinae, http://onlinelibrary.wiley.com/doi/10.1046/j.1365-8711.1999.02900.x/full
Sirius A & B optical, http://upload.wikimedia.org/wikipedia/commons/f/f3/sirius A and B Hub Sirius A & B X-ray, http://upload.wikimedia.org/wikipedia/commons/d/d6/sirius A %26 B X- ray.jpg VW Hyi, http://www.aavso.org/sites/default/files/images/vwhyilc2.gif lcsn, http://www.aavso.org/sites/default/files/images/lightcurves/sn1987a.jpg lcn, http://www.aavso.org/sites/default/files/images/lightcurves/v2467cyg.jpg lcrn, http://www.aavso.org/sites/default/files/images/lightcurves/rsoph.jpg lcdn, http://www.aavso.org/sites/default/files/images/lightcurves/ugem.jpg sirart, http://www.sciencephoto.com/media/331231/enlarge Waves, http://space.mit.edu/ kcooksey/special/images/vaulin.jpg fieldlines, http://scienceblogs.com/startswithabang/upload/2010/10/some matter is
From Hellier on cataclysmic variables: Vary irregulary Robert Kraft: They are made up out of two stars, where material flows from one to the other. One a compact object: a white dwarf and the other a red dwarf. WD was about a solar mass star, which due to Hydrogen shell burning became a red giant. The radiation pressure will overcome the gravitational force: Layers are expelled: Planetary nebula (LOOK UP!!). Primary: The compact object M 1. Secondary: the companion M 2 The flow goes through the Lagrangian point (show point figures: equipotential en the potential fig 2.4 of white en red dwarves. The material can t flow immediatly to the WD, due to the spinning motion of the system. (V lagrangian 10km/s and v rot 100km/s. So the material will rotate, untill it hits its own stream: turbelence: heating of the medium: energy is radiated away: smaller orbits are possible, however Angular momentum must be conserved. So mass moves inward & outward while interacting. The outward moving material will at some point come into the tidal influence sphere of the red dwarf, transfering its excess angular momentum to the red