Transcript CVs
Cataclysmic Variable Stars A Brief Overview What is a Cataclysmic Variable? • Close binary system – Primary = White Dwarf – Secondary = (usually) Red Dwarf • • • • • 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? • • • • • 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 • • • • • 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 • • • • 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 • • • • • 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 • • • • 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 • • • • 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 • • • • 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)