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.