Cataclysmic variables

Transcription

1 Cataclysmic variables Sander Bus Kapteyn Astronomical Institute Groningen October 6, 2011

2 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

3 Types of cataclysmic stars Supernovae Ia Novae Recurent Novae Dwarf Novae

4 Supernovae Ia Very violent event involving the destruction of a star, 20 magnitudes or more increase in luminosity.

5 Novae Sudden nuclear ignition of accreted matter from a solar type star onto a white dwarf, 7 to 16 magnitudes increase.

6 Recurrent novae Similar to the nova, only these events happen multiple times during the observation history, magnitudes are a bit lower.

7 Dwarf novae This system involves a WD and a red dwarf in close orbit, 2 to 6 magnitudes increase in luminosity.

8 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

9 WD-RD binary system

10 Problems How to get the stars in close orbit? How to accrete matter? How to have periodicity? How to have different lightcurve shapes?

11 How to get the stars in close orbit?

12 Roche geometry

13 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

14 Common envelope

15 Cataclysmic configuration

16 How to loose angular momentum? gravitational radiation magnetic braking

17 Gravitional radiation Angular momentum loss due to gravitational radiation J J M 1M 2 M a 4

18 Magnetic braking Ingredients Stellar wind Stellar magnetic field

19 Magnetic braking

20 Orbital period distribution

21 Kepler s law P 2 a 3 M 1 + M 2 Roche lobe geometry

22 Feature: long-period cutoff

23 Feature: period gap

24 Feature: short-period cutoff

25 Time evolution of orbital period

26 Dwarf novae What is the outburst mechanism?

27 Two outburst mechanism theories Osaki Instability in the disk Bath Instability in the secondary

28 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.

29 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

30 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?

31 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 Outburst state: α

32 Magnetic turbulence: Balbus-Hawley instability

33 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

34 Thermal instability

35 Lightcurve of a Dwarf Nova

36 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

37 High accretion rate The outburst will emit in the extreme UV

38 Low accretion rate Siphon effect: The corona handles accretion onto the WD, the corona will emit in X- and γ-rays

39 Measurements Outburst: 2.8 ± counts/s Quiescent: 8.1 ± counts/s for OY carinae by ROSAT

40 Different lightcurve shapes

41 Mass distribution after an outburst

42 Burst lightcurves

43 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

44 Long burst lightcurve: plateau Entire disk is sustained in the outburst

45 Long burst lightcurve: cooling wave Cooling wave moves inward Outer annuli will become quiescent first They don t radiate anymore

46 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

47 Thank you for your attention! Do you have questions?

48 chandrasekhar 31, Sn1a collissions/accretion 10, life on a HR, samenvatting WD characteristics, jaj/ast162/lectures/noteswl22.pdf measures of OY carinae,

49 Sirius A & B optical, A and B Hub Sirius A & B X-ray, A %26 B X- ray.jpg VW Hyi, lcsn, lcn, lcrn, lcdn, sirart, Waves, kcooksey/special/images/vaulin.jpg fieldlines, matter is

50 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