Cataclysmic Variables Mark Cropper Mullard Space Science Laboratory University College London 1
Course Structure 1. Brief historical introduction and context 2. Types and classes. 3. Non-magnetic systems 4. Magnetic systems: Polars and Intermediate Polars 5. Short and ultra-short period systems 6. Formation, evolution 7. The role of CVs within the larger scheme of astrophysical studies. 2
Cataclysmic Variables: what are they? 3
CVs: what are they? • Cataclysmic Variables are – semi-detached binaries accreting – from a red dwarf main-sequence-like secondary star – to a more massive white dwarf primary star • Roche potential: the gravitational potential around two orbiting point masses – resultant force on a test mass: Centre of Mass 2 credit: csep 10. phys. utk. edu 4
Roche Lobe Overflow • Semi-detached secondary star fills its Roche lobe so that it is distorted into a pear shape. 2 credit: csep 10. phys. utk. edu • At Lagrangian 1 (L 1) point, gravitational and centrifugal forces cancel and material is lost from the secondary star into the primary Roche lobe. • Material falls towards the white dwarf in a stream • The 4 other stationary points L 2 – L 5 are important for orbit theory credit: www. genesismission. org 5
CVs: main types non-magnetic Intermediate Polar 6
CVs: Role of the Magnetic Field (1) • magnetic field on primary <106 G (100 T) non-magnetic CV • accretion takes place through a disk • via boundary layer on white dwarf 7
CVs: Role of the Magnetic Field (2) • • NO DISK: accretion takes place via a stream and accretion column directly onto white dwarf • 8 magnetic field > 107 G (1000 T) polar/AM Her system the magnetic field controls the flow from some threading region
CVs: Role of the Magnetic Field (3) • • accretion takes place through a hollowed-out disk and then via accretion columns onto the white dwarf • 9 magnetic field ~106 G intermediate polar/DQ Her system magnetic field controls the flow in the final stages
CVs: Some background by Hevelius • First European observation of a CV, Nova Vulpecula, were made in 1670 by a monk Pére Dom Anthelme (2 nd magnitude) • Another nova, Nova Oph 1848 discovered by John Russell Hind • The first Dwarf Nova, U Gem, also discovered by J. R. Hind in 1855: noted that it was blue – most unusual for variable stars (mv~13. 5); substantial body of observations accrued and early harmonic analyses (Whittaker 1911) Shara, Moffat & Webbink (1985) • Next Dwarf Nova to be discovered was SS Cyg, in 1896, by Louisa Wells (Harvard College Plates) 10 credit: Warner (1995)
• SS Cyg observed almost continuously since 1896 (AAVSO) • Brightness history shows a variation on a timescale of ~50 days, • Unequal length maxima, no strict periodicity but remarkably regular timescale • varying between mv~8 and mv~12 (a factor of 40) • Less than the amplitude seen in novae, hence known as Dwarf Novae Warner (1995) from J. Cannizzo Historical light curves: SS Cyg
Light Curves: Novae • Typical amplitudes of Nova outbursts are larger, perhaps 10– 15 magnitudes (factor 104– 106) and recur on at least very long timescales different mechanism is operating • This is now known to be thermonuclear burning of the accreted material on the white dwarf Hellier (2001) from AAVSO data 12
Light Curves: Z-Cam stars Warner (1995) from J. Mattei/AAVSO • Some CVs show variability which is different from that of other Dwarf Novae: sometimes the brightness remains at a constant level before outbursts resume • Mean level during outburst phases is similar to that during constant phases • Called Z Cam stars after prototype 13
CV Optical Spectra • • First Dwarf Novae U Gem and SS Cyg observed in 1891 and 1897. • Spectral characteristics vary during outburst • Characterised by strong broad Balmer lines in absorption or emission, with Helium I and II • Other stars with similar spectra, but no outbursts known as Novalike variables. • 14 Spectroscopy of CVs started in 1860 s (Huggins), mostly Novae It was recognised that these might be explained by stars that were stuck in outburst, as in the non-outbursting episodes of the Z Cam stars Hessman et al (1984)
The CV Zoo: subtypes • Cataclysmic Variables (non-magnetic) – Novae large eruptions 6– 9 magnitudes – Recurrent Novae previous novae seen to repeat – Dwarf Novae regular outbursts 2– 5 magnitudes › SU UMa stars occasional Superoutbursts › Z Cam stars show protracted standstills › U Gem stars all other DN – Nova-like variables › VY Scl stars show occasional drops in brightness › UX UMa stars all other non-eruptive variables • Intermediate Polars/DQ Her stars • Polars/AM Her stars 15 magnetic systems
CVs: How do we know they are binaries? • • Joy (1956) found that SS Cyg had composite spectrum, and that radial velocity variations occurred on a timescale of 6 h 38 m again suggesting it was double • Walker (1954) found DQ Her to be an eclipsing binary • Kraft (1962) suggested all CVs might be binaries • Crawford & Kraft (1956) found that the secondary of AE Aqr occupied its “zero velocity surface” (Roche Lobe) so that some gas might be lost at the L 1 point • Kuiper (1941) had suggested for other binaries that turbulent gas would have angular momentum and swirl around the primary • Greenstein & Kraft (1959) found line profile changes through eclipse of DQ Her which result from the eclipse first of one side of the disk then the other confirmed this was the eclipse of a prograde rotating disk • Kraft (1961) found a spectroscopic variation on the orbital period resulting from impact region of the stream on the disk • 16 Joy (1940) found that RU Peg (Dwarf Nova) had a G 3 absorption spectrum, as well as an emission spectrum, suggesting it was double Large survey by Kraft (1962) on Palomar 200” telescope found orbital motion in almost all CVs, indicating they are close binaries with a white dwarf primary and a low mass main sequence secondary
Eclipsing CVs: light curves • If the system is seen edge-on the secondary star will cover the disk and primary star, causing an eclipse • This occurs on the orbital period Patterson et al (2000) IY UMa • Also evident is the bright hump resulting from the accretion stream impact region 17
Eclipsing CVs: light curves • Successive orbital phases cover different parts of the disk and primary star obscuring it from view from the observer Horne (1985) 18
Eclipsing CVs: light curves • This permits the different components of the system to be deconvolved bright spot disk white dwarf primary Wood et al (1986) Z Cha 19
Disk changes during outburst • At quiescence the contributions from the white dwarf and accretion spot are clearly evident • As outburst starts the disk component becomes more important • At maximum the light from the system is mostly from the disk, as evident in the strong U shaped eclipse 20 OY Car Warner (1995) from Vogt (1983)
Disk brightness profiles from eclipses • From the eclipse shadows the brightness of each part of the disk can be determined • Behaviour can be followed through an outburst • Changes can be seen in the structure of the disk • In bright states the disk temperature is T r – 3/4 • In faint states the disk temperature has a flatter relation and the effect of the hot spot from the stream impact is evident 21 EX Dra Baptista & Catalàn (2000)
Disk Models • In the simplest sense, disks can be modelled as a sum of annular blackbodies, each with the appropriate weighting for its area: this gives T r – 3/4 dependence • Temperature set by local dissipation, for example through Shakura & Sunyaev parameter for thin disks • More detailed models can be sums of atmospheres etc. La Dous (1989) 22
CV Disks through outburst Baptista & Catalàn (2000) • By following the X-ray and optical brightness of the disk during an outburst (needs coordinated observations) it can be seen that the X-ray brightness lags the optical brightness • This is because the outburst starts in the middle/outer parts of the disk, then propagates inwards • This is the result of an instability, or because of increased mass accretion rates from the secondary (Bath/Osaki debate) Mason et al (1978) 23
Disks: X-rays in outburst Wheatley, Mauche & Mattei (2003) optical • During outburst, outer regions of disk brighten first giving optical emission UV • When region of high dissipation reaches inner parts of disk X-ray emission starts to increase • As accretion rate increases, inner region becomes optically thick and able to radiate very efficiently: temperature drops so that emission is mostly in the Ultra-Violet 24 X-ray
Disks in quiescence: the boundary layer • X-rays are emitted in region at the inner edge of the disk, called the boundary layer optical • Can be explored through eclipse studies in the X-rays and UV/optical UV • Non-magnetic CVs are relatively faint in X-rays so only recently have observations achieved sufficient count rates to resolve the emission region • Eclipses are sharp, indicating that the emission is from a region only the width of the white dwarf, and not extended into the disk X-ray • Some indication that emission is from the polar regions: weak magnetic fields or obscuration 25 Ramsay et al (2001)
Non-magnetic CVs: X-ray Spectra • Only recently has it been possible to obtain X-ray spectra of high resolution of nonmagnetic systems in quiescence • Spectra show strong emission lines characteristic of optically thin emission from collisionally ionised plasmas • Boundary layer model is consistent with this • Rotational broadening is appropriate for white dwarf spin but not the inner Keplarian orbit of the disk 26 Pandel, Còrdova & Howell (2003) VW Hyi
Horne & Marsh (1986) Lines from disks • Each part of the disk has a particular velocity and brightness • Line profiles can be constructed from adding contributions from each part • Conversely, the brightness from each velocity zone can be determined from the profile: Doppler Tomography • If there is a relationship between the velocity and the location in the disk, (such as Keplerian motion, or freefall within the Roche lobe, then can map further from velocity to spatial coordinates 27
Doppler Tomograms of Disks • Using phase-resolved spectra (corresponding to different views of the system) the emission from the different emission regions can be mapped (similar to medical tomograms) • The location of the impact region can also be seen in Doppler tomograms • Since highest velocities in disks are at the centre and lower velocities outwards, the maps need to be “inverted” in the transformation to spatial coordinates for the disk component Spruit & Rutten (1998) 28
Superhumps • Some non-magnetic CVs, the SU UMa class of Dwarf Novae have occasional large amplitude outbursts, followed by more normal outbursts • During these large outbursts, a hump appears in the light curve: called superhumps • These humps evolve with time as outbursts decay. • Superhump period generally slightly longer than orbital period V 1159 Ori adapted fom Patterson et al (1995) by Hellier (2001) 29
Superhumps • Superhumps are thought to be caused by tidal interactions in the outer disk • • There fluid elements in the disk experience the attraction of the secondary star • Individual orbits are perturbed • 30 Disk is larger during outburst, reaching towards the edge of the Roche Lobe Global disk precession set up Hellier (2001)
Spiral Waves • Emission line structures can sometimes show strange structures at outburst Harlaftis, et al (2000) IP Peg • Doppler Tomograms indicate arc-shaped patterns, indicative of spiral structures • Can be reproduced using spiral structures in model data • Thought to be a spiral shock induced by the secondary as a result of noncircular orbits in disk Steeghs & Stehle (1999) 31
Polars 32
Magnetic systems: history • AM Her is a V~13. 5 variable star which Berg & Duthie (1977) suggested is the optical counterpart of a source in the Uhuru catalog (also a SAS-3 source) with a period of ~3. 1 hr • Lightcurve was unlike that of other CVs: too variable for Dwarf Novae and Nova-like CVs, no eclipses • In 1976, polarimetry obtained by Tapia, showed large circular polarisation variations on orbital period. • Largest circular polarisation seen in any celestial object: hence ‘Polar’ 33 AM Her Tapia (1977)
Polars: Evidence for magnetically confined emission • To achieve large levels of circular polarisation, radiation process must be largely cyclotron emission • For emission to be in the optical, require high values of magnetic field strength, ~30 MGauss (~3000 Tesla) • Expect this to disrupt the disk • Eclipse lightcurves show very sharp (seconds) drops and rises in brightness, consistent with a small spot on the white dwarf • Evidence for magnetically controlled accretion regions 34 HU Aqr Bridge et al (2002)
Polars: magnetically controlled accretion • Two accretion regions evident in some systems (UZ For) • No evidence of white dwarf • Weak accretion stream Perryman et al (2001) UZ For 35
Polars: magnetically controlled accretion • During eclipse, different parts of the system are successively eclipsed and uncovered • In some systems, accretion stream between stars can be very bright accretion spots + accretion stream + secondary only HU Aqr 36 Bridge et al (2002)
Polars • Magnetic field is too strong for a disk to form • material falls directly from secondary to primary • At some point material in stream threads onto magnetic field 37 • Subsequent accretion is quasiradial onto white dwarf
Rothschild et al (1981) Polars: X-ray emission • Polars/AM Her stars were found to be strong soft X-ray emitters (~1033 erg/s) in early surveys • X-ray emission characterised by thermalised free-fall velocities from a white dwarf so emission was from a hot region close to the white dwarf surface: post-shock • Cyclotron emission must also be from a hot region (otherwise narrow cyclotron emission lines rather than continuum) 38 AM Her
Polars: Spectral Energy Distribution • Most of the energy from these systems is a result of accretion • 3 main components: cyclotron radiation from accretion column 39 soft X-ray emission, from heated surface of primary Beuermann (1998) hard X-ray emission, also from accretion column
Polars: Radial Accretion • Infalling material is forced to follow the magnetic field lines • Gas is initially in free-fall but then it encounters a shock front • Shock converts kinetic energy into thermal energy (bulk motion into random motion) temperature increases to ~50 ke. V • Velocity drops by 1/4 and density increases by 4 • Material radiates by cyclotron and bremstrahlung and gradually settles on white dwarf 40 cold supersonic flow optical/IR cyclotron radiation shock hot postshock flow white dwarf hard X-rays soft X-rays/ extreme UV
Polars: accretion flow hydrodynamics • Equations of density velocity pressure – mass continuity – conservation of momentum height – conservation of energy • 1 -dimensional accretion • An analytical solution can be found to generate solutions in a step-wise scheme 41 cooling term bremsstrahlung cooling cyclotron cooling
Polars: accretion region hydrodynamics • Solutions to equations produce run of hydrodynamic variables (Temperature, Pressure etc) from which emissivity as a function of height can be calculated… 42
Cropper, Wu & Ramsay (2000) Polars: Emission from post-shock flow • Given the run of temperature and density, and assuming collisional ionisation (for this density regime) it is possible to show that the emission region is optically thin to X -rays • Also possible to calculate the ionisation fraction of any ion species, and therefore the emissivity, as a function of height in the post-shock flow as well as parameters such as – mass of the white dwarf – accretion rate • Predictions can be matched to spectra and continuum emission to derive fundamental parameters – X-ray calorimetry 43 Wu, Cropper & Ramsay (2001)
Polars: Energy Balance • The form of the spectral energy distribution, and particularly the relationship between the direct component of emission (X-ray thermal bremsstrahlung + cyclotron) and the reprocessed component been the topic of much debate. • Until recently measurements have indicated that the soft X-rays are much stronger than the direct emission, in contradiction to the basic model for magnetically confined accretion • One possibility has been that the accretion flow is not smooth, known as “blobby” accretion: – gives rise to flares in light curve (c. f. VV Pup) – blobs bury themselves deep in the white dwarf, so no visible post -shock flow for direct emission soft X-ray excess AM Her Frank, King & Raine (1992) Beuermann (1998) 44 King (1995)
Polars: energy balance revisited • New measurements have been made of ~40 polars (60% of known systems) to examine the energy balance using XMM-Newton which covers both hard and soft X-ray bands and also the optical/UV • Generally coverage of most of orbital; good fits achieved using stratified accretion column model GG Leo Ramsay et al (2003) 45 EU UMa
Polars: Energy Balance • Recent results find that most Polars have a direct emission/reprocessing balance (hard X-ray+cyclotron/soft X-rays) which is consistent with the standard view of magnetic accretion • A minority of systems have a soft excess • Still not clear what causes this (magnetic field? ) • Difference to past studies ascribed to – better instrumentation – better wavelength coverage (simultaneous) – better calibration – better models 46 Ramsay & Cropper (2003)
Polars: Spectral Characteristics • Optical spectra are dominated by strong emission lines of Hydrogen and Helium • General slope influenced by Balmer and Paschen continua • UV spectra show strong CIV, Si. IV and NV lines (plus others) • Strong lines indicative of substantial emission from the accretion stream • In high state generally no signature of the secondary or primary 47 Tanzi et al (1980) Schacter et al (1991)
Feigelson, Dexter & Liller (1978) Polars: Long-term light curves • Unlike disk-dominated CVs, polars tend to emit at an approximately constant level, with occasional drops to fainter levels • There is a continuum of levels, but the states tend to be called “high”, “intermediate” and “low” • In low states the accretion rate drops, no reservoir in a disk, so the underlying stars can become visible: important for measuring the system parameters 48
Polars: Shorter timescale variability • Light curves are strongly variable on orbital timescales • If accretion is occurring mainly near one magnetic pole, then, depending on the latitude of the accretion region, this can come into view and disappear over the limb as the synchronised binary rotates: “bright” and “faint” phases VV Pup Cropper & Warner (1986) • Many systems show strong variability due to flaring on timescales of tens of seconds • Some systems show quasiperiodic oscillations on a timescale of seconds: this is due to an intrinsic instability in the cooling gas accreting onto the white dwarf 49 Larsson (1992) V 834 Cen
Polars: Synchronisation • All of the variability in Polars occurs at a single period: the orbital period – radial velocity curves of the secondary – X-ray light curves from the primary – polarisation variations the white dwarf/red dwarf are locked into the same orientation: synchronised rotation • • As relative spin rate of primary decreases, locking can occur due to the dipole-dipole magnetostatic interaction between primary and (weaker) secondary magnetic field • Some Polars not quite in synchronism; in these systems it typically takes 5– 50 days for white dwarf orientation to repeat itself • 50 The mechanism for synchronisation is the dissipation due to the magnetic field of the primary being dragged through the secondary Very useful systems to study the effect of orientation of magnetic field on the accretion process
Polars: Asynchronous Systems • Changes from night to night on EUVE 2115 (7 -day slip period) 1 X-ray 2 3 4 5 6 7 51 Cropper, Ramsay & Marsh (2003) UV
Polars: optical spectra orbital variation orbital phase Hb velocity He. II 4686 Cropper, Ramsay & Marsh (in prep) 5 velocity orbital phase 52 velocity 3 orbital phase 1 • Doppler Tomograms showing resultant maps from changes in line emission night 5 orbital phase • Time sequence of spectra on different nights folded on orbital period for EUVE 2115 night 3 orbital phase night 1 velocity
Polars: measuring the magnetic field • The magnetic field can be measured in two main ways – cyclotron harmonics: measures the magnetic field in the cyclotron emitting region – Zeeman splitting: measures the field over the whole visible white dwarf (flux weighted) Thomas et al (2000) 3 RX 1313– 32 4 2 53
Polars: measuring the magnetic field • Zeeman split lines can sometimes be seen in low-state spectra (see also the signature of the secondary star) AM Her Schmidt, Stockman & Margon (1981) 54 Latham, Liebert & Steiner (1981)
Polars: measuring the magnetic field • Sometimes both techniques can be used on the same star MR Ser Schwope et al (1993) • • 55 And often two separate field strengths can be determined field strengths at the two poles are different; decentred dipole, or more complex field (superposition of multipoles) Generally accretion takes place preferentially near lower-field pole
Polars: Polarised Optical Emission • Cyclotron emission is elliptically polarised – linearly polarised viewed perpendicular to field – circularly polarised viewed along the field – position angle traces magnetic field line projected on sky • Diagnostic power very strong Harrop-Allin, Potter & Cropper (2001) ST LMi 56 Cropper (1996) HU Aqr
Polars: modelling the cyclotron radiation • The cyclotron radiation pattern (intensity and polarisations) has been calculated by several groups as a function of viewing angle, wavelength: various levels of sophistication • From this the total polarised emission from any surface element can be calculated • It is possible to iterate the map of emission points to gradually converge to the right map for the observed polarisations (Stokes Imaging, Potter, Hakala) • Produces a map of cyclotron emission Potter, Cropper & Hakala (2000) 57 V 347 Pav blue red
Polars: Eclipse mapping studies • It is possible to use eclipse mapping to calculate the stream brightness as a function of position HU Aqr • Then can use the brightness to generate the lightcurve from the stream only, and isolate cyclotron emitting region Harrop-Allin, Potter & Cropper (2001) HU Aqr 58
Polars: Stream mapping • More recent work even allows the emission regions not to follow a prescribed trajectory – allows trajectory to be determined freely • Emitting regions are defined and allowed to be located anywhere in Roche lobe of primary • Emission “swarm” evolved to produce a good fit • Fits which follow some linear configuration are preferred (self-organising maps – SOM) EP Dra 59 Bridge et al (2003)
Intermediate Polars 60
Intermediate Polars • After Polars were identified, Charles et al (1979) found an X-ray emitting V~13 star, AO Psc, with an optical spectrum like that of Polars, but without any identifiable polarisation • They also showed variability on three different timescales now known to be – the orbital, – the spin period of the white dwarf & – the mixture of the two (beat/synodic period) 61 Cropper et al (2002) AO Psc
Intermediate Polars: another example Cropper et al (2002) X-Ray 62 FO Aqr
Intermediate Polars: Power Spectra • By performing a Fourier Transform of the previous data, the main periodicities can be identified – orbital period Evans et al (2003) UV – white dwarf spin – beat (very faint in this system) • Also evident are harmonics when the variations are non -sinusoidal (2 , 3 , 2 ) • The variation of X-rays at a higher frequency suggested that due to magnetically controlled accretion – but with a lower field than Polars/AM Her systems 63 X-ray
Intermediate Polars: folded light curves • Can now fold the data on the main periods, to derive the phase relationship between different wavebands FO Aqr UV UV FO Aqr X-ray 64 Evans et al (2003)
Intermediate Polars • Since the magnetic field is not as strong as in Polars, a disk can form; field hollows out central parts of the disk • From the inner part of the disk, accretion occurs down field lines similar to that in Polars – so get radial accretion around both magnetic poles • Because field is lower, cyclotron radiation is less strong unpolarised (generally) 65
Intermediate Polars: more evidence • Classic observations by Nather (1978) and Patterson, Robinson & Nather (1978) found that DQ Her had a 72 sec oscillation that went away during eclipse • Also found that the phase of the oscillation changed through the centre of the eclipse, as a result of first one side of the disk being eclipsed then the other • Interpreted as resulting from the reprocessing of an X-ray beam sweeping the disk like a “light house” • Only recently seen in X-rays (scattered) Nather (1978) 66 Patterson, Robinson & Nather (1978)
Intermediate Polars: models • Intermediate Polars spin variability can be explained in several ways (much debate on this over the years) – visibility of the accretion region on the white dwarf – visibility of the accretion “curtains” – reprocessing of flux on the disk (optical/UV) stream Adapted from Hellier (2001) • From studies of the relative phasing in different wavelength bands and including to the absorption effects now known to be a combination of the above models leading to the complex behaviour in Intermediate Polar light curves 67
Intermediate Polars: X-ray Spectra • X-ray spectra of Intermediate Polars generally show just the multi-temperature thermal bremsstrahlung component from the hot radial accretion flow – no soft reprocessed component from the white dwarf • Main explanation is likely to be the larger area over which accretion takes place, but also photoelectric absorption is important AO Psc Cropper et al (2002) 68 FO Aqr
Intermediate Polars: radial accretion flow • Very strong evidence for radial accretion can be obtained by using the same accretion flow model (Stratified Accretion Column) shown earlier for Polars, and applying it to Intermediate Polars • This provides a detailed fit to high resolution Xray spectra of systems such as EX Hya • Single or even add-hoc 3 -temperature thermal bremsstrahlung models do not provide sufficiently good fits 69 Cropper et al (2002) EX Hya
Intermediate Polars: Fast-spinning systems • Some Intermediate Polars have very short spin periods, for example DQ Her: 71 sec, AE Aqr: 33 sec • In this case the magnetic field is thought to be small, so that the hollowed-out part of the disk is also small and the Keplarian velocities in the field coupling region large • AE Aqr is a special case: the white dwarf spin period is decreasing rapidly, and it can be calculated that almost all of the luminosity of the system arises as a result of the spindown energy • Spinning white dwarf kicks the gas from the stream out of the system – propellor • Much of the optical and radio emission from the system arises in colliding shocks in the ejecta 70 Wynn, King & Horne (1997)
Cataclysmic Variable Evolution • The white dwarf in CVs is the relic of the more massive star in the binary, already past a giant phase • The secondary star will have spent some time in the envelope of the primary red giant perhaps accreting material from it • Dynamical friction reduces the separation of the remnant core of the primary and the secondary, causing the secondary to spiral inwards and perhaps contributing to the ejection of the envelope • After this phase have a detached hot white dwarf with a main sequence secondary; such stars are known (eg BE UMa) and show strong reflection effects from the hot primary illuminating the secondary • Further angular momentum losses can shrink the binary separation until the secondary comes into contact with its Roche lobe, and mass transfer starts, giving rise to a CV • Alternatively in longer period systems the secondary can evolve, increasing in size and filling its Roche lobe 71
Evolution (ctd) • Transfer of material from a lower mass star to a higher mass star (as in a CV) causes the orbital separation to increase, lengthening the period and causing the star to fall away from contact with its Roche Lobe and accretion to stop • Hence some other mechanism has to been in place for stable mass transfer • Main candidate for more than two decades has been the magnetic braking caused by wind material from the secondary being threaded by field lines to large distances (Zwaan & Verbunt) • This magnetic braking robs the binary of angular momentum, so the two stars move closer together, maintaining contact with the Roche Lobe and quasi-stable mass transfer • From this an expected population of CVs can be generated as a function of orbital period, given ages and mass transfer rates 72
CV Evolution (ctd) • Any prediction from population models needs to confront the statistics of the observed period distribution Ritter & Kolb (1998) • From 10 down to ~3 hrs the distribution is approximately correct, but then there is a lack of systems in the 2 -3 hr range, known as the “Period Gap” • More shorter period systems 73
CV Evolution (ctd) • Zwaan & Verbunt suggested that at 3 hr the magnetic braking switches off, because secondaries with this mass are fully convective, so stellar dynamo is quenched • Mass transfer rate then is reduced, so secondary falls away from Roche Lobe, and mass transfer stops, so these systems are not seen as CVs • Then gravitational radiation would continue to operate, slowly driving the two stars together, until contact with the Roche Lobe was reestablished and accretion restarted: the origin of the <2 hr binaries • While this has been the “standard” explanation for some time, questions have always remained about the real effect of fully convective secondaries on the generation of magnetic field by the dynamo • Some evidence for this mechanism clear from the absence of a significant gap in the magnetic systems, in which braking is affected by the field from the primary (in addition to that from the secondary) 74
Spin-orbit coupling in magnetic systems • The two stars in Polar systems rotate synchronously with the orbit by the magnetic interactions, except in the case of 4 systems which deviate slightly from synchronism • Intermediate Polar systems are not synchronised • Many IPs are found to have Pspin~0. 1 Porb due to field/stream interactions • As the binary evolves and the period decreases it is likely that some IPs evolve into Polars 75 Norton, Somerscales & Wynn (2003)
CVs: Short Period Systems • CVs with main sequence secondaries have minimum orbital period of 80 minutes • However, some accreting binaries are seen to have shorter periods the secondaries must be white dwarfs, or perhaps the degenerate core of a giant which has been stripped away • The optical spectra of these systems show only Helium (no Hydrogen) • The disk behaviour is different because of the different ionisation of He compared to He; interesting astrophysical laboratories • Orbital periods from ~65 min to ~15 min • Known as the AM CVn systems Marsh, Horne & Rosen (1991) 76 GP Com
Ultra-short period Binaries • 3 systems have recently been discovered with binary periods <10 min (600 sec)! Small enough to fit within Jupiter RXJ 1914. 4+2457 (V 407 Vul) 9. 5 min Cropper et al (1998) RXJ 0806. 3+1527 5. 4 min Ramsay, Hakala & Cropper (2002) KUV 01584– 0939 (ES Cet) 77 Israel et al (2002) 10. 3 min Warner & Woudt (2002)
Observational Characteristics Cropper et al (1998) • X-ray light curve shows sharp rise to maximum and slower decline • X-rays essentially “off” for ~50% of the cycle small emission region • RXJ 1914. 4+2457 and RXJ 0806. 3+1527 X-ray light curves are almost identical 78 RXJ 1914. 4+2457 Burwitz & Reinsch (2000) RXJ 0806. 2+1527
Observational Characteristics Only one period in power spectra; if this is the orbital period then the secondary has to be degenerate (a white dwarf) RXJ 1914. 4+2457 X-ray optical window prewhitened ROSAT HRI data Cropper et al (1998) 79 NOT optical data Ramsay et al (2000) prewhitened
Observational Characteristics (3) 3) Optical/Infrared Colours RXJ 1914. 4+2457 Ramsay et al (2000) • No evidence for a late main-sequence secondary star • Spectral flux distribution in optical is that of a hot (~50000 Kelvin) black body 80
Observational Characteristics • Source is not constant in X-ray or optical/IR most likely some sort of accretion or similar mechanism Ramsay et al (2000) Long-term light-curve: RXJ 1914. 4+2457 81
Observational Characteristics (6) • Phasing of X-ray and optical/IR light curves argues against a single star • simplest explanation is a reprocessing source in a binary system where the X-rays from the primary are reprocessed from the secondary reprocessed X-ray RXJ 1914. 4+2457 Ramsay et al (2002) 82
Observational Characteristics (8) • Spectra of RXJ 0806. 3+1527 show no H emission – all emission is He, C and N and weak RXJ 0806. 2+1527 83 Israel et al (2002)
Models • Four models exist for these systems a) Intermediate Polar Motch et al (1996) Norton et al (2002) b) Double Degenerate Polar Cropper et al (1998) c) Double Degenerate Algol Marsh & Steeghs (2002) Ramsay et al (2002) d) Electric Star Wu et al (2002) a) Of these models, three involve magnetic fields b) Neutron star models are ruled out because of the absence of a hard tail of X-ray emission; also they need to be extremely close (too close for the observed level of reddening) 84
Model 4: Electric Star • Model introduced by Wu et al (2002) to deal with absence of polarisation and emission lines in RXJ 1914. 4+2457 • Scaled-up version of Jupiter-moon interactions heating Jupiter’s atmosphere at footpoints of magnetic field lines threading moon, BUT: a) energy release boosted by stronger magnetic field b) shorter period c) secondary larger than a moon • Up to 1036 ergs/s available (depending on degree of asynchronism) • Star shines by electrical resistive heating: new mechanism after 1. nuclear fusion 2. accretion 3. magnetic (magnetars) HST image (NASA) 85
Model 4: Electric Star (ctd) • Conductor (secondary) moving in magnetic field of primary has EMF induced across it (as in any generator) • EMF drives a current in 2 circuits if low density plasma in system • Emission takes place from two small hotspots at the base of the field lines on primary Wu et al (2002) 86
Model Comparison IP DD Polar DD Algol Electric Star (ES) Absence of Hard X-rays x √ √ √ Only one modulation period x √ √ √ Optical-IR colours x √ √ √ Phasing of X-ray and Optical ~ √ Shape of X-ray modulation √ √ Absence of Polarisation √ ~ √ √ Absence of H x √ √ √ Absence of strong emission lines x ~ ~ √ Period decreasing ~ x x √ Long-term variability √ √ • On current knowledge, only strongly ruled out model is the IP, mainly because of lack of photometric evidence for secondary 87
Further thoughts: ES Model • ES model suffers from being ‘unconventional’ but nevertheless: a) provides the best interpretation of the data b) evidently works in astrophysical systems (Jupiter) • Predicts the right shape of X-ray light curve (retrograde spin on primary WD) • Lifetime of the system is ~106 years, short but not particularly short – NOTE: the 103 years quoted by Marsh & Steeghs (2002) is a misinterpretation of the text in Wu et al (2002) • It is an expected end point for double WD pairs if one has a modest (<1 MG) field; secondary is not necessarily in contact with Roche Lobe – expect many of these systems? • Predicts decreasing orbital period, as observed for both systems 88
Gravitational Radiation • Ultra-short period binaries are expected to be strong sources of gravitational radiation. These will be the calibrators for LISA RXJ 0806. 3+1527 RXJ 1914. 4+2457 KUV 01584– 0939 89 Figure from NASA/JPL; strain from Nelemans et al (2001)
CVs in the grander scheme of things • Cataclysmic variables are fairly common systems, the later stages of much of binary evolution • They produce the low-level background of discrete sources in galactic X-ray emission – fainter but much more numerous than neutron-star or Black hole X-ray binaries • They are highly important laboratories for studies of accretion – disk behaviour › instabilities, › stream impacts, › warps, › tidal resonances, › spiral waves etc. – magnetically dominated accretion › accretion columns, › emission from post-shock flow, › shocks, instabilities etc. • Multi-wavelength emission (polarised in many cases) allows a multi-wavelength approach, providing very strong observational constraints on the interpretation of data 90
CVs in the grander scheme of things (ctd) • Important for investigations on how material interacts with a magnetic field: – threading region in Polars, – inner region of disk in Intermediate Polars, – Dwarf Nova oscillations in non-magnetic CVs • In general, the balance of: – visibility of underlying system (to provide the context) & – the emission (X-ray, optical) has been fundamental to making enormous progress in understanding a wide range of astrophysics • It is a field which incorporates fluid dynamics, MHD, a full range of emission processes, stellar evolution, gravitational radiation etc. • A large number of important observational techniques have been developed in the context of CVs and then used elsewhere: – Doppler tomography, – eclipse mapping of disks and streams, – Stokes imaging, – timing analyses • Many well-known astronomers/astrophysicists have worked in this field, developing their theoretical understanding and observational & interpretational skills before carrying these into other fields 91
CVs: Open Issues • This has been an extremely active field in the last 3 decades. Many scientists addressing the issues, large amount of time devoted on observational facilities, large and small, on ground and in-orbit • Much is now understood about these systems, and many of the fundamental issues in these systems have been addressed (energy balance in Polars, disk-instabilities in non-magnetics, accretion curtains in IPs) • Mature field, which is developing around its boundaries: ultra-compact systems are an important and exciting new development • However: the effect of new facilities, particularly in the X-rays (XMM-Newton and Chandra) and UV (FUSE, XMM-OM) is only now beginning to be felt, also greater access to 8 m telescopes on the ground we can still expect to learn a great deal 92
CVs: Open Issues • Many aspects deserve further investigation: here are some – boundary layer in non-magnetics – the base of the post-shock accretion flow in magnetics and the way this diffuses into the white dwarf – heating of the atmosphere around the accretion region in magnetics, and effect on overall energy distribution – low accretion rate regimes in magnetics, whether this results in a bombardment solution (no shock) – disk-magnetosphere interaction in IPs: important in a number of contexts – disk-stream interactions in non-magnetics – magnetosphere-stream interactions in Polars – irradiation of the stream and secondary by X-ray flux – more astrophysics in the post-shock flow models (such as the separation of electron and ion fluids) • Combinations of high quality data (eg. eclipse mapping of spectra) and new astrophysical fluid computations will transform the field and allow ever more intricate understandings of accretion phenomena to be achieved 93
Resources • Books: Brian Warner: Cataclysmic Variable Stars (Cambridge University Press, 1995, ISBN: 0521412315) Coel Hellier: CVs - How and Why They Vary (Praxis Publishing, 2001, ISBN: 1852332115) Frank, King & Raine, Accretion Power in Astrophysics (Cambridge University Press, 3 rd edition) • North American Workshops on Cataclysmic Variables • Magnetic Cataclysmic Variable Workshops Acknowledgements: These lectures drew partly from material in Warner (1995) and Hellier (2001) 94
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