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Cataclysmic Variables: 10 Breakthroughs in 10 Years

Cataclysmic Variables: 10 Breakthroughs in 10 Years. Christian Knigge University of Southampton. P. Marenfeld and NOAO/AURA/NSF . Outline. Introduction Cataclysmic variables: a primer 10 breakthroughs in 10 years (a personal and hugely biased perspective...) Evolution

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Cataclysmic Variables: 10 Breakthroughs in 10 Years

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  1. Cataclysmic Variables: 10 Breakthroughs in 10 Years Christian Knigge University of Southampton P. Marenfeld and NOAO/AURA/NSF

  2. Outline • Introduction • Cataclysmic variables: a primer • 10 breakthroughs in 10 years (a personal and hugely biased perspective...) • Evolution • Accretion • Outflows • The Role of UV Astronomy • Summary 98 7 654321 35 minutes XX XXXXX Links to Other Systems (BH/NS LMXBs)

  3. Cataclysmic Variables: A PrimerThe Physical Structure of CVs • White dwarf primary • UV bright • “Main-sequence” secondary • 75 mins < Porb < 6 hrs • Roche-lobe overflow • Accretion usually via a disk • UV-bright • Disk accretion is unstable if below critical rate • dwarf novae • Mass transfer and evolution driven by angular momentum loss • Evolution is (initially) from long to short periods Red Dwarf White Dwarf Accretion Disk Credit: Rob Hynes

  4. Cataclysmic Variables: A PrimerThe Orbital Period Distribution and the Standard Model of CV Evolution • Clear “Period Gap” between 2-3 hrs • Suggests a change in the dominant angular momentum loss mechanism: • Above the gap: • Magnetic Braking • Fast AML  High • Below the gap: • Gravitational Radiation • Slow AML  Low • Minimum period at Pmin ≈ 80 min • donor transitions from MS  BD • beyond this, Porb increases again • This disrupted magnetic braking scenario is the standard model for CV evolution Knigge 2006

  5. Breakthrough I: EvolutionDisrupted Angular Momentum Loss at the Period Gap Howell et al. 2001 • Standard model prediction • The period gap is caused by a disruption in AML when the donor becomes fully convective • Magnetic braking drives high above the gap • Donor is slightly out of TE and thus oversized • At , donor becomes fully convective • MB ceases (or is severely reduced) • drops --> donor relaxes (shrinks) to TE radius • Donor loses contact with RL • CV evolves through gap as detached binary • Residual AML (e.g. GR) shrinks orbit (and RL) • Contact with donor re-established at • Observational reality pre-2005 • No direct empirical support for this picture (other than the existence of the gap itself)

  6. Breakthrough I: EvolutionDisrupted Angular Momentum Loss at the Period Gap Patterson et al. (2005), Knigge (2006) • Donors are significantly larger than MS stars both above and below the gap • Clear discontinuity at M2 = 0.20 M☼, separating long- and short-period CVs! • Direct evidence for disrupted angular momentum loss! M-R relation based on eclipsing and “superhumping” CVs

  7. Breakthrough II: EvolutionReconstructing CV Evolution Empirically • We can even use the donor relation to quantitatively reconstruct CV evolution • CV Donors are significantly larger than MS stars because they are bloated by mass loss • Higher Larger • So we can use the degree of donor bloating at given to infer • Above the gap: slightly reduced “standard” MB recipes work well • Below the gap: need enhanced AML,  significant revision of the standard model! Knigge(2006) Knigge, Baraffe& Patterson (2011)

  8. Breakthrough III: EvolutionPeriod Bouncers with Brown Dwarf Secondaries • Standard model predictions • 99% of CVs should be found below the period gap • A full 70% should be “period bouncers” with brown dwarf secondaries • Observational reality pre-2006 • Not a single definitive period bouncer • Only ~10 candidates out of ~1000 CVs • No secondary with a well-established mass below the H-burning limit • Is this a selection effect or model failure? Howell et al. 2001

  9. Breakthrough III: EvolutionPeriod Bouncers with Brown Dwarf Secondaries Littlefair et al. 2006, Science, 314, 1578 • SDSS has yielded a deep new sample of ~200 CVs (Szkody et al. 2002-9)... • ...including a sub-set of faint, WD-dominated systems near Pmin(Gaensicke et al. 2009; see later) • A few of these are eclipsing, allowing precise system parameter determinations • At least 3 of these have M2 < 0.072 M☼(Littlefair et al. 2006, 2008) At least some post-period-minimum systems with brown dwarf donors do exist! But one of them is very strange…

  10. Breakthrough III: EvolutionPeriod Bouncers with Brown Dwarf Secondaries Littlefair et al. (2007) Patterson et al. (2008) • SDSS J1507 is one of the three eclipsing CVs with sub-stellar donors… • … but its for other CVs • Two ideas: • J1507 is young -- born with a sub-stellar donor (Littlefair et al. 2007) • J1507 is a low metallicityhalo CV (Patterson et el. 2008)  How can we test which is correct? • UV astronomy to the rescue! • FUV spectroscopy shows that [Fe/H] = -1.2 • SDSS J1507 is an eclipsing period bouncer in the Galactic halo! • Rosetta stone for studying effects of metallicity on accretion and evolution? Littlefair et al. (2007) Stehle et al. (1999) Uthas et al. (2011)

  11. Breakthrough IV: EvolutionThe Period Spike at Pmin • Standard model prediction • The number of CVs found in a particular Porb range is inversely proportional to the speed with which they evolve through it • So there should be a spike at Pmin, in the period distribution since • Observational reality pre-2009 • No convincing spike anywhere near Pmin in the CV Porb distribution Barker & Kolb 2003

  12. Breakthrough IV: EvolutionThe Period Spike at Pmin Gaensicke et al. 2009 • Boris Gaensicke and collaborators have obtained orbital periods for most of the new SDSS CVs • The resulting period distribution does show a spike at Pmin for the first time (Gaensicke et al. 2009) CVs do in fact “bounce” at Pmin! Previously known CVs SDSS CVs

  13. White Dwarf Breakthrough V: EvolutionCVs in Globular Clusters • CV space density: (e.g. Pretorius & Knigge 2007, 2011) • Effective volume of MW: • Expected # of CVs in MW: • Fraction of MW mass in GCs: • # of GCs in MW: • → expected # of CVs per GC: • A typical GC should contain ~100 CVs purely based on its stellar mass content • But bright X-ray binaries are overabundant in GCs by ~100x (Clark 1975, Katz 1975) • New dynamical formation channels are available in GCs • tidal capture (Fabian, Pringle & Rees 1976) • 3- and 4-body interactions • Could CV numbers also be enhanced? • Theory says yes, but “only” by a factor of ~2 (di Stefano & Rappaport 1994, Davies 1995/7, Ivanovaet al. 2006) • There should be hundreds of accreting WDs in GCs! • Important and useful: • Large samples of CVs at known distances • Drivers and tracers of GC dynamical evolution • Are GCs SN Ia factories?(Shara & Hurley 2006) So where are they? 3-body exchange encounter

  14. Breakthrough V: EvolutionCVs in Globular Clusters • Early searches typically found only a handful per GC (e.g. Shara et al. 1996, Bailyn et al. 1996, Cool et al. 1998) • Are CVs not formed or maybe even destroyed in CVs? • Significant implications for GC dynamics! • Selection effects? • Survey depth? • Dwarf nova duty cycle? • X-rays would be a great way to find CVs in GCs • But this used to be really hard! • Chandra has revolutionized the field • Deep X-ray surveys typically find tens per cluster • Numbers scale with collision rate  dynamical formation matters! GCs do harbour significant populations of dynamically-formed CVs! Shara et al. (1996) Shara et al. (1996) Difference Imaging of the Core of 47 Tuc 47 Tuc with Chandra (Grindlay et al. 2001; Heinke et al. 2005) 47 Tuc with Chandra (Grindlay et al. 2001; Heinke et al. 2005) 47 Tuc with the ROSAT HRI (Hasinger et al. 1994) Pooley & Hut (2006)

  15. Breakthrough V: EvolutionCVs in Globular Clusters • UV astronomy has also played a key role • Efficient way of finding new CVs and confirming X-ray-selected candidates (Knigge et al. 2002, Dieball et al. 2005, 2009, 2010, Thomson et al. 2012) • Even slitless multi-object spectroscopic identification/confirmation is possible! • Still many key unsolved questions! • Are there enough CVs in GCs? • Are they different from field CVs? • Where are the double WDs? • Are there SN Ia progenitors? The core of 47 Tuc: FUV (~1500A) The core of 47 Tuc: U-band Knigge et al (2002, 2003, 2008)

  16. Breakthroughs VI and VII: Accretion /OutflowsOutburst Hysteresis and Jets GX339: Gallo et al. (2004) SS Cyg: Koerding et al. 2008, Science • Both CVs (dwarf novae) and XRBs (X-ray transients) exhibit outbursts • Thermal/viscous disk instability • XRBs • Outbursts trace a q-shape in the X-ray hardness vs intensity plane (Fender, Belloni & Gallo 2004)  hysteresis • Collimated (radio) jets are seen (almost only) in the hard state • Hard-soft transition produces a powerful jet ejection episode • CVs (pre-2008) • No evidence for collimated jets in any CV • Constraint on theories of jet formation (e.g. Livio 1999)? • No constraints on outburst hysteresis • ElmarKoerding et al. (2008) • Do dwarf novae also execute a q-shaped outburst pattern? • Yes they do! • Best chance to see a powerful jet is during the “hard-to-soft” transition during the rise to a dwarf nova outburst • Discovery of the first CV radio jet! Dwarf nova eruption (optical): SS Cyg Wheatley et al (2003) Gallo et al. 2004 X-ray transient outburst (X-ray): GX 339 Gallo et al (2004) Adapted from Fender, Belloni & Gallo 2004

  17. Breakthrough VIII: Accretion Periodic Variability: Oscillations • Both XRBs and CVs often exhibit (quasi-)periodic oscillations on short (~dynamical) time-scales • Origin is poorly understood, but intimately connected to accretion/outflow processes in the innermost disk regions • Key result in XRBs (accreting NSs and BHs): • strong correlations between different types of oscillations, especially LKO and HBO • CVs also exhibit two types of oscillations • Is there a direct connection to LMXBs?s • Yes!(Warner & Woudt [2002...2010], Mauche [2003]) • DNOs : QPOs in CVs ↔ LKOs : HBOs in LMXBs • Universality of accretion physics extends to periodic variability • Models relying on ultra-strong gravity or B-fields are ruled out Warner & Woudt 2004 Psaltis, Belloni & van der Klis 1999 NS & BH LMXBs 26 CVs DNOs in VW Hyi Woudt & Warner (2002)

  18. Breakthrough IX: Accretion Non-Periodic Variability: The RMS-Flux Relation A CV (Pretorius & Knigge 2007) Black Hole XRB (Uttley & McHardy 2001) MV Lyr (Scaringi et al. 2011) • What about non-periodic accretion-induced variability (“flickering”)? • Stochastic variability has been closely studied in XRBs • Key discovery: the “rms-flux relation” (Uttley & McHardy 2001) • Rules out “additive” models (e.g. shot-noise) • What about CVs? • Non-trivial to study: variability time-scales are much longer, so need high-cadence, uninterrupted long-term light curves --> Kepler! • CVs also show the rms-flux relation! (Scaringi et al. 2011) • Accretion-induced variability is universal! • Key properties shared by supermassive BHs, stellar-mass BHs, NSs and WDs MV Lyr (Scaringi et al. 2011) An XRB (Churazov et al. 2003) AGN (Vaughan et al. 2011) Neutron Star XRB (Uttley & McHardy 2001) NGC 4051 (Seyfert 1)

  19. Breakthrough X: Evolution / Accretion / Outflows Do all CVs go nova? • We all “know” that CVs burn accreted matter explosively (Fujimoto, Iben, Starrfield, Shaviv, Shara, Townsley, Bildsten, Yaron...) → Nova Eruptions (typical recurrence time ~10,000 yrs) • But all known novae were actually discovered as such • How can we establish the general link empirically ? • Ejected nova shells may be detectable for ~1000 yrs! • So Shara et al. (2007) searched for resolved nebulae around ordinary CVs in the GALEX imaging archive.... • ...and disovered an ancient nova shell around the proto-typical dwarf nova Z Cam → ordinary CVs do undergo nova eruptions! • Postscript: Chinese astronomers would have disagreed with the classification of Z Cam as an “ordinary CV”... Shara et al. 2007, Nature 446, 159 r

  20. Summary The last decade has seen several breakthroughs in our understanding of CVs, many of which were made possible by ultraviolet observations • Evolution • The basic disrupted-angular-momentum-loss picture of CV evolution is correct ! • We know how to reconstruct CV evolution from both primary and secondary properties • CVs do exist in significant numbers in GCs • CVs not discovered as novae can still have nova shells --> all CVs experience nova eruptions • Accretion, Outflows and Links to Other Systems • CV outbursts exhibit hysteresis (“turtlehead” diagram) – just like XRBs and AGN • CVs can drive radio jets – just like XRBs and AGN • Accretion-induced oscillations in CVs are… – just like those in XRBs • Stochastic variability in CVs follows an rms-flux relation – just like XRBs and AGN The physics of disk accretion is universal CVs provide excellent, nearby, bright accretion laboratories

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