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

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Presentation on theme: "Christian Knigge University of Southampton School of Physics & Astronoy Cataclysmic Variables: 10 Breakthroughs in 10 Years P. Marenfeld and NOAO/AURA/NSF."— Presentation transcript:

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

2 Christian Knigge University of Southampton School of Physics & Astronoy 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 35 minutes XX XXXXX Links to Other Systems (BH/NS LMXBs)

3 Christian Knigge University of Southampton School of Physics & Astronoy Cataclysmic Variables: A Primer The Physical Structure of CVs White Dwarf Accretion Disk Red Dwarf White dwarf primary –UV bright “Main-sequence” secondary 75 mins < P orb < 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 Credit: Rob Hynes

4 Christian Knigge University of Southampton School of Physics & Astronoy Cataclysmic Variables: A Primer The Orbital Period Distribution and the Standard Model of CV Evolution Knigge 2006

5 Christian Knigge Breakthrough I: Evolution Disrupted Angular Momentum Loss at the Period Gap 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) University of Southampton School of Physics & Astronoy Howell et al. 2001

6 Christian Knigge University of Southampton School of Physics & Astronoy Patterson et al. (2005), Knigge (2006) Donors are significantly larger than MS stars both above and below the gap Clear discontinuity at M 2 = 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 Breakthrough I: Evolution Disrupted Angular Momentum Loss at the Period Gap

7 Christian Knigge University of Southampton School of Physics & Astronoy Breakthrough II: Evolution Reconstructing CV Evolution Empirically Knigge (2006)Knigge, Baraffe & Patterson (2011)

8 Christian Knigge Breakthrough III: Evolution Period 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? University of Southampton School of Physics & Astronoy Howell et al. 2001

9 Christian Knigge Breakthrough III: Evolution Period Bouncers with Brown Dwarf Secondaries SDSS has yielded a deep new sample of ~200 CVs (Szkody et al )......including a sub-set of faint, WD-dominated systems near P min (Gaensicke et al. 2009; see later) A few of these are eclipsing, allowing precise system parameter determinations At least 3 of these have M 2 < 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… University of Southampton School of Physics & Astronoy Littlefair et al. 2006, Science, 314, 1578

10 Christian Knigge Stehle et al. (1999) Breakthrough III: Evolution Period Bouncers with Brown Dwarf Secondaries University of Southampton School of Physics & Astronoy Littlefair et al. (2007)Patterson et al. (2008) Littlefair et al. (2007) Uthas et al. (2011)

11 Christian Knigge Breakthrough IV: Evolution The Period Spike at P min Standard model prediction –The number of CVs found in a particular P orb range is inversely proportional to the speed with which they evolve through it –So there should be a spike at P min, in the period distribution since Observational reality pre-2009 –No convincing spike anywhere near P min in the CV P orb distribution University of Southampton School of Physics & Astronoy Barker & Kolb 2003

12 Christian Knigge Previously known CVs SDSS CVs Breakthrough IV: Evolution The Period Spike at P min Boris Gaensicke and collaborators have obtained orbital periods for most of the new SDSS CVs The resulting period distribution does show a spike at P min for the first time (Gaensicke et al. 2009) CVs do in fact “bounce” at P min ! University of Southampton School of Physics & Astronoy Gaensicke et al. 2009

13 Christian Knigge Breakthrough V: Evolution CVs in Globular Clusters 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, Ivanova et 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? University of Southampton School of Physics & Astronoy 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: 3-body exchange encounter White Dwarf

14 Christian Knigge Breakthrough V: Evolution CVs in Globular Clusters University of Southampton School of Physics & Astronoy Shara et al. (1996) Difference Imaging of the Core of 47 Tuc 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! 47 Tuc with the ROSAT HRI (Hasinger et al. 1994) Shara et al. (1996) Pooley & Hut (2006) 47 Tuc with Chandra (Grindlay et al. 2001; Heinke et al. 2005) 47 Tuc with Chandra (Grindlay et al. 2001; Heinke et al. 2005)

15 Christian Knigge 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: U-band The core of 47 Tuc: FUV (~1500A) University of Southampton School of Physics & Astronoy Knigge et al (2002, 2003, 2008) Breakthrough V: Evolution CVs in Globular Clusters

16 Christian Knigge Dwarf nova eruption (optical): SS Cyg Wheatley et al (2003) X-ray transient outburst (X-ray): GX 339 Gallo et al (2004) Adapted from Fender, Belloni & Gallo 2004 Breakthroughs VI and VII: Accretion / Outflows Outburst Hysteresis and Jets University of Southampton School of Physics & Astronoy Gallo et al 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 Elmar Koerding 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!

17 Christian Knigge 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 [ ], 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 University of Southampton School of Physics & Astronoy Breakthrough VIII: Accretion Periodic Variability: Oscillations Psaltis, Belloni & van der Klis 1999 Warner & Woudt 2004 NS & BH LMXBs 26 CVs DNOs in VW Hyi Woudt & Warner (2002)

18 Christian Knigge An XRB (Churazov et al. 2003) A CV (Pretorius & Knigge 2007) University of Southampton School of Physics & Astronoy Breakthrough IX: Accretion Non-Periodic Variability: The RMS-Flux Relation Black Hole XRB (Uttley & McHardy 2001) Neutron Star XRB (Uttley & McHardy 2001) AGN (Vaughan et al. 2011) NGC 4051 (Seyfert 1) 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)

19 Christian Knigge Shara et al. 2007, Nature 446, 159 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”... University of Southampton School of Physics & Astronoy r Breakthrough X: Evolution / Accretion / Outflows Do all CVs go nova?

20 Christian Knigge University of Southampton School of Physics & Astronoy 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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