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Disk Stability vs Close Binary Evolution

Disk Stability vs Close Binary Evolution. Christian Knigge University of Southampton. Rob Hynes (LSU). Outline. Introduction The evolution of CVs and LMXBs Disk Stability vs Close Binary Evolution The Link Between Binary Evolution and Disk Stability Black Holes Neutron Stars

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Disk Stability vs Close Binary Evolution

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  1. Disk Stability vs Close Binary Evolution Christian Knigge University of Southampton Rob Hynes (LSU)

  2. Outline • Introduction • The evolution of CVs and LMXBs • Disk Stability vs Close Binary Evolution • The Link Between Binary Evolution and Disk Stability • Black Holes • Neutron Stars • White Dwarfs • Open Questions • Summary

  3. The Evolution of Compact Accreting Binaries • The Simplest Possible Case • WD/NS/BH primary • Unevolved, low-mass ZAMS secondary • Roche-lobe overflow • Accretion usually via a disk • 75 mins < Porb < 24 hrs • Mass transfer and evolution entirely driven by angular momentum losses • Evolution is (initially) from long to short periods Red Dwarf WD/NS/BH Accretion Disk Credit: Rob Hynes

  4. The CV Orbital Period Distribution and The Standard Model of CV Evolution Howell et al. (2001) • 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 and LMXB evolution Knigge(2006) Magnetic Braking GR GR

  5. Testing Evolution Theory Littlefair et al. 2006, Science, 314, 1578 • Over the last decade, the standard model has survived significant empirical scrutiny • Confirmation of discontinuity in donor M-R relation (Patterson et al. 2005, Knigge 2006) • Discovery of CVs with sub-stellar donor stars (Littlefair et al. 2006 --2008) • Discovery of CV period spike at (Gaensicke et al. 2009, Woudt & Warner 2012) • Reconstruction of CV evolution • From WDs (Townsley & Gaensicke 2009) • From donors: (Knigge, Baraffe & Patterson 2011) • Bottom line: • Standard model works well • May need enhanced AML below the gap, but “only” Knigge, Barraffe & Patterson 20011 Patterson et al. (2005), Knigge (2006) Gaensicke et al. (2009), Warner & Woudt (2012)

  6. The Link Between Binary Evolution and Disk (In)Stability • In the SM, all systems quickly join a unique evolution track, • According to the DIM, depends primarily on and • varies by only • depends primarily on  So there is also a unique stability line, J. Smak Y. Osaki J. Faulkner J. Cannizzo F. Meyer E. Meyer-Hofmeister J.-P. Lasota J.-M. Hameury … Kolb & Baraffe (1999)

  7. The Link Between Binary Evolution and Disk (In)Stability • and may cross! (Naïve) Predictions • Sharp transition from stable to unstable • Stable and unstable systems should not co-exist at the same • Evolution should set crossing point location Does any of this work in practice?

  8. Disk Stability vs Binary Evolution: Neutron Stars • Naïve Prediction • All NS LMXBs should be unstable (transient) • Observation • NS LMXBs include both stable and unstable systems • Stable and unstable systems coexist at all but the longest  So what’s wrong? Data from Coriat et al. (2012) King, Kolb & Burderi (1992)

  9. Disk Stability vs Binary Evolution: Neutron Stars Podsiadlowski et al. (2002) Two New Ingredients • Irradiation (van Paradijs 1996) • Irradiated disks are hotter: • Hot disks are more stable  But now all NS LMXBs should be stable (persistent) • Evolved Donors (King et al. 1996, 1997) • Slightly evolved donor stars are larger (at fixed mass) • lower mass at fixed period • reduced MB and GR rates • Allows also for transient NS LMXBs  Evolved donors may actually be expected (King & Kolb 1997) End of MS King, Kolb & Burderi (1992) Middle of MS Start of MS

  10. Disk Stability vs Binary Evolution: Neutron Stars • The existence of systems with evolved donors destroys the uniqueness of • is now a “2nd parameter” (usually unknown)  Cannot test DIM against naïve predictions! • Have to use observationally estimated instead (e.g. from (van Paradijs 1996; Coriat et al. 2012) • DIM passes such tests! Persistent Transient Coriat et al. (2012)

  11. Disk Stability vs Binary Evolution: Black Holes King, Kolb & Burderi (1996) • Consequences of changing  • due to MB is reduced • due to GR is increased • in DIM is increased BH NS

  12. Disk Stability vs Binary Evolution: Black Holes Data from Coriat et al. (2012) • Naïve Predictions • Without irradiation: All BH LMXBs should be unstable (transient) • With irradiation: All BH LMXBs should be stable (persistent) • Observation • Almost all BH LMXBs are unstable (transient) • But irradiation should matter!  So what’s wrong? King, Kolb & Burderi (1996)

  13. Disk Stability vs Binary Evolution: Black Holes • Let’s again test the DIM by comparing to empirical estimates (van Paradijs 1996; Coriat et al. 2012) • DIM again passes these tests! Persistent High-Mass XRBs (ignore) Transient Coriat et al. (2012)

  14. Disk Stability vs Binary Evolution: Black Holes • So why did the naïve evolution model not predict this? • Absence of stellar surface might change irradiation geometry (King, Kolb & Szuszkiewicz 1997) BUT • Evolved donor stars • Possible, but formation of short-period BH LMXBs is poorly understood (Podsiadlowski et al. 2003): • Low-mass secondaries can’t eject the massive envelope of the BH progenitor (but see Podsiadlowski et al. 2010) • Intermediate-mass secondariesdon’t have MB, so systems don’t reach short (but see Justham et al. 2006) • There may be a 3rd way (see later)

  15. Disk Stability vs Binary Evolution: White Dwarfs • Naïve Prediction (Shafter et al. 1986, 1992, Knigge et al. 2011) • CVs below the gap: unstable • CVs directly above the gap: unstable • CVs well above the gap: stable • CVs should become increasingly stable at longer above the gap • Transition at • Observations (Shafter et al. 1986, 1992, Knigge et al. 2011) • CVs below the gap: unstable • CVs directly above the gap: stable • CVs become increasingly unstable at longer above the gap • Stable and unstable systems co-exist at longer • So what’s wrong? Knigge, Baraffe & Patterson (2011) Stable Unstable Unstable X X DN Fraction ?

  16. Disk Stability vs Binary Evolution: White Dwarfs Schreiber & Lasota (2007) • We can again test the DIM more directly with measured • DIM passes this test also • Except what’s up with SS Cyg??? (we’ll come back to this later today…) • So why do the naïve predictions fail even for CVs?  Let’s look first at coexistence of DNe and NLs at same

  17. Disk Stability vs Binary Evolution:White Dwarfs The coexistence of stable and unstable systems at long • The mix of DNe and NLs implies a spread in • This has also been inferred more directly, via spread in But CVs should follow a unique evolution track!? Warner (1987) Patterson (1984)

  18. Disk Stability vs Binary Evolution:White Dwarfs The coexistence of stable and unstable systems at long Possibility 1: Mass-transfer cycles • Two possible mechanisms: • Irradiation: varies (King, Ritter, Kolb, Frank, Wu… 95+) • But it’s not trivial to induce cycles in CVs • Could also matter in NS and BH LMXBs (Buening & Ritter 2004) • Novae: varies • ML dominates  hibernation (Shara et al. 1986) • AML dominates high states (MacDonald 1986) Schenker et al. (1998) King et al. (1995) McCormick & Frank (1998)

  19. Disk Stability vs Binary Evolution:White Dwarfs The coexistence of stable and unstable systems at long Podsiadlowski et al. (2002) Possibility 2: Evolved Donors • Evolved donors are larger at fixed • Reduced GR and MB rates • Spread in  Spread in ? • Only explains lower-than-standard • For , evolved donors are actually expected (and observed Beuermann et al. 1998; Knigge 2006) • Probably explains spread in at long Podsiadlowski et al. (2003) End of MS Middle of MS Start of MS

  20. Disk Stability vs Binary Evolution:White Dwarfs The coexistence of stable and unstable systems at long Possibility 3: Natural Spread • CV evolution path and DIM stability criteria are not really unique  does affect and • So DNe and NLs can naturally coexist when • Underappreciated, but important ! • Can’t explain lack of DNe just above gap Shafter (1992) Theory Shafter, Wheeler & Cannizzo (1986) Data Shafter (1992)

  21. Disk Stability vs Binary Evolution:White Dwarfs The dominance of stable systems directly above the gap • Can any of the explanations for coexistence of DNe and NLs also explain dominance of stable, high- systems at ? • Mass transfer cycles • Maybe: • If the donor response to irradiation changes at (why?) • Evolved donors • Maybe: • If evolved donors contribute substantially to CV population already at • If is not actually required • But what about the SW Sex stars (e.g. Rodriguez-Gil et al. 2007) ? • Natural spread • No!

  22. Disk Stability vs Binary Evolution:White Dwarfs The dominance of stable systems directly above the gap A Neglected Model Revisited • Zangrilli et al. (1997) proposed a dual “boundary layer + envelope” dynamo model for MB • BL dynamo is strongest directly above gap, but shuts down just below it • Local maximum in directly above the gap • Could explain dominance of stable, high- CVs in the range • BUT: should also apply to single stars • M3-M4 dwarfs should be unusually active  (not) observed? Zangrilli et al. (1997)

  23. Disk Stability vs Binary Evolution:White Dwarfs One last puzzle… • What about systems inside the period gap? • In the standard model, they must have been borne there with an already fully convective donor • So they should evolve like short-period CVs, with AML driven just by GR • So they should all be unstable! Howell et al. (2001)

  24. Disk Stability vs Binary Evolution:White Dwarfs One last puzzle… • But that is not what we see! • DNe fraction actually declines monotonically from 100% to 0% through the gap • Interpretation??? • Low systems invading gap from above? (Stehle et al. 1997, Webbink & Wickramasinghe 2002) • Evolved systems? • New Physics? Knigge, Baraffe & Patterson (2011)

  25. Summary • The connection between DIM and binary evolution is more complicated than one might naively expect • Irradiation • Nuclear evolved donors • Natural spread in and • When these things are correctly taken into account, DIM generally works well • Key open questions • Why are CVs directly above the period gap all stable? • Why does the DN fraction decline monotonically through the period gap?

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