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Outrageous Outbursts: Accretion Disc Formation and Stability in Long Period CVs

Outrageous Outbursts: Accretion Disc Formation and Stability in Long Period CVs. RS Ophiuchi: A case study How is mass transferred from the red giant to the white dwarf? How is mass accreted by the white dwarf? How are the outbursts triggered? Wow ... symbiotic stars are important!.

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Outrageous Outbursts: Accretion Disc Formation and Stability in Long Period CVs

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  1. Outrageous Outbursts: Accretion Disc Formation and Stability in Long Period CVs • RS Ophiuchi: A case study • How is mass transferred from the red giant to the white dwarf? • How is mass accreted by the white dwarf? • How are the outbursts triggered? • Wow ... symbiotic stars are important! Graham Wynn Fergus Wilson, Auni Somero, Julian Osborne, Kim Page University of Leicester, UK Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  2. Long Period CVs, Recurrent Novae & RS Ophiuchi • Of 10 known recurrent novae 4 are “RS Oph like” • RS Oph, T CrB, V3890 Sgr and V745 Sco all have M giant secondaries and orbital periods > 100 days • The novae outbursts are thought to be driven by a thermonuclear runaway (TNR) on the white dwarf – just as in classical novae • The short recurrence times (~ 20 years for RS Oph) imply a massive, hot WD and high accretion rate: good supernovae type Ia candidates • These long period systems have the potential to contain very large and very massive accretion discs: how does accretion proceed in RS Oph? Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  3. RS Ophiuchi: Properties (See poster by Auni Somero) • Orbital Period: • White dwarf (primary) mass: • Red Giant (secondary) mass: • Assmue circular orbit • Binary Separation: • Distance from the WD to the L1 point: Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  4. How is mass transferred from the RG to the WD? • Roche lobe overflow (RLO) or stellar wind capture (SWC)? • For RLO we have good estimates of the total mass loss rate of the RG and the angular momentum (AM) of the transferred gas • The picture has been less clear for SWC Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  5. Mass Transfer Rates • Roche lobe Overflow (RLO) • Mass loss estimate for a lobe filling giant (eg Ritter 1999) • All of this mass is gravitationally captured by the WD • The WD accretion rate is not necessarily constant at the transfer rate • Stellar Wind Capture (SWC) • RG wind loss estimates are similar (eg Bohigas et al 1989) • The mass lost by the RG is not the same as that captured by the WD • Bondi-Hoyle capture rate (eg Livio et al 1986) Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  6. Is there a disc in RS Oph? • RLO: • we know the AM of the gas • A disc must form at • Wind capture: • AM of wind flow is less certain • Disc formation depends on the details of wind capture • Expect disc to form at • In both cases the disc is expected to viscously spread to Roche lobe overflow in RS Oph Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  7. Wind accretion Models (See poster by Fergus Wilson) no rotation prograde rotation retrograde rotation Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  8. Wind Accretion in RS Oph: Simulation Results Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  9. Is there a disc in RS Oph? ... Yes! • Disc size: • For RLO: • For SWC: • Tidal limit • Is the disc thermally-viscously stable? • Is the disc gravitationally stable? • Is the disc tilted or warped? Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  10. How are RS Oph outbursts triggered? • How does the mass required for the TNR get to the white dwarf? • TNR requires giving a mean • This is close to total RLO and SWC transfer rates • For SWC this would seem to require v < 50 km/s and slow/prograde rotation • There are no DN outbursts in between Novae • accretion must be direct impact (Livio, Truran & Webbink 1986), or • the disc must be thermally stable in quiescence • There are 2 possibilities for a stable disc in quiescence: • 1: The disc is cold and stable in quiescence • Here the disc as a mass reservoir in quiescence with little accretion • The thermal viscous instability would eventually trigger DNe-like outbursts • These DNe outbursts must trigger the novae • 2: The disc is hot and stable in quiescence • The white dwarf accretes at a high, steady rate between novae Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  11. Hot state inter-outburst accretion • Steady accretion at a high rate in quiescence via a stable, hot disc • Accretion rate must be outside the nuclear burning band or a supersoft phase would persist in quiescence • Mass transfer estimates are close to the lower end of the burning band • Accretion rate must be high enough to keep the disc stable and hot eg Fujimoto 1982 eg King, Rolfe and Schenker 2003 Is quiescence quiescent? Is an accretion rate this close to Eddington consistent with observations? Why is the disc so small? To remain hot and stable the disc must be at least a factor 10 smaller than the smallest predicted formation radius. Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  12. Hot solution: a massive yet attractive disc? • Why is the disc so small? • For disc to be hot and stable: • ~ 0.1 of smallest disc formation radius! • What stops the disc from reaching the tidal limit? • In binaries with orbital periods > 1 yr the outer disc can become gravitationally unstable. • If R(disc) -> R(tide) ... it could form planets! • Could the disc size/density profile be gravitationally limited/moderated? • Not according to systems like GRS1915+105! • Quiescence isn’t quiescent? • Accretion is a significant fraction of Eddington • Is this consistent with quiescent observations? • Embedded in RG wind? Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  13. Cold state inter-outburst accretion • A of disc of radius must undergo DNe-type outbursts • Mass for the TNR would have to be delivered to the WD during a ‘disc outburst’ • Disc mass is a function of radius • The hot state viscous timescale at this radius is: • The outburst accretion rate would be >> Eddington • Inefficient super-Eddington accretion would seem to make it very difficult to supply the mass for a TNR in a DN outburst • Rather than TNR, are the outbursts of RS Oph simply huge dwarf novae? Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  14. Cold solution: where novae meet dwarf novae • The observed super-soft X-ray phase requires nuclear burning. • Piro and Bildsten 2004: material joins the white dwarf over a spreading layer with a small azimuthal spreading angle • This reduces the mass required to start nuclear burning - find an ignition timescale ~ 50 days to accrete this mass • Burning begins under non-degenerate conditions – no TNR - transient SSS! • The disc will be irradiated by the central X-rays, prolonging the outburst • Radiation fields ~ Eddington will drive extremely strong outflows and winds • Accretion in the burning band over the SSS phase (~ 80 days) is consistent with this mass ( ) and seems to be consistent with the observed X-ray luminosity ( ) • The mass required for burning corresponds to a radius of • The observed outburst and recurrence times are consistent with this radius Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  15. Cold solution: evidence? • The outburst amplitude – recurrence time relation for DNe and RNe • if correct this strongly suggests the oubursts are related to the disc • Highly collimated radio jets within days of the 2006 outburst (Sokoloski 2006) • Suggests a disc must be present • Can a disc survive a nova? ... (next talk) • Naturally explains the faint quiescent X-ray flux (Mukai, 2008) • A disc outburst with radiation driven outflows would explain the bipolar wind structure • Irradiation models predict the outburst decay to be (bolometrically) linear over a viscous time – close to observations • The white dwarf mass is crucial. Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  16. The wider picture – symbiotic stars are really interesting • There are around 200 known symbiotic stars with orbital periods between 200 days and decades • A good fraction (~ all) of these must contain large and massive discs • These discs must be unstable and the systems transient – dwarf (!) novae • Some of these discs will supply mass in the burning band or higher • What would these systems look like? Transient SSSs? Novae? • These ‘not so dwarf novae’ make better type Ia supernovae progenitor candidates than TNR systems! • No WD mass loss via novae explosions to (unlike RNe) • All of the mass of the donor’s envelope is available for accretion (unlike SSSs) • What is the population of symbiotics? • How well do we know progenitor, current and remnant systems? • Remnant systems: e.g. low & high mass WD in a long period binary, single low mass He core WD! Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  17. Summary • Accretion in RS Oph takes place through a disc for RLO or SWC and this is likely to be true for most symbiotic stars • Lack of DN outbursts: accretion disc is stable between novae • Accretion via a stable, hot disc is only possible if the disc is smaller than any plausible formation radius. • Are the outbursts of RS Oph the huge dwarf novae? • How would a super-Eddington DNe outburst in a massive disc proceed? • How would it appear observationally? • The symbiotic stars have very long periods and large discs • These discs must undergo outbursts and many may be excellent SNe type Ia progenitors • These would be single degenerate type Ia’s in old stellar populations • An understanding of the population of symbiotics and their parameters would give the type Ia rate from this channel Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  18. Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  19. The Thermal-Viscous Instability • Thermal equilibrium in the disc is determined by local density (viscous heating) and surface temperature (radiative cooling) • Positive gradients: the disc is stable to increases in density • A negative gradient branch is caused by the sharp opacity change due to H ionisation • S-curve forces the disc to follow a thermal limit cycle between hot and coolstates • Heating is assumed to be accompanied by an increase in viscosity • Local Thermal Equilibrium Curve • Disc Temp • Local Disc Density Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  20. Dwarf Nova Outbursts • outburst timescales • duty cycle • X-ray/UV delay • super-outbursts • quiescent disc physics Truss, Murray, Wynn, Edgar, MNRAS 2000 Truss, Murray, Wynn, MNRAS, 2001 Truss, Wynn, Wheatley, MNRAS, 2004 Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

  21. The Thermal-Viscous Disc Instability in SPH • Disc instability is approximated by a ‘viscous switch’ triggered by local surface density • Local viscosity and sound speed are altered according to 2 surface density triggers • Changes occur on the local thermal timescale Truss, Murray, Wynn & Edgar, 2000 Wild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009

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