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

Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Outrageous Outbursts: Accretion Disc Formation and Stability in Long Period CVs

Graham Wynn

Fergus Wilson, Auni Somero, Julian Osborne, Kim Page University of Leicester, UKRS 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!

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

Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Long Period CVs, Recurrent Novae & RS OphiuchiOf 10 known recurrent novae 4 are RS Oph likeRS 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?


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009RS 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:


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009How 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


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Mass 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 WDThe 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 WDBondi-Hoyle capture rate (eg Livio et al 1986)


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Is 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 certainDisc formation depends on the details of wind captureExpect disc to form at

In both cases the disc is expected to viscously spread to

Roche lobe overflow in RS Oph


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Wind accretion Models (See poster by Fergus Wilson)no rotationprograde rotationretrograde rotation


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


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Is 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?


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009How 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 ratesFor SWC this would seem to require v < 50 km/s and slow/prograde rotation

There are no DN outbursts in between Novaeaccretion must be direct impact (Livio, Truran & Webbink 1986), orthe 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 quiescenceHere the disc as a mass reservoir in quiescence with little accretionThe thermal viscous instability would eventually trigger DNe-like outburstsThese DNe outbursts must trigger the novae

2: The disc is hot and stable in quiescenceThe white dwarf accretes at a high, steady rate between novae


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Hot state inter-outburst accretionSteady 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 1982eg King, Rolfe and Schenker 2003Is 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.


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Hot 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 isnt quiescent?

Accretion is a significant fraction of EddingtonIs this consistent with quiescent observations?Embedded in RG wind?


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Cold state inter-outburst accretionA 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?


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 angleThis 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 outburstRadiation 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 radiusOutrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Cold solution: where novae meet dwarf novae


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Cold solution: evidence? The outburst amplitude recurrence time relation for DNe and RNeif 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 presentCan 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.


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009The wider picture symbiotic stars are really interestingThere 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 discsThese discs must be unstable and the systems transient dwarf (!) novaeSome of these discs will supply mass in the burning band or higherWhat 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 donors 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!


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009SummaryAccretion 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 discsThese discs must undergo outbursts and many may be excellent SNe type Ia progenitorsThese would be single degenerate type Ias in old stellar populationsAn understanding of the population of symbiotics and their parameters would give the type Ia rate from this channel


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009The Thermal-Viscous InstabilityLocal Disc Density

Disc Temp

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 cool states

Heating is assumed to be accompanied by an increase in viscosityLocal Thermal Equilibrium Curve


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009Dwarf Nova Outburstsoutburst timescalesduty cycleX-ray/UV delaysuper-outburstsquiescent disc physics

Truss, Murray, Wynn, Edgar, MNRAS 2000Truss, Murray, Wynn, MNRAS, 2001Truss, Wynn, Wheatley, MNRAS, 2004


Outrageous OutburstsWild Stars in the Old West II, Tucson, Arizona, 15-19 March 2009The 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


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

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