white dwarf mergers: am cvn, sdb and r crb · pdf filewhite dwarf mergers: am cvn, sdb and r...
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White Dwarf mergers: AM CVn, sdB and R CrB connections
Simon Jeffery Armagh Observatory
many, many colleagues, but principally: Phil Hill, Uli Heber and Hideyuki Saio
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White Dwarf mergers: AM CVn, sdB and R CrB connections
• WD-WD binaries and WD-WD mergers• AM CVn stars• He+He WD mergers - EHe / sdB / sdO stars ?• CO+He WD mergers - EHe / RCrB / SNIa ?• CO+CO WD mergers - ?• What actually happens in a WD merger ?
– Angular Momentum ?– Disk / Envelope / Core ?– Hydrodynamics ?– Nucleosynthesis ?
• Lies, Damned Lies
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Origin of Binary White Dwarfs
Nelemans et al. 2001 A&A 365, 491 (inter alia)
HE+HE CO+CO
CO+HE HE+CO
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Binary White Dwarf Stability
Dynamically stable mass transfer for:
Nelemans et al. 2001 A&A 368, 939
Direct impact accretion
super-Eddington accretion
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Binary White Dwarf Stability
Dynamically stable mass transfer for:
Nelemans et al. 2001 A&A 368, 939
Direct impact accretion
super-Eddington accretion
Allegedly
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What happens in the unstable zone?
HE+HE
CO+HE
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white-dwarf white-dwarf binaries
period distribution:(Nelemans et al. 2001, Maxted et al. 2002, also Deloye’s talk)
merger timescales:
τm=107 (P/h)8/3 µ-1 (M/M)-2/3
yr (Landau & Lifshitz 1958)
CO+He merger frequency: ν ∼ 4.4 10-3 yr-1 (Neleman’s
et al. 2001) ν ∼ 2.3 10-3 yr-1 (Iben et al.)
q>5/6q>2/3
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white-dwarf merger models: old question!
• He+He ⇒ He ignition ⇒ HeMS or sdB star ⇒ CO WD
(Nomoto & Sugimoto 1977, Nomoto & Hashimoto 1987, Kawai, Saio & Nomoto 1987, 1988, Iben 1990)
• He+CO ⇒ RCrB star OR SNIa ?
(Webbink 1984, Iben & Tutukov 1984, Iben 1990)
• CO+CO ⇒ C ignition ⇒ O+Ne+Mg WD OR explosion ?
(Hachisu et al. 1986a,b, Kawai, Saio & Nomoto 1987, 1988, Nomoto & Hashimoto 1987, Mochkovitch & Livio 1990, Saio & Nomoto 1998)
• results critically sensitive to WD temperature AND accretion rate
• what do the products look like between merger and end-state?
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white dwarf merger models: basic approach
Saio & Jeffery ….
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He+He WD mergers
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hypothesis
He+He white dwarf formed
orbit decays
less massive WD disrupted when Porb ~4 minutes
super-Eddington accretion:
forms thick disk?
more massive WD accretes material from disk
⇒model
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hypothesis
He+He white dwarf formed
orbit decays
less massive WD disrupted when Porb ~4 minutes
super-Eddington accretion:
forms thick disk?
more massive WD accretes material from disk
⇒model
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hypothesis
He+He white dwarf formed
orbit decays
less massive WD disrupted when Porb ~4 minutes
super-Eddington accretion:
forms thick disk?
more massive WD accretes material from disk
⇒model
0.4 Msun He-WD accretes He at 10-5 Msun/yr
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hypothesis
He+He white dwarf formed
orbit decays
less massive WD disrupted when Porb ~4 minutes
super-Eddington accretion:
forms thick disk?
more massive WD accretes material from disk
⇒model
helium ignites in shell at core-envelope boundary 0.4 Msun He-WD
accretes He at 10-5 Msun/yr
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hypothesis
He+He white dwarf formed
orbit decays
less massive WD disrupted when Porb ~4 minutes
super-Eddington accretion:
forms thick disk?
more massive WD accretes material from disk
⇒model
helium ignites in shell at core-envelope boundary
helium-burning shell forces star to expand to yellow giant, ~103 yr
0.4 Msun He-WD accretes He at 10-5 Msun/yr
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hypothesis
He+He white dwarf formed
orbit decays
less massive WD disrupted when Porb ~4 minutes
super-Eddington accretion:
forms thick disk?
more massive WD accretes material from disk
⇒model
helium ignites in shell at core-envelope boundary
helium-burning shell forces star to expand to yellow giant, ~103 yr
accretion turned off at selected final mass
0.4 Msun He-WD accretes He at 10-5 Msun/yr
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hypothesis
He+He white dwarf formed
orbit decays
less massive WD disrupted when Porb ~4 minutes
super-Eddington accretion:
forms thick disk?
more massive WD accretes material from disk
⇒model
helium ignites in shell at core-envelope boundary
helium-burning shell forces star to expand to yellow giant, ~103 yr
accretion turned off at selected final mass
0.4 Msun He-WD accretes He at 10-5 Msun/yr
shell burns inwards in series of mild flashes; lifts degeneracy
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hypothesis
He+He white dwarf formed
orbit decays
less massive WD disrupted when Porb ~4 minutes
super-Eddington accretion:
forms thick disk?
more massive WD accretes material from disk
⇒model
helium ignites in shell at core-envelope boundary
helium-burning shell forces star to expand to yellow giant, ~103 yr
accretion turned off at selected final mass
0.4 Msun He-WD accretes He at 10-5 Msun/yr
shell burns inwards in series of mild flashes; lifts degeneracy
Helium core-burning star (sdB?) formed as shell reaches centre
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V652 Her
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sdB stars
• Four types:– sdB+MS (F-G) long-period– sdB+MS (M) short-period– sdB+WD (He) short-period– sdB single
• Four origins:– Stable RLOF– CE– Stable RLOF + CE– HeWD+HeWD merger
Greenstein & Sargent 1974
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sdO sdB HB
HeM
S
MS
WD
ZAHB
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sdB stars: helium abundance and He+He mergers ?
Edelmann et al. 2004, Winter 2006, O’Toole 2008
NHe ~ 0.001-0.10 NHe ~ 0.0001-0.02
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Helium-rich sdB/O’s: He, C, and N abundances
Stroeer et al. 2004, Hirsch et al. 2008
NHe ~ 0.1-0.99
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He+He WD merger
Ahmad et al. 2004, see also Justham et al. ???
He-sdB’s: merger or flasher?
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He+He WD merger
Ahmad et al. 2004, see also Justham et al. ???
He-sdB’s: merger or flasher?
PG1544+488: HesdB+HesdB binary ??
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CO+He WD mergers
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0.6 M CO-WD accretes He at 10-5 M /yr
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helium ignites in shell at core-envelope boundary
0.6 M CO-WD accretes He at 10-5 M /yr
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helium ignites in shell at core-envelope boundary
helium-burning shell forces star to expand to yellow giant, ~103 yr
0.6 M CO-WD accretes He at 10-5 M /yr
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helium ignites in shell at core-envelope boundary
helium-burning shell forces star to expand to yellow giant, ~103 yr
accretion turned off at selected final mass
0.6 M CO-WD accretes He at 10-5 M /yr
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helium ignites in shell at core-envelope boundary
helium-burning shell forces star to expand to yellow giant, ~103 yr
accretion turned off at selected final mass
0.6 M CO-WD accretes He at 10-5 M /yr
0.5 M CO-WD
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helium ignites in shell at core-envelope boundary
helium-burning shell forces star to expand to yellow giant, ~103 yr
accretion turned off at selected final mass
0.6 M CO-WD accretes He at 10-5 M /yr
0.5 M CO-WD
0.6 M , X=0.001
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CO+He merger: EHes and RCrBs
EHe starsCO+He mergers
solid: 0.6MCO+Hedashed: 0.5MCO+He
light: accretionheavy: contraction
EHes
Baade radii from pulsating EHes
HD168476
LS IV-1 2
RCrB stars
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Extreme Helium StarsR Coronae Borealis Stars
Hydrogen-Deficient Carbon Giants
2-nH
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Extreme Helium StarsR Coronae Borealis Stars
Hydrogen-Deficient Carbon Giants
2-nH
Spectral Type R2
C2(Swan)
CN(violet)
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The RCrB – EHe – O(He) – WD sequence
• RCrB / HdC
• EHe
• HesdO+
• O(He)
Surface abundances:
H < 1:105
N (from CNO cycle)
C (from 3α process)
O (α-capture on 12C)
Ne (2α-capture on 14N)
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Photospheric Abundancesa) Proxies for metallicity (Ni,Mn,Cr,Fe) ⇒ -2 < [Fe/H] < 0b) Overabundant light elements (Mg,Si,S,…) ??
Pandey, Lambert, Jeffery & Rao 2006, ApJ 638, 454
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Photospheric Abundancesc) [N/Fe] ∝ [(C+N+O)/Fe] OK
d) [O/Fe] >> 0 ??
e) [s/Fe] >> 0 AGB intershell ??
f) [Ne/Fe] >> 0 ??
Pandey, Lambert, Jeffery & Rao 2006, ApJ 638, 454
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Photospheric Abundances
g) F ?? Pandey (2007)
h) Li ??
i ) 18O >> 16O α-capture on N14 : but when? Clayton et al. (2007)
j) 12C >> 13C substantial 3α processing
Clayton et al. 2007
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Photospheric Abundances
g) F ?? Pandey (2007)
h) Li ??
i ) 18O >> 16O α-capture on N14 : but when? Clayton et al. (2007)
j) 12C >> 13C substantial 3α processing
Predicted by Brian Warner in 1967 !!
Clayton et al. 2007
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The merger process
Angular momentumDisk / Envelope / Core
HydrodynamicsNucleosynthesis
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What actually happens in a WD merger?
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What actually happens in a WD merger?
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SPH Simulations: 0.8+0.6 T Isern & Guerrero 2002, WD13 Naples
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SPH Simulations: 0.8+0.6 T Isern & Guerrero 2002, WD13 Naples
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Yoon et al. 2007, Also Benz et al. 1990ab, Segretain et al. 1997
evolution of a 0.9+0.6 M CO WD
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Yoon et al. 2007
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Yoon et al. 2007:
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Clayton et al. 2007: evolution of a CO+He WD merger
Considered a one-zone high-entropy envelope, for two cases (MHe = 0.2 and 0.4 M).
Computed temperature, density from 1d hydrodynamic evolution, including nucleosynthesis.
Found dramatic production of 18O.
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Phases in a DD merger
• Tidal disruption
• “Disk” formation
• Prompt nucleosynthesis in disk?
• Angular momentum dissipation
• High-entropy envelope forms
• Envelope “accreted” onto primary: dMenv/dt < dMedd/dt
• Helium (carbon) – burning starts
• Star expands, but high-S envelope remains
• Outer layers convective
• Accretion continues to completion
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Lies, Damned Lies, and ….
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Lies, Damned Lies, and …. Statistics
!! Warning !!
The statistics are due to Gijs Nelemans
The lies are entirely my own
All are still under discussion
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CO+He mergers: number densities
• 20% of all WD pairs include CO+He WD (Neleman’s et al 2001)
• CO+He WD merger rate: ν ∼ 4.4 10-3 yr-1 (Neleman’s et al. 2001) (Iben et al. give 2.3 10-3 yr-1)
• Heating rates between 10 000 and 40 000 K are 10 - 100 K yr-1, or evolution timescales: τ ∼ 300 - 3000 yr
• Merger rate × timescales gives number of EHes (N) in Galaxy between 1.3 and 13
• There are 17 known EHes in this temperature range
• Stars cooler than 10 000 K have τ ∼ 105 yr, ⇒ N = ν τ ∼ 30 - 300 cool CO+He merger products
• There are an estimated 200-1000 RCrBs in galaxy (Lawson et al. 1990), although only 33 are known (Alcock et al. estimate 3000 RCrBs)
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Observed mass distribution Predicted
Mass distributions look OK
Galactic distribution ??
Observed – bulge and thick disk
Predicted – thin disk
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Conclusions
• A significant number of DDs merge (a few/galaxy/century)• He+He WDs ⇒ EHe – sdO / sdB sequence
• CO+He WDs ⇒ RCrB – EHe – O(He) – WD sequence
• Physics of merger is really really interesting – surface abundances require hot mergers
• Predicted birth-rates and mass-distribution compatible with observed numbers
Questions: • Can observed merger products account for all DDs formed ?• Are any DDs left over to become stellar AM CVne ?• What happens to AM CVne when they ignite helium ?