The progeny of binary white dwarf mergers extreme helium stars, R CrB stars and subdwarf B stars

1
The progeny of binary white dwarf mergers
extreme helium stars, R CrB stars and subdwarf
B stars

• Simon Jeffery, Armagh Observatory, N Ireland
  Hideyuki Saio, Tohoku University, Sendai, Japan
  The British Council

based on work published in MNRAS 313, 671 and
MNRAS 333, 121,and also MNRAS 321,111, AA 376,
497, .
2
Outline

• white dwarf - white dwarf binaries
• merged binary white dwarf models
• HeHe
• COHe
• COCO
• V652 Her as an HeHe merger
• EHes and RCrBs as COHe mergers
• sdB and sdO stars as mergers
• Conclusions

3
white-dwarf white-dwarf binaries

• Star Type P (h) Reference
• WD0135-052 Saffer et al. (1988)WD2032248 Rin
  gwald (1988)WD0957-666 Bragaglia et al.
  (1990)PG1101364 Marsh (1995)WD1713332 Mars
  h et al. (1995)WD1241-010WD1317453WD2331290H
  S05070434 Jordan et al. (1998)PG0922162 Fin
  ley Koester (1997)WD0136768 Maxted et al.
  (2002)WD1204450
• Feige 55 Holberg et al. (1995)
• CE315 Ruiz et al. (2001)HE1414-0848 Napiwotsk
  i et al. (2002)RX J0806.31527 Israel et al.
  (2002)RX J0439.8-6809 van Teeseling etal.
  (1997)RX J191424 Cropper et al. (1998)
• X1850-087 Homer et al. (1996)G61-29 Nather
  et al. (1981)EUVE J143975.0 Vennes et al.
  (1999)LB 11146 Schmidt et al. (1998)

4
white-dwarf white-dwarf binaries

• birth rate 0.05 yr-1 Galaxy-1 (Nelemans et
  al. 2001)
• type 20 are COHe WD (Nelemans et al
  2001)
• number 2.5 108 Galaxy-1 (Nelemans et al.
  2001)
• period distribution(Nelemans et al. 2001,
  Maxted et al. 2002)
• merger timescales ?m107 (P/h)8/3 ?-1
  (M/M?)-2/3 yr (Landau Lifshitz 1958)
• COHe merger frequency ? ? 4.4 10-3 yr-1
  (Nelemans et al. 2001) ? ? 2.3 10-3 yr-1 (Iben
  et al.)

5
white-dwarf merger models

• He-He (Nomoto Sugimoto 1977, Nomoto Hashimoto
  1987, Kawai, Saio Nomoto 1987, 1988, Iben 1990)
  
• ? He ignition ? HeMS or sdB star ? CO WD
• HeCO (Webbink 1984, Iben Tutukov 1984, Iben
  1990)
• ? RCrB star OR SNIa ?
• COCO (Kawai, Saio Nomoto 1987, 1988, Nomoto
  Hashimoto 1987, Mochkovitch Livio 1990, Saio
  Nomoto 1998) ? C ignition ? ONeMg WD OR
  explosion ?
• results critically sensitive to WD temperature
  AND accretion rate
• what do the products look like between merger and
  end-state?

6
Hydrodynamic simulations
t27.2s
/109 cm

• Benz et al. 1990, ApJ 342, 986, ApJ 348, 647
• Segretain et al. 1997, ApJ 481, 355 SPH -
  0.90.6 M

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Progeny of WDWD mergers ?

• Extreme Helium stars
• RCrB stars
• subdwarf B stars
• sdBWD / sdBdM / sdBG
• sdB / He-sdB
• subdwarf O stars
• He-sdO
• PG1159 stars?
• H1504065 (Werner 2002)

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HeHe WD merger
accretion turned off at selected final mass
shell burns inwards in series of mild flashes
lifts degeneracy
helium-burning shell forces star to expand to
yellow giant, 103 yr
Helium core-burning star (sdB?) formed as shell
reaches centre
helium ignites in shell at core-envelope boundary
Saio Jeffery 2000
9
V652 Her the pulsating helium star
CNO-processed surface Some H (1) M 0.7
MSun, L 103 Lsun, dP/dt ? rapid contraction
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V652 Her
11
pulsation properties linear analysis of
evolutionary models gives fundamental pulsation
period dP/dt, derivative of period wrt time (or
dP/dn) also obtained evolution track through
P-dP/dn diagram looks good !
log dP/dt
V652 Her
12
COHe WD merger
0.6 M? , X0.001
accretion turned off at selected final mass
helium-burning shell forces star to expand to
yellow giant, 103 yr
0.5 M? CO-WD
helium ignites in shell at core-envelope boundary
Saio Jeffery 2002
13
Extreme Helium Stars

• A and B supergiants
• low mass
• dimensions post-AGB stars
• no planetary nebulae
• no binaries
• weak or absent hydrogen lines
• strong carbon lines (most)
• rare (3 in HD catalogue)

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COHe merger model Test 2
COHe mergers solid 0.6M?COHe dashed
0.5M?COHe light accretion heavy
contraction EHes Baade radii from pulsating
EHes Masses from log g
(0.94 M?)
EHe stars
(0.79 M?)
15
COHe merger model Test 4EHe contraction
HD160641
BD-9 4395
BD-1 3438
HD168476
16
COHe merger model Test 5surface abundances
Abundances log ni c, ? log ?ini 12.15
17
Observational tests for merged binary white dwarf
models
not bad!

• HeHe V652 Her
• mass ?
• radius ?
• luminosity ?
• pulsation period ?
• dP/dt ?
• composition ?

• COHe EHe stars
• absence of binaries ?
• radii and masses ?
• gravity measurements ?
• contraction rates ?
• surface abundances ?
• number densities ?

18
sdB and sdO stars

• Helium Main-Sequence stars
• Some apparently single
• Some extremely helium-rich

19
Helium-rich sdB stars and related objects

• Ahmad, Jeffery Woolf 2002, poster paper

20
Helium-rich subdwarfs as WD mergers?

• Ahmad, Jeffery Woolf 2002, poster paper

21
FAQs

• Stable or Unstable Mass Transfer ?
• If q gt 0.6, the increase of stellar radius
  exceeds the increase of Roche radius.
• M-R relation for WD is due to requirement of
  hydrostatic balance, hence radius will increase
  on a dynamical timescale (s), leading to runaway
  mass transfer. SPH calculations by Benz et al.
  (1990) show formation of thick disk around the
  more massive WD.
• If q lt 0.6, stable mass transfer may produce an
  AM CVn type system
• Conservation of Angular Momentum ?
• When the surface velocity is close to the Kepler
  velocity, angular momentum is transported
  efficiently from the star to the disk so that
  accretion continues as long as matter around the
  star exists (Paczynski 1990, Pophan Naryan
  1990)

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Conclusions

• The HeHe WD model provides a very good
  explanation for the origin of V652 Her. Some
  single sdB stars could be formed through this
  channel.
• The COHe WD model provides an excellent fit for
  the observed luminous Extreme Helium stars and,
  by association, the RCrB, luminous He-sdO and
  O(He) stars.
• He-sdB and low-luminosity He-sdO stars may be
  formed either through HeHe or through OHe
  mergers. More work is needed.
• Work is required to fully match the detailed
  surface abundances in all cases. The
  hydrodynamics of the merger event must also be
  explored fully.