White dwarf mergers and the progeny of helium stars

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White dwarf mergers and the progeny of helium
stars

• Simon Jeffery Armagh Observatory

H.Saio, Tohoku University, Sendai, Japan
The British Council D.Pollacco, QUB,
R.Starling, MSSL, P.W.Hill, St Andrews
University, V.Woolf, Armagh
PPARC
based on work published in MNRAS 313, 671, MNRAS
321,111 and submitted to MNRAS
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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)

• what are they?
• where did they come from?
• where are they going?

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proposals
early ideas mass-loss on AGB hot
bottom burning final-flash a white dwarf
evolves Iben et al.1984 back to the
AGB white dwarf binary merger one white dwarf
breaks up, Webbink 1984, Iben Tutukov
1984 falls onto its companion and forms a
giant quantitative models? observational tests?
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hypothesis HeHe white dwarf formed from binary
star evolution (observed) orbit decays through
gravitational, tidal and magnetic
interaction less massive WD disrupted when Porb
4 minutes and forms thick disk more massive WD
accretes material from disk ?model
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V652 Her
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
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internal details
inward migration of helium-burning shell and
response of surface to shell flashes
extent of shell and surface convection zones
during first five shell flashes
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V652 Her the pulsating helium star

• Period change (Kilkenny Lynas-Gray 1982 - 1996)
  
• corresponds to R/R0 2 .10-4 yr-1 (xdx/dt)
  but not-linear (R, R)
• Nonadiabatic linear pulsation models (Saio 1983 -
  1995)
• 1993 OPAL and OP opacities gt Z-bump opacity
  driving
• Nonlinear models (Fadeyev Lynas-Gray 1996)
• Best agreement for M0.72 MSun, Teff23 500K, L
  1062 LSun, Z0.0156
• Stellar atmosphere analysis (Jeffery et al. 1999)
• 1 H, Fe/H0, N-rich, C and O poor, log g 3.7
  / 0.1, Teff 24 500 / 500 K

• Extreme helium star (Berger Greenstein 1953)
• comparable to other helium stars HD124448,
  HD168476, BD10 2179
• Light variations (Landolt 1973)
• P0.108 days similar to ? Cepheids, but obviously
  not Pop I MS
• Radial velocity variations (Hill et al. 1981, )
• amplitude 70 km/s, rapid acceleration
  free-fall
• Radius measurement from Baades method
  (Lynas-Gray et al. 1984)
• M g.R2 0.7 / 0.3 Msun, L 103 Lsun

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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 !
V652 Her
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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)

• what are they?
• where did they come from?
• where are they going?

V652 Her may be the exception. What about more
luminous C-rich helium stars?
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hypothesis COHe white dwarf formed from binary
star evolution orbit decays through
gravitational, tidal and magnetic
interaction He-WD disrupted at contact and forms
thick disk CO WD accretes material from
disk ?model
About the movie...
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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
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Temporal evolution of accreting WD
He ignition
H ignition
Mi0.6X0.001
convection zone
hydrogen-burning shell
helium-burning shell
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Surface composition of accreting WD
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Observational tests for COHe merged binary
white dwarf models
1. Radius measurements for pulsating stars 2.
Gravity measurements 3. Contraction rates 4.
Surface abundances
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Radius measurement (Baades method)
method IUE SWPLWP LORES fluxes model
atmospheres ?Teff and ?? integrating radial
velocities ??R R?. ?R/ ? ? L R2Teff4 M
gR2/G PV Tel 2 others
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COHe merger model Test 1
COHe mergers solid 0.6M?COHe dashed
0.5M?COHe light accretion heavy
contraction EHes Baade radii from pulsating EHes
EHe stars
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COHe merger model Test 2
HD168476
HD160641
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Contraction measurement
150 IUE LORES spectra over 17-year baseline Teff
and ? measured from SWPLWP image pairs
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vectors represent predicted temperature evolution
over 10 years for 0.7 and 0.9 (solid) Msun helium
stars respectively dT/dt significant
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COHe merger model Test 3
HD160641
BD-9 4395
BD-1 3438
HD168476
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observational tests for EHe models
3 methods for estimating masses of EHes Ms
spectroscopic mass Mc-Ls g Mp pulsation mass ?
g Md direct mass ?R, ?, ??, g
contraction rates with masses will discriminate
between evolution models
and the surface composition provides a fossil
record of internal evolution . However errors
in M remain unacceptably large
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Conclusions
The HeHe WD model provides a very good
explanation for the origin of V652 Her. 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 and
luminous He-sdO stars. Work is still required to
match the detailed surface abundances in both
V652 Her and EHes. The hydrodynamics of the
merger event itself must be explored properly.
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spectrum of V652 Her
MgII
H?
HeI
HeI
WHT 1998 spectrum (histogram) plus model
(curve) Jeffery, Woolf Pollacco (in preparation)
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light and radial velocity curves
P 0.108 days
SAAO 1.0m StAPCCD V-band data from Kilkenny
(priv. comm.)
WHTISIS 1998 Jeffery, Woolf Pollacco (in
preparation)
?
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V652 Her evolutionary models
Constraints CNO-processed surface with some H M
0.7 MSun, L 103 Lsun,dP/dt ? rapid contraction
helium horizontal branch (Jeffery 1984) He core
with luminous shell contracts onto Helium Main
Sequence, reproduces M, L and dP/dt BUT no
plausible progenitor mixed red giant branch
(Sweigart 1996) Physics implausible, and see
above binary mass transfer (case BB Iben
Tutukov 1984) V652 not a binary Final-flash
white dwarf Luminosity too high for V652
Her Carbon abundance too high for V652 Her
merged white dwarf models HeHe (Iben 1990)
dM/dt gt Eddington ? sdB star HeCO (Iben
1990) dM/dt gt Eddington ? RCrB star Evolution
critically sensitive to WD temperature at
merger COCO (Saio Nomoto 1998) HeHe (Saio
Jeffery 2000) dM/dt half Eddington HeCO
(Saio Jeffery ...)
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models for helium stars with CO cores
Saio 1988
equilibrium models with degenerate CO core and
helium envelope, allowed to contract as He-shell
burns out
contraction rates for helium stars
Mc-Ls relation for helium stars
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IUE observations of helium stars
predicted contraction rates up to 100 K/yr 150
observations of seventeen helium stars from 1979
to 1995 effective temperatures and angular
diameters measured by fitting model atmospheres
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models for helium stars with CO cores
Saio 1988
equilibrium models with degenerate CO core and
helium envelope, allowed to contract as He-shell
burns out
contraction rates for helium stars
Mc-Ls relation for helium stars
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pulsations in helium stars
19 IUE LWPSWP observations of three pulsating
helium stars Teff and ?,best-fit periods,
amplitude gives ?? Radial velocities from SAAO
1.9m amplitude gives ?R hence stellar radius R
?R ?/?? with surface gravity g, M gR2/G
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vectors represent predicted temperature evolution
over 10 years for 0.7 and 0.9 (solid) Msun helium
stars respectively dT/dt significant