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, AA 376, 497 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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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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spectrum of V652 Her
MgII
H?
HeI
HeI
WHT 1998 spectrum (histogram) plus model
(curve) Jeffery, Woolf Pollacco 2001, AA 376,
497
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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 (2001)
?
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V652 Her the pulsating helium star

• 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
• Time-dependent atmosphere analysis (Jeffery et
  al. 2001)
• most precise radial velocity curve
• self-consistent Teff/log g around pulsation cycle
• improved measurement of EB-V0.060.01
• updated chemical abundances
• reanalysis of total flux variations
• revised measurements of radius and
  massltRgt2.310.02R? M
  0.590.18M ?
• line-broadening at minimum radius
• Tests for stellar structure, evolution and
  pulsation models

• Extreme helium star (Berger Greenstein 1953)
• Light variations (Landolt 1973)
• P0.108 days
• Radial velocity variations (Hill et al. 1981,
  Jeffery Hill 1986)
• structure in v curve?
• shock at minimum radius?
• Radius measurement from Baades method
  (Lynas-Gray et al. 1984)
• M g.R2 0.7 0.4/ 0.3 M?
• Period change (Kilkenny Lynas-Gray 1982 - 1996)
  
• R/R? 2 .10-4 yr-1 ,(R, R)
• Pulsation models (Saio 1983 - 1995, Fadeyev
  Lynas-Gray 1996, Montanes Rodriguez Jeffery
  2001)
• 1993 OPAL and OP opacities gt Z-bump opacity
  driving

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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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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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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
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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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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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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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
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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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vectors represent predicted temperature evolution
over 10 years for 0.7 and 0.9 (solid) Msun helium
stars respectively dT/dt?expected
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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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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 remain large
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models for COHe mergers
EHe stars ?
Saio Jeffery .
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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
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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 5. Number densities
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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 EHe masses
3 methods for estimating masses of EHes Ms
spectroscopic mass Mc-Ls g Mp pulsation mass ?
g Md direct mass ?R, ?, ??, g
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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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COHe merger model Test 3EHe contraction
HD160641
BD-9 4395
BD-1 3438
HD168476
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COHe merger model Test 3 EHe contraction
contraction rates with masses will discriminate
between evolution models
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COHe merger model Test 4surface abundances
A simple recipe (mass fractions)
1. Modelcompromised by assumption of
homogenerous accretion 2. A simple recipe a.
assume realistic composition of different layers
of progenitor white dwarfsb. mix together in
proportion to expected layer masses 3.
Observations of EHe, and RCrB stars
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COHe merger model Test 5number densities

• 20 of all WD pairs include COHe WD (Nelemans
  et al 2001)
• COHe WD merger rate ? ? 4.4 10-3 yr-1
  (Nelemans 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 ? ? 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 10000 K have ? ? 105 yr, ? N
  ? ? ? 30 - 300 cool COHe 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).
• Model builders reckon anything within a factor
  three is excellent!

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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 5. Number densities
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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,
luminous He-sdO and O(He) 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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