White dwarf mergers and the progeny of helium stars

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White dwarf mergers and the progeny of helium
stars

• Hideyuki Saio, Tohoku University
• Simon Jeffery, Armagh Observatory

The British Council, the Japan Society for the
Promotion of Science and the NI Dept of Culture
Arts and Leisure D.Pollacco, QUB, R.Starling,
MSSL, P.W.Hill, St Andrews Universitybased on
work published in MNRAS 313, 671, MNRAS 321,111
and accepted by 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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V652 Her

• HeHe WD merger
• mass ?
• radius ?
• luminosity ?
• pulsation period ?
• dP/dt ?
• composition ?

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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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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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Observational tests for COHe merged binary
white dwarf models
1. Binarity 2. R, M and L measurements for
pulsating stars 3. Gravity measurements 4.
Contraction rates 5. Surface abundances 6. Numbers
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Binarity
COHe merger model Test 1

• No extreme helium star has been found to be a
  binary
• (radial velocity searches)
• (IR excess searches)
• (UV excess searches)

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Radius measurement (Baades 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 2 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 2
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 3
HD168476
HD160641
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Contraction measurement
COHe merger model Test 4
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?expected
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COHe merger model Test 4
HD160641
BD-9 4395
BD-1 3438
HD168476
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COHe merger model Test 4 EHe contraction
contraction rates with masses will discriminate
between evolution models
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COHe merger model Test 5surface abundances
1. Model assumed homogeneous 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 6number 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. Absence of binaries ? 2. Radii and masses
for pulsating stars ? 3. Gravity measurements
? 4. Contraction rates ? 5. Surface
abundances ? 6. Number densities ?
not bad!
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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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Observations required
More direct measurements of pulsation masses More
sensitive measurements of contraction rates Very
high S/N spectroscopy over extended intervals
with stable telescope spectrograph
systems. Current success due to high stability of
IUE over a 17 year baseline and the use of UV
spectrophotometry to measure Teff. We could not
get time on UES/UCLES for such a project in the
mid 1990s, due to scheduling limitations. Now
the telescopes/spectrograph configurations are
being dismantled. Essential to get very good
data from a telescope / spectrograph
configuration that will be stable for a long (20
year) period. The 8m-class telescopes are
appropriate and some have suitable
spectrographs, so long as good data sequences can
be established now. Q-scheduling allows
observations that will enable pulsations to be
disentangled from evolution changes.
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Conclusions
The HeHe WD model provides a very good
explanation for the origin of V652 Her. Single
sdB stars could also be formed through this
channel. The COHe WD model provides an
excellent fit for all of the observed properties
luminous Extreme Helium stars and, by
association, the RCrB and luminous He-sdO stars.
It is the only model to do this. Work is still
required to match the detailed surface abundances
in both V652 Her and EHes. The hydrodynamics of
the merger event itself must also be explored
properly.