White dwarf mergers

1
White dwarf mergers

• Simon Jeffery Armagh Observatory

Hideyuki Saio, Tohoku University, Sendai,
Japan The British Council, JSPS, PPARC
based on work published in MNRAS 313, 671, MNRAS
321,111, AA 376, 497, MNRAS 333, 121
2
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.)

3
white-dwarf merger models

• HeHe (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 (Hachisu et al. 1986a,b, 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?
• COHe
• HeHe

4
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

5
SPH Simulations 0.80.6 T
Isern Guerrero 2002, WD13 Naples
6
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
7
Temporal evolution of accreting WD
He ignition
H ignition
Mi0.6X0.001
convection zone
hydrogen-burning shell
helium-burning shell
8
Extreme Helium Stars
How do merger models compare with observations of
extreme helium stars? Binarity Contraction
rates R, M and L measurements (pulsators) Gravity
measurements Surface abundances Number densities
9
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)
• Therefore they must have been binaries once.
• (Jeffery, Drilling Heber 1987)

10
COHe merger model Test 2
HD168476
COHe mergers solid 0.6M?COHe dashed
0.5M?COHe light accretion heavy
contraction EHes Baade radii from pulsating EHes
EHe stars
LS IV-1 2
11
COHe merger model Test 3gravities
HD168476
HD160641
solid 0.6 M? COHe dashed 0.5 M?
COHe dotted 0.7 M? HeHe
12
COHe merger model Test 4contraction
HD160641
BD-9 4395
BD-1 3438
HD168476
13
COHe merger model Test 5surface abundances
Abundances log ni c, ? log ?ini 12.15
14
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 10 000 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!

15
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
16
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
17
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
18
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 ?
• surface gravities ?
• contraction rates ?
• surface abundances ?
• number densities ?

19
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 COHe mergers. More work
is needed.
Work is also required to match the detailed
surface abundances in all cases. The
hydrodynamics of the merger event must also be
explored in more detail.
20
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