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 ReferenceWD0135-052 Saffer et al.
  (1988)WD2032248 Ringwald (1988)WD0957-666 B
  ragaglia et al. (1990)PG1101364 Marsh
  (1995)WD1713332 Marsh et al.
  (1995)WD1241-010WD1317453WD2331290HS0507043
  4 Jordan et al. (1998)PG0922162 Finley
  Koester (1997)WD0136768 Maxted et al.
  (2002)WD1204450HE1414-0848 Napiwotski et al.
  (2002).

4
white-dwarf white-dwarf binaries

• Star ReferenceWD0135-052 Saffer et al.
  (1988)WD2032248 Ringwald (1988)WD0957-666 B
  ragaglia et al. (1990)PG1101364 Marsh
  (1995)WD1713332 Marsh et al.
  (1995)WD1241-010WD1317453WD2331290HS0507043
  4 Jordan et al. (1998)PG0922162 Finley
  Koester (1997)WD0136768 Maxted et al.
  (2002)WD1204450Feige 55 Holberg et al.
  (1995)CE315 Ruiz et al. (2001)HE1414-0848 Na
  piwotski 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)

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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.)

6
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?

7
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
• O/Ne WDs?
• H1504065 (Werner 1991/2002)

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HeHe WD merger
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
10
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
11
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
12
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

13
V652 Her the pulsating helium star
CNO-processed surface Some H (1) M 0.6 M?,
L 103 L ?, R 2.3 R ? dP/dt ? rapid
contraction
14
V652 Her
15
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
16
V652 Her

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

17
(No Transcript)
18
COHe WD merger
EHe stars ?
19
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
20
Temporal evolution of accreting WD
He ignition
H ignition
Mi0.6X0.001
convection zone
hydrogen-burning shell
helium-burning shell
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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)

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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
25
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 Masses from log g
(0.94 M?)
EHe stars
(0.79 M?)
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COHe merger model Test 3
HD168476
HD160641
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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 4EHe contraction
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
Abundances log ni c, ? log ?ini 12.15
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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!

33
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 ?

34
sdB and sdO stars

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

35
Helium-rich sdB stars and related objects

• Ahmad, Jeffery Woolf 2002, poster paper

36
Helium-rich subdwarfs as WD mergers?

• Ahmad, Jeffery Woolf 2002, poster paper

37
COCO WD mergers
38
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 required to match the
detailed surface abundances in all cases. The
hydrodynamics of the merger event must also be
explored in more detail.
39
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)
40
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