Cannibals in space

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Cannibals in space

• Simon Jeffery Armagh Observatory

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cannibal n. a. 1. n. Person who eats human
flesh animal that eats its own species hence
ISM (2) n.
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What is the most important force in the universe?

• Every object in the universe is pulling every
  other object in the universe towards itself, with
  a force inversely proportional to the square of
  the distance between the two objects.

GRAVITY
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collisions / captures / mergers

• galaxies
• stars in clusters
• stars in binaries
• white dwarfs
• neutron stars
• black holes

the cosmic scrapyard
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Credit Chris Mihos, Lars Henquist and Frank
Summers
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galaxy collisions
2 galaxy collisions multiple galaxy
collisions Stephans quintet galactic
wrecks wreckage in the Hubble deep field wreckage
in closeup starbirth triggers
thats what I call a wreck
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Credit Joshua Barnes (University of Hawaii)
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NGC 4038/4039 Antennae Galaxy
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star-eyed cannibals
Mass exchange Roche lobe overflow (case
A/B/C) Algol binaries cataclysmic binaries X-ray
binaries
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stellar collisions
common envelope evolution blue stragglers red
giant - white dwarf collisions white dwarf
mergers neutron star mergers
stars that go bump in the night
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Credit Matthew Bate (IoA, Cambridge)
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Credit Stephan Rosswog (Univ.Leicester), Richard
West (UKAFF)
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Credit Stephan Rosswog (Univ.Leicester), Richard
West (UKAFF)
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the boring bit

• Oh no, not helium stars again?

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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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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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models for COHe mergers
EHe stars ?
Saio Jeffery .
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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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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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Observational tests for COHe merged binary
white dwarf models
1. Absence of binaries ? 2. Radius
measurements for pulsating stars ? 3. Gravity
measurements ? 4. Contraction rates ? 5.
Surface abundances ? 6. Number densities ?
not bad!
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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 an even more unusual extreme helium
star!
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V652 Her the pulsating helium star

• Time-dependent atmosphere analysis (2001)
• most precise radial velocity curve
• self-consistent Teff/log g around pulsation cycle
• revised measurements of radius and
  massltRgt2.310.02R? M
  0.590.18M ?
• Tests for stellar structure, evolution and
  pulsation models
• Credits Berger Greenstein 1953, Landolt 1973,
  Hill et al. 1981, Jeffery Hill 1986, Lynas-Gray
  et al. 1984, Kilkenny Lynas-Gray 1982 - 1996,
  Saio 1983 - 1995, Fadeyev Lynas-Gray 1996,
  Montanes Rodriguez Jeffery 2001, Jeffery et al.
  1999, Jeffery et al. 2001

• Extreme helium star (1953)
• Light variations (1973)
• Radial velocity variations (1981)
• P0.108 days
• Radius measurement from Baades method (1984)
• M g.R2 0.7 0.4/ 0.3 M?
• Period change (1982)
• R/R? 2 .10-4 yr-1 ,(R, R)
• Pulsation models (1993,2002)
• OPAL and OP opacities gt Z-bump opacity driving
• Stellar atmosphere analysis (1999)
• 1 H, Fe/H0, N-rich, C and O poor, log g 3.7
  / 0.1, Teff 24 500 / 500 K

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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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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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Can mergers really happen?
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)
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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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SPH 0.80.6 D
Isern Guerrero 2002, WD13 Naples
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SPH 0.80.6 T
Isern Guerrero 2002, WD13 Naples
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extreme helium stars

• products of white dwarf mergersCOHe white
  dwarfsHeHe white dwarfs

a new breed of stellar cannibal
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black holes
the black hole at the galactic center black holes
in other galaxies black holes adrift
cosmic asteroraptors
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NGC 4438 (Virgo cluster galaxy)
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if the comet doesnt get you, the black hole will