Binary Star Evolution

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Binary Star Evolution

• Cevin Kroxall
• Stellar Atmospheres à la Pilachowski

2
Binaries

• To zeroth order all stars are members of multiple
  systems!
• Really makes a difference when stars are
  interacting binaries
• 30-50 of all stars
• i.e. one of the stars fills its Roche lobe
• More likely to encounter this after the main
  sequence phase
• Half the stars in the sky have yet to experience
  this
• Half of those stars will be interacting binaries
  in the future

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Binary Stars
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Binary Origins

• At least 50 of PMS objects are multiple systems
• Binaries are created before the disks clear and
  bonafide PMS stars are revealed
• Start with a molecular cloud
• No B-field
• Lots of physics
• And 1 key assumption its similar to single star
  formation

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Are Formation Times Related to Periods?

• Collapse starts
• Gas becomes opaque
• Ionization
• Total Ionization of H
• Quasi Equilibrium

• Tohline, J 2002 ARAA, 40, 349

6
Formation Mechanisms

• Capture
• Need favorable three body encounters
• Very improbable (maybe in globular cores)
• Prompt Fragmentation
• Only homologous collapses work
• No scrambling of mass, proportionally collapsed
• Does NOT occur before at least 1 free-fall time
• Can occur in rotating clouds on the 1st quasi
  equilibrium state
• Mixed Results very dependant on initial
  conditions and resolution in models
• fragments but binaries?

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Formation Mechanisms

• Delayed Break-Up
• Free-fall and formation of a massive accretion
  disk which becomes unstable
• Get Dumb-bells and pears
• We assume!!!
• No one has actually done it! Rely on stability
  tests
• Leave out viscosity and get compressible
  ellipsoids
• Doesnt seem to lead to binaries

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Binary Formation Summary

• In conclusion binary formation is the primary
  branch of the star formation process. Obviously
  nature knows how to form binary star systems.
  Hopefully, in the coming decade we will find
  one or more fully convincing ways to do so as
  well.

Tohline, J 2002 ARAA, 40, 349
9
Binary Evolution

• The theory of binary star evolution relies
  heavily on models of single stars
• The theory of binary star evolution has a much
  different character then does the theory of
  single star evolution.
• There are no beautiful sequences of
  mathematically impeccable binary star models to
  which one can point with pride and compare
  successfully with observations.

• Iben, 1991, ApJSS, 76, 55

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Roche Lobes

• First basic concept in binary evolution theory
• A unique surface of constant potential which
  consists of two separate lobes, each enclosing
  on of the stellar components
• A particle inside a lobe experiences a force in
  the direction of the enclosed star
• Assumptions
• Centrally concentrated star
• Rotation is synchronous with orbit
• Circularized orbit

L2
Irradiated Roche Lobe - Podsialdlowskix http//
www-astro.physics.ox.ac.uk/podsi/podsiadlowski1.h
tml

• Iben, 1991, ApJSS, 76, 55

VV Cephei systems e?0.5
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Roche Lobe OverflowHow to fill your lobe

• Growth of a component due to internal changes
• Orbital shrinkage due to loss of angular momentum
• Swelling due to rejection of accreated matter or
  nuclear ignition
• A hardening collision between the binary and
  another star

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Remnant Mass Composition

• Second major concept/assumption in binary
  evolution
• The remnant of the of the donor will have the
  same mass and composition as the core of the
  donor when it first fills its Roche lobe

• Iben, 1991, ApJSS, 76, 55

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Mass Transfer Mass Conservation

• 3rd Concept - Function of the structure of the
  component at the moment of overflow, the degree
  of mass angular momentum conservation, and the
  response of the companion
• dM/dt -M/?th
• If the donor does not posses a deep convective
  envelope, then mass angular momentum
  conservation is acceptable

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Common Envelope

• 4th Concept - Secondary may form a hot expanding
  layer which then fills its Roche lobe
• Provides a frictional interaction between
  embedded cores and shedding material

• Leads to a tighter bound orbit
• One of the least understood phases of binary
  evolution
• One of the most important phases of binary
  evolution

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Common Envelope Problems

• Donor has a mass larger than 70 of the accretor
• Theoretically most red giant donors should
  experience this phase
• Observationally this is not true
• Mass loss by stellar wind prior to mass transfer?
• When can the CE be ejected? When do we get a
  complete merger?

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Orbital Angular Momentum Loss

• 5th Concept - Angular momentum loss can drive or
  sustain Roche Lobe filling (keep a component in
  conact with its Roche lobe despite it shrinking)
• Mechanisms to get rid of angular momentum
• Magnetic stellar wind
• Gravitational wave radiation
• Tidal torques
• 3-body interactions

What happens to magnetic breaking when a donor
becomes fully convective? (dynamo is killed)
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General Classification

• Based upon the evolutionary stage of the mass
  donor at the beginning of mass transfer
• Case A - main-sequence
• Case B - post-main-sequence, pre-helium-ignition
• Case C - post-helium-buring
• Two modes of mass transfer
• Quasi-conservative
• donor has a radiative envelope, orbital periods
  increase
• Dynamical
• Donor is giant with a deep convective envelope,
  orbits shrink
• Either an ejection of the common envelope leaving
  a tight binary or tidal destruction of binary
  components leading to a merger

SN 1987A over production? - Mass loss w/o spiral
in? - radiative common envelope?
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The Summary Iben Figures
The adventure begins
The CV sequence
The most likely end
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The Summary Iben Figures
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Blue Stragglers (BS)

• Found above the turnoff
• Found in both (and nearly all) open and globular
  clusters from 108 - 1013 years old
• First seen in M3 by Sandage (1953)
• Some are almost definitely NOT binaries
• Often centrally concentrated in clusters
• Li under abundant
• Slow rotators
• All are at least slightly evolved
• 4 are eclipsing binaries!!!!
• Only 0.1 of main sequence stars in globulars are
  eclipsers

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More BS
Johnson Sandage 1955, ApJ, 121, 616

• UBIQUITOUS BUT RARE
• Only a few percent of the stars that previously
  populated that area of the HR diagram

Blue staggler-hood afflicts relatively few stars
in a typical cluster
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Creating BS

• Stellar Merger Remnants
• Contraction of longer period binaries into
  contact binaries from angular momentum loss
• Related to timescales which we dont know

• Stellar Collision Remnants
• Requires binary - binary collisions
• Could be responsible for 10-20
• High numbers of eclipsers?

BOTH REQUIRE MANY PRIMORDIAL BINARIES
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Future BS

• Better statistics
• Constrain ages
• More BS are binaries in open clusters
• Is this real? (? - mergers ? - collisions)
• Radial distributions
• Why are they slow rotators?
• Need masses
• Faint main sequence proto-BS?

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Short Period Tidally Locked Binaries

• Preserve fragile elements like Li by freezing
  mixing in surface layers due to tidal torques

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Binary Summary

• Even though a star may be single now, it may
  well have been a member of a binary system in the
  past. Indeed, whenever one is confronted with a
  new stellar phenomenon, it is probably adviable
  to first thoroughly explore the possibility of a
  binary interaction as a cause of the phenomenon
  before starting to adjust the input physics in
  the stellar calculation.

P. Podsiadlowski (emphasis added)