Binary Star Evolution

Cevin KroxallStellar Atmosphere’s à la Pilachowski

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

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

Are Formation Times Related to Periods?

Tohline, J 2002 ARAA, 40, 349

A) Collapse startsB) Gas becomes

opaqueC) IonizationD) Total Ionization

of HE) Quasi

Equilibrium

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?

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

Doesn’t seem to lead to binaries

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

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

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

Iben, 1991, ApJSS, 76, 55

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Irradiated Roche Lobe -

Podsialdlowskix

http://www-astro.physics.ox.ac.uk/~podsi/podsiadlowski1.html

VV Cephei systems e0.5

L2

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

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

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

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

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?

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)

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?

The Summary Iben FiguresThe adventure begins

The most likely end

The CV

sequence

The Summary Iben Figures

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

More BS

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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”

Creating BS

Stellar Merger Remnants

• Contraction of longer period binaries into contact binaries from angular momentum loss

• Related to timescales which we don’t know

Stellar Collision Remnants

• Requires binary - binary collisions

• Could be responsible for 10-20%

• High numbers of eclipsers?

BOTH REQUIRE MANY PRIMORDIAL BINARIES

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?

Short Period Tidally Locked Binaries

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

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)