Binary Star Evolution

• Half of all stars are in binary systems - stellar evolution in binaries is important

• Roche Lobe: 3-D boundary where the gravity of 2 stars is equal; if a star expands beyond this boundary some of its matter accretes onto the other star

• Matter that transfers from one star to another spirals onto the other star through an accretion disk

• As the matter gets closer to the object, it moves faster and gets hotter because of friction, and produces X-rays

• Nova: the detonation of accumulated hydrogen in an accretion disk around a white dwarf

• Type 1a Supernova: collapse and explosion of a white dwarf that has accreted enough mass to go overcome electron degeneracy

Includes material from:• Lisker (

http://www.astro.unibas.ch/~tlisker/science/talks/kiel2004.ppt)

• Luhman (http://www.astro.psu.edu/users/kluhman/a1/Lec21.ppt)

• Orsela de Marco (http://www.ncac.torun.pl/~pngdansk/presentations/orsola_de_marco_talk.ppt)

• Gänsicke (http://deneb.astro.warwick.ac.uk/phsdaj/PX387/BinaryStars.ppt)

• Belyanin (http://faculty.physics.tamu.edu/belyanin/lecture notes 17.ppt)

• And references as noted on the slides

The Theoretical HR Diagram

Turn-off age Mass

Post-main sequence evolution

1-2: main sequence (core H-burning) 2-3: overall contraction 3-5: H burning in thick shell 5-6: shell narrowing 6-7: red giant branch7-10: core He burning 8-9: envelope contraction

RGB: R ~100-300 Ro

AGB: R ~ 500-1500 Ro

Common Envelope:A twice-in-a-lifetime opportunity

R

R

Roche LobesLagrange points are gravitational balance points where the attraction of one star equals the attraction of the other. The balance points in general map out the star’s Roche lobes. If a star’s surface extends further than its Roche lobe, it will lose mass.

• L1 - Inner Lagrange Point – in between two stars

– matter can flow freely from one star to other

– mass exchange

• L2 - on opposite side of secondary – matter can most easily leave system

• L3 - on opposite side of primary

• L4, L5 - in lobes perpendicular to line joining binary

• Roche-lobes: surfaces which just touch at L1

– maximum size of non-contact systems

•L1 – L3 are unstable - a small perturbation will lead the material to leave the L-point•L4&5 are stable, i.e. material will return to its initial position following a small perturbation

L1: SOHO

L2: Gaia, WMAP, JWT

Earth-Sun

Binary configurations and mass transfer

Binary star configurations and mass transfer

Detached: mass transfer via wind

Semidetached: mass transfer via Roche lobe overflow

Contact

1

2

Interactions in close binaries – 3 effects1. Distortion of the star(s) from

spherical shape: ellipsoidal modulation (bright when seen from sides)

2. Gravity darkening

3. Irradiation & heating: reflection effect

WD

Donor

hotter

lower gravity

eclipse

light variations due to secondarydistortion and gravity darkening

Common envelope Unstable Roche Lobe overflow

Depending on the efficiency of the energy transfer from the companion to the CE (), one might get:

A short-period binary, or… a merged star

The existence of a CE phase is inferred by the presence of evolved close binaries: CVs, Type Ia SN, LMXB, post-RGB sdB binaries, and binary CSPN, with P < 3-5 yr

unstable mass transfer - the Roche-lobe of the mass donor shrinks as a consequence of its mass loss, increasing the rate at which it loses mass

stable mass transfer - the Roche-lobe of the mass donor grows as a consequence of its mass loss, stopping the mass transfer

AccretionIf a star overflows its Roche lobe through the Lagrange point, gas will go into orbit around the companion. The gas will stay in the plane of the system and form an accretion disk.

If a red giant overflows its Roche lobe so that it engulfs the companion, its outside may be stripped away, leaving only its hot core.

Mass Loss

RS CVn Stars

● Two cool, partly-evolved MS stars with orbital periods of a few days

● Rotational period locked to the orbit● Generally, non-contact, mass

transfer by winds● High rotation (due to tidally locked

orbits) leads to high level of chromospheric activity

– Spots– Flares– Coronae, chromospheres

BY Dra and FK Comae Stars

● BY Dra stars are related to RS CVn stars but with lower mass primaries (K and M spectral type)

● FK Comae stars are also related to RS CVn statrs but with more evolved, subgiant primaries

● Fast rotation and high level of chromospheric activity than stars of similar spectral type

Gondoin et al.2002, A&A 383, 919-932

Ritter Obs. archive

● Prototype: Algol - a close double star whose components orbit each other every 2.9 days

● A B8 V star of about 3.7 solar masses and a K2 subgiant with 0.8 solar masses – paradox!

● K2 IV star was originally the primary, but has transferred much of its mass to the former secondary.

● Mass transfer rate from K2 to B8 about 5 x 10-7 solar masses per year

● Algol is an eclipsing system, but not-eclipsing systems have also been identified

● Some Be stars have been reclassified as Algols ● Long period Algols have accretion disks, but in

shorter period systems, gas flows onto the primary.

Richards & Albright

Algol Binaries

W Ursa Majoris Stars● Main sequence contact binaries

● Outer gas envelopes of the stars are in contact (overflowing their Roche lobes)

● Essentially share a common photosphere despite having two distinct nuclear-burning cores

● Separations of 0.01 AU (106 km)

● Highly circular orbits (e~ 0) with periods of only 0.3 – 1 day

● 1/500 of FGK stars in the solar vicinity (maybe 1% overall)

Blue Stragglers

• Sandage (1953) noted that a few stars in M3 appeared blue-ward and above MSTO

• Apparently normal MS stars of luminosity and mass greater than those currently evolving toward the red giant phase

• Common in globular clusters

• Origins?– HB stars crossing the

MS?– More recent star

formation?– Mergers

• Mass transfer• Binary coalescence• Collisions

Buonanno et al. 1994, A&A, 290, 69

Anomalous (or Dwarf) Cepheids

May be causally related to blue stragglers

● Found primarily in dwarf spheroidals (and globular clusters)

● Pulsation periods less than 1.5d

● Absolute magnitudes 0.5 > MV > -1.5

● Period-luminosity (P-L) relations differ significantly from those of Population I and II Cepheids

● ACs might have formed as a result of mass transfer (and possibly coalescence) in a close binary system of mass up to about 1.6 MSun

McCarthy & Nemec 1997, ApJ, 482, 203

Mass Transfer Binaries The more massive star in a binary

evolves to the AGB, becomes a peculiar red giant, and dumps its envelope onto the lower mass companion

● Ba II stars (strong, mild, dwarf)● CH stars (Pop II giant and subgiant)● Dwarf carbon stars● Nitrogen-rich halo dwarfs● Li-depleted Pop II turn-off stars

McClure et al 1980, ApJL 238, L35

Symbiotic Stars● A red giant and a small hot star, such as a white dwarf,

surrounded by nebulosity.

● Combined spectrum includes TiO molecular absorption plus emission lines of high ionization species (He II4686 Å and [O III]5007 Å)

● Three emitting regions: the individual stars themselves and the nebulosity that surrounds them both.

● The nebulosity originates from the red giant, which is in the process of losing mass quite rapidly through a stellar wind or through pulsation

● Short-lived phase so symbiotic stars are rare objects.

1. Pulsating red giant star and a compact, hot white dwarf star binary

2. The red giant is losing mass. The white dwarf concentrates the wind into an accretion disk

3. Nova outburst. The hot gas forms a pair of expanding bubbles above and below the equatorial disk.

4. Process repeats

Munari & Zwitter 2002, A&A 383, 188

RR Tel

Extreme Blue HB stars and sdB

Binaries

● Subdwarf B (sdB) stars are core helium burning stars of mass 0.5 with a very thin hydrogen-rich envelope

● Mass loss on RGB is strong enough to prevent the helium flash● Single-star evolution can’t account for the very small hydrogen envelope mass ● Close binary evolution may explain their origin

– Unstable mass transfer results in CE, which is ejected after a spiraling-in of both stars sdB+MS or sdB+WD

– Stable Roche-lobe overflow, no CE phase > larger orbital separation and periods– two He-WDs merge to ignite core helium burning - only scenario that produces

single sdB stars● Many sdB stars are members of binary systems with cool companions

NGC 6791[Fe/H[ = +0.4Age > 8 Gyr

Formation of a white dwarf/main sequence binary

2 CE: Formation of a

millisecond pulsar

2CE: Formation of WD-WD binaries

WD-WD merger: supernova type Ia

Binary star zoologyM1>M2, M1 evolves first. Wide binary? No interaction, evolve as single stars.

y

common envelope

common envelope

wind accretion

“High mass X-ray binary” (HMXB), P~days - months

detached WD/NS/BH + MS

binaryP~days - years

common envelope

WD+WDP~hours - days

WD+BD binary

NS+NS

red giantmass donor

“symbiotic stars”P~weeks - years

RLOF,wind

y

y

WD+MS binary“cataclysmic variable”

P~80min – 1day

NS/BH+MS binary“low mass X-ray binary”

(LMXB), P~1h - days

RLOFy

SNIay

SNIa

y

-ray bursts (GRB)

y

n