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
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