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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.

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Presentation on theme: "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."— Presentation transcript:

1 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

2 Includes material from: Lisker ( Luhman ( Orsela de Marco ( arco_talk.ppt) arco_talk.ppt Gänsicke ( Belyanin ( notes 17.ppt) notes 17.ppt And references as noted on the slides

3 The Theoretical HR Diagram Turn-off ageMass

4 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 branch 7-10: core He burning 8-9: envelope contraction

5 RGB: R ~ R o AGB: R ~ R o Common Envelope: A twice-in-a-lifetime opportunity R R

6 Roche Lobes Lagrange 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. L 1 - Inner Lagrange Point – in between two stars – matter can flow freely from one star to other – mass exchange L 2 - on opposite side of secondary – matter can most easily leave system L 3 - on opposite side of primary L 4, L 5 - in lobes perpendicular to line joining binary Roche-lobes: surfaces which just touch at L 1 – maximum size of non-contact systems L 1 – L 3 are unstable - a small perturbation will lead the material to leave the L-point L 4&5 are stable, i.e. material will return to its initial position following a small perturbation L 1 : SOHO L 2 : Gaia, WMAP, JWT Earth-Sun

7 Binary configurations and mass transfer

8 Binary star configurations and mass transfer Detached: mass transfer via wind Semidetached: mass transfer via Roche lobe overflow Contact 1 2

9 Interactions in close binaries – 3 effects 1.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 secondary distortion and gravity darkening

10 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

11 Accretion If 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

12 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

13 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, Ritter Obs. archive

14 ● 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 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

15 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 (10 6 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)

16 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

17 Anomalous (or Dwarf) Cepheids May be causally related to blue stragglers ● Found primarily in dwarf spheroidals (and globular clusters) ● Pulsation periods less than 1.5 d ● Absolute magnitudes 0.5 > M V > -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 M Sun McCarthy & Nemec 1997, ApJ, 482, 203

18 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

19 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

20 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[ = +0.4 Age > 8 Gyr

21 Formation of a white dwarf/main sequence binary

22 2 CE: Formation of a millisecond pulsar

23 2CE: Formation of WD-WD binaries WD-WD merger: supernova type Ia

24 Binary star zoology M 1 >M 2, M 1 evolves first. Wide binary? No interaction, evolve as single stars. y common envelope wind accretion “High mass X-ray binary” (HMXB), P~days - months detached WD/NS/BH + MS binary P~days - years common envelope WD+WD P~hours - days WD+BD binary NS+NS red giant mass 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 RLOF y SNIa y y  -ray bursts (GRB) y n

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