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)
Gänsicke (
Belyanin ( notes 17.ppt)
And references as noted on the slides

3. The Theoretical HR Diagram
Turn-off age
Mass

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. A twice-in-a-lifetime opportunity
Common Envelope:
A twice-in-a-lifetime opportunity R
AGB: R ~ Ro
And this one … R
RGB: R ~ Ro

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.
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
Earth-Sun
L1: SOHO
L2: Gaia, WMAP, JWT

7. Binary configurations and mass transfer

8. Binary star configurations and mass transfer1 2
Detached: mass transfer via wind
Semidetached: mass transfer via Roche
lobe overflow
Contact

9. Interactions in close binaries – 3 effects
Distortion of the star(s) from spherical shape: ellipsoidal modulation (bright when seen from sides) WD
Donor
Gravity darkening
lower gravity
light variations due to secondary
distortion and gravity darkening
eclipse
hotter
Irradiation & heating: reflection effect

10. Unstable Roche Lobe overflow
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
Common envelope
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
Depending on the efficiency of the energy transfer from the companion to the CE (), one might get:
stable mass transfer - the Roche-lobe of the mass donor grows as a consequence of its mass loss, stopping the mass transfer
A short-period binary, or… a merged star

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.
Mass Loss
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.

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. Algol Binaries
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

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 (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)

16. Blue Stragglers
Buonanno et al. 1994, A&A, 290, 69
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

17. Anomalous (or Dwarf) Cepheids
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
May be causally related to blue stragglers
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
Mass
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.
Munari & Zwitter 2002, A&A 383, 188
RR Tel
Pulsating red giant star and a compact, hot white dwarf star binary
The red giant is losing mass. The white dwarf concentrates the wind into an accretion disk
Nova outburst. The hot gas forms a pair of expanding bubbles above and below the equatorial disk.
4. Process repeats

20. Extreme Blue HB stars and sdB Binaries
NGC 6791
[Fe/H[ = +0.4
Age > 8 Gyr
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

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
M1>M2, M1 evolves first. Wide binary? No interaction, evolve as single stars. y
common envelope n
common envelope
wind accretion
“High mass X-ray binary”
(HMXB), P~days - months
common envelope
WD+WD
P~hours - days y
red giant
mass donor
“symbiotic stars”
P~weeks - years
RLOF,wind y
NS+NS
detached
WD/NS/BH + MS binary
P~days - years
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
-ray bursts (GRB) y
SNIa y
WD+BD binary