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_m
arco_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 branch
7-10: core He burning
8-9: envelope contraction
Common Envelope:
A twice-in-a-lifetime opportunity
R
AGB: R ~ 500-1500 Ro
R
RGB: R ~100-300 Ro
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
Earth-Sun
L1: SOHO
L2: Gaia, WMAP, JWT
– 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
Binary configurations and mass transfer
1
2
Binary star configurations
and mass transfer
Detached: mass transfer via wind
Semidetached: mass transfer via Roche
lobe overflow
Contact
Interactions in close binaries – 3 effects
1. Distortion of the star(s) from
spherical shape: ellipsoidal
modulation (bright when seen
from sides)
2.
Donor
WD
Gravity darkening
lower gravity
light variations due
to secondary
distortion and
gravity darkening
eclipse
hotter
3.
Irradiation & heating: reflection effect
Unstable Roche
Lobe overflow
Common envelope
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
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.
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
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 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
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
Buonanno et al. 1994, A&A, 290, 69
• Sandage (1953) noted that a
few stars in M3 appeared blue- • Origins?
ward and above MSTO
– HB stars crossing the
MS?
• Apparently normal MS stars of
– More recent star
luminosity and mass greater
formation?
than those currently evolving
– Mergers
toward the red giant phase
• Mass transfer
• Binary coalescence
• Common in globular clusters
• Collisions
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
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.
RR Tel
1.
Munari & Zwitter 2002, A&A 383, 188
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
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
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 zoology
M1>M2, M1 evolves first. Wide binary?
y
No interaction, evolve as single stars.
n
common envelope
wind accretion
common envelope
y
common envelope
WD+WD
P~hours - days
y
“High mass X-ray binary”
(HMXB), P~days - months
RLOF,wind
red giant
mass donor
“symbiotic stars”
P~weeks - years
detached
WD/NS/BH + MS
binary
P~days - years
y
RLOF
WD+MS binary
NS/BH+MS binary
“cataclysmic variable” “low mass X-ray binary”
P~80min – 1day
(LMXB), P~1h - days
y
NS+NS
SNIa
y
SNIa
y
WD+BD binary
-ray bursts (GRB)