mass transfer in binaries - university of...
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Mass Transfer in BinariesPhilipp Podsiadlowski (Oxford)
• Understanding the physics of mass transfer is essential
for understanding binary evolution
• Simplest assumption: stable, “conservative” mass
transfer in a circular system from a synchronized,
Roche-lobe-filling donor with a ‘sharp’ surface boundary
I. Observational Constraints
II. Some basic principles
III. Key Issues
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Observational Constraints
Symbiotic Binaries (S-type)
• should not exist
⊲ orbital periods are not explained
by simple binary evolution
⊲ tend to have mass ratios that
should lead to dynamically
unstable mass transfer
Hot Subdwarfs (sdBs)
• H-deficient, He-core burning,
low-mass stars (0.5 M⊙) with
well-defined history
→ ideal for testing both stable (wide
sdBs) and unstable (short-periods
sdBs) mass transfer
X-ray binaries
• observed X-ray luminosities much larger
than expected (irradiation effects?)
• the case of Cygnus X-2: an
intermediate-mass X-ray binary that
survived mass transfer with M ∼> 103 ˙MEdd
• the origin of low-mass black-hole binaries
• Super-Eddington accretion
Mass transfer in eccentric binaries
• VV Cephei systems: stable mass transfer
from red to blue supergiants with e ∼> 0.5
• recent: wide sdB binaries (post-RLOF
systems) have moderate eccentricities
(Østensen & Van Winckel [2012]; Deca
[2012]; Wade, Barlow [2012])
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Some Basic Principles
The radius evolution
• M is determined by the relative evolution
of the donor’s radius and the Roche-lobe
radius (or equivalent)
⊲ difference between stars with radiative
and convective envelopes → different
response to rapid mass loss
⊲ RRL depends on mass ratio and
angular-momentum loss
Mass-driving mechanisms
• Evolutionary-driven mass loss
⊲ nuclear evolution (slow phases)
⊲ thermal evolution (Hertzsprung gap;
donors forced out of thermal
equilibrium)
⊲ irradiation-driven evolution
(mass-transfer cycles in L/IMXBs?)
• Evolution driven by systemic
angular momentum loss
⊲ gravitational radiation (well
understood)
⊲ magnetic braking (poorly
understood)
Angular Momentum
• accounting for the angular
momentum of all the components
(donor, accretor, disk, systemic mass
loss) is essential for understanding
the evolution of binaries (orbital
evolution, stability of mass transfer)
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convective
radiative
radiative
convective
Podsiadlowski (2002)
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Podsiadlowski et al. (2002)
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Podsiadlowski et al. (2002)
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The role of non-conservative masstransfer
• mass transfer is often very
non-conservative
• angular-momentum loss affects orbital
evolution
⊲ different prescriptions give very
different outcomes (e.g. can
stabilize/destabilize mass transfer)
⊲ no good theoretical model, weak
observational constraints
• sdB binaries: mass transfer in stable
systems has to be very non-conservative
to produce short-period sdB binaries
with WD companions (Han et al.
2002/2003)
• observed mass loss modes:
⊲ bipolar mass loss from the accreting
component (also Cyg X-2)
⊲ disk-like outflow (from accretion disk
or system?)
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The criterion for dynamical mass transfer
• dynamical mass transfer is caused by a
mass-transfer runaway (giant expands, Roche lobe
shrinks)
⊲ for n = 1.5 polytrope:
q > qcrit = Mdonor/Maccretor = 2/3
• real stars have core-envelope structures
(Hjellming & Webbink 1987; Ge et al. 2010)
• the outer layer is non-adiabatic (e.g., Tauris,
Podsiadlowski, Han, Chen, Passy)
⊲ real stars: qcrit ≃ 1.1 − 1.3 for
(non-conservative; much smaller qcrit for
conservative case [Chen & Han 2008])
• tidally enhanced mass loss (CRAP) (Eggleton,
Tout)
• break-down of mixing-length theory before mass
transfer becomes dynamical (Paczynski &
Sienkiewicz 1972; → Pavlovskii)
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.
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Common-envelope evolution and ejection
• dynamical mass transfer leads to a CE and
spiral-in phase
• if envelope is ejected → short-period binary
(Paczynski 1976)
• CE ejection criterion?
• qualitatively: αCE |∆Eorb| > Eenv
• energy criterion (necessary, but not sufficient)
• other possible energies
⊲ recombination energy
⊲ accretion energy
⊲ nuclear energy (possibility of explosive CE
ejection)
• long-lived initial phase in synchronized binary
→ pre-expansion?
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Sawada et al. (1984)
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Atmospheric RLOF
• some symbiotics show ellipsoidal light
curve variations (Miko lajewska,
Gromadzki)
→ Roche-lobe filling (or at least close)
despite large mass ratio (∼> 3)
• M ∝ exp[−(RRL − R)/Ratm] (e.g.
Ratm = HP; Ritter 1988)
• real giants: Ratm ≫ HP
• RLOF of extended atmosphere (e.g.
Pastetter & Ritter 1989)
• short-lived phase (up to 105 yr)
• important to understand for estimating
rates of symbiotics
symbiotic phase
Chen et al. (2010)
⊲ MRG = 1.5 M⊙, MWD = 0.75 M⊙
⊲ Pinorb = 300 d
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The Orbital Period Distribution of S-TypeSymbiotics with WDs
• orbital period range: 200 – 1400 d
Problem:
⊲ these systems must have experienced a previous
mass-transfer phase
⊲ most likely dynamically unstable mass transfer
(common-envelope [CE] phase) → spiral-in phase →
much closer orbits expected
⊲ or stable mass transfer, which should led to a
widening of the systems
• need stable mass transfer with a lot of mass loss and
little orbital shrinkage (Webbink 1986)
• the role of circumbinary disks (formation?)
Main Goal:
• understand the evolutionary connection between
different types of binaries
e.g.: AGB mass transfer → circumbinary disks → post-AGB
binaries (pre-symbiotics) → S-type symbiotics → Type
Ia supernovae?
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Quasi-dynamical mass transfer?
• need a different mode of mass
transfer (Webbink, Podsiadlowski)
• very non-conservative mass transfer
but without significant spiral-in
• also needed to explain the properties
of double degenerate binaries
(Nelemans), υ Sgr, etc.
• transient CE phase or circumbinary
disk (Frankowski, Dermine)?
Transient Common-Envelope Phase
(Podsiadlowski et al. 1992)
• q ∼> qcrit: temporary (∼ 104 yr) CE phase
with moderate spiral-in (no differential
rotation!) (similar to γ-mechanism
proposed by Nelemans)
⊲ moderate shrinking of orbit (as implied
by observations; Miko lajewska)
⊲ accretion of RG/AGB material?
(observations!)
⊲ formation of circumbinary disk (→
eccentric post-AGB binaries, barium
stars [Dermine & Jorissen]) (outflow
from L2/L3 or left-over CE)
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Pols (1994)
The Early Case B Problem
• mass transfer in the Hertzsprung
gap (radiative envelopes) is
dynamically stable for large mass
ratios: qcrit ∼ 3 − 4 (e.g., Eggleton,
Han, Podsiadlowski, . . .)
• but: the accretor cannot ‘accept’
transferred mass (Pols 1994;
Wellstein & Langer 2001, . . .) →
contact phase even for q quite close
to 1
• → transient contact phase or
merger?
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Non-Synchronicity
• for large mass ratio, synchronization
is impossible
• origin of the Darwin instability
• modified ‘Roche-lobe’ radius (e.g.
Avni 1982)
• but: depends on angular momentum
transport inside the tidally forced
star
Eccentricity
• post-RLOF sdBs have moderate
eccentrities
• incomplete circularization even for
q ∼< 2?
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Kippenhahn & Meyer-Hofmeister (1977)
Petrovic, Langer & van der Hucht (2005)
The Role of the Accreting Star
• the accreting star expands if
tacc > tenvtherm (depends on entropy of
the accreted material; e.g. Shaviv;
Stahler [80s])
• a star only has to accrete a few % of
its total mass to be spun up to
critical surface rotation (Packet
1981)
• what happens to the angular
momentum?
⊲ angular momentum transport
inside accretor
⊲ mass loss from the system
(Langer et al.)
⊲ feedback to the orbit: the role of
the disk (e.g. Paczynski; Marsh)
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The Symbiotic Binary Mira AB
• wide binary (Porb ∼ 400 yr), consisting of
Mira A (Ppuls ≃ 330 d) and an accreting
white dwarf
• M ∼ 10−7 M⊙ yr−1
Observations:
• soft X-rays (Chandra, Karovska et al.
2005) from both components (shocks in
the wind of Mira A and from accretion
disk)
• the envelope of Mira is resolved in X-rays
and the mid-IR (Marengo et al. 2001)
⊲ the slow wind from Mira A fills its
Roche lobe (RRL ∼ 25 AU)
⊲ but: radius of Mira A: 1 – 2 AU
• a new mode of mass transfer(?): wind
Roche-lobe overflow
• important implications for D-type sym-
biotics
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Wind Roche-Lobe Overflow
• a new mass-transfer mode for wide
binaries
• high mass-transfer fraction (compared
to Bondi-Hoyle wind accretion) → more
efficient accretion of s-process elements
for the formation of barium stars
(without circularization)
• accretion rate in the regime where WDs
can accrete? → increase the range for
SN Ia progenitors (but may not be
efficient enough)
• asymmetric system mass loss →
formation of circumstellar disks and
bipolar outflows from accreting
component (e.g. OH231.8+4.2)
→ shaping of (proto-)planetary nebulae
⊲ binaries with longer orbital periods
important
Case D Mass Transfer
• extension of case C mass transfer,
but potentially more important
(possibly larger orbital period range)
• also: massive, cool supergiants with
dynamically unstable envelopes (e.g.
Yoon & Langer)
• large mass loss just before the
supernova?
• possible implications for Type II-L,
IIb supernovae (increases rate
estimates), SN 2002ic
• delays onset of dynamical mass
transfer
→ produces wider S-type
symbiotic binaries (i.e. solve
orbital period problem)
→ solve the problem of black-hole
binaries with low-mass
companions