Mass Transfer in Binaries - University of...
Transcript of Mass Transfer in Binaries - University of...
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
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])
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)
convective
radiative
radiative
convective
Podsiadlowski (2002)
Podsiadlowski et al. (2002)
Podsiadlowski et al. (2002)
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?)
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)
.
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?
Sawada et al. (1984)
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
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?
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)
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?
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?
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)
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
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