Transcript iptfsummer2014bildsten

Unusual Binaries
made via
Interactions
Lars Bildsten
Kavli Institute for Theoretical
Physics
University of California Santa
Barbara
70% of Massive Stars in Binaries
Kobulnicky et al. ‘14
Stars get Bigger as they Evolve
Paxton et al ‘11
Eventually exceeding the Roche
Radius
Stellar Response vs. Roche Radius
• Since the more massive star evolves first, it
will be the one to first fill the Roche Radius
• The resulting transfer is thus from massive to
less massive star (e.g. a 2M to a 1M ).
• Secular stability requires that the donor star
stay within the Roche radius as mass is lost.
• If not stable, then likely some excitement. .
Evolved Stars: Red Giant Branch
Paxton et al. ‘11
• M< 2 M develop degenerate Helium cores that
increase in mass with time until ignition in a flash
=> lifting degeneracy => stable He burning in core
Binary Evolution Expectations
• Binaries
tight enough
so that the <2M
primary fills the Roche
lobe on the first ascent
of the red giant branch
will have a degenerate
He core of mass 0.150.48 M
• Large orbital period
range relevant for this
to occur, from a few
days to over a year.
de Kool ‘92; de Kool & Ritter ‘93; Iben & Tutukov ’93. Politano ‘96
Changes in Orbital Parameters
• Presume that mass and angular momentum are
preserved during mass transfer. Ignoring spin, the
total angular momentum is:
• This leads to a simple change in separation due to
mass transfer of, from 2=> 1 , and
• So, since M2>M1 and M2 decreases, orbit shrinks!
Changes in Roche Radius
• Using the simpler form for the Roche Radius, we
get:
• The resulting change in the Roche Radius around
donor star (2), is then
• When M2>5M1/6 and M2 decreases, Roche Lobe
shrinks. . . Star MUST shrink even more, otherwise
a runaway situation occurs ! !
What’s the Story?
• More massive star evolves to become a red giant, and
fills the Roche Lobe
• To diagnose the stability of this mass transfer, we need to
know how the RGB responds relative to Roche Radius
• IF RGB is ‘in equilibrium’ during mass transfer, then the
radius is nearly constant and
• IF RGB star expands like the adiabat, then the constraint
is stricter
• In either case, let’s only consider the case where the
mass ratio is intrinsically unstable
Common Envelope Evolution
• The runaway mass transfer
leads to the construction of
a ``common” envelope
• Calculation is to unbind the
envelope with the energy
lost due to spiral-in of the
two ‘core’ stars, in this case
the He WD core and the MS
star
• Remnant is an exposed He
core
The New Helium WD
• Due to cooling, the challenge can be to find the
Helium WD while the companion star remains
bright.
Around Millisecond Radio Pulsars .
..
Around Millisecond Radio Pulsars . . .
• Derived period-core
mass relation
implies a prolonged
period of mass
transfer, likely due
to NS being massive
enough to allow for
stable mass transfer
Around Other White Dwarfs
Transit of the He WD
Eclipse of the He WD
by the MS A star
Properties of the HE WD: Low Mass
PTF: Look how bright these are!!!!
An iPTF Challenge to be met by fast
cadence?
What Happens Next for These?
• He WD cools, continues to transit the star, but
the other part of the eclipse becomes less
important. . . Find these in iPTF?
• The lower mass star then evolves, and a similar
unstable mass transfer will occur, revealing yet
another He WD, most likely in a very tight binary.
• Gravitational waves can then bring these
together. . . see later
• How can iPTF probe all of these populations in a
more robust manner than WASP due to depth
and higher cadence?
Double White Dwarfs Galore
SDSS revealed a large population of Helium WDs
• These stay bright
due to a stably
burning H envelope
(Panei et al ’07)
• Many found to be
with WDs in tight
orbits (Badenes,
Brown, Kilic, Mullally,
Steinfadt. .
• Many will reach
contact in 10 Gyr
• Note lack of binaries
with Mt>1.4
Kaplan, L.B. & Steinfadt ‘12
Gravitational Waves Drive the Objects into
Contact: Direct Impact destabilizes many of
the Double WDs
Marsh, Nelemans & Steeghs ‘04
Kaplan, Bildsten & Steinfadt ‘12
Mass Transfer for Roche Lobe Filling
Low Mass Helium WDs=> AM CVns
MWD=0.8
Summary
• Roche Lobe filling during evolution creates unusual
binaries, you will hear from Paula and me on CVs, a
class of stable mass transfer of H Rich material
• Galactic Variability surveys can find the ‘long-lived’
precursors to binary types that may become
explosive later in life (e.g. Type Ia SNe), or
eventually merge to create unusual stars (e.g. R
Cor Bor).
• As noted in the EL CVns discussion, I hope that iPTF
and, later, ZTF can sample binaries at adequate
depth to see the ‘same’ system at different stages
of evolution.
What’s up with this Constellation?
Canes Venatici
• AM CVn: Interacting
Doubel WDs
• RS CVn: interacting
binaries with
magnetic activity
• EL CVn: New
binaries. ..
Bootes Hunting Dogs!