lars bildsten kavli institute for theoretical physics university of california santa barbara unusual...
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Lars BildstenKavli Institute for Theoretical
PhysicsUniversity of California Santa
Barbara
Unusual Binaries made via
Interactions
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70% of Massive Stars in Binaries
Kobulnicky et al. ‘14
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Stars get Bigger as they EvolvePaxton et al ‘11
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Eventually exceeding the Roche Radius
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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. .
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Evolved Stars: Red Giant Branch
• 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
Paxton et al. ‘11
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Binary Evolution Expectations
de Kool ‘92; de Kool & Ritter ‘93; Iben & Tutukov ’93. Politano ‘96
• 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 0.15-0.48 M • Large orbital period range relevant for this to occur, from a few days to over a year.
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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!
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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 ! !
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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
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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
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The New Helium WD
• Due to cooling, the challenge can be to find the Helium WD while the companion star remains bright.
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Around Millisecond Radio Pulsars . . .
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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
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Around Other White Dwarfs
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Eclipse of the He WDby the MS A star
Transit of the He WD
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Properties of the HE WD: Low Mass
PTF: Look how bright these are!!!!
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An iPTF Challenge to be met by fast cadence?
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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?
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Double White Dwarfs Galore
Kaplan, L.B. & Steinfadt ‘12
• 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
SDSS revealed a large population of Helium WDs
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Gravitational Waves Drive the Objects into Contact: Direct Impact destabilizes many of
the Double WDs Marsh, Nelemans & Steeghs ‘04
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Mass Transfer for Roche Lobe Filling Low Mass Helium WDs=> AM CVns
Kap
lan,
Bild
sten
& S
tein
fadt
‘12
MWD=0.8
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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.
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What’s up with this Constellation?Canes Venatici
• AM CVn: Interacting Doubel WDs
• RS CVn: interacting binaries with magnetic activity
• EL CVn: New binaries. ..
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Bootes Hunting Dogs!