Lars BildstenKavli Institute for Theoretical

PhysicsUniversity of California Santa

Barbara

Unusual Binaries made via

Interactions

70% of Massive Stars in Binaries

Kobulnicky et al. ‘14

Stars get Bigger as they EvolvePaxton 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

• 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

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.

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

Eclipse of the He WDby the MS A star

Transit of the He WD

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

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

Gravitational Waves Drive the Objects into Contact: Direct Impact destabilizes many of

the Double WDs Marsh, Nelemans & Steeghs ‘04

Mass Transfer for Roche Lobe Filling Low Mass Helium WDs=> AM CVns

Kap

lan,

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‘12

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!