Philippe Thébault Planet formation in binaries

Planet formation in binaries why bother? a majority of solar-type stars in multiple systems >90 detected exoplanets in binaries Test bench for planet-formation scenarios

Outline I Introduction - exoplanets and circumstellar discs in binaries - orbital stability II Planet formation: the different stages that can go wrong - disc truncation / grain condensation - embryo formation III Planetesimal accretion: the stage that goes really wrong IV Light at the end of the tunnel? V Circumbinary planets

>12% of detected extrasolar planets in multiple systems But... (Raghavan et al., 2006, Roel et al., 2012) Exoplanets in Binaries ~2-3% ( 4-5 systems ) ”interesting” cases in close binaries with a b ≈20AU I

(Raghavan et al., 2006) Gliese 86 HD 41004A γ Cephei (circumstellar) Exoplanets in Binaries I

more massive planets on short-period orbits around close (<100AU) binaries Desidera&Barbieri, 2007 short period planets Statistical analysis Are planets-in-binaries different? Roel et al., 2012 Different formation process?? (Duchene, 2010) Roel et al., 2012 I

Long-term stability analysis (Holman&Wiegert, 1999) (David et al., 2003) (Fatuzzo et al., 2006) I

M 1 /M 2 =0.25 a b = 19 AU e b =0.41 Stability regions: a few examples a P = 2 AU e P =0.12 M 1 /M 2 =0.35 a b = 21 AU e b =0.42 a P = 0.11 AU e P =0.05 M 1 /M 2 =0.56 a b = 18 AU e b =0.40 a P = 2.6 AU e P =0.48  Cephei HD Gl 86 I

Statistical distribution of binary systems (Duquennoy&Mayor, 1991) a 0 ~30 AU ~50% binaries wide enough for stable Earths on S-type orbits ~10% close enough for stable Earths on P-type orbits I

The « standard » model of planetary formation How could it be affected by binarity? Step by Step scenario: 1-protoplanetary disc formation √ 4-Planetesimal accretion √ 5-Embryo accretion √ 2-Grain condensation  3-formation of planetesimals x 6-Later evolution, resonances, migration √ II

Grain condensation (Nelson, 2000) II

Is there enough mass left to form planet(s)? Shorter viscous lifetime for discs in binaries Protoplanetary discs in binaries: theory tidal truncation of circumprimary & circumbinary discs Müller & Kley (2012) II Artymowicz & Lubow (1994)

Protoplanetary discs in binaries: observations Most single stars have 3-5 Myr to form giant planets, but most (but not all!) tight binaries have <1 Myr Different formation process?? (Kraus et al., 2012) Discs in close binaries do have shorter lifetimes and are fainter

Last stages of planet formation: embryos to planets (Guedes et al., 2008) Possible in almost the whole dynamically stable region (Barbieri et al. 2002, Quintana et al., 2002, 2007, Thebault et al. 2004, Haghighipour& Raymond 2007, Guedes et al., 2008,...) it takes a lot to prevent large embryos from accreting II

very last stages of planet formation: planetary core migration (Kley & Nelson, 2008) “under the condition that protoplanetary cores can form …, it is possible to evolve and grow a core to form a planet with a final configuration similar to what is observed” II

It doesn’t take much to stop planetesimal accretion V esc (1km) ~ 1-2m/s V ero (1km on 1km) ~ 10-20m/s dV runaway accretion V esc accretion (slowed down) V erosion erosion (no-accretion) 3 possible regimes : planetesimal accretion: Crucial parameter: impact velocity distribution III

(e,a) evolution: purely gravitational case secular oscillations with phased orbits no increase untill orbit crossing occurs  V  (e 2 + i 2 ) 1/2 V Kep III

M 2 =0.5M 1 e 2 =0.3 a 2 =20AU (Thebault et al., 2006)) III

(e,a) evolution: with gas t final =5x10 4 yrs 1km<R<10km differential orbital phasing according to size III

5km planetesimals 1km planetesimals Differential orbital alignement between objects of different sizes typical gas drag run dV increase (Thebault et al., 2006) III (Thebault et al., 2006))

distribution at 1AU from α Cen A and at t=10 4 yrs high as soon as R 1 ≠R 2 (Thebault et al., 2008) III

Benz&Asphaug, 1999 Critical fragmentation Energy (Q*) conflicting estimates III

Accretion/Erosion behaviour V ero2 <dV erosion V ero1 <dV<V ero2 unsure V esc <dV<V ero1 perturbed accretion V esc <dV<V ero1 ”normal” accretion (Thebault et al., 2008) III at 1AU from α Cen A and at t=10 4 yrs

 Centauri B ”nominal case” erosion unsure perturbed accretion ”normal” accretion III (Thebault et al., 2009) HZ New Planet ! 0.04AU

HD III (Thebault, 2011) PARADOX? Planet At least 2 exoplanets are located in accretion-hostile regions

“big” (10-50km) planetesimals ? at 1AU from the primary and at t=10 4 yrs IV

large initial planetesimals? how realistic is a large « initial » planetesimals population? depends on planetesimal-formation scenario -> maybe possible if quick formation by instabilities (for ex. model of Johanssen 2007) but how do instabilities. proceed in the dynamically perturbed environment of a binary? ->more difficult if progressive sticking always have to pass through a km-sized phase in any case, it cannot be « normal » (runaway) accretion -> « type II » runaway? (Kortenkamp, 2001) IV

evolving gas disc coupled hydro/N-body simulations always higher than in the axisymmetric gas disc case! Paardekooper, Thebault & Mellema, 2008 IV

The role of the gas disc’s gravity dv are increased with respect to the gas-drag-only cases High dv even for equal-sized planetesimals Fragner, Nelson & Kley (2011) Inclined disc & circular binary IV

outward migration after the formation of embryos Payne, Wyatt &Thébault (2009) IV

different initial binary configuration?  most stars are born in clusters early encounters and binary compaction/exchanges are possible: Initial and final (e,a) for binaries in a typical cluster (Malmberg et al., 2007) IV

different initial orbit for the binary? Thebault et al., 2009 IV

a slightly inclined binary might help (Xie & Zhou, 2009) IV Favours accretion-friendly impacts between equal-sized bodies

a slightly inclined binary might help….but IV Xie & Zhou, 2009 …collision *rates* decrease dramatically

“realistic” treatment of collisions Paardekooper & Leinhardt (2010) Collisions prevent the onset of size-phased orbits The production of collisional fragments favours growth by « dust » sweeping IV HZ

Conclusions Gas drag works against planetesimal accretion In coplanar systems, in-situ planet formation is difficult in the HZ of binaries with ~20AU separation Outward migration of embryos by  a/a ~ 0.25 is possible Moderate 1<i B <10 o helps, but slows down the accretion ~50% (?) chance that a 20AU binary was initially wider Fragment production and dust sweeping might help Different, binary-specific planet-formation scenario? Instabilities?

Circumbinary planets: observations Most planets are close to the inner orbital stability limit V

Circumbinary planets: modelling Paardekooper et al.(2012) no dust accretion with dust accretion even in the most favourable case, no in-situ accretion for the circumbinary planets …but inward type I or type II migration might solve the problem…and also explain the current location of the planets close to the inner stability limit V

FIN