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Planet formation in binaries Philippe Thébault 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 I Exoplanets in Binaries >12% of detected extrasolar planets in multiple systems But... ~2-3% (4-5 systems) ”interesting” cases in close binaries with ab≈20AU (Raghavan et al., 2006, Roel et al., 2012) I (circumstellar) Exoplanets in Binaries Gliese 86 HD 41004A γ Cephei (Raghavan et al., 2006) I Statistical analysis Are planets-in-binaries different? Roel et al., 2012 Roel et al., 2012 Desidera&Barbieri, 2007 more massive planets on short-period orbits around close (<100AU) binaries Different formation process?? (Duchene, 2010) short period planets I Long-term stability analysis acrit 0.46 0.38 0.63eb 0.59eb 0.15eb2 0.20eb2 ab (Holman&Wiegert, 1999) (David et al., 2003) (Fatuzzo et al., 2006) I M1/M2=0.56 ab= 18AU eb=0.40 Stability regions: a few examples aP= 0.11AU eP=0.05 Gl 86 M1/M2=0.35 ab= 21AU eb=0.42 M1/M2=0.25 ab= 19AU eb=0.41 aP= 2.6AU eP=0.48 aP= 2AU eP=0.12 HD196885 Cephei I Statistical distribution of binary systems a0 ~30 AU ~50% binaries wide enough for stable Earths on S-type orbits ~10% close enough for stable Earths on P-type orbits (Duquennoy&Mayor, 1991) II The « standard » model of planetary formation How could it be affected by binarity? •Step by Step scenario: √ 1-protoplanetary disc formation 2-Grain condensation x 3-formation of planetesimals √ 4-Planetesimal accretion √ 5-Embryo accretion √ 6-Later evolution, resonances, migration II Grain condensation (Nelson, 2000) II Protoplanetary discs in binaries: theory Artymowicz & Lubow (1994) tidal truncation of circumprimary & circumbinary discs Müller & Kley (2012) •Is there enough mass left to form planet(s)? •Shorter viscous lifetime for discs in binaries Protoplanetary discs in binaries: observations Discs in close binaries do have shorter lifetimes and are fainter (Kraus et al., 2012) Most single stars have 3-5Myr to form giant planets, but most (but not all!) tight binaries have <1 Myr Different formation process?? II Last stages of planet formation: embryos to planets (Barbieri et al. 2002, Quintana et al., 2002, 2007, Thebault et al. 2004, Haghighipour& Raymond 2007, Guedes et al., 2008,...) Possible in almost the whole dynamically stable region it takes a lot to prevent large embryos from accreting (Guedes et al., 2008) II very last stages of planet formation: planetary core migration “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” (Kley & Nelson, 2008) III planetesimal accretion: Crucial parameter: impact velocity distribution 3 possible regimes : •dV < Vesc => runaway accretion •Vesc< dV < Verosion => accretion (slowed down) •Verosion < dV => erosion (no-accretion) It doesn’t take much to stop planetesimal accretion •Vesc(1km) ~ 1-2m/s •Vero(1km on 1km) ~ 10-20m/s III (e,a) evolution: purely gravitational case secular oscillations with phased orbits V (e2 + i2)1/2 VKep no <dV> increase untill orbit crossing occurs III M2=0.5M1 e2=0.3 a2=20AU (Thebault et al., 2006)) III (e,a) evolution: with gas 1km<R<10km tfinal=5x104yrs differential orbital phasing according to size III typical gas drag run (Thebault et al., 2006) (Thebault et al., 2006)) 5km planetesimals 1km planetesimals Differential orbital alignement between objects of different sizes dV increase III <dV(R1,R2)> distribution (Thebault et al., 2008) high <dV> as soon as R1≠R2 at 1AU from α Cen A and at t=104yrs III Benz&Asphaug , 1999 Critical fragmentation Energy (Q*) conflicting estimates III Accretion/Erosion behaviour (Thebault et al., 2008) Vero2<dV erosion Vero1<dV<Vero2 unsure Vesc<dV<Vero1 perturbed accretion Vesc<dV<Vero1 ”normal” accretion at 1AU from α Cen A and at t=104yrs a Centauri B III New Planet ! erosion HZ perturbed accretion unsure ”normal” accretion 0.04AU (Thebault et al., 2009) ”nominal case” III HD196885 PARADOX? Planet At least 2 exoplanets are located in accretion-hostile regions (Thebault, 2011) IV “big” (10-50km) planetesimals ? at 1AU from the primary and at t=104yrs 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. 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) model IV coupled hydro/N-body simulations Paardekooper, Thebault & Mellema, 2008 <dv> always higher than in the axisymmetric gas disc case! evolving gas disc IV The role of the gas disc’s gravity Fragner, Nelson & Kley (2011) Inclined disc & circular binary • dv are increased with respect to the gas-drag-only cases • High dv even for equal-sized planetesimals 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) Favours accretion-friendly impacts between equal-sized bodies IV a slightly inclined binary might help….but Xie & Zhou, 2009 …collision *rates* decrease dramatically IV “realistic” treatment of collisions • Collisions prevent the onset of size-phased orbits • The production of collisional fragments favours growth by « dust » sweeping HZ HZ Paardekooper & Leinhardt (2010) 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<iB<10o 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? V Circumbinary planets: observations • Most planets are close to the inner orbital stability limit V Circumbinary planets: modelling Paardekooper et al.(2012) no dust accretion even in the most favourable case, no in-situ accretion for the circumbinary planets with dust accretion …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 FIN