Philippe Thébault

1. Planet formation in binaries Philippe Thébault

2. Planet formation in binaries why bother? • a majorityof solar-type stars in multiple systems • >90 detectedexoplanets in binaries • Test bench for planet-formation scenarios

3. Outline • I Introduction • - exoplanets and circumstellardiscs in binaries • - orbitalstability • IIPlanet formation: the different stagesthatcan go wrong • - disctruncation / grain condensation • - embryo formation • IIIPlanetesimalaccretion: the stagethat goes reallywrong • IVLight at the end of the tunnel? • VCircumbinary planets

4. I Exoplanets in Binaries >12% ofdetectedextrasolar 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)

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

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

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

8. I Stability regions: a few examples M1/M2=0.56 ab= 18AU eb=0.40 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  Cephei HD196885

9. Statistical distribution of binarysystems I 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)

10. 1-protoplanetary disc formation √ 2-Grain condensation  3-formation of planetesimals x 4-Planetesimal accretion √ 5-Embryo accretion √ 6-Later evolution, resonances, migration √ II The « standard » model of planetary formation How could it be affected by binarity? • Step by Step scenario:

11. II Grain condensation (Nelson, 2000)

12. Protoplanetary discs in binaries: theory II Artymowicz & Lubow (1994) tidaltruncationofcircumprimary & circumbinarydiscs Müller & Kley (2012) • Is thereenoughmassleftto form planet(s)? • Shorterviscouslifetime for discs in binaries

13. 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??

14. Last stages of planet formation: embryos to planets II (Barbieri et al. 2002, Quintana et al., 2002, 2007, Thebault et al. 2004, Haghighipour& Raymond 2007, Guedes et al., 2008,...) Possible in almost the wholedynamicallystable region it takesa lottopreventlarge embryos from accreting (Guedes et al., 2008)

15. very last stages of planet formation: planetary core migration II “under the condition that protoplanetarycores 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)

16. 3 possible regimes : • dV < Vesc=> runawayaccretion • Vesc< dV < Verosion=> accretion (slowed down) • Verosion < dV=> erosion (no-accretion) III planetesimalaccretion: Crucial parameter: impact velocity distribution It doesn’t take much to stop planetesimal accretion • Vesc(1km) ~ 1-2m/s • Vero(1km on 1km) ~ 10-20m/s

17. 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

18. III M2=0.5M1 e2=0.3 a2=20AU (Thebault et al., 2006))

19. III (e,a) evolution: withgas 1km<R<10km tfinal=5x104yrs differential orbital phasing according to size

20. dV increase 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

21. <dV(R1,R2)> distribution III (Thebault et al., 2008) high <dV> as soon as R1≠R2 at 1AU from α Cen A and at t=104yrs

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

23. Accretion/Erosion behaviour III (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

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

25. III HD196885 PARADOX? Planet At least 2 exoplanetsarelocated in accretion-hostile regions (Thebault, 2011)

26. “big” (10-50km) planetesimals ? IV at 1AU from the primary and at t=104yrs

27. 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)

28. evolving gas disc coupled hydro/N-body simulations IV Paardekooper, Thebault & Mellema, 2008 <dv> alwayshigherthan in the axisymmetric gas disccase!

29. IV The role of the gas disc’s gravity Fragner, Nelson & Kley (2011) Inclineddisc & circularbinary • dv areincreasedwithrespectto the gas-drag-onlycases • High dv even for equal-sizedplanetesimals

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

31. 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)

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

33. IV a slightly inclined binary might help (Xie & Zhou, 2009) Favoursaccretion-friendlyimpactsbetweenequal-sizedbodies

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

35. 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)

36. Conclusions • Gas drag worksagainstplanetesimalaccretion • In coplanar systems, in-situ planet formation is difficult in the HZ ofbinarieswith ~20AU separation • Outward migration of embryos by a/a ~ 0.25 is possible • Moderate 1<iB<10ohelps, butslows down the accretion • ~50% (?) chancethat a 20AU binarywasinitiallywider • Fragment production and dust sweepingmighthelp • Different, binary-specific planet-formation scenario? Instabilities?

37. Circumbinary planets: observations V • Most planets arecloseto the inner orbitalstability limit

38. Circumbinary planets: modelling V 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

39. FIN