Philippe Thébault Paris Observatory

1. Planet formation in binaries Philippe Thébault Paris Observatory

2. Planet formation in binaries why bother? • a majority of stars in multiple systems • >80 detected exoplanets in binaries • testbed for planet-formation scenarios

3. Outline • I Introduction • - exoplanets in binaries • - orbital stability • II Planet formation: the different stages that can go wrong • - disc truncation / grain condensation • - embryo formation • III Planetesimal accretion • IV Light at the end of the tunnel?

4. Exoplanets in Binaries ~80 planets in binaries (Desidera & Barbieri, 2007)

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

6. Exoplanets in Binaries ~23% of detected extrasolar planets in multiple systems But... ~2-3% (4-5 systems) in close binaries with ab<30AU (Raghavan et al., 2006, Desidera&Barbieri, 2007)

7. Statistical analysis Are planets-in-binaries different? Desidera&Barbieri, 2007 long period planets short period planets more massive planets on short-period orbits around ”close-in”(<75AU) binaries Duchene (2010)

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

9. 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

10. 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)

11. 1-protoplanetary disc formation √ 2-Grain condensation  3-formation of planetesimals x 4-Planetesimal accretion √ 5-Embryo accretion √ 6-Later evolution, resonances, migration √ The « standard » model of planetary formation How could it be affected by binarity? • Step by Step scenario:

12. Grain condensation (Nelson, 2000)

13. Protoplanetary discs in binaries: theory tidal truncation of circumprimary & circumbinary discs Jang Condell et al. (2008) • Is there enough mass left to form planet(s)? • Lifetime of a truncated disc?

14. (Jensen et al., 1996) (Andrews & Williams, 2005) model fit with Rdisc<0.4ab model fit with Rdisc<0.2ab but high fdust compact disc might be optically thick => Mdust fdust Protoplanetary discs in binaries: Observations Depletion of mm-flux for binaries with 1<a<50AU

15. Protoplanetary discs in binaries (Cieza et al., 2009) 10AU threshold for inner disc presence reduced disc frequency or reduced disc lifetime?

16. 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)

17. very last stages of planet formation: planetary core migration “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)

18. 3 possible regimes : • dV < Vesc=> runaway accretion • Vesc< dV < Verosion=> accretion (slowed down) • Verosion < dV=> erosion (no-accretion) planetesimal accretion: 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

19. (e,a) evolution: purely gravitational case secular oscillations with phased orbits V  (e2 + i2)1/2 VKep no <dV> increase untill orbit crossing occurs

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

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

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

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

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

25. 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 the primary and at t=104yrs

26. a Centauri B erosion perturbed accretion unsure ”normal” accretion ”nominal case”

27. simplifications • Staticaxisymmetric gas disc • Initial eplanetesimals=0 • tfinal=104yrs • i = 0 coplanarity • no treatment of collision outcomes

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

29. large initial planetesimals? • how realistic is a large « initial » planetesimals population? depends on planetesimal-formation scenario -> maybe possible if quick formation by instabilities but how do grav.inst. 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)

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

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

33. a slightly inclined binary might help Xie & Zhou, 2009

34. a slightly inclined binary might help….but Xie & Zhou, 2009

35. accretion in inclined binaries inclinations 1<iB < 10o helps segregating planetesimal orbits according to sizes less frequent high-v R1≠R2 impacts global collision outcome balance more favourable to accretion BUT... low collision rates => slow accretion timescale issue

36. evolving gas disc ”superbee” wave damping ”minmod” wave damping coupled hydro/N-body simulations Paardekooper, Thebault & Mellema, 2008 <dV> always higher than in the axisymmetric gas disc case!

37. coupled hydro/N-body simulations role of the disc’s gravity Kley & Nelson (2007) high e-oscillations induced by gravitational interactions with the eccentric gas disc

38. the next big thing: realistic treatment of collisions Paardekooper & Leinhardt, 2010

39. Detection of debris discs in binaries Trilling et al. (2007)

40. debris discs in binaries (Thebault et al., 2010) a companion star cannot truncate a collisionally active debris disc

41. 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 sweeping might help • Do planets