Planetesimal Accretion in alpha Centauri

1. Planetesimal Accretion in alpha Centauri • Philippe Thébault(Stockholm/Paris Observatories) • Francesco Marzari (Padua) • Hans Scholl (Nice) (Thébault, Marzari & Scholl, Icarus, 2006) Thébault, Marzari & Scholl, MNRAS, 2008

2. the a Centauri system a Cen B K1V MB= 0.93 M⊙ a = 23.4 AU e = 0.52 a Cen A G2V MA= 1.1 M⊙ No > 2.5MJup planet around any of the stars (Endl et al.2001)

3. long term orbital stability Holman &Wiegert (1997) the ~a<2.5AU region is safe in the coplanar case

4. embryos-to-planets phase possible in the ~a<2.5AU region Quintana et al.(2002) (Barbieri et al.2002, Guedes et al.,2008)

5. planetesimals-to-embryos phase Marzari&Scholl (2000) possible in the ~a<2 AU region BUT assuming single-size planetesimals !

6. Planetesimal accretion dynamically quiet stage: Runaway growth gravitational focusing factor: (vesc(R)/v)2 If v~ vesc(r) then things get out of hand…=> Runaway growth

7. 3 possible regimes (sort of) : • dV < Vesc => runaway accretion • Vesc< dV < Verosion => accretion (slowed down) • Verosion < dV => erosion/no-accretion CRUCIAL PARAMETER: ENCOUNTER VELOCITY DISTRIBUTION

8. our numerical approach • <dV> evolution among planetesimals of different sizes, under the influence of: companion star’s gravitational perturbations gaseous friction • Derive <dV>« maps » for all impactor/target pairs (R1,R2) • Use collision outcome prescriptions to Interpret <dV>(R1,R2) in terms of: unperturbed accretion perturbed accretion erosion

9. gas drag • Modelling • Gas density profile: axisymmetric disc (??!!) • Explored parameters -r0 -a

10. Set-up nominal set-up (parameters with  are explored in the runs)

11. (e,a) evolution: gas free case secular oscillations with phased orbits no <dV> increase untill orbit crossing occurs

12. (e,a) evolution: withgas 1km<R<10km tfinal=104yrs differential orbital phasing according to size

13. <dV(R1,R2)> distribution high <dV> as soon as R1≠R2 at 1AU from the primary and at t=104yrs

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

15. Accretion/Erosion behaviour Vero2<dV erosion Vero1<dV<Vero2unsure Vesc<dV<Vero1perturbed accretion Vesc<dV<Vero1”normal” accretion at 1AU from the primary and at t=104yrs

16. “Initial” planetesimal size-distribution • what is a « population of km-sized planetesimals »? • depends on planetesimal formation process progressive sticking? gravitational instabilities? • Our nominal distribution: Maxwellian with <R>=5km explore other distributions (Gaussian, power-laws, etc…) explore different size ranges (0.1-1km & 5-50km)

17. nominal case the a>0.5AU region is hostile to planetesimal accretion

18. Alternative size distributions accretion-friendly only for extremely peaked distributions

19. Alternative gas disc profiles accretion friendly only for ~gas free cases (for a<1.3AU)

20. “small” planetesimals population at 1AU from the primary and at t=104yrs

21. “big” planetesimals population at 1AU from the primary and at t=104yrs

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

23. simplifications • Staticaxisymmetric gas disc • Initial eplanetesimals=0 • Time scale? • i = 0 can only make things worse

24. ”superbee” wave damping ”minmod” wave damping a first go at coupled hydro/N-body simulations Crucial role of the numerical ”wave damping” procedure but <dV> alwayshigherthan in the axisymmetric gas disc case! (Paardekooper, Thebault, Mellema, 2008)

25. initial conditions/time scale <e0> = eforced 100% orbital dephasing <e0> = 0 quick relaxation (few 103yrs) of the initial conditions

26. possible solutions to our problems(?) • large (>25km) initial planetesimals? • outward migration of planets? • Different initial binary configuration? • Re-phasing after/during gas disc dissipation?

27. large initial planetesimals? at 1AU, mutual collisions result preferentially in accretion for planetesimals >25km...but: • how realistic is a large « initial » planetesimals population? ->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)

28. planet migration? • gas disc induced migration (I,II or III) : mostly inward (?)migration, makes things even worse • later, planetesimal-scattering induced migration (gas-free disc) « Nice model » scenario BUT, so far, tested for giant planets beyond 5AU Realistic for terrestrial bodies within 1AU?

29. different initial binary configuration? • most stars 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)

30. accretion after/during gas dispersion? • gas is removed after ~107yrs -But, differential <dV> acquired with gas cannot be easily erased -In addition: pure gravitational effect alone trigger high <dV> within a few 105yrs. • rephasing during gas dispersal (Xie & Zhou, 2008) - low <dV> after ~tdispers. (105yrs?) - But, not low enough for <5km planetesimals - But, long accretion-hostile period (t<tdisper.) => fragmentation of planetesimals into ever smaller debris => fast removal by gas-drag induced inward drift?

31. test run with sudden gas removal 1km<R<10km • gas suddenly disappears at t=104yrs • tfinal =105yrs <dV> stay at a high level

32. test run with progressive gas dispersal 1km<R<10km • dispersion starts at t=104yrs • tdisp. =105yrs • tfinal =2x105yrs progressive re-phasing BUT radial drift of small bodies

33. presence of planets cannot be ruled out planet migration? different initial binary configuration? Conclusions • a>0.5AU (0.75AU) region hostile to km-sized planetesimals accretion • robust with respect to size-distribution and gas disc profile • planetesimals->embryo phase more sensitive to binarity than embryo->planets in-situ planet formation in the habitable zone is difficult with the present binary configuration