Philippe Thébault Paris Observatory Planet formation in binaries

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

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?

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

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

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

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

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

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

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

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 √

Grain condensation (Nelson, 2000)

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

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

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

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

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”

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

(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

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

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

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

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

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

Accretion/Erosion behaviour at 1AU from the primary and at t=10 4 yrs 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)

 Centauri B ”nominal case” erosion unsure perturbed accretion ”normal” accretion

simplifications  Initial e planetesimals =0  Static axisymmetric gas disc  i = 0 coplanarity  no treatment of collision outcomes  t final =10 4 yrs

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

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)

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

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)

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

a slightly inclined binary might help Xie & Zhou, 2009

a slightly inclined binary might help….but

accretion in inclined binaries inclinations 1<i B < 10 o helps segregating planetesimal orbits according to sizes less frequent high-  v R 1 ≠R 2 impacts global collision outcome balance more favourable to accretion BUT... low collision rates => slow accretion timescale issue

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

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

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

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

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

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 sweeping might help Do planets