Planetesimal Accretion inalpha CentauriPhilippe Thbault (Stockholm/Paris Observatories)Francesco Marzari (Padua)Hans Scholl (Nice)(Thbault, Marzari & Scholl, Icarus, 2006)Thbault, Marzari & Scholl, MNRAS, 2008

a Cen BK1VMB= 0.93 M

the a Centauri systema Cen AG2VMA= 1.1 M

a = 23.4 AUe = 0.52

No > 2.5MJup planet around any of the stars (Endl et al.2001)

long term orbital stabilityHolman &Wiegert (1997)the ~a

embryos-to-planets phaseQuintana et al.(2002) (Barbieri et al.2002, Guedes et al.,2008)possible in the ~a

planetesimals-to-embryos phaseMarzari&Scholl (2000)BUT assuming single-size planetesimals !possible in the ~a

Planetesimal accretiondynamically quiet stage: Runaway growthgravitational focusing factor: (vesc(R)/v)2 If v~ vesc(r) then things get out of hand=> Runaway growth

CRUCIAL PARAMETER:ENCOUNTER VELOCITY DISTRIBUTION

our numerical approach evolution among planetesimals of different sizes, under the influence of:companion stars gravitational perturbations gaseous friction Derive maps for all impactor/target pairs (R1,R2) Use collision outcome prescriptions to Interpret (R1,R2) in terms of:unperturbed accretionperturbed accretionerosion

ModellingGas density profile: axisymmetric disc (??!!)Explored parameters-r0 -agas drag

Set-upnominal set-up (parameters with are explored in the runs)

(e,a) evolution: gas free casesecular oscillations with phased orbitsno increase untill orbit crossing occurs

(e,a) evolution: with gastfinal=104yrs1km

distributionat 1AU from the primary and at t=104yrshigh as soon as R1R2

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

Accretion/Erosion behaviourat 1AU from the primary and at t=104yrsVero2

Initial planetesimal size-distribution what is a population of km-sized planetesimals?

depends on planetesimal formation processprogressive sticking? gravitational instabilities?Our nominal distribution: Maxwellian with =5kmexplore other distributions (Gaussian, power-laws, etc)explore different size ranges (0.1-1km & 5-50km)

nominal casethe a>0.5AU region is hostile to planetesimal accretion

Alternative size distributionsaccretion-friendly only for extremely peaked distributions

Alternative gas disc profilesaccretion friendly only for ~gas free cases (for a

small planetesimals populationat 1AU from the primary and at t=104yrs

big planetesimals populationat 1AU from the primary and at t=104yrs

a Centauri Bnominal caseerosionunsureperturbed accretionnormal accretion

simplifications

Time scale?

Initial eplanetesimals=0

Static axisymmetric gas disccan only make things worse

i = 0

a first go at coupled hydro/N-body simulationsCrucial role of the numerical wave damping procedurebut always higher than in the axisymmetric gas disc case!(Paardekooper, Thebault, Mellema, 2008)

initial conditions/time scale = 0 = eforced100% orbital dephasingquick relaxation (few 103yrs) of the initial conditions

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

large initial planetesimals? how realistic is a large initial planetesimals population?->maybe possible if quick formation by instabilitiesbut how do grav.inst. proceed in the dynamically perturbed environment of a binary? ->more difficult if progressive stickingalways have to pass through a km-sized phaseat 1AU, mutual collisions result preferentially in accretion for planetesimals >25km...but: in any case, it cannot be normal (runaway) accretion-> type II runaway? (Kortenkamp, 2001)

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 scenarioBUT, so far, tested for giant planets beyond 5AURealistic for terrestrial bodies within 1AU?

different initial binary configuration? most stars born in clustersearly encounters and binary compaction/exchanges are possible:Initial and final (e,a) for binaries in a typical cluster (Malmberg et al., 2007)

accretion after/during gas dispersion? gas is removed after ~107yrs-But, differential acquired with gas cannot be easily erased-In addition: pure gravitational effect alone trigger high within a few 105yrs. rephasing during gas dispersal (Xie & Zhou, 2008)- low after ~tdispers. (105yrs?)- But, not low enough for fast removal by gas-drag induced inward drift?

test run with sudden gas removal gas suddenly disappears at t=104yrs1km

test run with progressive gas dispersal progressive re-phasing BUT radial drift of small bodiesdispersion starts at t=104yrs1km

Conclusions a>0.5AU (0.75AU) region hostile to km-sized planetesimals accretionrobust with respect to size-distribution and gas disc profilein-situ planet formation in the habitable zone is difficult with the present binary configurationplanetesimals->embryo phase more sensitive to binarity than embryo->planets

As with many other systems, the stage which has been studied in most details. here again, the Need collision outcome prescription, to derive V_erod we talked about earlierUnfortunately....Careful approach: 2 values for V_critthis the same graph as the B@W one, but translated in terms of acc/frag...here again only the diagonal is safe=> But even that is not enough => we need the size distribution to get the global balance between accreting and erosive impactseroding impacts dominate accreting onesSo HOW ROBUST are these results? => parameter explorationPretty robustPretty robustgets worse, not because of sign.different , but because for the same dV, smaller bodies are more affectedOf course, what we did is a very simple test study, with obvious limitations: 2 types : numerical and additional mechanisms left out.However, despite these limitations, we think our core results are relatively robust: accretion-hostile in the HZ.First of all the numerical simplificationsMOST OBVIOUS SIMPLIFICATIONS: AXISYM.DISC

now to the physical processes left out: could they solve the problem raised in our study?point 1 is not really a missing process but...*pas forcement...weve seen on Monday nice examples of outward migrationsthat might be an option!!! We tried test simulations with dispersal, and nothing was left after 10**5 yrs...