Planetesimal formation at water snow line Joanna Drążkowska (University of Zurich) [email protected] collaboration: Yann Alibert (University of Bern) Kees Dullemond (Heidelberg University) Why is the snow line a good place to form planetesimals? Context • the connection between dust evolution and planetesimal formation represents a major gap in the state-of-the-art planet formation models • direct growth from dust to planetesimals is inefficient because of fragmentation and radial drift • streaming instability is efficient in turning pebbles to planetesimals but it only happens under special conditions • I make 1-D models of dust evolution in a global protoplanetary disk to predict where and when planetesimals form Icy dust is more sticky than dry dust, so the pebbles outside of the snow line grow to larger sizes than inside. Because of their size, the icy pebbles drift inward quickly. refractory dust Part of the water vapor released by evaporating icy pebbles is returned outside of the snow line by diffusion and recondenses locally enhancing the density of solids (this is called the "cold finger" effect). solid ice water vapor The small aggregates inside of the snow line are well coupled to the gas and they do not drift fast. A "traffic jam" appears in the inner part of the disk, leading to significant dust-to-gas ratio increase, which propagates outwards thanks to diffusion and collective drift effect. Models distance to the star [AU] MMSN 10 -2 10 0 10 2 10 0 10 1 10 2 5 ⋅10 5 yrs 10 -2 10 0 10 2 3 ⋅10 5 yrs Σ [g cm -2 ] 10 -2 10 0 10 2 2 ⋅10 5 yrs 10 -2 10 0 10 2 10 5 yrs 10 -2 10 0 10 2 10 4 yrs gas dust planetesimals water ice water vapor 10 -2 10 0 10 2 1 yr Can planetesimals already form during the disk infall? 10 -2 10 0 10 2 Σ plts [g/cm 2 ] MMSN 10 0 10 1 10 2 r [au] 10 5 10 6 10 7 t [years] cold finger traffic jam +cold finger buildup stage class II stage 33 32 31 30 29 28 27 26 log ˙ Σ plts [g cm −2 s −1 ] 10 -2 10 0 10 2 Σ plts [g/cm 2 ] MMSN 10 0 10 1 10 2 r [au] 10 5 10 6 10 7 t [years] buildup stage class II stage 32 31 30 29 28 27 26 25 log ˙ Σ plts [g cm −2 s −1 ] Most of dust coagulation models start when the protoplanetary disk is already fully-formed. At the same time, observational studies suggest that planetesimal formation should start very early. In our models, we found that planetesimal formation during the disk buildup is only possible if the gas and water vapor redistribution is efficient (α v ≥ 10 -3 ) but the dust resides in a quiescent midplane (α t ≤ 10 -4 ). This favours the cold finger effect, which is the only way to create pile-up of pebbles during disk infall. The plausibility of such a setup in a realistic protoplanetary disk is unclear. standard model α v = α t = 10 -3 "dead zone" model α v = 10 -3 , α t = 10 -5 Setup • the gas disk model is from Bitsch et al. (2015), the initial mass of the disk is 0.1 M ⊙ • the initial dust-to-gas ratio is 0.03 • the dust evolution algorithm is based on the two-population model of Birnstiel et al. (2012) • planetesimal formation criterion is based on the midplane density ratio ρ dust (St ≥ 10 -2 ) / ρ gas ≥ 1 • fragmentation velocity outside of the snow line is v f = 10 m/s and inside of the snow line v f = 1 m/s Results • dust evolution is driven by radial drift in the outer part of the disk and by fragmentation in its inner part • the radial drift shifts mass inwards and the dust-to-gas ratio in the inner disk is enhanced • the "traffic jam" is more efficient than the "cold finger" effect • the maximum dust-to-gas ratio enhancement always occurs directly outside of the snow line and it spreads outwards thanks to the collective drift • planetesimal formation only takes place directly outside of the snow line • typical outcome is a massive annulus of planetesimals (Σ plts > Σ MMSN ) Further reading: Drążkowska & Dullemond "Can dust coagulation trigger streaming instability?", 2014, A&A 572, A78 Drążkowska, Alibert & Moore "Close-in planetesimal formation by pile-up of drifting pebbles", 2016, A&A 594, A105 Drążkowska & Alibert "Planetesimal formation starts at the snow line", 2017, A&A 608, A92 Drążkowska & Dullemond "Planetesimal formation during protoplanetary disk buildup", 2018, A&A under review Conclusions • planetesimal formation needs large pebbles and enhanced dust-to-gas ratio: pebble pile-ups • planetesimal formation is hindered by the growth barriers – even if it happens via the streaming instability, large enough pebbles need to grow first • it is very hard to trigger planetesimal formation in the outer part of the disk during its gas-rich phase • planetesimals do not form everywhere in the disk, the water snow line is one favorable place to trigger planetesimal formation both when the disk is already fully formed and during its buildup • a significant redistribution of solids must take place before the conditions necessary for planetesimal formation are met • solids-to-gas ratio is increased at the location where planetesimals form, which should facilitate faster accretion of the final planets • if planetesimals are formed already in the disk infall stage, their mass is much lower than the mass of planetesimals formed during the subsequent disk evolution, but due to their early occurrence they may be important for planet formation