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تسجيل مستخدم جديدFormation of dust-rich planetesimals from sublimated pebbles inside of the snow line
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Content: For up to a few millions of years, pebbles must provide a quasi-steady inflow of solids from the outer parts of protoplanetary disks to their inner regions. Aims: We wish to understand how a significant fraction of the pebbles grows into planetesimals instead of being lost to the host star. Methods:We examined analytically how the inward flow of pebbles is affected by the snow line and under which conditions dust-rich (rocky) planetesimals form. When calculating the inward drift of solids that is due to gas drag, we included the back-reaction of the gas to the motion of the solids. Results: We show that in low-viscosity protoplanetary disks (with a monotonous surface density similar to that of the minimum-mass solar nebula), the flow of pebbles does not usually reach the required surface density to form planetesimals by streaming instability. We show, however, that if the pebble-to-gas-mass flux exceeds a critical value, no steady solution can be found for the solid-to-gas ratio. This is particularly important for low-viscosity disks (alpha < 10^(-3)) where we show that inside of the snow line, silicate-dust grains ejected from sublimating pebbles can accumulate, eventually leading to the formation of dust-rich planetesimals directly by gravitational instability. Conclusions: This formation of dust-rich planetesimals may occur for extended periods of time, while the snow line sweeps from several au to inside of 1 au. The rock-to-ice ratio may thus be globally significantly higher in planetesimals and planets than in the central star.
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We present a study on the formation of planetary systems around low mass stars similar to Trappist-1, through the accretion of either planetesimals or pebbles. The aim is to determine if the currently observed systems around low mass stars favour one
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We develop a simple model to predict the radial distribution of planetesimal formation. The model is based on the observed growth of dust to mm-sized particles, which drift radially, pile-up, and form planetesimals where the stopping time and dust-to
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ic jam, because small grains drift inward at lower speeds than larger pebbles. Aims. We aim to evaluate the effect of high dust concentrations around the snowline onto the gas dynamics. Methods. Using numerical simulations, we model the global radial evolution of an axisymmetric protoplanetary disk. Our model includes particle growth, evaporation and recondensation of water, and the back-reaction of dust onto the gas, taking into account the vertical distribution of dust particles. Results. We find that the dust back-reaction can stop and even reverse the net flux of gas outside the snowline, decreasing the gas accretion rate onto the star to under $50%$ of its initial value. At the same time the dust accumulates at the snowline, reaching dust-to-gas ratios of $epsilon gtrsim 0.8$, and delivers large amounts of water vapor towards the inner disk, as the icy particles cross the snowline. However, the accumulation of dust at the snowline and the decrease in the gas accretion rate only take place if the global dust-to-gas ratio is high ($varepsilon_0 gtrsim 0.03$), if the viscous turbulence is low ($alpha_ u lesssim 10^{-3} $), if the disk is large enough ($r_c gtrsim 100, textrm{au}$), and only during the early phases of the disk evolution ($t lesssim 1, textrm{Myr}$). Otherwise the dust back-reaction fails to perturb the gas motion.
Context: The formation of rocky planetesimals is a long-standing problem in planet formation theory. One of the possibilities is that it results from gravitational instability as a result of pile-up of small silicate dust particles released from subl
imating icy pebbles that pass the snow line. Aims: We want to understand and quantify the role of the water snow line for the formation of rock-rich and ice-rich planetesimals. In this paper, we focus on the formation of rock-rich planetesimals. A companion paper examines the combined formation of both rock-rich and ice-rich planetesimals. Methods: We develop a new Monte Carlo code to calculate the radial evolution of silicate particles in a turbulent accretion disk, accounting for the back-reaction (i.e., inertia) of the particles on their radial drift velocity and diffusion. Results depend in particular on the particle injection width (determined from the radial sublimation width of icy pebbles), the pebble scale height and the pebble mass flux through the disk. The scale height evolution of the silicate particles, which is the most important factor for the runaway pile-up, is automatically calculated in this Lagrange method. Results: From the numerical results, we derive semi-analytical relations for the scale height of the silicate dust particles and the particles-to-gas density ratio at the midplane, as functions of a pebble-to-gas mass flux ratio and the $alpha$ parameters for disk gas accretion and vertical/radial diffusion. We find that the runaway pile-up of the silicate particles (formation of rocky planetesimals) occurs if the pebble-to-gas mass flux ratio is $> [(alpha_{Dz}/alpha_{acc})/3 times 10^{-2}]^{1/2}$ where $alpha_{Dz}$ and $alpha_{acc}$ are the $alpha$ parameters for vertical turbulent diffusion and disk gas accretion.
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