摘要：Context. 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 (α< 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.
关键词：planets and satellites: formation;planet-disk interactions;accretion, accretion disks
