  The formation of planetesimals is expected to occur via particle-gas
instabilities that concentrate dust into self-gravitating clumps. Triggering
these instabilities requires the prior pileup of dust in the protoplanetary
disk. Until now, this has been successfully modeled exclusively at the disk's
snowline, whereas rocky planetesimals in the inner disk were obtained only by
assuming either unrealistically large particle sizes or an enhanced global disk
metallicity. However, planetesimal formation solely at the snowline is
difficult to reconcile with the early and contemporaneous formation of iron
meteorite parent bodies with distinct oxidation states and isotopic
compositions, indicating formation at different radial locations in the disk.
Here, by modeling the evolution of a disk with ongoing accretion of material
from the collapsing molecular cloud, we show that planetesimal formation may
have been triggered within the first 0.5 million years by dust pileup at both
the snowline (at approximately 5 au) and the silicate sublimation line (at
approximately 1 au), provided turbulent diffusion was low. Particle
concentration at approximately 1 au is due to the early outward radial motion
of gas and is assisted by the sublimation and recondensation of silicates. Our
results indicate that, although the planetesimals at the two locations formed
about contemporaneously, those at the snowline accreted a large fraction of
their mass (approximately 60 percent) from materials delivered to the disk in
the first few 10,000 yr, whereas this fraction is only 30 percent for the
planetesimals formed at the silicate line. Thus, provided that the isotopic
composition of the delivered material changed with time, these two planetesimal
populations should have distinct isotopic compositions, consistent with
observations.
