  Collisional growth of dust occurs in all regions of protoplanetary disks with
certain materials dominating between various condensation lines. The sticking
properties of the prevalent dust species depend on the specific temperatures.
The inner disk is the realm of silicates spanning a wide range of temperatures
from room temperature up to sublimation beyond $1500\,\mathrm{K}$. For the
first time, we carried out laboratory collision experiments with hot levitated
basalt dust aggregates of $1\, \rm mm$ in size. The aggregates are compact with
a filling factor of $0.37 \pm 0.06$. The constituent grains have a wide size
distribution that peaks at about $0.6\,\mu \mathrm{m}$. Temperatures in the
experiments are varied between approximately $600\,\mathrm{K}$ and
$1100\,\mathrm{K}$. Collisions are slow with velocities between
$0.002\,\mathrm{m}\,\mathrm{s}^{-1}$ and $0.15\,\mathrm{m}\,\mathrm{s}^{-1}$,
i.e., relevant for protoplanetary disks. Aside from variations of the
coefficients of restitution due to varying collision velocities, the
experiments show low sticking probability below $900\,\mathrm{K}$ and an
increasing sticking probability starting at $900\,\mathrm{K}$. This implies
that dust can grow to larger size in hot regions, which might change planet
formation. One scenario is an enhanced probability for local planetesimal
formation. Another scenario is a reduction of planetesimal formation as larger
grains are more readily removed as a consequence of radial drift. However, the
increased growth at high temperatures likely changes planetesimal formation one
way or the other.
