Formation of silicon oxide grains at low temperature
  The formation of grains in the interstellar medium, i.e., at low temperature,
has been proposed as a possibility to solve the lifetime problem of cosmic
dust. This process lacks a firm experimental basis, which is the goal of this
study. We have investigated the condensation of SiO molecules at low
temperature using neon matrix and helium droplet isolation techniques. The
energies of SiO polymerization reactions have been determined experimentally
with a calorimetric method and theoretically with calculations based on the
density functional theory. The combined experimental and theoretical values
have revealed the formation of cyclic (SiO)$_k$ ($k$ = 2--3) clusters inside
helium droplets at $T$ = 0.37 K. Therefore, the oligomerization of SiO
molecules is found to be barrierless and is expected to be fast in the
low-temperature environment of the interstellar medium on the surface of dust
grains. The incorporation of numerous SiO molecules in helium droplets leads to
the formation of nanoscale amorphous SiO grains. Similarly, the annealing and
evaporation of SiO-doped Ne matrices lead to the formation of solid amorphous
SiO on the substrate. The structure and composition of the grains were
determined by infrared absorption spectroscopy, transmission electron
microscopy, and energy-dispersive X-ray spectroscopy. Our results support the
hypothesis that interstellar silicates \textbf{can be formed} in the low
temperature regions of the interstellar medium by accretion through barrierless
reactions.
