Laboratory experiments on the low temperature formation of carbonaceous
  grains in the ISM
  The life-cycle of cosmic dust grains is far from being understood and the
origin and evolution of interstellar medium (ISM) grains is still under debate.
In the ISM, the cosmic dust destruction rate is faster than the production rate
by stellar sources. However, observations of ISM refractory matter suggest that
to maintain a steady amount of cosmic grains, some supplementary production
mechanism takes place. In this context, we aimed to study possible re-formation
mechanisms of cosmic grains taking place at low temperature directly in the
ISM. The low temperature condensation of carbonaceous materials has been
investigated in experiments mimicking the ISM conditions. Gas-phase
carbonaceous precursors created by laser ablation of graphite were forced to
accrete on cold substrates (T about 10 K) representing surviving dust grains.
The growing and evolution of the condensing carbonaceous precursors have been
monitored by MIR and UV spectroscopy under a number of experimental scenarios.
It is demonstrated, for the first time, the possibility to form ISM
carbonaceous grains in "situ". The condensation process is governed by carbon
chains that first condense into small carbon clusters and finally into more
stable carbonaceous materials, which structural characteristics are comparable
to the material formed in gas-phase condensation experiments at very high
temperature. We also show that the so-formed fullerene-like carbonaceous
material is transformed into a more ordered material under VUV processing. The
cold condensation mechanisms here discussed can give fundamental clues to fully
understand the balance between the timescale for dust injection, destruction
and re-formation in the ISM.
