Laboratory experiments on the low temperature formation of carbonaceous grains in the ISM

Abstract: 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.