Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense
  clouds: a case of dust grain chemistry
  We present an extensive review of gas phase reactions producing
methylacetylene and propene showing that these relatively abundant unsaturated
hydrocarbons cannot be synthesized through gas-phase reactions. We explain the
formation of propene and methylacetylene through surface hydrogenation of C3
depleted onto interstellar ices, C3 being a very abundant species in the gas
phase.
