One possible origin of ethanol in interstellar medium: Photochemistry of mixed CO2-C2H6 films at 11Â K. A FTIR study

It has been predicted by theoretical models that ethane and ethanol are present in icy mantles covering dust particles in dense interstellar clouds. Laboratory spectra of ethanol embedded in astrophysically relevant ice matrices were compared to the Infrared Space Observatory and ground-based astronomical spectra of high mass protostars. From this comparison strict upper-limits of ethanol (compared to solid water) on interstellar grains could be derived that are below 1.2%. In dense star forming regions ethanol is observed in gas phase with an abundance which is many orders of magnitude in excess of predictions based on pure gas-phase chemistry. Ethane has not been observed in the interstellar gas or on grains. In contrast, ethane has been detected in several comets with a percentage of <1% relative to water ice. Only upper limit could be obtained for cometary ethanol. In order to investigate a possible pathway leading to icy ethanol, we have studied the reaction of atomic oxygen with condensed ethane films by insertion of an oxygen atom in a CH bond. We generated oxygen atoms in situ by photolysis of ozone and carbon dioxide. Carbon dioxide is an abundant ice in the interstellar medium and comets. We recorded a set of IR spectra of ethane ice (mixed with carbon dioxide or with ethanol) at 11 K and studied the photolysis of ethane with ultraviolet photons below 200 nm. We discuss our experimental results, production from irradiation of CO2 + C2H6, of CH3CH2OH and CH3CHO in addition to photoproducts of CO2 or C2H6 and their implications for interstellar/cometary chemistry.