2010A&A...519A..22M

Methyl formate (HCOOCH₃) is a complex organic molecule detected in hot cores and hot corinos. Gas-phase chemistry fails to reproduce its observed abundance, which usually varies between 10^–7 and 10^–9 with respect to H₂. Laboratory experiments were performed in order to investigate a solid-state route of methyl formate formation, to obtain an estimate of the amount that can be formed, and to verify whether it can account for the observed abundances. Several solid samples (16K) of astrophysical interest were analyzed by infrared spectroscopy in the 4400-400cm^–1 range. The infrared spectral characteristics of frozen methyl formate were studied by deriving their band strength values. The effects produced upon warm-up of the samples were analyzed comparing the spectra taken at different temperatures. In order to study the formation and destruction mechanism of methyl formate in the interstellar ices, a binary mixture of methanol (CH₃OH) and carbon monoxide (CO) ice and a sample of pure methanol were irradiated at 16K with 200keV protons. Methyl formate was identified through its fundamental mode (CH₃ rocking) at about 1160cm^–1. We present the mid-infrared methyl formate ice spectrum showing both the amorphous (16K) and the crystalline (110K) structure. We report novel measurements of the band strength values of the six main methyl formate bands. We prove the formation and the destruction of methyl formate after irradiation of CH₃OH and a CO:CH₃OH mixture. Extrapolating our results to the interstellar medium conditions we found that the production timescale of methyl formate agrees well with the evolutionary time of molecular clouds. The comparison with the observational data indicates that the amount of methyl formate formed after irradiation can account for the observed abundances. The present results allow us to suggest that gas phase methyl formate observed in dense molecular clouds is formed in the solid state after cosmic ion irradiation of icy grain mantles containing CO and CH₃OH and released to the gas phase after desorption of icy mantles.