Production of H3+ via photodissociation of organic molecules in interstellar clouds

Abstract

We present experimental results obtained from photoionization and photodissociation processes of abundant interstellar CH3-X type organic molecules like methanol (CH3OH), methylamine (CH 3NH2) and acetonitrile (CH3CN) as alternative route for the production of H3+ in interstellar and star-forming environments. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons with energies between 200 and 310 eV and time-of-flight (TOF) mass spectrometry. Mass spectra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross-sections for H3+ production by soft X-rays were determined. We have found that, among the channels leading to molecular dissociation, the H3+ yield could reach values up to 0.7 per cent for single photoionization process and up to 4 per cent for process involving double photoionization. The H3+ photoproduction cross-section due to the dissociation of the studied organic molecules by photons over the C1s edge (200-310 eV) were about 0.2-1.4 × 10 -18 cm2. Adopting the typical X-ray luminosity L X ≳ 1031 erg s-1 which best fit the observational data for AFGL 2591 we derive an estimative for the H 3+ photoproduction rate due to methyl compound dissociation process. The highest value for the H3+ column density from methanol dissociation by soft X-rays, assuming a steady state scenario, was about 1011 cm2, which gives the fraction of the photoproduced H3+ of about 0.05 per cent, as in the case of dense molecular cloud AFGL 2591. Despite the extreme small value, this represent a new and alternative source of H3+ into dense molecular clouds and it is not been considered as yet in interstellar chemistry models. © 2007 RAS.