Production of H${}_3^{+}$ via photodissociation of organic molecules in interstellar clouds

Abstract

We present experimental results obtained from photoionization and photodissociation processes of abundant interstellar CH${}_3$-X type organic molecules like methanol (CH${}_3$OH), methylamine (CH${}_3$NH${}_2$) and acetonitrile (CH${}_3$CN) as alternative route for the production of H${}_3^{+}$ 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 mass spectrometry. Mass spectra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross sections for H${}_3^{+}$ production by soft X-rays were determined. We have found that, among the channels leading to molecular dissociation, the H${}_3^{+}$ yield could reach values up to 0.7% for single photoionization process and up to 4% for process involving double photoionization. The H${}_3^{+}$ 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 $\times$ 10${}^{-18}$ cm${}^2$. Adopting the typical X-ray luminosity $L_X\gtrsim 10^{31}$ erg s${}^{-1}$ which best fit the observational data for AFGL 2591 (Stauber et al. 2005) we derive an estimative for the H${}_3^{+}$ photoproduction rate due to methyl-compounds dissociation process. The highest value for the H${}_3^{+}$ column density from methanol dissociation by soft X-rays, assuming a steady state scenario, was about $10^{11}$ cm${}^2$, which gives the fraction of the photoproduced H${}_3^{+}$ of about 0.05%, as in the case of dense molecular cloud AFGL 2591. Despite the extreme small value, this represent a new and alternative source of H${}_3^{+}$ into dense molecular clouds and it is not been considered as yet in interstellar chemistry models.

The paper contains 7 pages, 3 figures and 3 tables. Accepted to be published on MNRAS on 2006 December 14. Received 2006 December 14; in original form 2006 September 19