Photoproduction of H$_3^+$ from gaseous methanol inside dense molecular clouds

TL;DR

This study experimentally investigates how soft X-ray induced photodissociation of methanol in dense molecular clouds can produce H$_3^+$ ions, offering a new potential source for interstellar chemistry models.

Contribution

It provides the first experimental quantification of H$_3^+$ production from methanol via soft X-ray photodissociation in dense clouds.

Findings

H$_3^+$ production cross sections were measured.

Estimated H$_3^+$ column density from methanol photodissociation.

Photoproduction accounts for about 0.05% of observed H$_3^+$ in a dense cloud.

Abstract

We present experimental results obtained from photoionization and photodissociation processes of abundant interstellar methanol (CH$_3$OH) as an alternative route for the production of H$_3^+$ in dense clouds. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS) employing soft X-ray and time-of-flight mass spectrometry. Mass spectra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross sections for the production of H$_3^+$ due to molecular dissociation of methanol by soft X-rays (C1s edge) were determined. The H$_3^+$'s photoproduction rate and column density were been estimated adopting a typical soft X-ray luminosity inside dense molecular and the observed column density of methanol. Assuming a steady state scenario, the highest column density value for the photoproduced H$_3^+$ was about 10$^{11}$ cm$^2$, which gives the ratio photoproduced/observed of about 0.05%, as in the case of dense molecular cloud AFGL 2591. Despite the 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.