Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments

TL;DR

This study experimentally investigates UV-induced formation of complex organic molecules in methanol-rich ices, revealing efficient production pathways that explain observed protostellar abundances and depend on experimental conditions.

Contribution

It provides quantitative data on formation rates and yields of complex organics in methanol ices, advancing understanding of astrochemical processes in star-forming regions.

Findings

Complex organics form readily during irradiation and warm-up.

Product ratios depend on UV fluence, temperature, and ice composition.

Yields help estimate photodissociation branching ratios and radical diffusion barriers.

Abstract

(Abridged) Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. This study aims to experimentally quantify the broad-band UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K under ultra-high vacuum conditions. The reaction products are mainly identified by RAIRS and TPD experiments. Complex organics are readily formed in all experiments, both during irradiation and during a slow warm-up of the ices to 200 K after the UV lamp is turned off. The relative abundances of photoproducts depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or CH3OH:CH4/CO ice mixtures are used. C2H6, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3, HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. The derived product-formation yields and their dependences on different experimental parameters, such as the initial ice composition, are used to estimate the CH3OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. The experiments show that ice photochemistry in CH3OH ices is efficient enough to explain the observed abundances of complex organics around protostars and that ratios of complex molecules can be used to constrain their formation pathway.