Formation and fate of methyl formate in space upon ion irradiation and its astrophysical relevance

The Universe is molecular in nature, as over 200 molecules have been detected in the gas-phase in the interstellar and circumstellar medium (ISM/CSM). Starting from molecular hydrogen, many species including H₂O, CO₂, NH₃, CH₄, CH₃OH, and other complex organic molecules (COMs) have been shown to be formed efficiently on the surface of interstellar ice grains throughout the star-formation process. Interstellar ices are believed to be the main carriers of prebiotic molecules that have been included in the outer Solar System’s ice objects such as moons, comets, and Kuiper Belt Objects (KBOs). Therefore, understanding how COMs form and evolve in space is of pivotal importance to study the potential link between species in space and life on Earth.

COMs like the isomers of C₂H₄O₂, i.e., glycolaldehyde (HCOCH₂OH), acetic acid (CH₃COOH), and methyl formate (HCOOCH₃), have been observed abundantly around the Galactic centre, in dark clouds, and hot cores of the interstellar medium (ISM), as well as in some comets of the Solar System (e.g., Favre et al. 2011; Bockelee-Morvan et al. 2000). However, their exact gas-grain formation and destruction pathway is still unclear (Balucani et al. 2015). According to El-Abd et al. (2019), the observed column densities of methyl formate and acetic acid are well-correlated, and are likely simply tracking the relative total gas mass in star forming regions. Methyl formate and glycolaldehyde, however, display a stark dichotomy in their relative column densities. The latter finding implies that different formation/destruction routes are at play for the three isomers.

To date, there is a strong laboratory evidence for an efficient production of glycolaldehyde, methyl formate, and acetic acid in the ISM through energetic processing of methanol-rich interstellar ices (Gerakines et al. 1996; Bennett and Kaiser 2007; Oberg et al. 2009; Modica and Palumbo 2010; de Barros et al. 2011; Modica et al. 2012). However, so far models and laboratory studies cannot fully reproduce the observed mutually exclusive presence of specific isomers in certain star formation regions. Understanding the formation of the C₂H₄O₂ isomers is an important step to verify the formation of yet more complex molecules that are necessary for life. In this talk, I will present our latest results obtained at the ion accelerator ATOMKI facility in Debrecen (Hungary) using the novel ultrahigh vacuum ICA end station. Following a systematic approach, we have exposed mixtures of CO:CH₃OH (1:1, 1:2, 2:1) to 200 keV and 1 MeV H+ at 20 K. Ices are monitored by means of FTIR spectroscopy and results compared with those from the analogue irradiation of a series of pure species including CO, CO₂, CH₄, CH₃OH, methyl formate, and acetic acid. Results will be discussed in light of upcoming JWST mission.

 

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