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

Abstract

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. REFERENCESFavre et al. HCOOCH3 as a probe of temperature and structure in Orion-KL. A&A 532, A32, 2011Bockelee-Morvan et al. New molecules found in comet C/1995 O1 (Hale-Bopp). Investigating the link between cometary and interstellar material. A&A, v.353, p.1101-1114,  2000Balucani et al.  Formation of complex organic molecules in cold objects: the role of gas-phase reactions, Monthly Notices of the Royal Astronomical Society: Letters, Volume 449, Issue 1, 01, Pages L16–L20, 2015El-Abd et al. Interstellar Glycolaldehyde, Methyl Formate, and Acetic Acid. I. A Bimodal Abundance Pattern in Star-forming Regions. ApJ, 883:129 (24pp), 2019Gerakines et al. Ultraviolet processing of interstellar ice analogs: I. Pure ices. Astronomy and Astrophysics 312(1):289-305, 1996Bennett and Kaiser. On the Formation of Glycolaldehyde (HCOCH2OH) and Methyl Formate (HCOOCH3) in Interstellar Ice Analogs. ApJ 661 899, 2007Oberg et al. Formation rates of complex organics in UV irradiated CH_3OH-rich ices. I. Experiments. A&A, Volume 504, Issue 3, pp.891-913, 2009Modica and Palumbo. Formation of methyl formate after cosmic ion irradiation of icy grain mantle. A&A 519, A22,  2010de Barros et al. Cosmic ray impact on astrophysical ices: laboratory studies on heavy ion irradiation of methane. A&A 531, A160, 2011Modica et al. Formation of methyl formate in comets by irradiation of methanol-bearing ices. Planetary and Space Science. 73(1):425–429, 2012