Destruction of C2H4O2 isomers in ice-phase by X-rays: Implication on the abundance of acetic acid and methyl formate in the interstellar medium

Author(s)

Rachid, Marina G. ; Faquine, Karla ; Pilling, S.

• Document type: Journal article
• Source: Planetary and Space Science; v. 149, p. 11-pg., 2017-12-01.

Abstract

The C2H4O2 isomers methyl formate (HCOOCH3), acetic acid (CH3COOH) and glycoaldehyde (HOCH2CHO) have been detected in molecular clouds in the interstellar medium, as well as, hot cores, hot corinos and around protostellar objects. However, their abundances are very different, being methyl formate more abundant than the other two isomers. This fact may be related to the different destruction by ionizing radiation of these molecules. The goal of this work is experimentally study the photodissociation processes of methyl formate and acetic acid ices when exposed to broadband soft X-ray from 6 up to 2000 eV. The experiments were performed coupled to the SGM beamline in the Brazilian Synchrotron Light Source (LNLS/CNPEM) at Campinas, Brazil. The simulated astrophysical ices (12 K) were monitored throughout the experiment using infrared vibrational spectroscopy (FTIR). The analysis of processed ices allowed the determination of the effective destruction cross sections of the parent molecules as well as the effective formation cross section of daughter molecular species such as CO, CO2, H2O, CH4 and H2CO (only for methyl formate) and the hydrocarbons C2H6 and C5H10 (only for acetic acid). The half-lives of molecules at ices toward young stellar objects (YSOs) and inside molecular clouds (e.g. Sgr B2 and W51) due to the presence of incoming soft X-rays were estimated. We determined the effective formation rate and the branching ratios for assigned daughter species after the establishment of a chemical equilibrium. The main product from photodissociation of both methyl formate and acetic acid is CO, that can be formed by recombination of ions, formed during the photodissociation, in the ice surface. The relative abundance between methyl formate and acetic acid (N-CH3COOH/N-HCOOCH3) in different astronomical scenarios and their column density evolution in the presence of X-rays were calculated. Our results suggest that such radiation field can be one of the factors that explain the difference in the C2H4O2 isomers abundances. (AU)

• FAPESP's process: 14/08643-0 - Experimental investigation on the photolysis of astrophysical ices containing acetic acid and its isomers (methyl formate and glycolaldehyde)
• Grantee: Karla Faquine Rodrigues
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• FAPESP's process: 09/18304-0 - Synthesis and degradation of prebiotic molecular species at planetary atmospheres, comets and interstellar ice analogs
• Grantee: Sergio Pilling Guapyassu de Oliveira
• Support Opportunities: Research Grants - Young Investigators Grants
• FAPESP's process: 16/11334-5 - Experimental simulation of the effects of energetic particles (ions and electrons) on the surface of icy bodies in the Solar System: physicochemical transformation of ices mixtures containing H2O,CO2,CH4 (10:1:1)
• Grantee: Marina Gomes Rachid
• Support Opportunities: Scholarships in Brazil - Master
• FAPESP's process: 12/17248-2 - Production of molecules with astrobiological interest in experiments simulating the moon Enceladus in the presence of UV photons, X-rays, electrons, and fast ions
• Grantee: Alexandre Bergantini de Souza
• Support Opportunities: Scholarships in Brazil - Doctorate