Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Abstract

Context. Methanol and its precursor formaldehyde are among the most studied organic molecules in the interstellar medium and are abundant in the gaseous and solid phases. We recently developed a model to simulate CO hydrogenation via H atoms on interstellar ice surfaces, the most important interstellar route to H 2 CO and CH 3 OH, under laboratory conditions. Aims. We extend this model to simulate the formation of both organic species under interstellar conditions, including freeze-out from the gas and hydrogenation on surfaces. Our aim is to compare calculated abundance ratios with observed values and with the results of prior models. Methods. Our model utilises the continuous-time, random-walk Monte Carlo method, which -unlike other approaches -is able to simulate microscopic grain-surface chemistry over the long timescales in interstellar space, including the layering of ices during freeze-out. Results. Simulations under different conditions, including density and temperature, have been performed. We find that H 2 CO and CH 3 OH form efficiently in cold dense cores or the cold outer envelopes of young stellar objects. The grain mantle is found to have a layered structure with CH 3 OH on top. The species CO and H 2 CO are found to exist predominantly in the lower layers of ice mantles where they are not available for hydrogenation at late times. This finding is in contrast with previous gas-grain models, which do not take into account the layering of the ice. Some of our results can be reproduced by a simple quasi-steady-state analytical model that focuses on the outer layer. Conclusions. Observational solid H 2 CO/CH 3 OH and CO/CH 3 OH abundance ratios in the outer envelopes of an assortment of young stellar objects agree reasonably well with our model results, which also suggest that the large range in CH 3 OH/H 2 O observed abundance ratios is due to variations in the evolutionary stages. Finally, we conclude that the limited chemical network used here for surface reactions apparently does not alter the overall conclusions.

Surface formation routes of interstellar molecules: hydrogenation reactions in simple ices

“…Also several new reaction routes were revealed. The experimental outcome can be used in astrochemical models with the intent to simulate the formation of new species in the solid phase on astronomical timescales (e.g., Cuppen et al 2009), extending the laboratory results beyond experimental constraints.…”

“…The work presented in the next section follows a bottom-up approach and summarizes a representative sample of relevant experiments (e.g., Watanabe and Kouchi 2002;Watanabe et al 2004Watanabe et al , 2006Fuchs et al 2009;Miyauchi et al 2008;Ioppolo et al 2008Ioppolo et al , 2010Ioppolo et al , 2011aMatar et al 2008;Oba et al 2009Oba et al , 2010Cuppen et al 2010;Mokrane et al 2009;Romanzin et al 2011;Ö berg et al 2009). These experiments prove that species like H 2 CO, CH 3 OH and H 2 O can be formed at low temperatures by simple hydrogenation (i.e., without the need for thermal, UV or cosmic ray processing) and provide the basic molecular data to simulate their formation on astronomical timescales (e.g., Cuppen et al 2009), even though the ice as a whole is not representative for a realistic astronomical ice. Figure 3 shows the schematic representation of the reaction network that is presented here.…”

“…This is probably due to the reduced residence-time of H atoms into the ice at those temperatures, which decreases the probability that hydrogenation reactions occur. Since the rate of formation of molecules depends on a sequence of events: deposition of H atoms, diffusion and then reaction, reaction barriers can only be determined using a detailed model to fit the experimental data (e.g., Fuchs et al 2009;Cuppen et al 2009 used in an astrochemical model to investigate the formation of formaldehyde and methanol under interstellar conditions (see Sect. 5).…”

“…Simulations for different interstellar parameters, including density and temperature, have been performed in Cuppen et al (2009). Formaldehyde and methanol were found to form efficiently in cold dense cores or the cold outer envelopes of young stellar objects.…”

Detecting Weak Spectral Lines in Interferometric Data through Matched Filtering

“…Similarly, lines of a known species can be used as a filter for an undetected but chemically related molecule that is presumed to be co-spatial. The bottom row panels of Figure 2 present observations of H 2 CO around TW Hya (Öberg et al 2017) which could be used as a filter for CH 3 OH emission, due to their linked formation pathways (e.g., Cuppen et al 2009;Qi et al 2013;Walsh et al 2014;Loomis et al 2015). Details of the observations and kernel generation are presented in Appendix D.…”