A new model for the formation of grain mantles in Prestellar Cores

Details A new model for the formation of grain mantles in Prestellar Cores A new model for the formation of grain mantles in Prestellar Cores

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.348t&link_type=abstract

The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole

Astronomy and Astrophysics

Astronomy

Scientific paper

The understanding of the formation of Complex Organic Molecules (COMs), observed in large quantities in massive hot cores since two decades (Blake et al. 1987), has gained a renewed interest in the last few years after the detection of abundant COMs also in solar type protostars, specifically in hot corinos (Cazaux et al. 2003), and in the clouds of the Galactic Center (Requena-Torres et al. 2007). Many of these COMs are assumed to be mainly formed on interstellar grains, which act as catalysts for chemical reactions (Tielens & Hagens 1982). This is the case of the methyl formate (HCOOCH_3), assumed to be formed via reactions between radicals such as CH_3O and HCO during the warm-up phase of the formation of protostars (Garrod & Herbst 2006). However, the state of art astrochemical models tend to underestimate the abundance of this COM, as shown in the figure below, where we report the observed and modelled HCOOCH_3/CH_3OH abundance ratios. Observations show that a ratio of 0.1 and 1 can be reached in hot-cores and in hot-corinos respectively whereas astrochemical models predict ratios up to 0.01 at most. These previous models assumed that radicals as CH_3O and HCO are produced by the photodissociation of species frozen onto the grain mantles by CR induced FUV emission, a difficult effect to quantify and, therefore, relatively uncertainty. Here, we have developed a new model which shows that radicals can be naturally trapped in grain mantles during the cold and dense pre-collapse phase, namely in prestellar cores. The new model is based on two key points: 1) the adoption of a layer/layer approach, where we distinguish the chemical processes that happen in the mantle inert bulk and on the reactive surface, as also suggested by the results of the Monte Carlo models by Cuppen and collaborators (Cuppen et al. 2009); 2) the influence of the porous structure of grains, which allows light particles to stay on the surface for a longer time than in non-porous grains, as also suggested by Perets & Biham (2006). We will present the results of this new model. Specifically, we will discuss the influence of the computed mantle composition on the assumed parameters that are either variable, like the density, or uncertain, like the porosity. Finally, we will compare our results with those from previous models, and with observations of ices.

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