Modeling the chemical evolution of a collapsing prestellar core in two spatial dimensions

Abstract

Context. The physical conditions in a collapsing cloud can be traced by observations of molecular lines. To correctly interpret these observations the abundance distributions of the observed species need to be derived. The chemistry in a collapsing molecular cloud is not in a steady state as the density and temperature evolve. We therefore need to follow chemical reactions, both in the gas phase and on dust grains, as well as gas-grain interactions, to predict the abundance distributions. Aims. Our aim is to model the abundances of molecules, in the gas phase and on grain mantles in the form of ice, from prestellar core collapse to disk formation. We want to investigate the need for grain surface reactions and compare our results with observed abundances, column densities, and ice-mantle compositions. Methods. We use a 2-dimensional hydrodynamical simulation as a physical model from which we take the density, temperature, and the flow of the gas. Trace particles, moving along with the gas, are used to follow the chemistry during prestellar core collapse and disk formation. We calculated abundance profiles and column densities for various species. The evolution of these abundances and the composition of ices on grain mantles were compared to observations and we tested the influence of grain surface reactions on the abundances of species. We also investigated the initial abundances to be adopted in more detailed modeling of protoplanetary disks by following the chemical evolution of trace particles accreting onto the disk. Results. Fractional abundances of HCO + , N 2 H + , H 2 CO, HC 3 N, and CH 3 OH from our model with grain surface reactions provide a good match to observations, while abundances of CO, CS, SO, HCN, and HNC show better agreement without grain surface reactions. The observed mantle composition of dust grains is best reproduced when we include surface reactions. The initial chemical abundances to be used for detailed modeling of a protoplanetary disk are found to be different from those in dark interstellar clouds. Ices with a binding energy lower than about 1200 K sublimate before accreting onto the disk, while those with a higher binding energy do not.

Physical Processes in the Interstellar Medium

“…The resulting chemical stratification significantly complicates the interpretation of molecular line observations, and again requires the use of sophisticated chemical models which have to be coupled to the dynamical evolution (e.g. Aikawa et al, 2008;van Weeren et al, 2009;Furuya et al, 2012). From the observational side, the freeze-out of many molecules makes it difficult to use their emission lines for probing the physical conditions in the inner regions of the cores.…”

Modeling the processing of interstellar ices by energetic particles

“…Roberts et al 2007;Semenov et al 2010;van Weeren et al 2009). In terms of this model it can be described as a porous layer, fully exposed to surface reactions.…”

Ortho-H2and the age of prestellar cores

“…Nilsson et al (2000) have shown, however, that this ratio depended on multiple factors, time evolution being only one of them, and other conditions such as the initial O/C + ratio would strongly influence the result making the SO/CS ratio an unreliable clock if the other parameters cannot be estimated with some accuracy. The existence of chemical clocks has not yet been established (van Weeren et al 2009) but the situation is now evolving: we recently presented a chemical model that clearly constrains the age of molecular clouds below 10 My (Pagani et al 2011).…”