Hydrodynamics with gas-grain chemistry and radiative transfer: comparing dynamical and static models

Abstract

We quantify if the chemical abundance gradients given by a dynamical model of core collapse including time-dependent changes in density and temperature differ greatly from abundances derived from static models, where the density and temperature structures of the core are kept fixed as the chemistry evolves. For this study we developed a new one-dimensional spherically symmetric hydrodynamics code that couples the hydrodynamics equations with a comprehensive time-dependent gas-grain chemical model, including deuterium and spin-state chemistry, and radiative transfer calculations to derive self-consistent time-dependent chemical abundance gradients. We applied the code to model the collapse of a starless core up to the point when the infall flow becomes supersonic. The abundances predicted by the dynamical and static models are almost identical during the quiescent phase of core evolution, but the results start to diverge after the onset of core collapse, where the static model underestimates abundances at high medium density (inner core) and underestimates them at low density (outer core), and this is clearly reflected in simulated lines. The static model generally overestimates deuteration, which is increasingly evident the more D atoms are substituted in the molecule. We also find that using a limited chemical network, or a limited set of cooling molecules, may lead to an overestimate of the collapse timescale, and in some cases may prevent the collapse altogether. In our model, most of the line cooling near the center of the core is due to HCN, CO, and NO. In conclusion, the use of a static physical model is not a reliable method of simulating chemical abundances in starless cores after the onset of gravitational collapse. The adoption of complex chemistry and a comprehensive set of cooling molecules is necessary to model the collapse adequately.

Accepted by A&A; 15 pages, 12 figures, 5 tables; arXiv abstract heavily modified and redacted from the original

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