Radial molecular abundances and gas cooling in starless cores

Department of Physics, PO Box 64, 00014 University of Helsinki, Finland
e-mail: olli.sipila@helsinki.fi

Received: 21 February 2012
Accepted: 15 May 2012

Abstract

Aims. We aim to simulate radial profiles of molecular abundances and the gas temperature in cold and heavily shielded starless cores by combining chemical and radiative transfer models. Attention is also given to the time-evolution of both the molecular abundances and the gas temperature.

Methods. A determination of the dust temperature in a modified Bonnor-Ebert sphere is used to calculate initial radial molecular abundance profiles. The abundances of selected cooling molecules corresponding to two different core ages are then extracted to determine the gas temperature at two time steps. The calculation is repeated in an iterative process yielding molecular abundances consistent with the gas temperature. Line emission profiles for selected substances are calculated using simulated abundance profiles.

Results. The gas temperature is a function of time; the gas heats up as the core grows older because the cooling molecules are depleted onto grain surfaces. The change in gas temperature associated with depletion is of the order of 1 K. The contributions of the various cooling molecules to the total cooling power change with time, but the main cooling molecule at all times, in the range of environments studied here, is CO. Radial chemical abundance profiles are non-trivial: different species present varying degrees of depletion and in some cases inward-increasing abundances profiles, even at t > 10⁵ years. Line emission simulations indicate that cores of different ages can present significantly different line emission profiles, depending on the tracer species considered.

Conclusions. Chemical abundances and the associated line cooling power change as a function of time. Most chemical species are depleted onto grain surfaces at densities exceeding  ~10⁵ cm^-3. Notable exceptions are NH₃ and N₂H ⁺ ; the latter is largely undepleted even at n_H ~ 10⁶ cm^-3. On the other hand, chemical abundances are not significantly developed in regions of low gas density even at t ~ 10⁵ years, revealed by inward-increasing abundance gradients. Except in high-density regions where the gas-dust coupling is significant, the gas temperature can be significantly different from the dust temperature. This may have implications on core stability. Owing to the potentially large changes in line emission profiles induced by the evolving chemical abundance gradients, our models support the idea that observed line emission profiles can, to some extent, be used to constrain the ages of starless cores.