On the stability of nonisothermal Bonnor-Ebert spheres
II. The effect of gas temperature on the stability
Max-Planck-Institute for Extraterrestrial Physics (MPE),
2 Department of Physics, PO Box 64, 00014 University of Helsinki, Finland
Received: 27 May 2014
Accepted: 23 August 2015
Aims. We investigate the stability of nonisothermal Bonnor-Ebert spheres in the context of a model that includes a self-consistent calculation of the gas temperature. In this way, we can discard the assumption of equality between the dust and gas temperatures and study the stability as the gas temperature changes with the chemical evolution of the cooling species.
Methods. We use a gas-grain chemical model to calculate the chemical evolution. The model includes a time-dependent treatment of depletion onto grain surfaces, which strongly influences the gas temperature as the main coolant molecule CO depletes from the gas. The dust and gas temperatures are solved with radiative transfer calculations. For consistent comparison with previous work, we assume that the cores are deeply embedded in a larger external structure, corresponding to visual extinction AVext = 10 mag at the core edge. We also study the effect of lower values of AVext.
Results. We find that the critical nondimensional radius ξ1 derived here, which determines the maximum density contrast between the core center and the outer boundary, is similar to our previous work where we assumed Tdust = Tgas. Here, the ξ1 values lie below the isothermal critical value ξ0 ~ 6.45, but the difference is less than 10 %. We find that chemical evolution does not notably affect the stability condition of low-mass cores (< 0.75 M⊙), which have high average densities and a strong gas-grain thermal coupling. In contrast, for higher masses the decrease in cooling due to CO depletion causes substantial temporal changes in the temperature and in the density profiles of the cores. In the mass range 1−2 M⊙, ξ1 decreases with chemical evolution, whereas above 3 M⊙, ξ1 instead increases with chemical evolution. We also find that decreasing AVext strongly increases the gas temperature, especially when the gas is chemically old, and this causes ξ 1 to increase with respect to models with higher AVext. However, the derived ξ1 values are still close to ξ0. The density contrast between the core center and edge derived here varies between 8 and 16 depending on core mass and the chemical age of the gas, compared to the constant value ~14.1 for the isothermal BES.
Key words: radiative transfer / ISM: clouds / astrochemistry / ISM: molecules
© ESO, 2015