Radiative Heating and Cooling in Circumstellar Envelopes

Peter Woitke

Dissertation, Technische Universität Berlin, 1997

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Abstract:

This thesis investigates the thermal state of diluted gases being exposed to stellar radiation fields. On the basis of a steady-state non-LTE description, the radiative heating and cooling rates of the gas are determined, considering the typical densities present in circumstellar envelopes.

The following radiative processes are examined: line transitions of neutral and singly ionized atoms, vibrational and rotational transitions of polar diatomic and linear molecules, respectively, quadrupole transitions of H_2, bound-free transitions from the electronic ground states and (in case of hydrogen) from excited electronic levels, photodissociation and free-free transitions.

A thermodynamic description of the gas is developed which allows for a time-dependent determination of the temperature structure in the circumstellar envelopes of cool and warm stars and can be included into more complex, e.g. hydrodynamic, model calculations. Three applications of this description are presented:

First, the stable radiative equilibrium states of the gas are calculated for the circumstellar envelopes of R Coronae Borealis (RCB) stars. It is found that the condition of radiative equilibrium is not sufficient in order to determine the temperature of the gas. More than one temperature solution may exist for fixed conditions of pressure and radiation field. Thus, a spatial coexistence of hot, atomic and cool, molecular phases is principally conceivable ("thermal bifurcations").

Second, the relaxation process towards radiative equilibrium is studied in the circumstellar envelopes of C-stars. The character of the thermal relaxation behind propagating shock waves, which are caused by a pulsation of the central star, is discussed. It is found that the gas must be sufficiently dense in order to be capable to reestablish radiative equilibrium after the passage of such shocks. For densities <10^8cm^{-3}, the behavior of the gas becomes more and more adiabatic, so that finally the condition of radiative equilibrium looses its significance concerning the determination of the temperature structure.

Third, the time-dependent behavior of the gas in the circumstellar envelopes of pulsating RCB stars is investigated more detailed. A model for shock levitated atmospheres is developed, where the gas is periodically heated and compressed by shock waves and re-expands between the shocks. Within a distinct density interval the gas is found to undergo a two-step cooling process, consisting of radiative cooling at high temperatures followed by adiabatic expansion at low temperatures. In this case a considerable supercooling of the gas occurs, temporarily producing temperatures below 1500K (far below the values expected from radiative equilibrium) at radial distances as small as 1.5-3R_*, despite of the high effective temperatures of these stars. Thus, this thesis states the hypothesis that the onset of dust formation close to RCB stars is caused by shock waves, which might trigger the spectacular decline events.