2. The nature of the instability
We consider radiative/thermal instabilities in a dust forming system as illustrated in Fig. 1. The following assumptions on the important interactions in the system are made:
As will be discussed in Sect. 3, an increase of the opacity may either cause a decrease (negative feedback) or an increase (positive feedback) of the mean intensity via radiative transfer effects, depending on the circumstances. In any case, a control loop is constituted (see Fig. 1) which acts on relatively short time scales, where the slowest process inside the loop likely is the dust formation (see Sect. 5). If the latter influence is negative, the overall feedback in the control loop is positive (). In such a case the control loop is self-amplifying, i.e. unstable against small perturbations which may arise e.g. from fluctuations. This kind of instability possibly causes a spatial structuring of the dust forming medium. In the opposite case, initial perturbations are damped by the dust forming system. According to the assumptions outlined, the stability of dust forming regions is hence controlled by the non-local effect of opacity variations on the mean intensity, which is investigated below.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000