ASTRONOMY AND ASTROPHYSICS Monte Carlo transition probabilities

Transition probabilities governing the interaction of energy packets and matter are derived that allow Monte Carlo NLTE transfer codes to be constructed without simplifying the treatment of line formation. These probabilities are such that the Monte Carlo calculation asymptotically recovers the local emissivity of a gas in statistical and radiative equilibrium. Numerical experiments with one-point statistical equilibrium problems for Fe II and Hydrogen confirm this asymptotic behaviour. In addition , the resulting Monte Carlo emissivities are shown to be far less sensitive to errors in the populations the emitting levels than are the values obtained with the basic emissivity formula.