NUMERICAL-CALCULATIONS OF SIO MASER EMISSION .1. INTENSITY AND VARIABILITY

A model for the excitation and emission of SiO masers in evolved stars , that includes a non-local treatment of the radiative transfer, is pr esented. The approximations made and their effects, the sources of num erical noise, and in general the reliability of the model are discusse d. The code is shown to give an important amount of information on the theoretical expectations for SiO maser emission. The masers are suppo sed to be placed in an inner quasi-stationary layer of the circumstell ar envelope, the properties of which are discussed in view of the avai lable information. The numerical predictions are generally in good agr eement with the observational data. In particular, the calculated inte nsity attains the measured level for a wide range of physical conditio ns. The maser pump is found to be dominantly radiative, acting through the DELTAv=1 SiO vibrational transitions. Collisional pump is also po ssible, but probably gives only a marginal contribution to the total m aser output. The radiative maser pump naturally explains the observed dependence of the maser intensity on the stellar infrared continuum, b oth from object to object and during the stellar variability cycle. Ho wever some observational features, like the well known low intensity o f the v=2 J=2-1 line and the possible spatial coincidence of the J=1-0 v=1 and v=2 masers, are not reproduced by the numerical results; poss ible explanations to this disagreement are discussed. This is the firs t paper of a series, of which the second part deals with the predicted spatial extent of the SiO maser emission.