Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...285..953b&link_type=abstract
Astronomy and Astrophysics 285, 953-970 (1994)
Astronomy and Astrophysics
Astrophysics
62
Masers, Stars: Asymptotic, Post-Asymptotic Giant Branch (Agb, Post-Agb), Stars: Circumstellar Matter, Stars: Late Type, Radio Lines: Stars
Scientific paper
A model for the excitation and emission of SiO masers in evolved stars, that includes a non-local treatment of the radiative transfer, is presented. The approximations made and their effects, the sources of numerical noise, and in general the reliability of the model are discussed. The code is shown to give an important amount of information on the theoretical expectations for SiO maser emission. The masers are supposed to be placed in an inner quasi-stationary layer of the circumstellar envelope, the properties of which are discussed in view of the available information. The numerical predictions are generally in good agreement with the observational data. In particular, the calculated intensity attains the measured level for a wide range of physical conditions. The maser pump is found to be dominantly radiative, acting through the {DELTA}v=1 SiO vibrational transitions. Collisional pump is also possible, but probably gives only a marginal contribution to the total maser output. The radiative maser pump naturally explains the observed dependence of the maser intensity on the stellar infrared continuum, both from object to object and during the stellar variability cycle. However some observational features, like the well known low intensity of 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; possible explanations to this disagreement are discussed. This is the first paper of a series, of which the second part deals with the predicted spatial extent of the SiO maser emission.
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