SIMULATING H2O MASER EMISSION IN THE MASS OUTFLOWS FROM EVOLVED STARS

The combination of a time-dependent spherically symmetric hydrodynamic model of stellar atmosphere pulsation and a radiation transport code, which incorporates maser saturation theory, enabled us to synthesise maps and spectra of H2O maser emission from the circumstellar envelope s of long period variable stars.The synthetic maps and spectra compare favourably with observed 22, 321 and 325 GHz H2O maser emission. As i s observed in H2O maser regions the peak emission occurs between 3-8 s tellar radii from the star. The calculated H2O maser regions are in co nditions of n(H2) = 10(6) - 10(8) cm(-3) assuming a fractional abundan ce of 10(-4); kinetic temperatures of 550-3000 K; dust ensemble temper atures of 500-1200 K and an accelerating velocity held, The IR radiati on held is explicitly included in the radiation transport model, incor porating the latest absorption efficiency data for silicates from Drai ne. We reproduce the features seen in high angular resolution MERLIN s pectral line datacubes. This shows that a mass outflow model which ext ends the photosphere using pulsations and incorporates radiation press ure on silicate based dust particles can produce the observed data on small (10-mas) angular scales.