A MODEL FOR THE CIRCUMSTELLAR ENVELOPE OF WX-SER

A model is presented for the circumstellar envelope of the M-giant Mira variable star, WX Ser, at maximum photometric light. The model is based upon a comprehensive analysis of the transfer of radiation through the star's circumstellar dust envelope. Optical constants for amorphous silicate grains derived from laboratory measurements of olivine grains are tested against high signal-to-noise ratio observations of WX Ser, and a dust model is produced for its circumstellar envelope. In the 8-23 mum region the published optical constants for silicate dust grains are found to give a poor fit to the observations of WX Ser, and a new composite dielectric function is presented for the silicate grains in the shell of WX Ser which gives a better fit to the observations in that region of the spectrum. The best model for WX Ser assumes a stellar temperature of 1960 K, and a dust-to-gas ratio of 3.0 X 10(-3), if the gas mass-loss rate is 2.2 X 10(-7) M. yr-1. A dust condensation temperature of 1 000 K is found to give the best fit to observations of the star. In the 20-100 mum region the energy distribution of WX Ser may be best understood if the far-IR emissivity of silicate dust grains is allowed to vary as lambda-1.5.