MIDINFRARED IMAGING OF AGB STAR ENVELOPES .1. COLORS, CLASSIFICATION AND MASS-LOSS ESTIMATES

Mid-infrared images obtained with the TIRCAM camera are presented for a sample of 16 sources including both O-rich and C-rich AGB and post-A GB stars. Photometry and colors are derived using 10% band with filter s at 8.8, 9.8, 11.7 and 12.5 mu m. These are compared with photometric data and Low Resolution Spectra (LRS) from IRAS. The observations for m the basis for developing photometric criteria suitable to discrimina te between O-rich and C-rich sources and to derive mass loss rates. Th e evolutionary status of the observed stars and the consequences for t he mass loss history on the AGB are discussed. Finally, for the C-rich star CW Leo we show evidence for a geometrical extension of the envel ope at 12.5 mu m.