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Session 114 - Circumstellar Material & Nebulae.
Oral session, Saturday, January 10
Georgetown,
The IRAS [12]-[25], [25]-[60] two-color diagram (van der Veen and Habing 1988 Aamp;A 194, 125) has proved to be a valuable tool for the study of the dust/gas envelopes produced by mass loss from cool luminous AGB stars. The data for these stars form a sequence of incresing [12]-[25] color, which models show to be a sequence of increasing mass loss.
A survey of CO(3-2) emission from circumstellar envelopes was performed to produce observations of a magnitude-limited sample of stars in each region of the two-color diagram. These observations provide stellar radial velocities and (with the use of a line-formation model) mass loss rates. The observations confirm that the sequence of incresing [12]-[25] color is a sequence of increasing mass loss rate, and connects the sequence to the stellar progenitor mass by measuring the velocity dispersion as a function of location in the two-color diagram. Because the distances to many of these stars are very poorly determined, and because galactic rotation contributes to the radial velocities, the analysis was done using a new maximum-likelihood technique which incorporates the relationship between velocity dispersion and scale height.
The velocity dispersion was found to decrease strongly with increasing mass loss rate: the diagram therefore shows not an evolutionary sequence but a sequence of stellar initial mass. Low mass (solar mass) stars have low mass loss rates while higher progenitor-mass stars have higher mass loss rates, probably as a result of the larger core mass and hence luminosity at the beginning of AGB evolution. The stas with the lowest mass loss rates are an exception; they have an anomalously low velocity dispersion, possibly indicating a mixture of stellar masses biased towards high-mass stars by magnitude selection effects.
