The Color Evolution of Carbon Stars

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Session 46 -- Late Type Stars
Display presentation, Wednesday, 9:20-6:30, Heller Lounge Room

[46.15] The Color Evolution of Carbon Stars

S. Josephine Chan (Institute of Astronomy, National Central University and Department of Physics and Astronomy, University of Calgary)

The discovery of far infrared excesses in visual carbon stars (Thronson et al.~1987, ApJ, 322, 770) has led to the theory that visual carbon stars descend from mass-losing oxygen-rich stars and the excess is due to the remnant of the oxygen-rich silicate dust shell (Chan and Kwok 1988, ApJ, 334, 362). This distribution of visual carbon stars in the IRAS color-color diagram can be successfully explained by this `detached-shell' model. On the other hand, infrared carbon stars (stars with circumstellar SiC features) have a distribution in the color-color diagram along the blackbody line, and they are currently undergoing large-scale mass loss (Chan and Kwok 1990, A\&A, 237, 354). Assuming that visual carbon stars evolve into infrared carbon stars, there remains a group of transition objects distributed in the shape of a `C' between the visual and infrared carbon stars in the IRAS color-color diagram.

A set of self-consistent model calculations of the evolution from visual carbon stars to infrared carbon stars is presented. A two-shell system model, the Interrupted Mass Loss Model (oxygen-rich detached shell and newly-forming SiC dust shell) has been developed (Chan 1993, PASP, submitted). The model tracks successfully explain a `C' shaped distribution of carbon stars in the IRAS color-color diagram. A new mass-loss formula for a newly-forming SiC shell has been introduced. The model spectra produce excellent fits to the observed energy distributions of $\sim$~110 transition objects. The existence of these transition objects with infrared properties intermediate between visual and infrared carbon stars, lends further supports to the idea of an evolutionary link between visual and infrared carbon stars.

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