Mixing and Nucleosynthesis in Low- and Intermediate-Mass AGB Stars
The existence of carbon stars brighter than M$\sb {\rm bol}$ = $-4$ can be understood in terms of dredge up in thermally pulsing asymptotic giant branch (AGB) stars. Models in this study include the effect that carbon recombination has upon opacity, and it is shown that a low-metallicity (Z = 0.001), low-envelope mass (0.1 M$\sb\odot$), low-core mass (0.6 M$\sb\odot$) AGB star can transform into a carbon star with a brightness $-5.0$ $<$ M$\sb{\rm bol}$ $<$ $-4.2$. In models that use a mixing length to pressure scale height ratio of 1.5 (with no overshoot), the transport of $\sp $C outward stops (a few) $\cdot$ 10$\sp{-5}$ M$\sb\odot$ below the convective envelope. If a limited amount of convective overshoot is included in these models, dredge up of interior matter will occur when the post-pulse luminosity is $-4.9$ $<$ M$\sb{\rm bol}$ $<$ $-4.1$.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign