CNO abundances in Cepheids and supergiants - Theoretical implications
Abstract
An effort is made to explain the observed nitrogen enhancement and carbon and oxygen depletions in yellow supergiants and Cepheids by means of models of a nine-solar mass star. Although the alteration of the normal or near normal abundances of CNO for the main sequence stars, when they become red giants with deeply mixed convective envelopes prior to core He ignition, results in N enhancement and C reduction, it is noted that the observed CNO abundance changes are much larger. Additional mechanisms that must then be considered include the effects of primordial abundances, mass loss and mass transfer, convective overshoot, rotation, and magnetic fields. A simplified three-zone model is proposed for the mixing between the convective H-burning core and the inner part of the radiative envelope, where the additionally mixed material is later dredged up during the red giant phase.
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