19 - Chemical abundances

Summary

Progenitors of PN play an important role in the chemical enrichment of the Galaxy because He and significant amounts of C and N are produced by nucleosynthesis during the MS, RG, and AGB stages of their evolution. These elements are brought to the surface of the star by convection events called dredge ups (Iben, 1991). The first dredge up occurs in all stars on the RGB following the exhaustion of H in the core. Convection extending into the interior brings material processed in the CNO cycle to the surface. The second dredge-up occurs only in higher mass (≥3 M_⊙) stars during the early AGB phase and results in an increase in the surface abundance of ⁴ He and ¹⁴ N at the expense of ¹² C and ¹⁶ O. The third dredge-up occurs on the AGB after each He-shell flash, resulting in He, C, and s-process elements being brought to the surface (Renzini and Voli, 1981). This repeated enhancement of C abundance in the stellar surface may in some stars bring the C/O ratio to be greater than unity. Since the less abundant of these two elements is completely tied up in the CO molecule, the photospheric spectrum of the star will change from O-based (e.g., TiO and VO) to C-based (e.g., C₂ and CN), and a carbon star is created (see Section 10.2). Stellar winds during the AGB eject these elements from the stellar surface and therefore enrich the interstellar medium.