An abundance analysis of two carbon-rich proto-planetary nebulae IRAS Z02229+6208 and iras 07430+1115

In this paper, we present an LTE abundance analysis of two new proto-planet ary nebulae, IRAS Z02229+6208 and IRAS 07430+1115, based on high-resolution (R approximate to 55,000) optical echelle spectra. Results show that both stars are metal-poor ([Fe/H] = -0.5) and overabundant in C, N, and s-proces s elements. The average elemental abundances are [C/Fe] = +0.8, [N/Fe] = +1 .2, and [s-process/Fe] = +1.4 for IRAS Z02229+6208, and [C/Fe] = +0.6, [N/F e] = +0.4, and [s-process/Fe] = +1.6 for IRAS 07430+1115. These abundances suggest that the stars have experienced nucleosynthesis on the asymptotic g iant branch (AGB), and the resultant products of CNO, 3 alpha, and s-proces s reactions were brought to the photosphere during shell flashes and deep m ixing episodes during the AGE phase of their evolution. Of major significan ce is the measurement of a high Li abundance in both stars, log epsilon(Li) approximate to 2.3 and 2.4 for IRAS Z02229 + 6208 and IRAS 07430 + 1115, r espectively. This may be the result of hot bottom burning, below the deep c onvective zone. We also present an analysis of the circumstellar molecular (C-2 and CN) and atomic (Na I and K I) absorption spectra of both stars. We derive rotational temperatures, column densities, and envelope expansion v elocities using molecular C-2 Phillips and CN Red system bands. The values derived for expansion velocities, 8-14 km s(-1) are typical of the values f ound for post-AGE stars. IRAS 07430+1115 is unusual in that it shows P Cygn i-shaped C-2 emission profiles in the spectra of the circumstellar envelope . A minimum distance for IRAS Z02229+6208, determined from interstellar Na I lines, suggests that it is evolved from an intermediate-mass star. Includ ing these two stars, the number of post-AGE stars for which clear C, N, and s-process elemental overabundances are found rises to eight. IRAS Z02229+6 208 is known to possess the 21 mu m emission feature in its mid-infrared sp ectrum; these results support the idea that all 21 mu m emission stars are carbon-rich post-AGE stars.