An Abundance Analysis of Two Carbon‐rich Proto–Planetary Nebulae: IRAS Z02229+6208 and IRAS 07430+1115

Abstract

In this paper, we present an LTE abundance analysis of two new proto-planetary nebulae, IRAS Z02229+6208 and IRAS 07430+1115, based on high-resolution (R ≈ 55,000) optical echelle spectra. Results show that both stars are metal-poor ([Fe/H] = -0.5) and overabundant in C, N, and s-process 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/Fe] = +0.4, and [s-process/Fe] = +1.6 for IRAS 07430+1115. These abundances suggest that the stars have experienced nucleosynthesis on the asymptotic giant branch (AGB), and the resultant products of CNO, 3α, and s-process reactions were brought to the photosphere during shell flashes and deep mixing episodes during the AGB phase of their evolution. Of major significance is the measurement of a high Li abundance in both stars, log (Li) ≈ 2.3 and 2.4 for IRAS Z02229+6208 and IRAS 07430+1115, respectively. This may be the result of hot bottom burning, below the deep convective zone. We also present an analysis of the circumstellar molecular (C2 and CN) and atomic (Na I and K I) absorption spectra of both stars. We derive rotational temperatures, column densities, and envelope expansion velocities using molecular C2 Phillips and CN Red system bands. The values derived for expansion velocities, 8-14 km s-1, are typical of the values found for post-AGB stars. IRAS 07430+1115 is unusual in that it shows P Cygni-shaped C2 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. Including these two stars, the number of post-AGB stars for which clear C, N, and s-process elemental overabundances are found rises to eight. IRAS Z02229+6208 is known to possess the 21 μm emission feature in its mid-infrared spectrum; these results support the idea that all 21 μm emission stars are carbon-rich post-AGB stars.