The transition from AGB to post-AGB evolution as observed by AKARI and Spitzer

TL;DR

This study uses AKARI and Spitzer data to analyze the spectral energy distributions of nearly 200 post-AGB stars, revealing diverse dust compositions and suggesting a common presence of carbon-rich dust in post-AGB winds.

Contribution

It provides a comprehensive spectral analysis of post-AGB stars, highlighting dust composition diversity and proposing that post-AGB winds are universally carbon-rich regardless of prior AGB chemistry.

Findings

Diverse dust compositions including oxygen-rich, carbon-rich, and intermediate types.

Presence of crystalline dust features in some oxygen-rich stars.

Evidence that post-AGB winds are always carbon-rich regardless of previous AGB chemistry.

Abstract

The AKARI and Spitzer satellites provided an unique opportunity to observe a variety of stars, which are considered as departing from the Asymptotic Giant Branch (AGB) and have started their post-AGB evolution recently. Most of these stars are absent optically and are bright in the mid-IR wavelength range. Spectra of close to 200 objects have been obtained. For all of them the 1-60 micron spectral energy distribution has been constructed using photometric data from various surveys. We report here on the results of Spitzer observations of 88 IRAS selected post-AGB candidates and discuss them in comparison to the results of the AKARI observations of post-AGB candidates reported elsewhere in these proceedings. The dust compositions can be divided broadly in oxygen- and carbon-rich types, but a variety of intermediate types have been found. Among the oxygen-rich stars amorphous dust prevails, but a few sources show emission features from crystalline dust. The spectra from carbon-rich shells may be completely featureless, may show emission features from PAHs or a molecular absorption line from C2H2. We found also sources with a neon emission line at 12.8 micron. More than a third of all sources show a near-infrared excess at < 5 micron and almost all of them show evidence of C-rich dust in their shells. We postulate that the emerging post-AGB wind after the end of AGB evolution contains always carbon-rich dust irrespective of the chemistry of the former AGB star.