A thermal-infrared and millimeter-wave study of evolved stars, and proto-planetary and planetary nebulae

Abstract

The evolution of low- and intermediate-mass stars (1M(⊙) - 8M(⊙)) from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PN) phase is a poorly understood phase of stellar evolution. We have observed a sample of AGB stars, Proto-Planetary and Planetary Nebulae (PPNe and PNe) at thermal-infrared and millimeter wavelengths. The thermal-infrared emission comes from the warm dust (T(dust) ≈ 100-300 K) circumstellar shells. Images at these wavelengths provide a unique "close-up" look at the morphology of these sources, and therefore allow us to constrain the geometry and/or mass loss rates on short dynamical timescales. The millimeter-wave observations probe the extended circumstellar molecular envelopes; therefore they provide valuable spatial and kinematical information on the larger scales (and over longer dynamical timescales) than the mid-IR images. Our results show that the dust shells of all the planetary and proto-planetary nebulae (PPNe and PNe) in our study are not spherical; those that are well-resolved appear to be bipolar and can be modeled with axially symmetric models. They strongly suggest that the evolution from AGB to PN is often accompanied by higher mass loss rates in the equatorial plane than in the polar regions, as predicted by binary star (common envelope) evolution models, or models involving stellar rotation. At least one bipolar nebula (M 4-18) appears to be a single, low-mass star. From our mid-IR images at feature wavelengths we find that the spatial distribution of the carbonaceous and silicate grains varies over the surface of the nebulae. IC 5117 shows evidence for stratification of carbon- and oxygen-based grains and suggests that some PNe go through temporal changes in circumstellar chemistry (from oxygen-rich to carbon-rich) as they evolve off the AGB. The molecular envelopes of the sources in our study can be modeled with spherically symmetric models though two sources (IRC + 10216 and AFGL 2343) show kinematical and spatial evidence for departures from spherical symmetry. Our millimeter observations of IRC + 10216 also provide confirmation that the interstellar UV photons initiate a variety of photochemical reactions in circumstellar envelopes of AGB stars and PNe.