John C. Lattanzio

AGB Stars and the Observed Abundance of Neon in Planetary Nebulae

Asymptotic giant branch stars are expected to produce 22 Ne through the combined H and He burning that operates during their thermally pulsing evolution. However, observationally there is a fairly tight correlation between the O and Ne abundances as measured in planetary nebulae in various populations. In this paper we use recent detailed stellar evolutionary calculations for compositions appropriate to the Galaxy and the Large Magellanic Cloud, in an attempt to determine if the models are consistent with the observed abundances. We show that there is only a narrow range in stellar mass, about 2 to 4 M� (lower for lower (Fe/H)) where 22 Ne is produced in sufficient quantities to affect the total observed elemental neon abundance, which is mostly 20 Ne. The models appear to be consistent with the observations, but a more thorough analysis is required.

The evolution and C, N and O yields of intermediate-mass Z = 10 -5 stars in isolation and in close binary systems

We have computed the evolution of Z = 10−5 stars of masses between 4 and 9 M⊙, from their main sequence till the late TP-(S)AGB phase. We use a recent Monash version of the Mount Stromlo Stellar Evolution code, in which molecular opacities include the effects of variable C=O abundances ratio, [1]. By computing hundreds (or thousands) of thermal pulses, we have been able either to remove the bulk of the stellar envelopes or to obtain stellar cores very close to MCh. Using [2] prescription for the mass loss rates the computed stars lose their envelopes before their cores reach MCh. This would prevent the occurrence of SN 1.5 for Z = 10−5 stars. Nevertheless the results by [3] suggest that the former prescription might overestimate the mass-loss rates. Therefore we have decreased the rates by [2]. For all the cases we present, even a decrease of one order of magnitude let the stellar cores reach MCh before the envelope is lost. Therefore the occurrence of SN1.5 at Z = 10−5 and their potential contribution to...

DIVISION IV / WORKING GROUP ABUNDANCES IN RED-GIANTS

1. Meetings The main activity of the WG on Abundances in Red Giants has been to propose a JD for the IAU GA in 2009. The increasing evidence for distinct populations within globular clusters is leading to the view that there is a continuum between globular clusters and the smallest of the galaxies. Our JD was designed to investigate this link. However, our JD was incorporated into IAU Symposium No. 266 Star Clusters: Basic Building Blocks throughout Time and Space for the IAU XXVII in Rio de Janeiro, 2009. We will be responsible for organising one session in the Symposium to cover the agenda put forward in our JD proposal. On a similar topic, many researchers associated with the WG will be attending a MaxPlanck Workshop on the similar topic Chemical Evolution of Dwarf Galaxies and Stellar Clusters in July 2008. Members of the WG were closely involved in the organisation of a Royal Astronomical Society specialist discussion meeting on Super-AGB stars, in London, February 2008. The conclusion of this meeting was that the single biggest problem for these stars is mass-loss, because it is both crucial and unknown.

DIVISION IV / WG: ABUNDANCES IN RED GIANTS

Unfortunately the Business Meeting clashed with interesting sessions on stellar convection theory that were very relevant to most members of this Working Group. Hence the attendance was very small, and some preliminary discussions were later followed up by email among the Organising Committee members.

How binary stars affect galactic chemical evolution

At least 60% of stars appear to be binary and about half of these are close enough to interact. Because of the enormous expansion on the AGB, many of these interactions will involve an AGB star and a relatively compact companion, anything from a low-mass main-sequence star to a degenerate remnant. Mass loss plays the dominant role in determining the lifetime and the extent of nuclear processing of the AGB phase. Binary interaction will increase the mass loss from the AGB star and curtail its evolution, either through Roche-lobe overflow, common-envelope evolution or the driving of an enhanced stellar wind. These processes will tend to reduce the metals, particularly carbon, returned to the inter-stellar medium. On the other hand merged systems or companions that accrete a substantial amount of mass themselves evolve into AGB stars that can synthesize and return more carbon than the two individuals would have alone. By synthesizing large populations of stars, with nucleosynthesis and binary interaction, we estimate a reduction in carbon yield owing to binary star evolution of as much as 15%.