SINs

[ SINs Publications ]

Stellar Structure and Evolution

Monash is one of the world's leading centres for the study of stellar structure and evolution, and especially nucleosynthesis (see below). Studies concentrate on the Asymptotic Giant Branch (AGB) phase of evolution, the last phase in the lives of most stars (those with masses between about 0.8 and 10 times that of the Sun). Although a very short phase (perhaps just a few million years) this is one of the most important as these very bright stars dominate the light and energy output of many stellar populations, and their nucleosynthesis is proving essential to understanding the composition of the Galaxy and the Universe.

Recent work has seen us extend our calculations to one of the last unknown areas of stellar evolution, the fate of the Super-AGB stars. These ignite carbon degenerately but do not progress further in the nuclear burning phases. Hence they still experience thermal pulses on the AGB, and their nucleosynthesis may prove to be the missing link in many problems currently under investigation, such as anomalous abundances seen in globular cluster stars.

We have an active collaboration with Lawrence Livermore National Laboratory, and work to use their massively parallel 3D stellar hydrodynamics code Djehuty to investigate mixing and other hydrodynamic phenomena in stars. This work has led to the discovery of a mechanism that drives ``deep-mixing'', which was postulated to exist based on observational constraints. We discovered that the burning of 3He after first dredge-up creates a molecular weight inversion which drives mixing, and simultaneously destroys the 3He produced in the star, reconciling another problem with the observed abundance of 3He and the amount predicted from the Big Bang. Work with Djehuty continues, on various applications, including the common envelope phase of binary evolution.

Bubbles of 4He arise from the burning of 3He in a red-giant, driving mixing between the H-shell and the convective envelope. This work, with Djehuty, provides a mechanism to drive deep-mixing, solving a 30-year-old problem.

Nucleosynthesis

Nuclear reactions in stars of masses up to ~10 times the mass of the Sun are modelled using a sophisticated computational tool that uses information on the stellar structure, in particular the temperature, density, and convective motions in stellar interiors, to calculate the efficiency of nuclear reactions at each point mass in a star. The extended nuclear networks, of up to 400 nuclei and a few thousands of nuclear reactions, allow us to make detailed predictions for nuclear abundances of the elements and their isotopes from hydrogen to lead. These abundances are calculated as a function of time, from when the star is born to the time when it sheds most of its material into the interstellar medium and turns into a white dwarf, and compared to observational constraints coming from spectroscopic observations of abundances in the atmospheres of young and old stars and laboratory measurements of meteoritic star dust. These abundances are also needed as inputs in models of galactic chemical evolution and of stellar population synthesis to address problems such as the composition of the Sun, the evolution of stellar clusters and galaxies, and stars in the early Universe. We have close links to stellar spectroscopists as well as those involved in the laboratory study of pre-solar meteorite grains, a new and emerging science for the study of the composition of now extinct stars.

Schematic structure of an AGB star.

Neutron capture by the slow process is studied at Monash.

Binary Stars and the Early Universe

Extremely metal-poor stars in the Galactic halo are relics from a time when the Universe was very young. Their surface abundances represent a window on to the distant past offering us a tantalising glimpse of the chemical make-up of our Galaxy in its earliest stages of formation and evolution. As such, they are increasingly well studied by the observational community. Of these metal poor stars, around 20% are found to be rich in carbon, the so-called carbon-enhanced metal-poor (CEMP) stars. Many of these objects are also rich in s-process elements. Many of these stars show evidence for the existence of an unseen binary companion. This suggests that a possible formation mechanism for these CEMP stars is the transfer of carbon-rich material from an asymptotic giant branch (AGB) star to a low-mass companion in a binary system, via a stellar wind. The paradigm for forming extrinsic carbon stars at near-solar metallicity involves the transfer of material from a carbon-rich primary (the initially more massive and more evolved) star on to a low-mass companion (the secondary) in a binary system. The primary produced carbon during its AGB phase and it transfers this mass to the smaller star. This mechanism is believed to produce the Ba and CH stars which are seen among metal-rich populations.

To understand the CEMP stars, we need to understand three things: the AGB stars that produce the carbon (and other elements), how material is transferred from one star to another and what happens to that material once it has been transferred to the companion star. The evolution and nucleosynthesis of AGB stars is studied with a state-of-the-art code. Using this we can study how certain elements are synthesised (e.g. carbon, sodium, magnesium) inside these stars and also how these species can be brought to the surface. The models include stellar winds to determine how much of the material is ejected from the star and hence what can be transferred to the companion.

Material transferred to a the companion may not remain on the surface -- it may become mixed with the material of the companion. This will change the chemical signatures that we might expect to observe. In the simplest picture, we expect no mixing to occur until the star becomes a giant and develops a deep convective envelope, mixing in the accreted material as it does so. However, other mixing processes may be at work and could lead to the accreted material being mixed before the giant stage is reached. One such process is thermohaline mixing. This process allows material of higher mean molecular weight (i.e. the accreted material) to mix down through material of lower mean molecular weight (i.e. the pristine material of the companion). We therefore use the stellar evolution code to model the evolution of the companion to study the effects of mixing processes to enable us to predict the chemical signatures that we expect to observe in the CEMP stars.

A calculation of mass-transfer in binary stars.