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Register a new userOn the formation of Super-AGB stars in intemediate mass close binary systems
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The evolution of a star of initial mass 9 M_s, and Z = 0.02 in a Close Binary System is followed in the presence of different mass companions in order to study their influence on the final evolutionary stages and, in particular, on the structure and composition of the remnant components. We study two extreme cases. In the first one the mass of the secondary is 8 M_s, whereas in the second one the mass was assumed to be 1 M_s. For the first of those cases we have also explored the possible outcomes of both conservative and non-conservative mass-loss episodes. During the first mass transfer episode, several differences arise between the models. The system with the more extreme mass ratio is not able to survive the 1st. Roche lobe overflow, and spiral-in of the secondary onto the envelope of the primary is most likely. The system formed by two stars of comparable mass undergoes two mass transfer episodes in which the primary is the donor. We have performed two sets of calculations corresponding to this case in order to account for conservative and non-conservative mass transfer during the first mass loss episode. One of our main results is that for the non-conservative case the secondary becomes a Super-AGB. Such a star undergoes a final dredge-up episode, similar to that of a single star of comparable mass. The primary components do not undergo a Super-AGB phase, but instead a carbon-oxygen white dwarf is formed in both cases, before reversal mass transfer occurs. However, given the extreme mass ratios at this stage between the components of the binary system, the possibility of merger episodes remains likely. We also discuss the presumable final outcomes of the system and possible observational counterparts.
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Short period binary systems containing magnetic Ap stars are anomalously rare. This apparent anomaly may provide insight into the origin of the magnetic fields in theses stars. As an early investigation of this, we observed three close binary systems that have been proposed to host Ap stars. Two of these systems (HD 22128 and HD 56495) we find contain Am stars, but not Ap stars. However, for one system (HD 98088) we find the primary is indeed an Ap star, while the secondary is an Am star. Additionally, the Ap star is tidally locked to the secondary, and the predominately dipolar magnetic field of the Ap star is roughly aligned with the secondary. Further investigations of HD 98088 are planned by the BinaMIcS collaboration.
The evolution of a star of initial mass 10 $M_{odot}$, and metallicity $Z = 0.02$ in a Close Binary System (CBS) is followed from its main sequence until an ONe degenerate remnant forms. Restrictions have been made on the characteristics of the companion as well as on the initial orbital parameters in order to avoid the occurrence of reversal mass transfer before carbon is ignited in the core. The system undergoes three mass loss episodes. The first and second ones are a consequence of a case B Roche lobe overflow. During the third mass loss episode stellar winds may play a role comparable to, or even more important than Roche lobe overflow. In this paper, we extend the previously existing calculations of stars of intermediate mass belonging to close binary systems by following carefully the carbon burning phase of the primary component. We also propose different possible outcomes for our scenario and discuss the relevance of our findings. In particular, our main result is that the resulting white dwarf component of mass $1.1 M_odot$ more likely has a core composed of oxygen and neon, surrounded by a mantle of carbon-oxygen rich material. The average abundances of the oxygen-neon rich core are $X({rm O}^{16})=0.55$, $X({rm Ne}^{20})=0.28$, $X({rm Na}^{23})=0.06$ and $X({rm Mg}^{24})=0.05$. This result has important consequences for the Accretion Induced Collapse scenario. The average abundances of the carbon-oxygen rich mantle are $X({rm O}^{16})=0.55$, and $X({rm C}^{12})=0.43$. The existence of this mantle could also play a significant role in our understanding of cataclysmic variables.
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We describe our first attempt at modelling nucleosynthesis in massive AGB stars which have undergone core carbon burning, the super-AGB stars. We fit a synthetic model to detailed stellar evolution models in the mass range 9<=M/Msun<=11.5 (Z=0.02), and extrapolate these fits to the end of the AGB. We determine the number of thermal pulses and AGB lifetime as a function of mass and mass-loss prescription. Our preliminary nucleosynthesis calculations show that, for a reasonable mass-loss rate, the effect of hot-bottom burning in super-AGB stars on the integrated yield of a stellar population is not large. There are many uncertainties, such as mass-loss and convective overshooting, which prevent accurate yield calculations. However, as potential progenitors of electron-capture supernovae, these stars may contribute 7% of non-type-Ia supernovae.
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