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  Papers • Papers of 2004
A New Synthetic Model for AGB Stars
Izzard, Robert G.; Tout, Christopher A.; Karakas, Amanda I.; Pols, Onno R.
We present a synthetic model for thermally pulsing asymptotic giant branch (TPAGB) evolution constructed by fitting expressions to full evolutionary models in the metallicity range 0.0001<=Z<=0.02. Our model includes parametrizations of third dredge-up and hot-bottom burning with mass and metallicity. The Large Magellanic Cloud and Small Magellanic Cloud carbon star luminosity functions are used to calibrate third dredge-up. We calculate yields appropriate for galactic chemical evolution models for 1H, 4He, 12C, 13C, 14N, 15N, 16O and 17O. The initial-final mass relation is examined for our stars and found to fit to within 0.1 M of the observations. We also reproduce well the white dwarf mass function for masses above about 0.58 M. The new model is to be implemented in a rapid binary star evolution code.
Monthly Notices of the Royal Astronomical Society, Volume 350, Issue 2, pp. 407-426
Formation rates of SNe and GRBs
Izzard, Robert G.; Ramirez-Ruiz, Enrico; Tout, Christopher A.
Core collapse of massive stars with a relativistic jet expulsion along the rotation axis is a widely discussed scenario for gamma-ray burst (GRB) production. However the nature of the stellar progenitor remains unclear. We study the evolution of stars that may be the progenitors of long-soft GRBs - rotating naked helium stars presumed to have lost their envelopes to winds or companions. Our aim is to investigate the formation and development of single and binary systems and from this population evaluate the rates of interesting individual species. Using a rapid binary-evolution algorithm that enables us to model the most complex binary systems and to explore the effect of metallicity on GRB production, we draw the following conclusions. First, we find that, if we include an approximate treatment of angular momentum transport by mass loss, the resulting spin rates for single stars become too low to form a centrifugally supported disc that can drive a GRB engine, although they do have sufficiently massive cores to form black holes. Secondly, massive stars in binaries result in enough angular momentum - due to spin-orbit tidal interactions - to form a centrifugally supported disc and are thus capable of supplying a sufficient number of progenitors. This holds true even if only a small fraction of bursts are visible to a given observer and the GRB rate is several hundred times larger than the observed rate. Thirdly, low-metallicity stars aid the formation of rapidly rotating, massive helium cores at collapse and so their evolution is likely to be affected by the local properties of the interstellar medium (ISM). This effect could increase the GRB formation rate by a factor of 5-7 at Z= Z/200. Finally we quantify the effects of mass loss, common-envelope evolution and black-hole formation and show that more stringent constraints to many of these evolution parameters are needed in order to draw quantitative conclusions from population synthesis work.
Monthly Notices of the Royal Astronomical Society, Volume 348, Issue 4, pp. 1215-1228
A binary origin for low-luminosity carbon stars
Izzard, Robert G.; Tout, Christopher A.
We show that mass transfer from thermally pulsing asymptotic giant branch stars in binary systems naturally leads to a population of dim, extrinsic carbon stars in the Magellanic Clouds. This explains the existence of the dim tail in the carbon star luminosity functions of the clouds. We also conclude that the majority of these extrinsic stars are first giant-branch stars rather than early asymptotic giant branch stars.
Monthly Notices of the Royal Astronomical Society, Volume 350, Issue 1, pp. L1-L4
Duplicitous Nucleosynthesis
Izzard, Robert G.
The effect of a binary companion on stellar nucleosynthesis is considered. Synthetic stellar evolution and nucleosynthesis algorithms are outlined which allow the large binary-star parameter space to be explored by a population nucleosynthesis approach. This is demonstrated by a comparison of single- and binary-star stellar yields. Future prospects for the field of binary star nucleosynthesis are outlined.
Memorie della Societa Astronomica Italiana, v.75, p.754
Fully simultaneous calculations of AGB evolution
Stancliffe, R. J.; Izzard, R. G.; Tout, C. A.; Pols, O. R.
We present the first fully simultaneous calculations of asymptotic giant branch evolution. We find that third dredge-up occurs at lower core masses and is deeper than in non-simultaneous calculations. At a metallicity of Z=0.008 we form carbon stars at low core masses and hence lower luminosities. This allows us to reproduce the carbon star luminosity function for the Large Magellanic Cloud without any ad hoc adjustments to our models.
Memorie della Societa Astronomica Italiana, v.75, p.670
Stellardb: A Stellar Abundances Database
Odman, C. J.; Izzard, R. G.
We present Stellardb, a database of astronomical observations of various objects from stars to Lyman-alpha clouds. We describe its web-based interface and features. The database is designed to be an interactive and searchable compilation of published stellar abundance surveys. This project is work in progress.
Memorie della Societa Astronomica Italiana, v.75, p.631
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