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  Science • Binary Stars and Nucleosynthesis

What is the effect of duplicity on nucleosynthesis?
and
Why are binaries important?



From Izzard and Tout (2003):

For many years the study of stellar yields and galactic chemical evolution (GCE) has gone on assuming, mainly for simplicity, that stars are isolated objects (one exception being De Donder and Vanbeveren, 2002). Reality bites deeply into this picture with the observation that most stars are in multiple systems and that many of these systems are interacting. The state of the art in binary star nucleosynthesis is focused on explosive events such as type Ia supernovae and classical novae but other binary star processes contribute to pollution of the interstellar medium. Mass-transfer by Roche-lobe overflow (RLOF) occurs particularly when the stellar radius is growing rapidly and so commonly when one star is on the asymptotic giant branch (AGB).

To investigate the effect of a companion star on stellar evolution the binary_c code was developed. The code uses the BSE package at its core, with nucleosynthesis in parallel, to explore large parameter spaces in reasonable periods of time. Mass lost from the binary system contributes to the stellar yield and since the BSE code is based on the SSE code nucleosynthesis for populations of single stars is also calculated. The two are then compared to see if there is any difference.

Binaries are important:
  • The presence of a companion affects evolution by mass loss and gain. Good examples are RLOF due to interaction between a giant (GB/AGB) star and a MS star. Truncation of the GB/AGB phase may prevent dredge-up events and hence reduce the amount of nuclear processing material undergoes prior to explusion to the ISM. Common envelopes generally result and while the detailed physics is unclear it is likely that mass is ejected to the ISM from some of these stars. The most extreme example is a stellar merger, where two stars become one. What happens? We do not really know.
  • A companion star can be spun up and down by tides. This may be a good way to make rapidly rotating stars which become gamma-ray bursts, the most violent explosions observed in the Universe.
  • Novae and SNeIa only occur in binaries. What are the progenitors of SNeIa? We do not know - despite what you may have been told!


Binary evolution is plagued with uncertainy, even compared to our understanding of single-star evolution.
Effects which must be considered include:
  • Duplicity : single star or binary star.
  • Metallicity and, less importantly, initial abundance mix. The initial abundance mix depends on the galactic evolution history and even the solar mix is somewhat uncertain.
  • Initial distributions: What is the IMF? What is the initial distribution for primary mass, secondary mass, separation/period and eccentricity for binary stars?
  • Abundance changes at dredge-ups. These changes can depend on the input physics, especially in the case of third dredge-up. Calibration to observations is necessary in this case and leads to the introduction of free parameters to increase the amount of dredge-up. There is also great uncertainty with regard to the s-process isotopes, in particular the size of the C13 pocket during third dredge-up.
  • Wind loss rates. Mass loss due to stellar winds is a very dodgy affair - most prescriptions in current use are quite phenomenological and have little regard for actual physics. With this in mind it is important to test a range of prescriptions.
  • Common envelope parameters - the parameters αCE and λCE parameterise our ignorance of this complex process, mainly because the mechanism for driving off the stellar envelope is unclear (magnetic fields? friction? ionization? who knows!).
  • Eddington limit : should this be imposed during accretion processes?
  • HBB temperature : somewhat uncertain is the amount of HBB, this can be varied in the model
  • Black hole formation : what is the mass of a black hole forming from a given mass progenitor?
  • Supernova kicks : is there a kick at SN formation? What is the magnitude/distribution of this kick? Pulsar peculiar velocities give us an idea but are not necessarily the answer to the question.
  • Binary induced wind loss - see Chris Tout's PhD. Does the presence of a binary companion increase wind loss rates?
  • Time evolution of the yields. Even if the integrated yield up to (say) 15Gyr from a population of stars is similar when comparing binary and single stars, the time evolution probably is not. For example, nitrogen peaks far more quickly in single than in binary stars because massive (M>4Msun) TPAGB stars in binaries overflow their Roche lobes prior to HBB so C12 cannot be converted into N14.
  • Numerical resolution - requires careful consideration!

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