| 1. |
- Church, Ross P., et al.
(author)
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Detailed models of the binary pulsars J1141-6545 and B2303+46
- 2006
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In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 372:2, s. 715-727
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Journal article (peer-reviewed)abstract
- We have modelled the formation of the eccentric double-degenerate binaries J1141-6545 and B2303+46 using the Henyey-type full stellar evolution code STARS and the population synthesis code BSE. We find that the outcome depends strongly on the common envelope (CE) evolution efficiency parameter alpha(CE) and show that both systems can be modelled with a single value of alpha(CE). The final orbit of the system depends critically on the order of Roche lobe filling events. The phase space of progenitors and the different evolutionary pathways followed by binary stars that form eccentric double-degenerate binaries in the two codes are compared. We show that the pathways are similar between the codes and that the distribution of progenitors in mass and separation phase space is qualitatively the same, thus validating the use of BSE-like population synthesis for simulations of this type. The phase space of initial parameters is very different to that obtained using ad hoc arguments about the evolution, which shows that such arguments are insufficient to model evolutionary pathways of this complexity. There are some differences associated with the prescription adopted for CE evolution but these are not qualitatively significant. We investigate the dependence of the formation mechanism on wind mass loss and the CE efficiency parameter alpha(CE), showing that it depends strongly on the latter but rather less on the former.
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| 2. |
- Church, Ross P., et al.
(author)
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Mass transfer in eccentric binaries: the new oil-on-water smoothed particle hydrodynamics technique
- 2009
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In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 395:2, s. 1127-1134
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Journal article (peer-reviewed)abstract
- To measure the onset of mass transfer in eccentric binaries, we have developed a two-phase smoothed particle hydrodynamics (SPH) technique. Mass transfer is important in the evolution of close binaries, and a key issue is to determine the separation at which mass transfer begins. The circular case is well understood and can be treated through the use of the Roche formalism. To treat the eccentric case, we use a newly developed two-phase system. The body of the donor star is made up from high-mass water particles, whilst the atmosphere is modelled with low-mass oil particles. Both sets of particles take part fully in SPH interactions. To test the technique, we model circular mass-transfer binaries containing a 0.6 M-circle dot donor star and a 1 M-circle dot white dwarf; such binaries are thought to form cataclysmic variable ( CV) systems. We find that we can reproduce a reasonable CV mass-transfer rate, and that our extended atmosphere gives a separation that is too large by approximately 16 per cent, although its pressure scale height is considerably exaggerated. We use the technique to measure the semimajor axis required for the onset of mass transfer in binaries with a mass ratio of q = 0.6 and a range of eccentricities. Comparing to the value obtained by considering the instantaneous Roche lobe at pericentre, we find that the radius of the star required for mass transfer to begin decreases systematically with increasing eccentricity.
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| 3. |
- Davies, Melvyn B, et al.
(author)
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Stellar encounters involving massive stars in young clusters
- 2006
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In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 370:4, s. 2038-2046
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Journal article (peer-reviewed)abstract
- We model collisions between pre-main-sequence stars using a smoothed particle hydrodynamics method. Assuming that all collisions lead to simple mergers, we use derived merger cross-sections to calculate the time-scale to make a 50-M-circle dot star by collisions within the core of a stellar cluster as a function of stellar number density. We show that a 50-M-circle dot star may be produced in this manner within 106 yr beginning with a cluster core of 200 1-M-circle dot stars within a radius of 0.0025 pc. Encounters between one high-mass star and one low-mass star tend to result in the tidal shredding of the latter, producing a massive disc around the former. This disc spreads viscously and provided a much larger target than any star for subsequent collisions. If a star strikes the disc, it is likely to be captured, and so forms a binary with the other star. Subsequent encounters between the binary and single stars lead either to exchanges or to the formation of merged objects. The inclusion of this effect leads to a significant reduction in the time taken to produce a 50-M-circle dot star. We also consider the role played by primordial binaries. We show that the time-scale required to produce a 50-M-circle dot star decreases with increasing binary fraction. We find that the number of primordial binaries is reduced by encounters. The core of a cluster must therefore contain a very high binary fraction initially if a large fraction of the massive stars are to be contained within binaries when the 50-M-circle dot star is produced.
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