Evolution of massive binary stars in the LMC and its implications for radio pulsar population |
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| Authors: | N Rathnasree A Ray |
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| Institution: | (1) Theoretical Astrophysics Group, Tata Institute of Fundamental Research, 400005 Bombay |
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| Abstract: | The Hertzsprung-Russell diagram of the Large Magellanic Cloud compiled recently by Fitzpatrick & Garmany (1990) shows that
there are a number of supergiant stars immediately redward of the main sequence although theoretical models of massive stars
with normal hydrogen abundance predict that the region 4.5 ≤ logT
eff ≤ 4.3 should be un-populated (“gap”). Supergiants having surface enrichment of helium acquired for example from a previous
phase of accretion from a binary companion, however, evolve in a way so that the evolved models and observed data are consistent
— an observation first made by Tuchman & Wheeler (1990). We compare the available optical data on OB supergiants with computed
evolutionary tracks of massive stars of metallicity relevant to the LMC with and without helium-enriched envelopes and conclude
that a large fraction (≈ 60 per cent) of supergiant stars may occur in binaries. As these less evolved binaries will later evolve into massive X-ray
binaries, the observed number and orbital period distribution of the latter can constrain the evolutionary scenarios of the
supergiant binaries. The distributions of post main sequence binaries and closely related systems like WR + O stars are bimodal-consisting
of close and wide binaries in which the latter type is numerically dominating. When the primary star explodes as a supernova
leaving behind a neutron star, the system receives a kick and in some cases can lead to runaway O-stars. We calculate the
expected space velocity distribution for these systems. After the second supernova explosion, the binaries in most cases,
will be disrupted leading to two runaway neutron stars. In between the two explosions, the first born neutron star’s spin
evolution will be affected by accretion of mass from the companion star. We determine the steady-state spin and radio luminosity
distributions of single pulsars born from the massive stars under some simple assumptions. Due to their great distance, only
the brightest radio pulsars may be detected in a flux-limited survey of the LMC. A small but significant number of observable
single radio pulsars arising out of the disrupted massive binaries may appear in the short spin period range. Most pulsars
will have a low velocity of ejection and therefore may cluster around the OB associations in the LMC. |
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| Keywords: | stellar evolution binary stars Large Magellanic Cloud radio pulsar population |
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