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After the discovery in October of last year of a neutron star (PSR J1614-2230) with a mass equivalent to about two Suns, astronomers have begun to ask how such an object forms, given that almost all neutron stars found to date have masses close to 1.4 solar masses.
An error in the mass estimate can be excluded, because this can be measured very precisely via the very regular radio pulses received as the star rotates, 317 times a second.
If neutron stars that form as a by-product of supernova explosions tend to have masses of only 1.4 solar masses then it must have acquired mass at a later stage, and this could only happen at the expense of a companion star. As it happens, PSR J1614-2230 is gravitationally linked to another collapsed object, a white dwarf, and they orbit each other with a period of just 9 days.
According to Lorne Nelson and other researchers (Bishop's University, MIT, Oxford and UCSB) that have analysed this stellar system, it is most likely that the neutron star has stripped the outer layers off the white dwarf, reducing it to a nucleus of carbon and oxygen.
To understand exactly how this could have happened, Nelson a co-workers used a super-computer to simulate 40,000 possible initial configurations for the two stars, and then followed the evolution of the system for a time equal to the age of the Universe.
The results of these many simulations, just recently presented at the Canadian CASCA 2011 meeting, demonstrate that there are many configurations via which a neutron star can significantly increase its mass at the expense of a companion. The results will clearly lead some revision of the theories regarding neutron star formation and evolution.
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