The Binary Radio Pulsar PSR J161436.50-223031.08
It was the discovery that cast all exotic matter theories into doubt: the discovery of the pulsar PSR J1614-2230 (Zeeya 2010). PSR J1614–2230 is a millisecond pulsar (MSP), a neutron star (NS), of mass in a binary system with a white dwarf of mass (Demorest et al. 2010). It was the subject of a recent post by myself [PSR J1614-2230: A Neutron Star Born Massive?], but since then new developments have been made regarding its evolutionary model. Evolutionary models are extremely important in the understanding of stellar astrophysics: creating a chronological time-line of the main events associated to binary and single stellar systems.
In 2010 Demorest et al. (2010) used the Green Bank Telescope at the National Radio Astronomy Observatory to observe the system through a complete 8.7 day orbit, recording the pulse arrival times from PSR J1614–2230 over this period. After accounting for factors that would alter pulse arrival times from exactly matching its period of 3.1508076534271 milliseconds, sub-categorising this particular pulsar as an MSP. So we have a very high-mass highly rotating MSP: my opinions when writing the previous article on this particular object was that the neutron star grew as a result of accretion by the red giant companion star, which has later become the ~0.5 white dwarf. However, the maximum mass of a neutron star is 1.4, so this evolutionary prediction would need to account for 0.57 of extra solar mass, a combined total greater than the white dwarf companion!
Recent work by Lin et al. (2011) has in fact confirmed my gut reaction to the PSR J1614-2230 discovery. They confirmed that the degenerate companion star is mostly (i.e., 90%) Carbon and Oxygen (with a surrounding shell of Helium which comprises 10% of the white-dwarf mass), but there is also a thin outer envelope that is composed of ∼15% H to be of mass . However, starting with neutron stars of mass 1.4, Lin et al. (2011) were not able to produce the observed 1.97 NS with the requisite combination of orbital period (3.15ms) and white dwarf mass to match the PSR J1614-2230 system. They were, surprisingly able, to evolve neutron stars as massive as 2, with the correct orbital period just not in combination with a massive enough white dwarf (WD).
The difficulty with evolving high-mass NSs in orbit with massive WD companions is that the progenitors of these white dwarfs are initially substantially more massive than the NS, resulting in very rapid, thermal-timescale mass transfer (greatly in excess of the Eddington limit), and the neutron star is thereby prevented from accreting a significant fraction of the donor star’s mass (Rappaport et al. 1995; Tauris & Savonije 1999; Podsiadlowski & Rappaport 2000; Tauris et al. 2011). Therefore Lin et al. (2011) tentatively concluded that the initial mass of the NS would have to have been higher than the canonical value of 1.4.
By running some 700 supplementary models with initially higher-mass NSs, they found that to successfully produce a system like PSR J1614-2230, it is required to have minimum initial neutron star mass of at least , as well as initial donor masses for the WD of and an orbital period of the system of. The evolutionary simulations conducted here by Lin et al. (2012) has at least gone some way to establishing a new paradigm for the possible sequence of events which causes high-mass (>1.4) neutron stars to become extreme mass neutron stars like the one found in PSR J1614-2230.
- Demorest, P. B. et al. (2010). A 2 Neutron Star Measured Using Shapiro Delay. Nature, 467 (7319): pp.1081–1083.
- Lin, J. et al. (2011) LMXB & IMXB Evolution: I. The Binary Radio Pulsar PSR J1614-2230. The Astrophysical Journal, 732 (2): Article I.D. 70.
- Tauris, T.M et al. (2011) Millisecond Pulsar Formation With CO White Dwarf Companions – I. PSR J1614-2230: Evidence For A Neutron Star Born Massive. Monthly Notices Royal Astronomical Society, 416 (3): pp. 2130-2142.