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PSR J1614-2230: A Neutron Star Born Massive?

12/07/2011

The recent discovery of a 2 binary millisecond pulsar (Demorest et al. 2010) has not only important consequences for the equation-of-state of nuclear matter at high densities but also raises the interesting question if the neutron star PSR J1614-2230 was born massive. The answer is vital for understanding neutron star formation in core collapse supernovae.

Neutron stars are formed as compact remnants of massive stars (10−30) which explode in supernovae at the end of their stellar life. In order to better understand the mechanisms of the electron capture and core collapse supernovae knowledge of the distribution of birth masses of neutron stars is vital. However, in order to weigh a neutron star it must be a member of a binary system. This introduces an uncertainty in determining the original birth mass of the neutron star since these neutron stars are often observed in X-ray binaries or, at a later stage, as recycled pulsars and hence after they have undergone a phase of mass accretion from their companion star.

The most precisely measured masses of neutron stars are obtained in double neutron star systems via general relativistic effects. The related post-Keplerian parameters include periastron advance, redshift/time dilation, orbital period derivative and Shapiro delay (e.g. Will 2009). Shapiro delays of radio signals from pulsars (Stairs et al. 1998) have the advantage of being measurable also in low eccentricity systems if the orbital inclination is such that the pulses passes in the vicinity of its companion. This method yields the opportunity to weigh both neutron stars accurately – and hence also determine the mass of the last formed neutron star which has not accreted any material. So far, such measurements have revealed that even the most massive of these neutron stars (the non-recycled pulsars) do not exceed a mass of 1.39 (Thorsett & Chakrabarty 1999; Schwab et al. 2010).

There is, however, some evidence from neutron stars in X-ray binaries, e.g. Vela X-1, that suggests neutron stars can be born more massive than this value.

N.B. Mass determinations of Vela X-1 (Barziv et al. 2001; Rawls et al. 2011) suggest that this neutron star has an observed mass of 1.77 ± 0.08. The companion star to Vela X-1 is a B0.5 Ib supergiant (HD 77581) with a mass of about 23 which implies that the present mass of the neutron star is very close to its birth mass.

Binary millisecond pulsars are known to be key sources of research in fundamental physics. They host the densest matter in the observable Universe and possess very rapid spins as well as relativistic magnetospheres with out flowing plasma winds. Being ultra stable clocks they also allow for unprecedented tests of gravitational theories in the strongfield regime (Kramer & Wex 2009). Equally important, however, binary millisecond pulsars represent the end point of stellar evolution, and their observed orbital and stellar properties are fossil records of their evolutionary history. Thus one can use binary pulsar systems as key probes of stellar astrophysics.

Recent Shapiro delay measurements of PSR J1614−2230 (Demorest et al. 2010) allowed a precise mass determination of this record high-mass pulsar (neutron star) and its white dwarf companion. It is well established that the neutron star in binary millisecond pulsar systems forms first, descending from the initially more massive of the two binary stellar components. The neutron star is subsequently spun-up to a high spin frequency via accretion of mass and angular momentum once the secondary star evolves (Alpar et al. 1982; Radhakrishnan & Srinivasan 1982; Bhattacharya & van den Heuvel 1991).

In this recycling phase the system is observable as a low-mass X-ray binary (e.g. Nagase 1989) and towards the end of this phase as an X-ray millisecond pulsar (Wijnands & van der Klis 1998; Archibald et al. 2009). Although this formation scenario is now commonly accepted many aspects of the mass-transfer process and the accretion physics (e.g. the accretion efficiency and the details of non-conservative evolution) are still not well understood (Lewin & van der Klis 2006).

I would conclude that the neutron star in PSR J1614−2230 was born significantly more massive (1.7 ± 0.15) than neutron stars found in previously known radio pulsar binaries – a fact which is important for understanding stellar evolution of massive stars in binaries and the explosion physics of core collapse SNe. Finally, based on this high value for the neutron star birth mass I would argue that the progenitor star of PSR J1614−2230 had a zero-age main sequence (ZAMS) mass of 20−25 and did not loose its envelope before core helium exhaustion.

Journal References:

  • Demorest, P.B. et al., (2011) A 2 Neutron Star Measured Using Shapiro Delay. Nature, 467 (7319) pp.1081-1083.
  • Liu, W.M. & Chen, W.C. (2011) On The Progenitors Of Millisecond Pulsars By The Recycling Evolutionary Channel. Monthly Notices Royal Astronomical Society, 469 (1) pp.89-98.
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