Progenitor neutron stars of the lightest and heaviest millisecond pulsars

¹ N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
e-mail: fortin@camk.edu.pl; bejger@camk.edu.pl; haensel@camk.edu.pl; jlz@camk.edu.pl
² LUTh, UMR 8102 du CNRS, Observatoire de Paris, 92195 Meudon Cedex, France
³ Istituto Nazionale di Fisica Nucleare – Sezione di Roma, P.le Aldo Moro 2, 00185 Roma, Italy

Received: 3 September 2014
Accepted: 12 November 2015

Abstract

Context. The recent mass measurements of two binary millisecond pulsars, PSR J1614−2230 and PSR J0751+1807 with a mass M = 1.97 ± 0.04 M_⊙ and M = 1.26 ± 0.14 M_⊙, respectively, indicate a wide range of masses for such objects and possibly also a broad spectrum of masses of neutron stars born in core-collapse supernovae.

Aims. Starting from the zero-age main sequence binary stage, we aim at inferring the birth masses of PSR J1614−2230 and PSR J0751+1807 by taking the differences in the evolutionary stages preceding their formation into account.

Methods. Using simulations for the evolution of binary stars, we reconstruct the evolutionary tracks leading to the formation of PSR J1614−2230 and PSR J0751+1807. We analyse in detail the spin evolution due to the accretion of matter from a disk in the intermediate-mass/low-mass X-ray binary. We consider two equations of state of dense matter, one for purely nucleonic matter and the other one including a high-density softening due to the appearance of hyperons. Stationary and axisymmetric stellar configurations in general relativity are used, together with a recent magnetic torque model and observationally-motivated laws for the decay of magnetic field.

Results. The estimated birth mass of the neutron stars PSR J0751+1807 and PSR J1614−2230 could be as low as 1.0 M_⊙ and as high as 1.9 M_⊙, respectively. These values depend weakly on the equation of state and the assumed model for the magnetic field and its accretion-induced decay.

Conclusions. The masses of progenitor neutron stars of recycled pulsars span a broad interval from 1.0 M_⊙ to 1.9 M_⊙. Including the effect of a slow Roche-lobe detachment phase, which could be relevant for PSR J0751+1807, would make the lower mass limit even lower. A realistic theory for core-collapse supernovæ should account for this wide range of mass.