Equation of state of dense matter and the minimum mass of cold neutron stars

¹ N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland e-mail: jlz@camk.edu.pl
² Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
³ Centre de Recherche Astronomique de Lyon, ENS de Lyon, 69364 Lyon, France

Corresponding author: P. Haensel, haensel@camk.edu.pl

Received: 30 July 2001
Accepted: 9 January 2002

Abstract

Equilibrium configurations of cold neutron stars near the minimum mass are studied, using the recent equation of state SLy, which describes in a unified, physically consistent manner, both the solid crust and the liquid core of neutron stars. Results are compared with those obtained using an older FPS equation of state of cold catalyzed matter. The value of depends very weakly on the equation of state of cold catalyzed matter: it is for the SLy model, and for the FPS one. Central density at M_min is significantly lower than the normal nuclear density: for the SLy equation of state we get central density , to be compared with obtained for the FPS one. Even at M_min, neutron stars have a small liquid core of radius of about 4 km, containing some 2–3% of the stellar mass. Neutron stars with are bound with respect to dispersed configuration of the hydrogen gas, but are unbound with respect to dispersed . The effect of uniform rotation on the minimum-mass configuration of cold neutron stars is studied. Rotation increases the value of M_min; at rotation period of 10 ms the minimum mass of neutron stars increases to , and corresponds to the mass-shedding (Keplerian) configuration. In the case of the shortest observed rotation period of radio pulsars 1.56 ms, minimum mass of uniformly rotating cold neutron stars corresponds to the mass-shedding limit, and is found at for the SLy EOS and for the FPS EOS.