Equation of state of dense matter and the minimum mass of cold neutron stars
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 Mmin 0.09Msun depends very weakly on the equation of state of cold catalyzed matter: it is 0.094 Msun for the SLy model, and 0.088 Msun for the FPS one. Central density at Mmin is significantly lower than the normal nuclear density: for the SLy equation of state we get central density 1.7 1014 g/cm3, to be compared with 2.3 1014 g/cm3 obtained for the FPS one. Even at Mmin, neutron stars have a small liquid core of radius of about 4 km, containing some 2-3% of the stellar mass. Neutron stars with 0.09 Msun <M<0.17 Msun are bound with respect to dispersed configuration of the hydrogen gas, but are unbound with respect to dispersed Fe56. The effect of uniform rotation on the minimum-mass configuration of cold neutron stars is studied. Rotation increases the value of Mmin; at rotation period of 10 ms the minimum mass of neutron stars increases to 0.13 Msun, 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 0.61 Msun for the SLy EOS and 0.54 Msun for the FPS EOS.
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