Constraints from Cooling Neutron Stars on their Mass Distributions

Abstract

The neutron star mass distribution remains uncertain, with limited consensus on the number or location of its peaks. We attempt to constrain the distribution by simulating fixed-mass cooling trajectories for stars constructed with the V18 equation of state, varying 25 superfluid parameter combinations through a two-dimensional scaling of the proton 1S0 pairing gap, and assigning isolated neutron stars to masses within the physically permissible 1.1 to 2.3 solar mass interval using trajectories for both light and heavy envelope compositions. We compare these distributions to inferred mass distributions from the literature and to subpopulations of neutron stars in binaries using root-mean-square deviations and the Kolmogorov-Smirnov test. We find that cooling theory mass distributions exhibit concentrated peaks that are highly sensitive to proton 1S0 gap scaling and shift systematically with the parameter Sx, but do not reproduce the secondary peaks suggested by several bimodal distributions. We present the superfluid parameter combinations that best reproduce each inferred mass distribution and subpopulation of neutron stars in binaries. Our conclusions are severely limited by uncertainty in pulsar luminosity and age estimates, selection effects, and the superfluid properties of the V18 equation of state.