Limiting masses and radii of neutron stars and their implications

We combine the equation of state of dense matter up to twice nuclear saturation density n(sat) obtained using chiral effective field theory (chi EFT) and recent observations of neutron stars to gain insights about the high-density matter encountered in their cores. A key element in our study is the recent Bayesian analysis of correlated EFT truncation errors based on order-by-order calculations up to next-to-next-to-next-to-leading order in the chi EFT expansion. We refine the bounds on the maximum mass imposed by causality at high densities and provide stringent limits on the maximum and minimum radii of similar to 1.4 M-circle dot and similar to 2.0 M-circle dot stars. Including chi EFT predictions from n(sat) to 2 n(sat) reduces the permitted ranges of the radius of a 1.4 M-circle dot star, R-1.4, by similar to 3.5 km. If observations indicate R-1.4 < 11.2km, then our study implies that either the squared speed of sound c(s)(2) > 1/2 for densities above 2 n(sat) or that x EFT breaks down below 2 n(sat). We also comment on the nature of the secondary compact object in GW190814 with mass similar or equal to 2.6 M-circle dot and discuss the implications of massive neutron stars >2.1 M-circle dot (2.6 M-circle dot) in future radio and gravitational-wave searches. Some form of strongly interacting matter with c(s)(2) > 0.35 (0.55) must be realized in the cores of such massive neutron stars. In the absence of phase transitions below 2 n(sat), the small tidal deformability inferred from GW170817 lends support for the relatively small pressure predicted by chi EFT for the baryon density n(B) in the range 1-2 n(sat). Together they imply that the rapid stiffening required to support a high maximum mass should occur only when n(B) greater than or similar to 1.5-1.8 n(sat).