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 nₒ₀ₓ obtained using chiral effective field theory () 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 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 1. 40. 16em{0ex}M_ and 2. 00. 16em{0ex}M_ stars. Including predictions from nₒ₀ₓ to 20. 16em{0ex}nₒ₀ₓ reduces the permitted ranges of the radius of a 1. 40. 16em{0ex}M_ star, R₁. ₄, by 3. 50. 16em{0ex}km. If observations indicate R₁. ₄1/2 for densities above 20. 16em{0ex}nₒ₀ₓ or that breaks down below 20. 16em{0ex}nₒ₀ₓ. We also comment on the nature of the secondary compact object in GW190814 with mass 2. 60. 16em{0ex}M_ and discuss the implications of massive neutron stars >2. 10. 16em{0ex}M_0. 16em{0ex} (2. 60. 16em{0ex}M_) in future radio and gravitational-wave searches. Some form of strongly interacting matter with cₒ^2>0. 350. 16em{0ex} (0. 55) must be realized in the cores of such massive neutron stars. In the absence of phase transitions below 20. 16em{0ex}nₒ₀ₓ, the small tidal deformability inferred from GW170817 lends support for the relatively small pressure predicted by for the baryon density n₁ in the range 1--20. 16em{0ex}nₒ₀ₓ. Together they imply that the rapid stiffening required to support a high maximum mass should occur only when n₁1. 5--1. 80. 16em{0ex}nₒ₀ₓ.