We investigate how a phase transition from neutron-star matter to spin-polarized neutron matter affects the equation of state and mass-radius relation of neutron stars .
While general extension schemes for the equation of state allow for high pressures inside neutron stars , we find that a phase transition to spin-polarized neutron matter excludes extreme regimes .
Hence , such a transition limits the maximum mass of neutron stars to lie below 2.6 - 2.9 M _ { \odot } , depending on the microscopic nuclear forces used , while significantly larger masses could be reached without these constraints .
These limits are in good agreement with recent constraints extracted from the neutron-star merger GW170817 and its electromagnetic counterpart .
Assuming the description in terms of spin-polarized neutron matter to be valid in the center of neutron stars , we find that stars with a large spin-polarized domain in their core are ruled out by GW170817 .