Constant circulation sequences of binary neutron stars and their spin characterization

Understand

For isentropic fluids, dynamical evolution of a binary system conserves the baryonic mass and circulation; therefore, sequences of constant rest mass and constant circulation are of particular importance.

• In this work, we present the extension of our Compact Object CALculator (\cocal{}) code to compute such quasiequilibria and compare them with the well-known corotating and irrotational sequences, the latter being the simplest, zero-circulation case.
• The circulation as a measure of the spin for a neutron star in a binary system has the advantage of being exactly calculable since it is a local quantity.
• To assess the different measures of spin, such as the angular velocity of the star, the quasilocal, dimensionless spin parameter $J/M^2$, or the circulation $\mathcal{C}$, we first compute sequences of single, uniformly rotating stars and describe how the different spin diagnostics are related to each other.