Equilibrium sequences of synchronized and irrotational binary systems composed of different mass stars in Newtonian gravity

Abstract

We study the equilibrium sequences of close binary systems in circular orbits and composed of different mass stars with a polytropic equation of state in Newtonian gravity. The solution is a multidomain spectral method which we have recently developed. The computations are performed for both cases of synchronized and irrotational binary systems with adiabatic indices $\ensuremath{\gamma}=3,$ 2.5, 2.25, 2, and 1.8, and for three mass ratios: ${M}_{1}{/M}_{2}=0.5,$ 0.2, and 0.1. It is found that the equilibrium sequences always terminate at a mass shedding limit (the appearance of a cusp on the surface of the less massive star). For synchronized binaries, this contrasts with the equal mass case, where the sequences terminate instead by a contact configuration. Regarding the turning point of the total angular momentum (or total energy) along a sequence, we find that it is difficult to get it for small mass ratios. Indeed, we do not find any turning points for ${M}_{1}{/M}_{2}l~0.5$ in the irrotational case. However, in the synchronized case, it becomes easier again to find one for mass ratios much smaller than ${M}_{1}{/M}_{2}\ensuremath{\sim}0.2.$