Adiabatic evolution of mass-losing stars

Abstract

We investigate the evolution of the stellar structure, when a star fills and overflows its Roche lobe in a circular, equatorial orbit around a supermassive black hole. The stellarmass-loss timescale is anticipated to be long compared with the dynamical time-scale and short compared with the thermal time-scale of the star; so, the entropy as a function of enclosed mass is conserved. For a representative set of stars, we calculate how the stellar entropy, pressure, radius, density and orbital angular momentum vary when the star adiabatically loses mass. We also provide interpolated formulae of the stellar mean density in terms of the remaining stellar mass for different types of stars. As the stellar orbit changes with the stellar density, Sun-like stars, upper main-sequence stars and red giants will spiral inwards and then outwards, while lower main-sequence stars, brown dwarfs and white dwarfs will always spiral outwards. We discuss the validity and limitation of the adiabatic mass-loss assumption and show that such a mass-transfer process is always stable on dynamical time-scales when the mass ratio of the two objects is large. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.