A numerical modelling of rotating substellar objects up to mass-shedding limits

Abstract

ABSTRACT Rotation may affect the occurrence of sustainable hydrogen burning in very low-mass stellar objects by the introduction of centrifugal force to the hydrostatic balance as well as by the appearance of rotational break-up of the objects (mass-shedding limit) for rapidly rotating cases. We numerically construct the models of rotating very low-mass stellar objects that may or may not experience sustained nuclear reaction (hydrogen-burning) as their energy source. The rotation is not limited to being slow so the effect of the rotational deformation of them is not infinitesimally small. Critical curves of sustainable hydrogen burning in the parameter space of mass versus central degeneracy, on which the nuclear energy generation balances the surface luminosity, are obtained for different values of angular momentum. It is shown that if the angular momentum exceeds the threshold $J_0=8.85\times 10^{48}\, {\rm erg}~{\rm s}$ the critical curve is broken up into two branches with lower and higher degeneracy because of the mass-shedding limit. Based on the results, we model mechano-thermal evolutions of substellar objects, in which cooling, as well as mass/angular momentum reductions, are followed for two simplified cases. The case with such external braking mechanisms as magnetized wind or magnetic braking is mainly controlled by the spin-down time-scale. The other case with no external braking leads to the mass-shedding limit after gravitational contraction. Thereafter the object sheds its mass to form a ring or a disc surrounding it and shrinks.