The effect of irradiation on the Jeans mass in fragmenting self-gravitating protostellar discs

Abstract

When a self-gravitating disc is subject to irradiation, its propensity to fragmentation will be affected. The strength of self-gravitating disc stresses is expected to dictate disc fragmentation: as the strength of these torques typically decreases with increasing sound speed, it is reasonable to assume, to first order, that disc fragmentation is suppressed when compared to the non-irradiated case, although previous work has shown that the details are complicated by the source of the irradiation. We expand on a previous analysis of the Jeans mass inside spiral structures in self-gravitating discs, incorporating the effects of stellar irradiation and background irradiation. If irradiation is present, fragmentation is suppressed for marginally unstable discs at low accretion rates (compared to the no-irradiation case), but these lower accretion rates correspond to higher mass discs. Fragmentation can still occur for high accretion rates, but is consequently suppressed at lower disc surface densities, and the subsequent Jeans mass is boosted. These results further bolster the consensus that, without subsequent fragment disruption or mass-loss, the gravitational instability is more likely to form brown dwarfs and low-mass stars than gas giant planets.