Non instantaneous mixing: Deuterium burning in very low mass stars and brown dwarfs

We present results from detailed computations focused on chemical mixing in convective regions of very low mass stars (VLMs) and brown dwarfs (BDs) during deuterium burning. Computations are made both in the instantaneous and in the coupled–diffusive mixing schemes to check which mixing is required to treat these phases. The convective timescale is much shorter than the nuclear time scales for the burning of the deuterium present in the forming cloud during the pre–MS phase, and instantaneous mixing is a satisfactory approximation. This is confirmed by explicit model computation. During the following MS phase, the two mixing schemes lead to different D–profiles along the structure; differences in terms of total stellar luminosity remain however negligible. We conclude that, so far as we are dealing with VLMs and BDs evolutionary models, the treatment of chemical mixing does not influence either the pre-MS initial D-burning phase nor the hydrogen burning minimum mass and luminosity of the MS.