Abstract

I will present evolutionary models for young low mass stars and brown dwarfs taking into account episodic phases of accretion at early stages of evolution, a scenario supported by recent large surveys of embedded protostars. An evolution including short episodes of vigorous accretion can explain the observed luminosity spread in HR diagrams of star forming regions at ages of a few Myr, for objects ranging from a few Jupiter masses to a few tenths of a solar mass. The gravitational contraction of these accreting objects strongly departs from the standard Hayashi track at constant Teff. The obtained luminosity spread at 1 Myr in the HR diagram is equivalent to what can be misinterpreted as a ~ 10 Myr age spread for non-accreting objects. I will show that a consistent scenario based on the coupling between accretion burst models and stellar evolution calculations can also explain observations of Fu Ori objects, and in particular the expansion of their central star. Our results suggest a link between the fraction of accretion energy absorbed by the protostar and the accretion burst intensity, suggesting the existence of a threshold in accretion rate where a transition from ”cold” to ”hot” accretion occurs. This transition may stem from a change in the accretion geometry, like a transition from magnetospheric or thin disk to thick disk accretion. These calculations bear important consequences on our understanding of star formation and early stages of evolution and on the determination of the IMF for young (≤ a few Myr) clusters.