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We present evolutionary models for young low mass stars and brown dwarfstaking into account episodic phases of accretion at early stages of theevolution, a scenario supported by recent large surveys of embedded protostars.An evolution including short episodes of vigorous accretion ($\mdot \ge 10^{-4}\msolyr$) followed by longer quiescent phases ($\mdot < 10^{-6} \msolyr$) canexplain the observed luminosity spread in HR diagrams of star forming regionsat ages of a few Myr, for objects ranging from a few Jupiter masses to a fewtenths of a solar mass. The gravitational contraction of these accretingobjects strongly departs from the standard Hayashi track at constant $\te$. Thebest agreement with the observed luminosity scatter is obtained if most of theaccretion shock energy is radiated away. The obtained luminosity spread at 1Myr in the HR diagram is equivalent to what can be misinterpreted as a $\sim$10 Myr age spread for non-accreting objects. We also predict a significantspread in radius at a given $\te$, as suggested by recent observations. Thesecalculations bear important consequences on our understanding of star formationand early stages of evolution and on the determination of the IMF for young($\le$ a few Myr) clusters. Our results also show that the concept of a stellarbirthline for low-mass objects has no valid support.

Episodic accretion at early stages of evolution of low mass stars and brown dwarfs: a solution for the observed luminosity spread in HR diagrams?