  The pre-main-sequence evolution of low-mass stars and brown dwarfs is studied
numerically starting from the formation of a protostellar/proto-brown dwarf
seed and taking into account the mass accretion onto the central object during
the initial several Myr of evolution. The stellar evolution was computed using
the STELLAR evolution code developed by Yorke & Bodenheimer with recent
modifications by Hosokawa et al. The mass accretion rates were taken from
numerical hydrodynamics models of Vorobyov & Basu computing the circumstellar
disk evolution starting from the gravitational collapse of pre-stellar cloud
cores of various mass and angular momentum. The resulting stellar evolution
tracks were compared with the isochrones and isomasses calculated using
non-accreting models. We find that mass accretion in the initial several Myr of
protostellar evolution can have a strong effect on the subsequent evolution of
young stars and brown dwarfs. The disagreement between accreting and
non-accreting models in terms of the total stellar luminosity L_st, stellar
radius R_st and effective temperature T_eff depends on the thermal efficiency
of accretion, i.e., on the fraction of accretion energy absorbed by the central
object. The largest mismatch is found for the cold accretion case, in which
essentially all accretion energy is radiated away. The relative deviations in
L_st and R_st in this case can reach 50% for 1.0-Myr-old objects and remain
notable even for 10-Myr-old objects. In the hot and hybrid accretion cases, in
which a constant fraction of accretion energy is absorbed, the disagreement
between accreting and non-accreting models becomes less pronounced, but still
remains notable for 1.0-Myr-old objects. These disagreements may lead to the
wrong age estimate for objects of (sub-)solar mass when using the isochrones
based on non-accreting models (abridged).
