Effect of accretion on the pre-main-sequence evolution of low-mass stars and brown dwarfs

摘要

Aims. The pre-main-sequence evolution of low-mass stars and brown dwarfs is studied numerically starting from the formation of a protostellar or proto-brown dwarf seed and taking into account the mass accretion onto the central object during the initial several Myr of evolution. Methods. The stellar evolution was computed using the STELLAR evolution code with recent modifications. The mass accretion rates were taken from numerical hydrodynamics models by computing the circumstellar disk evolution starting from the gravitational collapse of prestellar 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. Results. 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 _(?) , stellar radius R _(?), and effective temperature T _(eff) depends on the thermal efficiency of accretion, that is, on the fraction of accretion energy that is 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 _(?) and R _(?) in this case can reach 50% for objects 1.0 Myr old, and they remain notable even for objects 10 Myr old. 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 objects 1.0 Myr old. These disagreements may lead to an incorrect age estimate for objects of (sub-)solar mass when using the isochrones that are based on non-accreting models, as has also been noted previously. We find that objects with strong luminosity bursts exhibit notable excursions in the L _(?) – T _(eff) diagram, but the character of these excursions is distinct for hybrid or hot and cold accretion scenarios. In particular, the cold accretion scenario predicts peak luminosities that are greater than those of known FU Orionis-type outbursts, which implies that cold accretion is physically less realistic. Conclusions. Mass accretion during the early stages of star and brown dwarf evolution is an important factor, but its effect depends on the details of how the accreted energy can further be distributed within the star. Efforts should now be made to better understand the character of accretion in young protostellar objects.