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An important shortcoming of the classical model of low-mass star formation is the so-called luminosity problem, whereby embedded protostars are observed to be under-luminous compared to what is expected from a steady mass accretion. An effect of this is observed in the Hertzsprung-Russell diagrams of star forming regions that show a large luminosity spread, which is incompatible with well-defined isochrones based on classic non-accreting protostellar evolution models. A possible solution is that accretion is not steady, but varies with time. It is not clear if the time evolution of accretion rates in deeply embedded protostars is best characterised by a smooth decline from early to late stages, or by intermittent bursts of high accretion. Furthermore, while an age can be defined for a star forming region, the ages of individual stars in the region will vary. I will discuss how we can trace the accretion history of young stars, how non-steady accretion impacts the evolution of the protostellar structure, and how the observed luminosity spread can be explained through the combined effect of a protostellar age spread, a consequence of sustained star formation in star forming regions, and time-varying protostellar accretion for individual protostars. |
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