Explaining the luminosity spread in young clusters: proto and pre-main
  sequence stellar evolution in a molecular cloud environment
  Hertzsprung-Russell diagrams of star forming regions show a large luminosity
spread. This is incompatible with well-defined isochrones based on classic
non-accreting protostellar evo- lution models. Protostars do not evolve in
isolation of their environment, but grow through accretion of gas. In addition,
while an age can be defined for a star forming region, the ages of individual
stars in the region will vary. We show how the combined effect of a
protostellar age spread, a consequence of sustained star formation in the
molecular cloud, and time-varying protostellar accretion for individual
protostars can explain the observed luminosity spread. We use a global MHD
simulation including a sub-scale sink particle model of a star forming region
to follow the accretion process of each star. The accretion profiles are used
to compute stellar evolution models for each star, incorporating a model of how
the accretion energy is distributed to the disk, radiated away at the accretion
shock, or incorporated into the outer layers of the protostar. Using a modelled
cluster age of 5 Myr we naturally reproduce the lumi- nosity spread and find
good agreement with observations of the Collinder 69 cluster, and the Orion
Nebular Cluster. It is shown how stars in binary and multiple systems can be
externally forced creating recurrent episodic accretion events. We find that in
a realistic global molecular cloud model massive stars build up mass over
relatively long time-scales. This leads to an important conceptual change
compared to the classic picture of non-accreting stellar evolution segmented in
to low-mass Hayashi tracks and high-mass Henyey tracks.
