An assessment of HR diagram constraints on ages and age spreads in star-forming regions and young clusters
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Pre-main sequence evolutionary theory is not well-calibrated to
observations. With care, the observed quantities can be converted into effective
temperature and luminosity (i.e. the Hertzsprung-Russell diagram) which the
theoretical calculations also predict as a function of stellar mass and age. For a
sample of nearby young stellar clusters and associations ranging in age from <1
Myr to >100 Myr, we have tested the loci of luminosity as a function of effective
temperature against various sets of predicted pre-main sequence isochrones. As
we found in Hillenbrand & White (2004) which tested stellar masses, here for
the stellar ages there are two conclusions: some evolutionary calculations fare
better than others in reproducing the empirical sequences, and systematic differences
between all pre-main sequence evolutionary calculations and the data
are apparent. We also simulate hypothetical clusters of varying star formation
history and compare the resulting HR diagram predictions to observed clusters.
Our efforts are directed towards quantitative assessment of apparent luminosity
spreads in star forming regions and young clusters, which are often erroneously
interpreted as true luminosity spreads indicative of true age spreads.
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