We present a critical assessment of commonly used pre-main-sequence
isochrones by comparing their predictions to a set of well-calibrated
colour-magnitude diagrams of the Pleiades in the wavelength range 0.4 to 2.5
microns. Our analysis shows that for temperatures less than 4000 K the models
systematically overestimate the flux by a factor two at 0.5 microns, though
this decreases with wavelength, becoming negligible at 2.2 microns. In optical
colours this will result in the ages for stars younger than 10 Myr being
underestimated by factors between two and three.
We show that using observations of standard stars to transform the data into
a standard system can introduce significant errors in the positioning of
pre-main-sequences in colour-magnitude diagrams. Therefore we have compared the
models to the data in the natural photometric system in which the observations
were taken. Thus we have constructed and tested a model of the system responses
for the Wide-Field Camera on the Isaac Newton Telescope.
As a benchmark test for the development of pre-main-sequence models we
provide both our system responses and the Pleiades sequence.Comment: 15 pages, 12 figures, 7 tables, accepted for publication in MNRAS.
All tables are available online at the Cluster Collaboration homepage
http://www.astro.ex.ac.uk/people/timn/Catalogues
