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Dire Warnings

1) Good uses for these isochrones.

Our primary aim in making these model isochrones available is to allow others to measure cluster ages from the pre-MS which are consistent with those we have derived. In addition, these models can also be used to identify possible new cluster members from their positions in CMDs which would then require spectroscopic follow-up. Finally, these models could also be used to calculate mass functions for stellar populations based on their positions in the CMD.

2) Bad uses for these isochrones.

We advise users that given the processes involved in creating these model isochrones, they should not be used to compare against new cluster photometry and then argue that, due to a mismatch between the models and the data, there is a particular problem with either the interior or atmospheric models. The semi-empirical pre-MS model isochrones presented here yield ages in agreement with other methods over the two decades of age from 6 to 600 Myr. Stepping outside this age range is not necessarily wrong, but arguments have to be made about why the extrapolation might work in the case in question.

3) Does your photometric system match ours?

In Paper I we made the argument very strongly that there can be significant errors in measuring ages due to the mismatch between the filters in which the observations were taken, and the filters used to create the model isochrones. For pre-main-sequence stars in particular, we also pointed out that using transformations between photometric systems (especially those created for main-sequence stars) can introduce significant errors. So if your observations were not taken with the Sloan Telescope, but were taken with filters designed to match the Sloan, be careful. You will get a significant improvement over conventional isochrones by using the ones presented here, but you really need to do a simulation to assess how close you will get. This dire warning applies especially to the Bessell filters.

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