CMD Frequently Asked Questions

NEW! Revised for CMD v3.0 on May 2018!
What is the meaning of the label (or stage) in the isochrones? The label was implemented with the PARSEC v1.2S tracks, to divide input tracks and output isochrones into rough "evolutionary sections". They have not been carefully checked for all masses and metallicities, and we know of the presence of some misclassified points in output. Nonetheless, many people (including ourselves) find these labels very useful. Use them with caution! Their approximate meaning is:
Is there a bug in the isochrones of log(age)=9.15? No. For this particular age, there is a pronounced peak in the production rate of AGB stars, that causes a transient red phase in the integrated SSP colours. The reason is explained in detail in Girardi et al. (2013), and in section 3.2.1 of Girardi & Bertelli (1998). It derives from a real physical effect, namely the sudden change in helium-burning lifetimes as stars of low mass develop electron-degenerate cores. Therefore, it is not a bug. The effect is well dealt by our algorithms, and appears either as a very populated single AGB sequence, or as multiple AGB sequences in the same isochrone, over a somewhat limited range of ages. However, this feature has caused problems to many users, especially when the isochrones are fed to population synthesis codes that do not deal properly with the (occasional) gaps in the mass distribution of AGB stars. In order to alleviate this specific problem:

In PARSEC-based isochrones: At the termination of the TP-AGB tracks we have inserted points with label=9 (which means post-AGB, see item above) and logL=-9.999. For most isochrones, this point represents a clear, single termination point, that can be safely ignored. However, for the isochrones with multiple TP-AGB sections, a couple of label=9 points will conveniently appear at the limits of the gap in initial mass between two TP-AGB sections. Therefore, these two label=9 points, with their neglegible luminosity, will prevent people from populating the gap with spurious, bright TP-AGB stars. The AGB-boosting effect will still be there, of course, but with its correct amplitude.

In pre-PARSEC versions of the isochrones: The problem was more evident at metallicities Z=0.008, and was mitigated by supressing the Z=0.008, 1.85 M_sun evolutionary track from the isochrone interpolation, then spreading the "overproduction" of AGB stars at log(age)=9.15 over a wider range of ages, and avoiding gaps in the mass distribution of AGB stars.

How do I refer to these isochrones? Use the references provided in the output header. These references however may not be complete and we strongly encourage you to look at the papers for the details!

In case you want to add a reference to this URL, use "". References to official papers are however to be preferred.

This service is mantained by Léo Girardi at the Osservatorio Astronomico di Padova.
Questions, comments and special requests should be directed to leo.girardi@oapd·inaf·it .
Last modified: May 14, 2018