A Library of Isochrones in the SDSS ugriz System with M67-based Empirical Corrections

Version 1.0:  March 12, 2009

The isochrones on this page are described in Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. II. Test of Theoretical Stellar Isochrones (ApJ, submitted, 2008) by Deokkeun An, Marc H. Pinsonneault, Thomas Masseron, Franck Delahaye, Jennifer A. Johnson, Donald M. Terndrup, Timothy C. Beers, Inese I. Ivans, and Zeljko Ivezic.

Stellar evolutionary tracks were generated using YREC (Sills, Pinsonneault, & Terndrup 2000, F. Delahaye & Pinsonneault, in preparation) for a wide range of compositions and ages. Similar models were employed in our previous studies in the Johnson-Cousins-2MASS filter system, but the new models include opacity data from the Opacity Project (OP).  Theoretical isochrones in the luminosity-T_eff plane were transformed onto the observed color-magnitude plane, using a set of synthetic spectra generated from MARCS (Gustafsson et al. 1998).

These models are in satisfactory agreement with the cluster data (An & Johnson, et al. 2008) over a wide range of metallicity, but a large discrepancy of model colors is found at the bottom of the main sequence for clusters at and above the solar metallicity.  To alleviate a large distance error from main-sequence fitting, we derived empirical corrections on the model color-T_eff relations based on M67, at the distance modulus of (m - M)₀ = 9.61 (835.6 pc), E(B - V) = 0.04, and [Fe/H] = 0.0 (see references in An et al. 2007).  We applied these color-T_eff corrections to models at [Fe/H] ≥ 0.0, but used a linear ramp between [Fe/H] = -0.8 and 0.0, so the corrections become zero at [Fe/H] = -0.8.  In u - g we applied the same corrections to models in all of the metallicity range.  Note that our color corrections are strictly valid at 4000 ≤ T_eff (K) ≤ 6000.

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Last modified: July 6, 2009 (dmt)