Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. II. Test of Theoretical Stellar Isochrones

TL;DR

This study tests theoretical stellar models against SDSS cluster data, finding good overall agreement but discrepancies at low temperatures, and demonstrates accurate distance and age estimations using multiple color indices.

Contribution

The paper introduces improved YREC-based isochrones for SDSS filters and validates their effectiveness in deriving cluster distances and ages, highlighting systematic errors and model-data discrepancies.

Findings

Models match main sequences across a range of metallicities.

Significant color discrepancies at low temperatures for metal-rich clusters.

Distance and age estimates are consistent with previous measurements.

Abstract

We perform an extensive test of theoretical stellar models for main-sequence stars in ugriz, using cluster fiducial sequences obtained in the previous paper of this series. We generate a set of isochrones using the Yale Rotating Evolutionary Code (YREC) with updated input physics, and derive magnitudes and colors in ugriz from MARCS model atmospheres. These models match cluster main sequences over a wide range of metallicity within the errors of the adopted cluster parameters. However, we find a large discrepancy of model colors at the lower main sequence (Teff < ~4500 K) for clusters at and above solar metallicity. We also reach similar conclusions using the theoretical isochrones of Girardi et al. and Dotter et al., but our new models are generally in better agreement with the data. Using our theoretical isochrones, we also derive main-sequence fitting distances and turn-off ages for five key globular clusters, and demonstrate the ability to derive these quantities from photometric data in the Sloan Digital Sky Survey. In particular, we exploit multiple color indices (g - r, g - i, and g - z) in the parameter estimation, which allows us to evaluate internal systematic errors. Our distance estimates, with an error of sigma(m - M) = 0.03-0.11 mag for individual clusters, are consistent with Hipparcos-based subdwarf fitting distances derived in the Johnson-Cousins or Stromgren photometric systems.