Galactic star clusters in the u'g'r'i'z' photometric system




Clem, James Lewis

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Although the Sloan Digital Sky Survey (SDSS) has compiled an unprecedented amount of photometric data on stellar populations in the Milky Way, it is not presently possible to accurately and consistently interpret these data due to the lack of precise fiducial stellar sequences and color-temperature relations for the new 'u'g'r'i'z' photo-metric system. In order to address these deficiencies, this study describes an extensive observational project that has obtained high-quality and homogeneous photometry for a number of different Galactic star clusters spanning a wide range in meta.llicity (-2.5 ≤ [Fe/H] ≤ +0.3), as observed in the u'g'r'i'z' passbands with the MegaCam wide-field imager on the Canada-France-Hawaii Telescope. By employing these purest of stellar populations, fiducial sequences have been defined from color-magnitude diagrams (CMDs) that. extend from the tip of the RGB down to approximately 4 magnitudes below the turnoff points: these have been accurately calibrated to the standard u'g'r'i'z` system via a set of secondary photometric standards located within these same clusters. Consequently, they can serve as a valuable set of empirical fiducials for the interpretation of stellar populations data in the system, as well as calibrators for the transformation of stellar isochrones to the u'g'r'i'z' bandpasses. In fact. when a new grid of theoretical color-Teff relations and bolometric corrections for the system is computed from Kurucz model atmospheres and employed to translate isochrones to the observed planes, generally very good and consistent fits to the CMDs of the metal-poor ([Fe/H] <0.0) globular clusters M 92, M 13, and M 71 are found, once reasonable estimates of the cluster parameters are assumed (the only obvious discrepancy is a shift of ~ 0.01 - 0.03 mag between the predicted and observed giant branches when the models are fitted to the main sequences). More-over, the same isochrone fits are entirely consistent with those inferred from cluster observations in other photometric systems - notably, the BV(RI) , and uvby systems. At the metal-rich end ([Fe/H] ≥ 0.0), however. these transformations fail to match the observed loci of cool, main-sequence stars (Teff ≤ 5000K) in the open clusters M 67 and NGC 6791. An exploration into possible causes of these discrepancies is provided.



astronomical photometry, star clusters