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2004AJ....127.1227C - Astron. J., 127, 1227-1256 (2004/February-0)

Empirically constrained color-temperature relations. II. uvby.

CLEM J.L., VANDENBERG D.A., GRUNDAHL F. and BELL R.A.

Abstract (from CDS):

A new grid of theoretical color indices for the Strömgren uvby photometric system has been derived from MARCS model atmospheres and SSG synthetic spectra for cool dwarf and giant stars having -3.0≤[Fe/H]≤+0.5 and 3000≤Teff≤8000 K. At warmer temperatures (i.e., 8000<Teff≤40,000 K), this grid has been supplemented with the synthetic uvby colors from recent Kurucz atmospheric models without overshooting (Castelli, Gratton, & Kurucz, published in 1997). Our transformations appear to reproduce the observed colors of extremely metal-poor turnoff and giant stars: the various uvby color-magnitude diagrams (CMDs) for the [Fe/H]~-2.2 globular cluster M92 can be matched exceedingly well down to MV~6 by the same isochrone that provides a very good fit to published BV data (see Paper I), on the assumption of the same distance and reddening. Due to a number of assumptions made in the synthetic color calculations, however, our color-Teff relations for cool stars fail to provide a suitable match to the uvby photometry of both cluster and field stars having [Fe/H]>-2.0. To overcome this problem, the theoretical indices at intermediate and high metallicities have been corrected using a set of color calibrations based on field stars having well-determined distances from Hipparcos, accurate Teff estimates from the infrared flux method, and spectroscopic [Fe/H] values. In contrast with Paper I, star clusters played only a minor role in this analysis in that they provided a supplementary constraint on the color corrections for cool dwarf stars with Teff≤5500 K. They were mainly used to test the color-Teffrelations and, encouragingly, isochrones that employ the transformations derived in this study are able to reproduce the observed CMDs (involving u-v, v-b, and b-y colors) for a number of open and globular clusters (including M67, the Hyades, and 47 Tuc) rather well. Moreover, our interpretations of such data are very similar, if not identical, with those given in Paper I from a consideration of BV(RI)Cobservations for the same clusters–which provides a compelling argument in support of the color-Teffrelations that are reported in both studies. In the present investigation, we have also analyzed the observed Strömgren photometry for the classic Population II subdwarfs, compared our final'' (b-y)-Teffrelationship with those derived empirically in a number of recent studies and examined in some detail the dependence of the m1index on [Fe/H].

Journal keyword(s): Stars: Color-Magnitude Diagrams - Galaxy: Globular Clusters: General - Galaxy: Globular Clusters: Individual: Messier Number: M92 - Galaxy: Globular Clusters: Individual: Messier Number: M3 - Galaxy: Globular Clusters: Individual: Name: 47 Tucanae - Galaxy: Open Clusters and Associations: General - Galaxy: Open Clusters and Associations: Individual: Messier Number: M67 - Galaxy: Open Clusters and Associations: Individual: Name: Hyades - Galaxy: Open Clusters and Associations: Individual: NGC Number: NGC 6791 - Stars: Atmospheres - Stars: Fundamental Parameters - Stars: General

VizieR on-line data: <Available at CDS (J/AJ/127/1227): table3.dat>