Empirically Constrained Color-Temperature Relations. II.uvby

Abstract

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 À3:0 ½Fe=H þ0:5 and 3000 T eA 8000 K. At warmer temperatures (i.e., 8000 < T eA 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 ½Fe=H $ À2:2 globular cluster M92 can be matched exceedingly well down to M V % 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-T eff relations for cool stars fail to provide a suitable match to the uvby photometry of both cluster and field stars having ½Fe=H > À2:0. 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 T eff 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 T eA 5500 K. They were mainly used to test the color-T eff relations and, encouragingly, isochrones that employ the transformations derived in this study are able to reproduce the observed CMDs (involving uÀv, vÀb, and bÀy 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 ) C observations for the same clusters-which provides a compelling argument in support of the color-T eff relations 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'' (bÀy)-T eff relationship with those derived empirically in a number of recent studies and examined in some detail the dependence of the m 1 index on [Fe/ H].

Calibration of Strömgrenuvby-Hβphotometry for late-type stars – a model atmosphere approach

“…We note in passing that Clem et al (2004) also found discrepancies between their calculated m 1 indices and observations. For their coolest models they had to apply upward corrections to their calculated m 1 indices of 0.1-0.3 mag.…”

“…15, the CN line strengthening which is expected for the red giants improves the fit to observed c 1 indices for the red giant models considerably. Clem et al (2004) applied semiempirical corrections also to their calculated (u − v) and (v − b) colours in order to fit observations. The resulting effect on their c 1 indices is typically less than 0.1 mag while we would need a downward correction of our c 1 indices of about 0.2 mag for the coolest models to fit (cf.…”

“…Following Clem et al (2004), we shall test our model colours versus observed colours for globular clusters. Unlike the approach of Clem et al, our main emphasis lies on exploring the possibilities and shortcomings of our theoretical model colours sooner than correcting them semiempirically to establish new calibrations.…”

“…Sometimes, semiempirically corrected fluxes from model atmospheres have also been used for calibrations of Strömgren photometry, see e.g. Lejeune et al (1999) and Clem et al (2004).…”

Stellar rotation, binarity, and lithium in the open cluster IC 4756

“…Colors are based on calibrations from ATLAS-9 spectra (Castelli & Kurucz 2003). Small but systematic differences are not surprising and are partly due to the isochrone calibration itself (see Clem et al 2004).…”