The calibration of Stroemgren uvby-Hβ photometry for late-type stars: a model atmosphere approach

The use of model atmospheres for deriving fundamental parameters of stars, such as Teff, log g and [Fe/H], will increase as we find and explore extreme stellar populations where empirical calibrations are not yet available. Moreover, calibrations for upcoming large satellite missions of new spectrophotometric indices, similar to the uvby-Hβ system, will be needed. I aim to test the power of theoretical calibrations based on a new generation of MARCS models by comparisons with observational photometric data. I calculate synthetic uvby-Hβ colour indices from synthetic spectra. A sample of 389 stars is used for a direct comparison of the synthetic indices versus empirical data and for scrutinizing the possibilities of theoretical calibrations for temperature, metallicity and gravity. I show that the temperature sensitivity of the synthetic (b-y) colour is very close to its empirical counterpart, whereas the temperature scale based on Hβ shows a slight offset. The theoretical metallicity sensitivity of the m1 index (and for G-type stars its combination with c1) is somewhat higher than the empirical one, based on spectroscopic determinations. The gravity sensitivity of the synthetic c1 index shows a satisfactory behaviour when compared with observations of F stars. For stars cooler than the sun, a deviation is significant in the c1-(b-y) diagram. The theoretical calibrations of (b-y), (v-y) and c1 seem to work well for Pop II stars and leads to effective temperatures for globular cluster stars supporting recent claims by Korn et al (2007) that atomic diffusion occurs in stars near the turn-off point of NGC 6397. Synthetic colours of stellar atmospheres can indeed be used, in many cases, to derive reliable fundamental stellar parameters. The deviations seen as compared with observational data could be due to incomplete line lists but are possibly also due to effects of assuming plane-parallel or spherical geometry and LTE. For a full length article, see Oenehag et al (2008, in preparation).