The ability of intermediate-band Strömgren photometry to correctly identify dwarf, subgiant, and giant stars and provide stellar metallicities and surface gravities ^*

Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, 22100 Lund, Sweden e-mail: [anna,sofia,ingemar]@astro.lu.se

Received: 25 October 2009
Accepted: 10 February 2010

Abstract

Context. Several large scale photometric and spectroscopic surveys are being undertaken to provide a more detailed picture of the Milky Way. Given the necessity of generalisation in the determination of, e.g., stellar parameters when tens and hundred of thousands of stars are considered it remains important to provide independent, detailed studies to verify the methods used in the surveys.

Aims. Our first aim is to critically evaluate available calibrations for deriving [M/H] from Strömgren photometry. Secondly, we develop the standard sequences for dwarf stars to reflect their inherent metallicity dependence. Finally, we test how well metallicities derived from ugriz photometry reproduce metallicities derived from the well-tested system of Strömgren photometry.

Methods. We evaluate available metallicity calibrations based on Strömgren uvby photometry for dwarf stars using a catalogue of stars with both uvby photometry and spectroscopically determined iron abundances ([Fe/H]). The catalogue was created for this project. Using this catalogue, we also evaluate available calibrations that determine log g. A larger catalogue, in which metallicity is determined directly from uvby photometry, is used to trace metallicity-dependent standard sequences for dwarf stars. We also perform comparisons, for both dwarf and giant stars, of metallicities derived from ugriz photometry with metallicities derived from Strömgren photometry.

Results. We provide a homogenised catalogue of 451 dwarf stars with 0.3 < (b–y)₀ < 1.0. All stars in the catalogue have uvby photometry and [Fe/H] determined from spectra with high resolution and high signal-to-noise ratios (S/N). Using this catalogue, we test how well various photometric metallicity calibrations reproduce the spectroscopically determined [Fe/H]. Using the preferred metallicity calibration for dwarf stars, we derive new standard sequences in the c_1,0 versus (b–y)₀ plane and in the c_1,0 versus (v–y)₀ plane for dwarf stars with 0.40 < (b–y)₀ < 0.95 and 1.10 < (v–y)₀ < 2.38.

Conclusions. We recommend the calibrations of Ramírez & Meléndez (2005) in deriving metallicities from Strömgren photometry and find that intermediate band photometry, such as Strömgren photometry, more accurately than broad band photometry reproduces spectroscopically determined [Fe/H]. Strömgren photometry is also better at differentiating between dwarf and giant stars. We conclude that additional investigations of the differences between metallicities derived from ugriz photometry and intermediate-band photometry, such as Strömgren photometry, are required.