Determination of the Effective Temperature and Surface Gravity of A and B-type Horizontal-branch Stars: The Effect of Metallicity

Abstract We present a study of the effect of metallicity on the determination of the effective temperature and surface gravity of A and B type horizontal-branch stars using the spectroscopic method based on fitting the Balmer line profiles. We used synthetic spectra derived from chemically homogeneous atmospheric models calculated with PHOENIX in the LTE mode at various metallicities [M/H] between −1.5 and +0.5 dex solar. Stars from globular clusters M3 and M13 known to have a low metallicity ([Fe/H] ≃ 1.5 dex solar), observed at the Calar Alto 3.5 m telescope, are used for this purpose. For each star, we determined the atmospheric parameters considering various metallicity values. For stars cooler than 11,500 K and model metallicities between −1.5 and −1.0 dex, the effective temperature and surface gravity differ by a maximum of 200 K and 0.1 dex, respectively. For stars hotter than 11,500 K, considering that their atmospheric metallicities are broadly enhanced due to atomic diffusion, metallicities at ±0.5 dex from solar are of more interest. In this low metallicity range, atmospheric parameters increase, with increasing metallicity, to a maximum of 500 K in effective temperature and 0.25 dex in surface gravity.