Exploration of the BaSeL stellar library for 9 F-type stars COROT potential targets -: Comparisons of fundamental stellar parameter determinations

The Basel Stellar Library (BaSeL models) is constituted of the merging of various synthetic stellar spectra libraries, with the purpose of giving the most comprehensive coverage of stellar parameters. It has been corrected for systematic deviations detected in respect to UBVRIJHKLM photometry at solar metallicity, and can then be considered as the state-of-the-art knowledge of the broad band content of stellar spectra. In this paper, we consider a sample of 9 F-type stars with detailed spectroscopic analysis to investigate the Basel Stellar Library in two photometric systems simultaneously Johnson (B - V, U - B) and Stromgren (b - y, m(1), and c(1)) The sample corresponds to potential targets of the central seismology programme of the COROT space experiment, which have been recently observed at CHP. The atmospheric parameters T-eff, [Fe/H], and log g obtained from the BaSeL models are compared with spectroscopic determinations as well as with results of other photometric calibrations. For a careful interpretation of the BaSeL solutions, we computed confidence regions around the best chi (2)-estimates and projected them on T-eff-[Fe/H], T-eff-log g, and log g-[Fe/H] diagrams. We first derive the 3 atmospheric parameters from the full photometric information available (Johnson and Stromgren data). The BaSeL library has only been calibrated for broad band UBVRIJHKLM photometry: and it presents therefore intrinsic limitations in respect to other photometric systems, especially with different bandwidth. Thus, using this combination (Johnson and Stromgren), BaSeL temperatures are systematically lower (similar to 130 K), and the discrepancy for the gravity and the metallicity can be quite large in comparison to the other methods. To disentangle these unexpected discrepancies, we fixed the temperatures at their spectroscopic values and studied the relative influence of each colour index in the log g-[Fe/H] diagrams. We find inconsistent results between Stromgren and Johnson synthetic colours. While the: Johnson colours give impressively good log g-[Fe/H] solutions, the combination of the mi and cl Stromgren synthetic indices does not provide reliable results. This is not, of course, due to any intrinsic superiority of the UBV over the Stromgren system, but to the properties of the BaSeL models. Finally, in order to simultaneously and accurately determine the stellar parameters T-eff, [Fe/H] and log g; we suggest to use the combination of the synthetic BaSeL indices B - V, U - B and b - y rather than B - V, U - B, b - y, m(1) and c(1).