Temperature and gravity scales in hot stars

Abstract

Precise determinations of effective temperatures and surface gravities are mandatory to derive not only reliable chemical abundances, but also parameters like distances, masses, radii and luminosities in OB stars. We have previously determined atmospheric parameters of 30 well-studied OB main sequence to giant stars via multiple ionization equilibria in NLTE, reaching uncertainties as low as ∼300 K for effective temperature and ∼0.05 dex for surface gravity. Based on the comparison of our spectroscopic parameters and reddening-independent quantities of the Johnson and Strömgren photometric systems, temperature and gravity calibrations are proposed to different photometric indices depending on the luminosity class. With these calibrations, effective temperatures can be determined at a precision of ∼400 K for luminosity classes III/IV and of ∼800 K for luminosity class V. Surface gravities can reach internal uncertainties as low as ∼0.08 dex. Our uncertainties are smaller than typical differences among other methods in the literature, which reach values up to ± 2000 K for temperature and ± 0.25 dex for gravity, and in extreme cases, + 6000 K and ± 0.4 dex, respectively.