Balmer lines as Teff and log g indicators for non-solar composition atmospheres -: An application to the extremely helium-weak star HR 6000

Although the importance of a correct abundance assumption in the determination of effective temperature and surface gravity of a star has been demonstrated in the literature, this determination is often still extremely simplified, neglecting the effects of non-solar chemical abundances. In this paper we show how the modeling of the profiles of H-delta and H-gamma, commonly used as T-eff and log g indicators, is affected when the chemical composition is far from the standard one. As a target for our study we selected the chemically peculiar star HR6000. Comparing the observed and synthetic profiles of H6(delta) and H-gamma we obtained T-eff=12950 K and log g=4.05; the atmospheric model has been computed with a metal opacity scale evaluated for [M/H]=-0.5 and He/H=0. A number of FeII lines have been used to infer the rotational velocity (v(e) sin i=0 km s(-1)) and the heliocentric radial velocity (RV=0.67 km s(-1)). By requiring that the abundance of iron is independent of the 96 measured equivalent widths, we determined the microturubulence velocity (xi=0 km s(-1)). The abundance pattern coming from our study is similar to the one inferred from UV lines by Castelli et al. (1985), with the exception of O, Al, Si, Sc and Ni. Possible causes for these descrepancies are discussed. With respect to the Sun, we found the iron peak elements to be normal or overabundant and the light elements, with the exception of Na and P, to be extremely underabundant. We find that HR 6000 is one of the most He-underabundant among the chemically peculiar stars.