Balmer lines as T$\mathsf{_{eff}}$ and log g indicators for non-solar composition atmospheres

Abstract

Abstract.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 δ and H γ , 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 HR 6000. Comparing the observed and synthetic profiles of H δ and H γ we obtained T eff = 12 950 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 (ξ = 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.

A refined analysis of the remarkable Bp star HR 6000

“…Table 3 is that obtained from the region longward of the Balmer discontinuity. Its value is the same as that presented by Catanzaro et al (2004 Zirconium: no Zr lines were observed in the spectrum. They are not predicted by assuming solar Zr abundance.…”

“…Helium: from the analysis of the He i lines 4713 Å and 5875 Å, Catanzaro et al (2004) derive an underabundance of (Bonifacio et al 1995). This kind of anomalous profiles was interpreted as evidence of helium stratification (Dworetsky 2004;Bohlender 2005).…”

“…They fixed T eff = 12 950 ± 50 K and log g = 4.05 ± 0.01 from ATLAS9 models computed for [M/H] = −0.5 and zero helium abundance. This particular grid of models yields the same parameters from both Balmer profiles, but we note that the parameters do not change if the grid computed with [M/H] = 0.0 and solar He abundance is used, provided that the different parameters from H γ ([12 750 K, 3.95] The last panel of Table 1 shows that the cooler model, as adopted by Catanzaro et al (2004) (model T12950g405k2M05nohe), does not reproduce the observed Strömgren indices. In fact, the difference between the observed and computed (b − y) 0 indices amounts to 0.020 mag, or even more, depending on the adopted reddening, while the difference between the c 0 indices is 0.122 mag.…”

“…We used the reddening relations E(V − I c ) = 1.25E(B − V) from Dean et al (1978) and Mathis (1990). Catanzaro et al (2004) were the first to derive parameters for HR 6000 from high resolution spectra, in particular from H γ and H δ Balmer profiles observed on FEROS spectra. They fixed T eff = 12 950 ± 50 K and log g = 4.05 ± 0.01 from ATLAS9 models computed for [M/H] = −0.5 and zero helium abundance.…”

“…Previously, optical spectra had only been studied by and who identified the spectra from 3323 Å to 5317 Å and discussed the abundances on the basis of the line intensities. It is remarkable that the stellar parameters derived by Catanzaro et al (2004) from the Balmer profiles (T eff = 12 950 ± 50 K, log g = 4.05 ± 0.01) are 1000 K lower than those previously adopted and deduced from the photometry by Castelli et al (1985) (T eff = 14 000 K, log g = 4.0), by Smith (1997) (T eff = 13 990 K, log g = 4.29) and by van den (T eff = 14 000 K, log g = 4.3).…”