Fundamental parameters of He-weak and He-strong stars

Abstract

Context. He-weak and He-strong stars are chemically peculiar AB objects whose He lines are anomalously weak or strong for their MK spectral type. The determination of fundamental parameters for these stars is often more complex than for normal stars due to their abundance anomalies. Aims. We discuss the determination of fundamental parameters: effective temperature, surface gravity, and visual and bolometric absolute magnitudes of He-weak and He-strong stars. We compare our values with those derived independently from methods based on photometry and model fitting. Methods. We carried out low resolution spectroscopic observations in the wavelength range 3400-4700 Å of 20 He-weak and 8 He-strong stars to determine their fundamental parameters by means of the Divan-Chalonge-Barbier (BCD) spectrophotometric system. This system is based on the measurement of the continuum energy distribution around the Balmer discontinuity (BD). For a few He-weak stars we also estimate the effective temperatures and the angular diameters by integrating absolute fluxes observed over a wide spectral range. Non-LTE model calculations are carried out to study the influence of the He/H abundance ratio on the emergent radiation of He-strong stars and on their T eff determination. Results. We find that the effective temperatures, surface gravities and bolometric absolute magnitudes of He-weak stars estimated with the BCD system and the integrated flux method are in good agreement between each other, and they also agree with previous determinations based on several different methods. The mean discrepancy between the visual absolute magnitudes derived using the hipparcos parallaxes and the BCD values is on average ±0.3 mag for He-weak stars, while it is ±0.5 mag for He-strong stars. ForHe-strong stars, we note that the BCD calibration, based on stars in the solar environment, leads to overestimated values of T eff . By means of model atmosphere calculations with enhanced He/H abundance ratios we show that larger He/H ratios produce smaller BD which naturally explains the T eff overestimation. We take advantage of these calculations to introduce a method to estimate the He/H abundance ratio in He-strong stars. The BD of HD 37479 suggests that the T eff of this star remains fairly constant as the star spectrum undergoes changes in the intensity of H and He absorption lines. Data for the He-strong star HD 66765 are reported for the first time.

Fundamental parameters of B supergiants from the BCD system

“…Since this excess of He contributes to the absorption in the stellar surface layers, the value of the Balmer discontinuity can be affected. It was shown by Cidale et al (2007) that in hot dwarf stars the larger the He abundance the smaller the value of D. In order to see in more detail the influence of the He/H abundance ratio on the emitted visual energy distribution, a grid of synthetic spectra for a range of effective temperatures was computed in non-LTE using the TLUSTY and SYNSPEC computing codes (Hubeny & Lanz 1995) and the references therein, assuming model atmospheres with He/H ratios of 0.1, 0.2, 0.5 and 1.0, and Z = 0.02. The atomic models we used are basically those provided on the TLUSTY website for H i (9 levels), He i (20 individual levels) and He ii (20 levels).…”

“…objects whose atmospheres can be modeled in the framework of hydrostatic and radiative equilibrium approximations. However, since both the photospheric and the circumstellar components of the BD are spectroscopically well separated, it can also be used to study some "peculiar" objects, like: i) Be stars (Divan & Zorec 1982;Zorec 1986;Zorec & Briot 1991;Chauville et al 2001;Zorec et al 2005;Vinicius et al 2006); ii) objects with the B[e] phenomenon (Cidale et al 2001); iii) chemically peculiar stars (He-W group) (Cidale et al 2007). …”

Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740

“…The UV Si iii line profiles can be better reproduced by models with an effective temperature higher by roughly 2000 K. The published temperature determinations from the optical region seem to indicate the need for an opposite shift of the effective temperature (Zboril et al 1997;Cidale et al 2007;Netopil et al 2008). However, because there is a relatively large uncertainty in the T eff determination (see Sect.…”

Open clusters

“…A&A 544, A64 (2012) and 5) it can be applied with high accuracy to normal and many peculiar classes of B-type stars (Zorec & Briot 1991;Cidale et al 2001;Zorec et al 2005;Cidale et al 2007), even if these objects display a second BD that could be either in emission or in absorption (Divan 1979). To determine the parameter D = log(F +3700 /F −3700 ) we normalized the observed energy distribution with a Planck function to rectify the energy distribution at both sides of the BD.…”