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.

No abstract

We aim to reach an analytic expression that describes the path of light rays through the Earth’s atmosphere in the particular situation in which an inferior mirage is occurring. To achieve our goal, we assume an exponential refractive index profile close to the ground, as suggested by empirical and theoretical studies on the state of air when an inferior mirage is taking place. We consider a parallel-plane atmosphere and assume that the laws of geometric optics apply. Since Fermat’s principle holds, we solve the Euler’s equation and, from the solution we obtain an analytic expression that describes the ray paths in a plane perpendicular to the ground. Given that we focus on the particular case of inferior mirages, we were able to find a very simple and easy-to-use expression to describe the ray paths, allowing us to extract additional valuable information with minimal computational effort. We take advantage of it to impose a limit to the thickness of the air layer next to the ground where appropriate conditions exist to bend the rays upwards, and produce an inferior mirage.

Assuming that V (x) ≈ (1 − µ) G1(x) + µL1(x) is a very good approximation of the Voigt function, in this work we analytically nd µ from mathematical properties of V (x). G1(x) and L1(x) represent a Gaussian and a Lorentzian function, respectively, with the same height and HWHM as V (x), the Voigt function, x being the distance from the function center. In this paper we extend the analysis that we have done in a previous paper, where µ is only a function of a; a being the ratio of the Lorentz width to the Gaussian width. Using one of the dierential equation that V (x) satises, in the present paper we obtain µ as a function, not only of a, but also of x. Kielkopf rst proposed µ(a, x) based on numerical arguments. We nd that the Voigt function calculated with the expression µ(a, x) we have obtained in this paper, deviates from the exact value less than µ(a) does, specially for high |x| values.