Fundamental Parameters of He-Weak and He-Strong Stars

TL;DR

This study uses spectrophotometry and modeling to determine fundamental parameters of He-weak and He-strong stars, revealing temperature overestimations in He-strong stars due to He/H ratios and proposing a correction method.

Contribution

It introduces a method to estimate He/H ratios in He-strong stars and assesses the accuracy of the BCD system for these stars compared to other methods.

Findings

Good agreement between BCD and flux integration methods for He-weak stars.

Overestimation of Teff in He-strong stars using BCD calibration.

He/H ratio influences the BCD parameter and Teff estimation.

Abstract

We carried out low resolution spectroscopic observations in the wavelength range 3400-4700 A of 20 He-weak and 8 He-strong stars to determine their fundamental parameters by means of the Divan-Chalonge-Barbier (BCD) spectrophotometric system. 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 Teff determination. 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. For He-strong stars, we note that the BCD calibration, based on stars in the solar environment, leads to overestimated values of Teff. 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 Teff 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 Teff 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.