A spectral analysis of the hydrogen-deficient star HD 144941.

Abstract

HD144941 is a B type hydrogen-deficient star. Previous analysis has suggested that its effective temperature and surface gravity are similar to the nitrogen rich H-deficient star V652 Her. However, its H/He ratio puts it between the classifications of an extreme and an intermediate helium star, and its low C and N abundances make it unique amongst known H-deficient stars. High-resolution, high-S/N, optical spectra have been obtained using the AAT. A grid of model atmospheres has been constructed for varying T_eff_, logg and H/He abundance. Ionisation equilibria for available metal ions, fitted Hei line profiles, and UV/visual flux distributions have been used to determine T_eff_, logg and H/He. With optimal values of T_eff_=23200K and logg=3.9 the abundances by number of the principal species are: n_H_=5.21%, n_He_=94.79%, n_C_=0.0017%, n_N_=0.00079%, n_O_=0.0024%, n_Mg_=0.00031%, n_Al_=0.000019%, n_Si_=0.00027%, n_Fe_=0.00066%. HD144941 lies at the high logg, high T_eff_ end of the group of EHe stars, but its n_H_/n_He_ ratio of 0.055 is above average for the group. The surface composition is also remarkable in having a considerable metal deficiency (Z=0.0003), probably of primordial origin. The low metallicity explains why substantial radial pulsations are not observed for a star with these surface properties and is a vindication of modern helium star pulsation models and opacities.