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Three papers on the composition and stability of
the hydrogen-deficient dwarf HD144941.

A spectral analysis of the hydrogen-deficient star HD144941

P.M.Harrison and C.S.Jeffery

School of Physics and Astronomy, University of St Andrews North Haugh, St. Andrews, Fife, KY16 9SS, Scotland.

1997, A&A 323, 177

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 Teff, logg and H/He abundance. Ionisation equilibria for available metal ions, fitted He I line profiles, and UV/visual flux distributions have been used to determine Teff, logg and H/He. With optimal values of Teff=23 200 K and $\logg=3.9$ the abundances by number of the principal species are: nH=5.21%, nHe=94.79%, nC=0.0017%, nN=0.00079%, nO=0.0024%, nMg=0.00031%, nAl=0.000019%, nSi=0.00027%, nFe=0.00066%.

HD144941 lies at the high gravity, high temperature end of the group of EHe stars, but its nH/nHe 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.

Keywords. stars: helium stars -- stars: individual stars (HD144941) -- stars: atmospheres -- stars: abundances -- stars: oscillations -- stars: evolution

The iron abundance in helium star HD144941

C.S.Jeffery1 and P.M.Harrison2

1Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
2Dept of Physics \& Astronomy, University of St Andrews, Fife KY16 9SS, Scotland

1997, A&A, 323, 393

Abstract. The iron abundance in the helium star HD144941 has been measured from ultraviolet Fe III lines. It is 1.9+/-0.2 dex below solar and confirms the low metallicity (Z=0.0003) previously determined from measurements of CNO and other light elements. The result is important because the low CNO metallicity must be reflected in the iron-group elements in order to explain why HD144941 does not show pulsations.

Keywords. stars: helium -- stars: individual (HD\,144941) -- stars: abundances

The UV iron spectrum. The ultraviolet spectrum of HD144941 in a region rich in Fe III absoprition lines was observed at high resoution with IUE. The observation is compared with synthetic spectra calculated using three different iron abundances.

A search for variability in the helium-rich subdwarf HD144941

C.S.Jeffery and P.W.Hill

University of St Andrews

1996, Observatory 116, 156

Abstract. Asearch for photometric variability in the helium-rich subdwarf HD144941 found no significant periodic variations with semi-amplitudes greater than 0.0053 mag. at frequencies between 0.02 and 3 day-1 or greater than 0.0023 mag. at frequencies between 5 and 220 day-1. A periodogram analysis suggests the presence of a signal with semi-amplitude 0.0016 mag. and period 0.0194d with a false alarm probability of 3%. These results help to define the instability strip boundary for low-luminosity helium-rich stars, and show that it is a sensitive function of metallicity.