Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://star.arm.ac.uk/~csj/abstracts/hd144941_abs.html
Дата изменения: Mon Feb 8 17:55:22 1999 Дата индексирования: Tue Oct 2 05:50:19 2012 Кодировка: Поисковые слова: universe |
Three papers on the composition
and stability of
the hydrogen-deficient dwarf HD144941.
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
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.
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.