Abundances of the Sup/3 He star 3 CEN A (B5 IIIp) from ultraviolet IUE spectra

Abstract

High resolution (0.010-0.025nm) IUE ultraviolet spectra of the peculiar Sup/3He star 3 Cen A were analyzed in the whole 120-320nm region. Abundances were derived from the comparison of the observed and computed spectra. For computing spectra a Kurucz ATLAS9 model with parameters T_eff_=17500K, logg=3.8, and microturbulent velocity ξ=0km/s was adopted. A rotational velocity vsini=0km/s was assumed. Colour indices, flux distribution, ultraviolet Si lines, and Balmer profiles were used to fix the model parameters. The observed spectra are reproduced at best with the following abundances: He [-0.55], B [-1.0], C [-0.75], N [-0.5], Mg [-0.5], Al [-1.5] underabundant, P [+1.6], Sc [+1.0], Mn [+1.4], Cu [+2.0] overabundant, Ga [+3.25], Kr [+3.3], Hg [+4.5] extremely overabundant. The comparison of the ultraviolet spectrum and of the abundances of 3 Cen A with those of the normal B-type star ι Her and of the Pop II peculiar B-type star Feige 86 has shown that the same elements contribute to the line spectrum of ι Her and 3 Cen A even if with different abundances, so that the spectra can be compared. In the case of Feige 86, the overabundance of heavy elements which are not observed in 3 Cen A (i.e. Pt II, Pt III, Au II, Au III, probably Mo III), the lower overabundance of Ga [+1.0], and the larger underabundance of the light elements C [-2.5] and N [-2.0] make the ultraviolet line spectra of Feige 86 somewhat different from those of both ι Her and 3 Cen A. Also the He I lines have different profiles in the two peculiar stars, although He is underabundant in both. With the specific abundances of 3 Cen A we computed an opacity sampling ATLAS12 model having the same parameters as the ATLAS9 model. We found that the energy distribution from the ATLAS9 model and solar abundances is not very different from that derived from the ATLAS12 model, the synthetic spectrum approach, and the specific abundances of 3 Cen A.