Abundances from B stars of the Magellanic Clouds.

Abstract

Model atmosphere analyses of two B giants, NGC 1818/D 12 and NGC 330/3, which are probably members of blue globular clusters of the LMC and SMC, respectively, are carried out. The analyses are based on CASPEC spectrograms (resolution ~20.000 S/N >/~ 50), on IUE low dispersion spectrograms (both SWP and LWP) and on UBVRI photometry. The analysis is carried out differentially with respect to the galactic star HR 3663 (B3III) which has been observed with the same instruments (CASPEC and IUE). The observations are best fitted by model atmospheres with effective temperatures (T_eff_) and surface gravities (log g) of 17,900 / 2.85 and 18,700 / 3.00 for the LMC and SMC star, respectively, and a microturbulent velocity of ξ = 10 km s^-1^ for both stars. A deficiency of the metals (without CNO) of {DELTA} log ɛ, = -0.8 and -1.1 is derived for the LMC and SMC star, respectively. The CNO pattern is surprising; whereas C behaves like the rest of the metals, N is considerably and O significantly overabundant. The abundances are compared with data of H II regions and of late type stars of the field and in clusters, found in the literature. From the comparatively well known distances and hence luminosities and the model parameters, the radius and the mass of both stars are derived. We obtain R = 22.8 R_sun_ and M = 15 M, for NGC 1818/D12 and R = 23.9 R_sun_ and M = 19 M_sun_ for NGC 330/3. These values agree within the uncertainties of our analyses with the values estimated from evolutionary tracks.