Ultraviolet observations of stellar winds in Be and ‘normal’ B non-supergiant stars

A homogeneous set of wind-velocity and column-density measurements is presented for 40 early-type (B0−B5) Be stars and 40 non-emission, non-supergiant B stars, based on model profile fits to high resolution IUE C IV, Si IV and Si III spectra. The characteristics of mass-loss in the Be and ‘normal’ B stars are compared, and related to the wind properties of O type stars. Stellar winds are evident in Be stars in our sample as late as B3 in C IV, Si IV and/or Si III. Weaker, superionized winds are also present in the most luminous ‘normal’ B stars. Discrete absorption components are apparent in more than 50 per cent of the Be stars in the sample, and their total ionic column density may be ≥ 70 per cent of that due to the ‘underlying’ winds. Maximum observed wind velocities of up to ∼ 1500 km s−1 are evident in Be stars, and are correlated with the projected rotation velocity. The corresponding ratios of maximum observed wind velocity to photospheric escape velocity increase as a function of effective temperature. The ‘normal’ B stars have wind velocities which do not, in general, exceed the photospheric escape velocity. The product of mass-loss rate and ion fraction (⁠|$\dot M q_\text i$|⁠) is a strong function of stellar luminosity for C3+ and Si3+ in Be stars; however, the slope of this dependence may be shallower than that noted for O type stars. These results highlight the role of radiation pressure driving in Be star winds, though additional mechanisms may provide the initial acceleration in cooler stars. The relative wind ion fractions and the UV photospheric absorption lines of Be and ‘normal’ B stars are also compared.