PHOTOSPHERIC VELOCITY-GRADIENTS IN B1 SUPERGIANTS

We present IUE spectra for a set of B1 supergiants. These data clearly demonstrate that the Si III lambda1300 multiplet (lower level 6.6 ev above ground) becomes progressively more asymmetric with increasing luminosity or, equivalently, mass loss. In the most luminous supergiants, the cores of the lambda1300 multiplets become displaced by up to -150 km s-1 relative to nearby, weaker lines. In addition, systematic velocity shifts and asymmetries are also present in the strong Si III singlets lambda1312 and lambda1417 in the more luminous stars. The obvious implication of these observations is that the outer photospheres of B supergiants are affected by radial expansion, and this may account for why even non-LTE line analyses of these stars require large micro-turbulent velocity fields, and why line blanketed LTE models underestimate their overall UV line blanketing.