Rotational-Velocity Effects in the uvby, Systems and on MK Spectral Types for B Stars*

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Abstract
Effects of stellar axial rotation on the intrinsic photometric indices of the uvby, β systems have been investigated for early-type members of the Orion OBI association using new photoelectric data and υe sin i values compiled from the literature. A comparison of the observational results with theoretical predictions for rotating stellar models is made for 78 apparently normal B stars. The deviations in the observed β index are in reasonably good agreement with those predicted from the models in that they show the |$({\upsilon}_\text{e}\,\text{sin}\,i)^{2}$| dependence expected from gravity darkening and rotational distortion: no systematic effects are present below |${\upsilon}_\text{e}\,\text{sin}\,i\,\approx\,250\,\text{km}\,\text{s}^{-1}$| The results show that rapid rotation affects the colour indices of stars near the top of the main sequence and that these effects should be considered when deriving evolutionary (nuclear) ages for stellar aggregates. Axial rotation also appears to affect MK spectral classification for late B stars through a dependence of the ratio He I λ 4471/Mg II λ 4481, as recently suggested by Collins. Additional υe sin i data are needed for unevolved stars in the range B7–B9 to strengthen this assertion.

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