The variability of the Hg II λ3984 line of the mercury-manganese star α Andromedae

The variability of the Hg II lambda3984 line in the primary of the binary star alpha And was discovered through the examination of high-dispersion spectra with signal-to-noise ratios greater than 500. This first definitively identified spectrum variation in any mercury-manganese star is not due to the orbital motion of the companion. Rather, the variation is produced by the combination of the 2.38236 day period of rotation of the primary that we determined and a nonuniform surface distribution of mercury that is concentrated in its equatorial region. If the surface mercury distribution exhibits long-term stability, then it is likely that a weak magnetic field operates in its atmosphere, but if changes are observed in the line pro le over a period of a few years, then these would constitute direct evidence for diffusion.