A Tight Upper Limit on Oscillations in the Ap Star ϵ Ursae Majoris from WIRE Photometry

Observations of ϵ UMa obtained with the star tracker on the Wide Field Infrared Explorer satellite during a month in mid-2000 are analyzed. This is some of the most precise photometry of an Ap star. The amplitude spectrum is used to set an upper limit of 75 parts per million for the amplitude of stellar pulsations in this star unless by coincidence it oscillates with a single mode at the satellite orbit, its harmonics, or their 1 day-1 aliases. This is the tightest limit put on the amplitude of oscillations in an Ap star. As the rotation period of ϵ UMa is relatively short (5.1 days), it cannot be argued that the observations were made at a wrong rotational phase. Our results thus support the idea that some Ap stars do not pulsate at all.