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The rapidly oscillating Ap star HD 99563 and its distorted dipole pulsation mode
We undertook a time-series photometric multisite campaign for therapidly oscillating Ap (roAp) star HD 99563 and also acquired mean lightobservations over four seasons. The pulsations of the star, which showflatter light maxima than minima, can be described with a frequencyquintuplet centred on 1557.653 μHz and some first harmonics of it.The amplitude of the pulsation is modulated with the rotation period ofthe star that we determine with 2.91179 +/- 0.00007 d from the analysisof the stellar pulsation spectrum and of the mean light data. We breakup the distorted oscillation mode into its pure spherical harmoniccomponents and find it is dominated by the l= 1 pulsation, and also hasa notable l= 3 contribution, with weak l= 0 and 2 components. Thegeometrical configuration of the star allows us to see both pulsationpoles for about the same amount of time; HD 99563 is only the fourthroAp star for which both pulsation poles are seen and only the thirdwhere the distortion of the pulsation modes has been modelled. We pointout that HD 99563 is very similar to the well-studied roAp star HR 3831.Finally, we note that the visual companion of HD 99563 is located in theδ Scuti instability strip and may thus show pulsation. We showthat if the companion was physical, the roAp star would be a2.03-Msolar, object, seen at a rotational inclination of44°, which then predicts a magnetic obliquity .
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