Discovery of the 'missing' mode in HR 1217 by the Whole Earth Telescope

Abstract

HR 1217 is a prototypical rapidly oscillating Ap star that has presented a test to the theory of non-radial stellar pulsation. Prior observations showed a clear pattern of five modes with alternating frequency spacing of 33.3 and 34.6 µHz, with a sixth mode at a problematic spacing of 50.0 µHz (which equals 1.5 x 33.3 µHz) to the high-frequency side. Asymptotic pulsation theory allowed for a frequency spacing of 34 µHz, but Hipparcos observations rule out such a spacing. Theoretical calculations of magnetoacoustic modes in Ap stars by Cunha predicted that there should be a previously undetected mode 34 µHz higher than the main group , with a smaller spacing between it and the highest one. In this letter, we present preliminary results from a multisite photometric campaign on the rapidly oscillating Ap star HR 1217 using the 'Whole Earth Telescope'. While a complete analysis of the data will appear in a later paper, one outstanding result from this run is the discovery of a newly detected frequency in the pulsation spectrum of this star, at the frequency predicted be Cunha.