Rapidly oscillating Ap stars

Rapid light variations with periods in the range of 6 to 12 min have been discovered in five cool magnetic Ap stars. All of the data for these stars are consistent with the hypothesis that they are pulsating in l = 1 or l = 2 non- radial p-modes of very high overtone, k, with their pulsation axes and magnetic axes aligned. By extension of the oblique rotator model we suggest the oblique pulsator model which predicts from the frequencies and amplitudes of pulsation the values of i, the inclination of the rotation axis to the line-of-sight, β, the inclination of the magnetic axis to the rotation axis, and Prot, the rotation period of the star. For all five cases now known the values of i, β, and Prot so determined are completely consistent with other determinations of those values using observations of magnetic and photometric variations with rotation period. The five rapidly oscillating Ap stars are: (1) HD 24712 (HR 1217) which oscillates in two l = 1 modes with periods near 6. 14 min, (2) HD 83368 (HR 3831) which oscillates in an l = 1 mode with a period of 11. 67 min and in an l = 2 mode with exactly half that period, (3) HD 137949 (33 Lib) which oscillates in a single mode with a period of 8. 27 min, (4) HD 101065 which oscillates in two modes with periods near 12. 14 min, and (5) HD 128898 (HR 5463, α Cir) which oscillates with periods near 6. 8 min. We estimate the overtones of the two l = 1 modes in HD 24712 to be k ≈ 60 from their frequency ratio. An improved rotation period for HD 24712 is |Pᵣot\ =\ 2. 8519 0. 0030| day. HD 83368 has a double wave mean light variation with a period of Prot = 2. 8519 ± 0. 0030 day. We predict |i\, =86^ \, , =\, 36^ \, , \ or\, i\, =\, 36^, \, =86^| for this star by application of the oblique pulsator model. Given |R\, =\, 1. 8R_\, and\, i\, =\, 86^| for HD 83368, we predict | \, sin\, i\, =\, 32\, km\, s^-1| We also predict a polarity reversing magnetic field in this star. Hundreds of hours of observations of these stars are presented which substantiate these claims. We discuss the possibility that these stars are δ Scuti stars or, alternatively, that pulsation in many high overtone modes is common among stars, but only some stars pulsate in just a few modes allowing us to detect their integrated variations. We suggest that either the magnetic field structure or density discontinuities in the atmospheres of these stars create a trap for just these few modes. We speculate on the possible temperature range of the rapidly oscillating Ap stars and discuss 21 Corn in this context. Due to the high overtones involved, these oscillations are essentially confined to the outer atmospheres of these stars and hence are not a strong argument against the diffusion hypothesis. The rapidly oscillating Ap stars provide the first body of data for the theoretical study of pulsation in the presence of strong global magnetic fields.