Abstract:
HR 1217 is one of the best-studied rapidly oscillating Ap (roAp) stars, with a frequency
spectrum of alternating even- and odd- modes that are distorted by the presence of a strong,
global magnetic field. Several recent theoretical studies have found that within the observable
atmospheres of roAp stars the pulsation modes are magneto-acoustic with significant frequency
perturbations that are cyclic with increasing frequency. To test these theories a Whole Earth
Telescope extended coverage campaign obtained 342 h of Johnson B data at 10-s time resolution
for the roAp star HR 1217 over 35 d with a 36 per cent duty cycle in 2000 November–December.
The precision of the derived amplitudes is 14 μmag, making this one of the highest precision
ground-based photometric studies ever undertaken. Substantial support has been found for the
new theories of the interaction of pulsation with the strong magnetic field. In particular, the
frequency jump expected as the magnetic and acoustic components cycle through 2π rad in
phase has been found. Additionally, comparison of the new 2000 data with an earlier 1986
multisite study shows clear amplitude modulation for some modes between 1986 and 2000.
The unique geometry of the roAp stars allows their pulsation modes to be viewed from varying
aspect with rotation, yielding mode identification information in the rotational sidelobes that
is available for no other type of pulsating star. Those rotational sidelobes in HR 1217 confirm
that two of the modes are dipolar, or close to dipolar; based on the frequency spacings and
Hipparcos parallax, three other modes must be either l= 0 or 2 modes, either distorted by the
magnetic field, or a mix of m-modes of given l where the mixture is the result of magnetic and
rotational effects. A study of all high-speed photometric Johnson B data from 1981 to 2000
gives a rotation period P rot = 12.4572 d, as found in previous pulsation and photometric studies,
but inconsistent with a different rotation period found in magnetic studies. We suggest that
this rotation period is correct and that zero-point shifts between magnetic data sets determined
from different spectral lines are the probable cause of the controversy over the rotation period.
This WET data set is likely to stand as the definitive ground-based study of HR 1217. It will
be the baseline for comparison for future space studies of HR 1217, particularly the MOST
satellite observations.
