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Nonlinear, Nonradial Pulsation in Rapidly Oscillating Ap Stars

Published online by Cambridge University Press:  12 April 2016

D. W. Kurtz
D. W. Kurtz*
Affiliation:
Department of Astronomy, University of Cape Town

Abstract

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The rapidly oscillating Ap stars pulsate in high-overtone, low degree p-modes with their pulsation axes aligned with their oblique magnetic axes. They show non-linearity in their pulsation in three ways:

  1. 1) The harmonics of the basic pulsation frequency are detectable.

  2. 2) The pulsation phase seems to vary stochastically on a time scale of days to years depending on the star.

  3. 3) The form of the nonradial surface distortion is not constant with time.

These three effects are illustrated with HR 3831, the best studied of the roAp stars. HR 3831 pulsates in distorted dipole mode which can be modelled as a linear sum of axisymmetric l = 0, 1, 2, and 3 spherical harmonics aligned with the magnetic axis. This gives rise to a 7-frequency multiplet split by exactly the rotation frequency. The form of the distortion shows small changes on a time-scale of years. HR 3831 shows a 5-frequency rotationally split first harmonic multiplet, a 3-frequency rotationally split second harmonic multiplet, and a single third harmonic frequency has probably been detected at an amplitude of 0.065 mmag. The first harmonic has changed its form significantly over the last 10 years. A technique for decomposing the fundamental frequency septuplet into its component spherical harmonics is used to fit the pulsation phase as a function of rotation phase. This allows a unique O-C to be defined for any length of light curve. The long term behaviour of the O-C diagram cannot be modelled adequately with a combination of periodic (Doppler shift) and quadratic (evolution) terms; there seems to be a significant stochastic component. The direction of the pulsation phase reversal at rotational phase 0.747 is indeterminate; sometimes it is a positive-going reversal, sometimes negative-going. At present it is not known whether this is a numerical artifact, or a physical effect in the star. If it is a physical effect, it means that small non-periodic differences in pulsation amplitude between the bipolar hemispheres have been detected.

Type
II. Observational Facts
Information
International Astronomical Union Colloquium , Volume 134: Nonlinear Phenomena in Stellar Variability , 1993 , pp. 207 - 214
Copyright
Copyright © Kluwer 1993

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