Rapidly oscillating Ap star
type of variable star
Rapidly oscillating Ap stars (roAp stars) are a subtype of the Ap star class that exhibit short-timescale rapid photometric or radial velocity variations. The known periods range between 5 and 23 minutes. They lie in the δ Scuti instability strip on the main sequence.
Discovery
The first roAp star to be discovered was HD 101065 (Przybylski's Star) in 1961. The oscillations were discovered by Donald Kurtz using the 20-inch (510 mm) telescope at the South African Astronomical Observatory, who saw 10–20-millimagnitude variations in the light curve of the star with a period of 12.15 minutes.
Classification
The roAp stars are sometimes referred to as rapidly oscillating α2 Canum Venaticorum variables. Both the roAp stars and some α2 CVn variables lie on the δ Scuti instability strip and are magnetic chemically peculiar stars, but the roAp stars have very short periods less than an hour.
Oscillations
The roAp stars oscillate in high-overtone, low-degree, non-radial pressure modes. The usual model that is used to explain the behavior of these pulsations is the oblique pulsator model. In this model the axis of pulsation is aligned with the magnetic axis, which can lead to modulation of the amplitude of the pulsation, depending on the orientation of the axis to the line of sight, as it varies with rotation.
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