Rapidly oscillating Ap 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.

The first roAp star to be discovered was HD 101065 (Przybylski's Star). The oscillations were discovered by Donald Kurtz using the 20 inch 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.

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.

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. The apparent link between the magnetic axis and the pulsation axis gives clues to the nature of the driving mechanism of the pulsations. As the roAp stars seem to occupy the main sequence end of the δ Scuti instability strip, it has been suggested that the driving mechanism may be similar, i.e. the opacity mechanism operating in the Hydrogen ionization zone. No standard pulsation model can be made to excite oscillations of the roAp type using the opacity mechanism. As the magnetic field appears to be important, research has taken this into account in deriving non-standard pulsation models. It has been suggested that the modes are driven by the suppression of convection by the strong magnetic field near the magnetic poles of these stars which would account for the alignment of the pulsation axis with the magnetic axis. An instability strip for the roAp stars has been calculated which agreed with the positions on the Hertzsprung–Russell diagram of the roAp stars discovered up to that point, but predicted the existence of longer period pulsators amongst the more evolved roAp stars. Such a pulsator was discovered in HD 177765 which has the longest pulsation period of any roAp star at 23.6 minutes.

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