Rapidly oscillating Ap stars (roAp stars) are a subtype of theAp star class that exhibit short-timescale rapidphotometric orradial velocity variations. The known periods range between 5 and 23 minutes. They lie in theδ Scutiinstability strip on themain sequence.

The first roAp star to be discovered was HD 101065 (Przybylski's Star) in 1961.^[1] The oscillations were discovered byDonald Kurtz using the 20-inch (510 mm) telescope at theSouth African Astronomical Observatory, who saw 10–20-millimagnitude variations in thelight 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.^[2] Both the roAp stars and someα² CVn variables lie on the δ Scuti instability strip and are magneticchemically 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.^[3][4][5] 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 themain sequence end of theδ Scutiinstability strip, it has been suggested that the driving mechanism may be similar, i.e. theopacity mechanism operating in thehydrogenionization 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,^[6] which would account for the alignment of the pulsation axis with the magnetic axis. An instability strip for the roAp stars has been calculated,^[7] which agreed with the positions on theHertzsprung–Russell diagram of the roAp stars discovered up to that point, but predicted the existence of longer-period pulsators among the more evolved roAp stars. Such a pulsator was discovered inHD 177765,^[8] which has the longest pulsation period of any roAp star at 23.6 minutes.

Most roAp stars have been discovered using small telescopes to observe the small changes in amplitude caused by the pulsation of the star. However, it is also possible to observe such pulsations by measuring the variations in radial velocity of sensitive lines, such asneodymium orpraseodymium. Some lines are not seen to pulsate, such asiron. It is thought that the pulsations are of highest amplitude high in the atmospheres of these stars, where the density is lower. As a result, thespectral lines that are formed byelements that are radiatively levitated high in the atmosphere are likely to be most sensitive to measuring the pulsation, whereas the lines of elements such asiron, which gravitationally settle, are not expected to exhibit radial velocity variations.

• Rapidly oscillating Ap stars
• A-type stars