Rapidly oscillating Ap stars (roAp stars) are a subtype of the Ap star class that exhibit short-timescale rapid photometric or

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

variations. The known periods range between 5 and 23 minutes. They lie in the δ Scuti

instability strip The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillati ...

on the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

Discovery

The first roAp star to be discovered was HD 101065 ( Przybylski's Star). The oscillations were discovered by

Donald Kurtz Donald Wayne Kurtz (1948 – ) is an astronomer known for his research into asteroseismology. He completed his PhD in astronomy at the University of Texas in 1976 before moving to Cape Town, where he became a professor at the University of Cape ...

using the telescope at the

South African Astronomical Observatory South African Astronomical Observatory (SAAO) is the national centre for optical and infrared astronomy in South Africa. It was established in 1972. The observatory is run by the National Research Foundation of South Africa. The facility's funct ...

, who saw 10–20-millimagnitude variations in the

light curve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

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 star In astrophysics, chemically peculiar stars (CP stars) are stars with distinctly unusual Metallicity, metal abundances, at least in their surface layers. Classification Chemically peculiar stars are common among hot main-sequence (hydrogen-burning) ...

s, 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. 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

end of the δ Scuti

, it has been suggested that the driving mechanism may be similar, i.e. the opacity mechanism operating in the

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...

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 The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosity, luminosities versus their stellar classifications or eff ...

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 in HD 177765, 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 as

neodymium Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes i ...

praseodymium Praseodymium is a chemical element with the Symbol (chemistry), symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered to be one of the rare-earth metals. It is a soft, silvery, malleable and ductile ...

. Some lines are not seen to pulsate, such as

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

. 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, the

spectral lines A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...

that are formed by elements that are radiatively levitated high in the atmosphere are likely to be most sensitive to measuring the pulsation, whereas the lines of elements such as

, which gravitationally settle, are not expected to exhibit radial velocity variations.

List of identified roAp stars

References

{{Variable star topics, state=collapsed * *Ap