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 21 min. They lie in the delta Scuti

Delta Scuti

Delta Scuti is a white, F-type giant star in the constellation Scutum. It is approximately 187 light years from Earth. Delta Scuti is the prototype of the Delta Scuti type variable stars. It is a high-amplitude δ Scuti type pulsator with light variations of about 0.15 minutes...

 instability strip on the main sequence

Main sequence

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 Hertzsprung and Henry Norris Russell...

.

Discovery

The first roAp star to be discovered was HD 101065 (Przybylski's Star

Przybylski's star

Przybylski's Star , or HD 101065, is a peculiar star that is located about 410 light years from the Sun in the constellation Centaurus....

). The oscillations were discovered by Donald Kurtz

Donald Kurtz

Donald Wayne Kurtz 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 Town...

 using the 20 inch telescope at the South African Astronomical Observatory

South African Astronomical Observatory

South African Astronomical Observatory is the national center 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 function is to conduct research in astronomy and astrophysics...

, who saw 10-20 millimagnitude variations in the lightcurve of the star with a period of 12.15 min.

Oscillations

The roAp stars oscillate in high-overtone, low-degree, non-radial pressure modes. The usual model that is used to explain the behaviour 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

Main sequence

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 Hertzsprung and Henry Norris Russell...

 end of the delta Scuti

Delta Scuti

Delta Scuti is a white, F-type giant star in the constellation Scutum. It is approximately 187 light years from Earth. Delta Scuti is the prototype of the Delta Scuti type variable stars. It is a high-amplitude δ Scuti type pulsator with light variations of about 0.15 minutes...

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

Opacity (optics)

Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

 mechanism operating in the Hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 ionization

Ionization

Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...

 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 116114 which has the longest pulsation period of any roAp star at 21 min.

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

Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...

 or Praseodymium

Praseodymium

Praseodymium is a chemical element that has the symbol Pr and atomic number 59. Praseodymium is a soft, silvery, malleable and ductile metal in the lanthanide group. It is too reactive to be found in native form, and when artificially prepared, it slowly develops a green oxide coating.The element...

. Some lines are not seen to pulsate, such as Iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

. 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 that are formed by elements

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 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 Iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

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

Identified roAp stars

Star Name	V magnitude	Spectral Type	Period (min)
AP Scl, HD 6532	8.45	Ap SrEuCr	7.1
BW Cet, HD 9289	9.38	Ap SrCr	10.5
BN Cet, HD 12098	8.07	F0	7.61
HD 12932	10.25	Ap SrEuCr	11.6
BT Hyi, HD 19918	9.34	Ap SrEuCr	14.5
DO Eri, HD 24712	6.00	Ap SrEu(Cr)	6.2
UV Lep, HD 42659	6.77	Ap SrCrEu	9.7
HD 60435	8.89	Ap Sr(Eu)	9.7
LX Hya, HD 80316	7.78	Ap Sr(Eu)	11.4-23.5
IM Vel, HD 83368	6.17	Ap SrEuCr	11.6
AI Ant, HD 84041	9.33	Ap SrEuCr	15.0
HD 86181	9.32	Ap Sr	6.2
HD 99563	8.16	F0	10.7
Przybylski's star Przybylski's star Przybylski's Star , or HD 101065, is a peculiar star that is located about 410 light years from the Sun in the constellation Centaurus.... , HD 101065	7.99	controversial	12.1
HD 116114	7.02	Ap	21.3
LZ Hya, HD 119027	10.02	Ap SrEu(Cr)	8.7
PP Vir, HD 122970	8.31	unknown	11.1
α Cir Alpha Circini Alpha Circini is a variable star in the constellation of Circinus, belonging to the class of rapidly oscillating Ap stars.-References:#... , HD 128898	3.20	Ap SrEu(Cr)	6.8
HI Lib, HD 134214	7.46	Ap SrEu(Cr)	5.6
β CrB Beta Coronae Borealis Beta Coronae Borealis , which has the traditional name Nusakan, is the second brightest star in the constellation of Corona Borealis. Although it appears to the naked eye to be a single star, it is actually a binary star system... , HD 137909	3.68	F0p	16.2
GZ Lib, HD 137949	6.67	Ap SrEuCr	8.3
HD 150562	9.82	A/F(p Eu)	10.8
HD 154708	8.76	Ap	8.0
HD 161459	10.33	Ap EuSrCr	12.0
HD 166473	7.92	Ap SrEuCr	8.8
HD 176232	5.89	F0p SrEu	11.6
HD 185256	9.94	Ap Sr(EuCr)	10.2
CK Oct, HD 190290	9.91	Ap EuSr	7.3
QR Tel, HD 193756	9.20	Ap SrCrEu	13.0
AW Cap, HD 196470	9.72	Ap SrEu(Cr)	10.8
γ Eql, HD 201601	4.68	F0p	12.4
BI Mic, HD 203932	8.82	Ap SrEu	5.9
MM Aqr, HD 213637	9.61	A(p EuSrCr)	11.5
BP Gru, HD 217522	7.53	Ap (Si)Cr	13.9
CN Tuc, HD 218495	9.36	Ap EuSr	7.4