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).[1] 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.


The roAp stars are sometimes referred to as rapidly oscillating α2 Canum Venaticorum variables.[2] 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.[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 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,[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 the Hertzsprung–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 in HD 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 as neodymium or praseodymium. Some lines are not seen to pulsate, such as iron. 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 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, which gravitationally settle, are not expected to exhibit radial velocity variations.

List of identified roAp stars

Star name V magnitude Spectral type Period (minutes)
HD 177765 9.1 Ap 23.6
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, 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, HD 128898 3.20 Ap SrEu(Cr) 6.8
HI Lib, HD 134214 7.46 Ap SrEu(Cr) 5.6
β CrB, 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


  1. ^ Kurtz, D. W. Information Bulletin on Variable Stars, vol. 1436 (1978).
  2. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  3. ^ Kurtz, D. W. Monthly Notices of the Royal Astronomical Society, vol. 200, p. 807 (1982).
  4. ^ Shibahashi, H. & Takata, M. Publication of the Astronomical Society of Japan, vol. 45, p. 617 (1993).
  5. ^ Bigot, L. & Dziembowski, W. Astronomy & Astrophysics, vol. 391, p. 235 (2002).
  6. ^ Balmforth, N. et al. Monthly Notices of the Royal Astronomical Society, vol. 323, p. 362 (2001).
  7. ^ Cunha, M. S. Monthly Notices of the Royal Astronomical Society, vol. 333, p. 47 (2002).
  8. ^ Alentiev et al., Monthly Notices of the Royal Astronomical Society, 2012, L398.
10 Aquilae

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The outer envelope of this star has an effective temperature of 7,550 K, giving it the yellow-white hued glow of an F-type star. It is a type of chemically peculiar star known as an Ap star, as indicated by the 'p' in the stellar classification.

10 Aql was described as a variable star in 1973 and a period of six days was suggested. Its small amplitude, period, chemical peculiarity, and position in the H–R diagram indicated that it may be an α2 CVn variable and it was given this classification in the General Catalogue of Variable Stars, along with the variable star designation V1286 Aquilae. Later studies showed that the period was spurious and several very short pulsation periods were found: 11.6, 12.0, and 13.4 minutes. This indicated that 10 Aql was a member of the new rapidly oscillating Ap star class.

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The two components are designated Beta Coronae Borealis A (officially named Nusakan , the traditional name of the system) and B


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Outline of astronomy

The following outline is provided as an overview of and topical guide to astronomy:

Astronomy – studies the universe beyond Earth, including its formation and development, and the evolution, physics, chemistry, meteorology, and motion of celestial objects (such as galaxies, planets, etc.) and phenomena that originate outside the atmosphere of Earth (such as the cosmic background radiation).

Przybylski's Star

Przybylski's Star (pronounced or ), or HD 101065, is a rapidly oscillating Ap star at roughly 355 light-years (109 parsecs) from the Sun in the southern constellation of Centaurus.


ROAP can mean:

Rights Object Acquisition Protocol

RTCWeb Offer/Answer Protocol

Rapidly oscillating Ap star (roAp star).

Star systems
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