In astrophysics, chemically peculiar stars (CP stars) are stars with distinctly unusual metal abundances, at least in their surface layers.
Chemically peculiar stars are common among hot main-sequence (hydrogen-burning) stars. These hot peculiar stars have been divided into 4 main classes on the basis of their spectra, although two classification systems are sometimes used: [1]
The class names provide a good idea of the peculiarities that set them apart from other stars on or near the main sequence.
The Am stars (CP1 stars) show weak lines of singly ionized Ca and/or Sc, but show enhanced abundances of heavy metals. They also tend to be slow rotators and have an effective temperature between 7000 and 10000 K.
The Ap stars (CP2 stars) are characterized by strong magnetic fields, enhanced abundances of elements such as Si, Cr, Sr and Eu, and are also generally slow rotators. The effective temperature of these stars is stated to be between 8000 and 15000 K, but the issue of calculating effective temperatures in such peculiar stars is complicated by atmospheric structure.
The HgMn stars (CP3 stars) are also classically placed within the Ap category, but they do not show the strong magnetic fields associated with classical Ap stars. As the name implies, these stars show increased abundances of singly ionized Hg and Mn. These stars are also very slow rotators, even by the standards of CP stars. The effective temperature range for these stars is quoted at between 10000 and 15000 K.
The He-weak stars (CP4 stars) show weaker He lines than would be expected classically from their observed Johnson UBV colours. A rare class of He-weak stars are, paradoxically, the helium-rich stars, with temperatures of 18000–23000 K. [2] [3]
It is generally thought that the peculiar surface compositions observed in these hot main-sequence stars have been caused by processes that happened after the star formed, such as diffusion or magnetic effects in the outer layers of the stars. [4] These processes cause some elements, particularly He, N and O, to "settle" out in the atmosphere into the layers below, while other elements such as Mn, Sr, Y and Zr are "levitated" out of the interior to the surface, resulting in the observed spectral peculiarities. It is assumed that the centers of the stars, and the bulk compositions of the entire star, have more normal chemical abundance mixtures which reflect the compositions of the gas clouds from which they formed. [1] In order for such diffusion and levitation to occur and the resulting layers to remain intact, the atmosphere of such a star must be stable enough to convection that convective mixing does not occur. The proposed mechanism causing this stability is the unusually large magnetic field that is generally observed in stars of this type. [5]
Approximately 5–10% of hot main sequence stars show chemical peculiarities. [6] Of these, the vast majority are Ap (or Bp) stars with strong magnetic fields. Non-magnetic, or only weakly magnetic, chemically peculiar stars mostly fall into the Am or HgMn categories. [7] [3] A much smaller percentage show stronger peculiarities, such as the dramatic under-abundance of iron peak elements in λ Boötis stars.
Another group of stars sometimes considered to be chemically peculiar are the 'sn' stars. These hot stars, usually of spectral classes B2 to B9, show Balmer lines with sharp (s) cores, sharp metallic absorption lines, and contrasting broad (nebulous, n) neutral helium absorption lines. These may be combined with the other chemical peculiarities more commonly seen in B-type stars. [8]
It was originally proposed that the unusual helium lines were created in a weak shell of material around the star, [9] but are now thought to be caused by the Stark effect. [8]
There are also classes of chemically peculiar cool stars (that is, stars with spectral type G or later), but these stars are typically not main-sequence stars. These are usually identified by the name of their class or some further specific label. The phrase chemically peculiar star without further specification usually means a member of one of the hot main sequence types described above. Many of the cooler chemically peculiar stars are the result of the mixing of nuclear fusion products from the interior of the star to its surface; these include most of the carbon stars and S-type stars. Others are the result of mass transfer in a binary star system; examples of these include the barium stars and some S stars. [6]
There are very few reports of the exoplanets whose host stars are the chemically peculiar stars. [10] [11] The young variable star HR 8799, which hosts four directly imaged massive planets, belongs to the group of λ Boötis stars. [12]
63 Andromedae is an Alpha2 Canum Venaticorum variable star in the constellation Andromeda. Its variable star designation is PZ Andromedae. With an apparent magnitude of about 5.6, it is bright enough to be seen by naked eye. Based upon an annual parallax shift of 8.53 mas, it is located 382 light years away.
56 Aquarii, abbreviated 56 Aqr, is a star in the constellation of Aquarius. 56 Aquarii is its Flamsteed designation. It is a sixth magnitude star, having an apparent visual magnitude of 6.36, and thus is a challenge to view with the naked eye. Based upon an annual parallax shift of 5.07 mas, it is located around 643 light years from the Earth. At that distance, the visual magnitude is diminished by an extinction of 0.12 due to interstellar dust. The star is moving closer with a heliocentric radial velocity of −28 km/s. It is a candidate runaway star showing a transverse peculiar velocity of 213.87 km/s.
Pi Boötis, Latinized from π Boötis, is a probable triple star system in the northern constellation of Boötes. It is visible to the naked eye with a combined apparent visual magnitude of 4.50. Based upon an annual parallax shift of 10.67 mas as seen from Earth, it is located roughly 310 light years from the Sun.
12 Canis Majoris is a variable star located about 707 light years away from the Sun in the southern constellation of Canis Major. It has the variable star designation HK Canis Majoris; 12 Canis Majoris is the Flamsteed designation. This body is just barely visible to the naked eye as a dim, blue-white hued star with a baseline apparent visual magnitude of +6.07. It is moving away from the Earth with a heliocentric radial velocity of +16 km/s. This is the brightest star in the vicinity of the open cluster NGC 2287, although it is probably not a member based on its proper motion.
HD 35520 is a single star in the northern constellation of Auriga. It has a white hue and is dimly visible to the naked eye with an apparent visual magnitude of 5.92. The distance to this star is approximately 2,200 light years based on parallax. The radial velocity for the star is, as yet, poorly constrained.
BN Camelopardalis is a suspected astrometric binary in the northern circumpolar constellation of Camelopardalis. It appears as a variable star that is visible to the naked eye as a dim, white-hued point of light with an apparent visual magnitude that fluctuates around 5.49. The system is located at a distance of around 310 light years from the Sun based on parallax, and is drifting further away with a radial velocity of +9 km/s.
46 Aquilae is a star in the constellation of Aquila, located to the north of Tarazed. 46 Aquilae is its Flamsteed designation. It is a dim, blue-white hued star that is a challenge to view with the naked eye, having an apparent visual magnitude of 6.33. This object is located approximately 830 light years from the Sun, based on parallax. It is moving closer to the Earth with a heliocentric radial velocity of −25 km/s.
V518 Carinae is a naked-eye variable star in the constellation Carina. It is a member of the bright open cluster IC 2602 near the Carina Nebula.
CU Virginis is a single star in the equatorial constellation of Virgo. It has an apparent visual magnitude of 4.99, which is bright enough to be faintly visible to the naked eye. The distance to this star can be estimated from its annual parallax shift of 13.9 mas, yielding a separation of 234 light years.
11 Persei is a single star in the constellation of Perseus, located about 418 light years away from the Sun. It is visible to the naked eye as a dim, blue-white hued star with an apparent visual magnitude of 5.76.
HD 137509 is a star in the southern constellation of Apus, positioned less than a degree from the northern constellation boundary with Triangulum Australe. It has the variable star designation of NN Apodis, or NN Aps for short, and ranges in brightness from an apparent visual magnitude of 6.86 down to 6.93 with a period of 4.4916 days. The star is located at a distance of approximately 647 light years from the Sun based on parallax, and is drifting further away with a radial velocity of +0.50 km/s.
36 Lyncis is a solitary variable star located around 620 light years away from the Sun in the northern constellation of Lynx. It has the variable star designation of EI Lyncis, while 36 Lyncis is the Flamsteed designation. This object is visible to the naked eye as a dim, blue-white hued star with an apparent visual magnitude of 5.30. It is moving further away from the Earth with a heliocentric radial velocity of 21 km/s.
HR 273 is a chemically peculiar spectroscopic binary system in the northern circumpolar constellation of Cassiopeia. It has an apparent visual magnitude of 5.9 making it faintly visible to the naked eye from dark suburban skies. Parallax measurements with the Hipparcos spacecraft put this system at a distance of roughly 350 light years.
Phi Phoenicis, Latinized from φ Phoenicis, is a binary star system in the southern constellation of Phoenix. It is faintly visible to the naked eye with an apparent visual magnitude of 5.1. Based upon an annual parallax shift of 10.185 mas as seen from Earth, it is located approximately 320 light years from the Sun. It is moving away with a heliocentric radial velocity of 10.4 km/s.
Theta Hydri, Latinized from θ Hydri, is the Bayer designation for a blue-white hued star in the southern constellation of Hydrus. It is faintly visible to the naked eye with an apparent visual magnitude of +5.53. Based upon an annual parallax shift of 6.50 mas as seen from Earth, is located approximately 502 light years from the Sun. At that distance, the visual magnitude of the star is diminished by an extinction of 0.10 due to interstellar dust. It is moving away from the Sun with a radial velocity of +12.3 km/s.
HD 142250 is a star in the constellation Scorpius. It has a visual apparent magnitude of 6.1, being visible to the naked eye only in excellent seeing conditions. From parallax measurements, it is located 486 light-years (149 parsecs) away from Earth. This distance, together with the star's proper motion, indicate that HIP 77900 is a member of the Upper Scorpius subgroup of the Scorpius–Centaurus association, the nearest OB association to the Sun. This subgroup is the youngest of the three of the association, with an estimated age of 11 million years.
Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type. Their helium lines place them in a later spectral type then their hydrogen lines.
HIP 79098 is a binary star in the constellation Scorpius. It has a visual apparent magnitude of +5.88, being visible to the naked eye under very dark skies. From parallax measurements by the Gaia spacecraft, it is located approximately 500 light-years from Earth.
2 Cassiopeiae is a white bright giant in the constellation Cassiopeia, about 2,800 light years away. It is a chemically peculiar Am star.
V686 Coronae Australis is a solitary, bluish-white-hued variable star located in the southern constellation Corona Australis. It has an apparent magnitude that ranges between 5.25 and 5.41, which makes it faintly visible to the naked eye. Gaia DR3 parallax measurements imply a distance of 480 light years and it is slowly receding with a heliocentric radial velocity of 1.3 km/s. At its current distance V686 CrA's average brightness is diminished by 0.35 magnitudes due to extinction from interstellar dust and it has an absolute visual magnitude of −0.24.
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