Chemically peculiar star

In astrophysics, chemically peculiar stars (CP stars) are stars with distinctly unusual metal abundances, at least in their surface layers.

Classification

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 K and 10 000 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 K and 15 000 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 10 000 K and 15 000 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 18,000 - 23,000 K.[2][3]

Cause of the peculiarities

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.

sn 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]

Other stars

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]

See also

References

  1. ^ a b Preston, G. W (1974). "The chemically peculiar stars of the upper main sequence". In: Annual Review of Astronomy and Astrophysics. Volume 12. (A75-13476 03-90) Palo Alto. 12: 257–277. Bibcode:1974ARA&A..12..257P. doi:10.1146/annurev.aa.12.090174.001353.
  2. ^ Gomez, A. E; Luri, X; Grenier, S; Figueras, F; North, P; Royer, F; Torra, J; Mennessier, M. O (1998). "The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars". Astronomy and Astrophysics. 336: 953. Bibcode:1998A&A...336..953G.
  3. ^ a b Netopil, M; Paunzen, E; Maitzen, H. M; North, P; Hubrig, S (2008). "Chemically peculiar stars and their temperature calibration". Astronomy & Astrophysics. 491 (2): 545. arXiv:0809.5131. Bibcode:2008A&A...491..545N. doi:10.1051/0004-6361:200810325.
  4. ^ Michaud, Georges (1970). "Diffusion Processes in Peculiar a Stars". Astrophysical Journal. 160: 641. Bibcode:1970ApJ...160..641M. doi:10.1086/150459.
  5. ^ Kochukhov, O; Bagnulo, S (2006). "Evolutionary state of magnetic chemically peculiar stars". Astronomy & Astrophysics. 450 (2): 763. arXiv:astro-ph/0601461. Bibcode:2006A&A...450..763K. doi:10.1051/0004-6361:20054596.
  6. ^ a b McClure, R. D (1985). "The carbon and related stars". Journal of the Royal Astronomical Society of Canada. 79: 277. Bibcode:1985JRASC..79..277M.
  7. ^ Bychkov, V. D; Bychkova, L. V; Madej, J (2009). "Catalogue of averaged stellar effective magnetic fields - II. Re-discussion of chemically peculiar a and B stars". Monthly Notices of the Royal Astronomical Society. 394 (3): 1338. Bibcode:2009MNRAS.394.1338B. doi:10.1111/j.1365-2966.2008.14227.x.
  8. ^ a b Saffe, C.; Levato, H.; Maitzen, H. M.; North, P.; Hubrig, S. (2014). "On the nature of sn stars. I. A detailed abundance study". Astronomy and Astrophysics. 562: A128. arXiv:1401.5764. Bibcode:2014A&A...562A.128S. doi:10.1051/0004-6361/201322091.
  9. ^ Abt, H. A.; Levato, H. (1977). "Spectral types in the Orion OB1 association". Publications of the Astronomical Society of the Pacific. 89: 797. Bibcode:1977PASP...89..797A.
109 Virginis

109 Virginis is a single, white-hued star in the zodiac constellation of Virgo, located some 134.5 light years away from the Sun. It is the seventh-brightest member of this constellation, having an apparent visual magnitude of +3.72.This is an A-type main-sequence star with a stellar classification of A0 V, and is a suspected chemically peculiar star. However, Abt and Morrell (1995) gave it a class of A0 IIInn, matching a giant star with "nebulous" lines. It is spinning rapidly with a projected rotational velocity of 285 km/s, which is giving the star an oblate shape with an equatorial bulge that is an estimated 31% larger that the polar radius. The star is 320 million years old with 2.58 times the mass of the Sun and about 2.7 times the Sun's radius. It is radiating 63 times the Sun's luminosity from its photosphere at an effective temperature of 9,683 K.

11 Orionis

11 Orionis is a solitary Ap star in the equatorial constellation of Orion, near the border with Taurus. It is visible to the naked eye with an apparent visual magnitude of 4.65, and it is located approximately 365 light years away from the Sun based on parallax. The star is moving further from the Sun with a heliocentric radial velocity of +16.8 km/s.This object is a chemically peculiar star, known as an Ap star, with enhanced silicon and chromium lines in its spectrum. It is an α² CVn variable, ranging from 4.65 to 4.69 magnitude with a period of 4.64 days. The magnetic field measured from metal lines has a strength of +160±390 G.

26 Vulpeculae

26 Vulpeculae is a close binary star system in the northern constellation of Vulpecula, around 644 light years away from the Sun. It is a challenge to view with the naked eye, having an apparent visual magnitude of 6.40. The star is moving closer to the Earth with a heliocentric radial velocity of −63 km/s, and is expected to come within 225 light-years in around 2.6 million years.This is a single-lined spectroscopic binary with an orbital period of 11 days and an eccentricity of 0.28. The visible component is a suspected chemically peculiar star with a stellar classification of A5 III, suggesting this is an evolved giant star. It has about 4.6 times the Sun's radius and is radiating 80 times the Sun's luminosity from its photosphere at an effective temperature of 7,888 K.

34 Cancri

34 Cancri is a star in the zodiac constellation of Cancer, located about 568 light years away from the Sun. It is a challenge to view with the naked even under good viewing conditions, having an apparent visual magnitude of 6.48. At the distance of this star, its visual magnitude is diminished by an extinction of 0.14 due to interstellar dust.This is an A-type main-sequence star with a stellar classification of A1 V. It is a chemically peculiar star, possibly of the magnetic-type (CP2), showing an abnormal abundance of strontium. The star has only a moderate projected rotational velocity of 18 km/s. It has an estimated 2.7 times the mass of the Sun and about 2.7 times the Sun's radius. The star is radiating 70 times the Sun's luminosity from its photosphere at an effective temperature of 9,661 K.

36 Aurigae

36 Aurigae is a single variable star located about 910 light years away from the Sun in the constellation Auriga. It has the variable star designation V444 Aurigae, while 36 Aurigae is the Flamsteed designation. This object is visible to the naked eye as a dim, white-hued star with a baseline apparent visual magnitude of 5.71. It is moving further from the Earth with a heliocentric radial velocity of +16 km/s.This is a magnetic chemically peculiar star that has been given stellar classifications of A1 Vp Si and B9.5p Si,Fe, indicating it is a late B- or early A-type star showing peculiarities of silicon and iron in the spectrum. It is an Alpha2 Canum Venaticorum variable that ranges in visual magnitude from 5.70 down to 5.74 with a period of 14.368 days. The star has 4.4 times the mass of the Sun and is radiating 724 times the Sun's luminosity from its photosphere at an effective temperature of 10,046 K.

46 Aquilae

46 Aquilae is a star in the constellation of Aquila, located to the north of Tarazed (γ Aquilae). 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.This body has a stellar classification of B9 III, matching a late B-type giant star. It is a chemically peculiar star of a weak Mercury-Manganese type (CP3), and is the most chromium–deficient star known. The star may possess a magnetic field with a strength greater than 2 kG. It is radiating 180 times the luminosity of the Sun from its photosphere at an effective temperature of 12,900 K.

49 Arietis

49 Arietis is a single star in the northern constellation of Aries. 49 Arietis is the Flamsteed designation. It is visible to the naked eye as a faint, white hued star with an apparent visual magnitude of 5.90. The star is located at a distance of about 218 light-years (67 parsecs) distant from Earth based on parallax.This object is classified as an Am star, or non-magnetic chemically peculiar star of the CP1 class, which means the spectrum displays abnormal abundances of certain heavier elements. It has a stellar classification of kA2hA6mA7, which means it has the calcium K line of an A2 class star, the Hydrogen lines of an A6 star, and the metal lines of an A7 star. 49 Arietis has a moderately high rate of spin, showing a projected rotational velocity of 52 km/s, and is radiating 15 times the luminosity of the Sun from its photosphere at an effective temperature of 8,424 K.

56 Arietis

56 Arietis (abbreviated 56 Ari) is a star in the northern constellation of Aries. 56 Arietis is the Flamsteed designation. It is faintly visible to the naked eye with an apparent visual magnitude is 5.79. The estimated distance to this star is approximately 500 light-years (150 parsecs). This is a magnetic, chemically peculiar star of the silicon type and it has a rapid rotation period of 17.5 hours.56 Arietis is also known by its variable star designation SX Arietis. 56 Arietis is the prototype of a class of variable stars known as SX Arietis variables, which are rotationally variable stars with strong magnetic fields.

5 Aquarii

5 Aquarii is a single star in the zodiac constellation of Aquarius, located about 830 light years away from the Sun, based on parallax. 5 Aquarii is the Flamsteed designation. It is visible to the naked eye as a faint, blue-white hued star with an apparent visual magnitude of 5.55. This object is moving closer to the Earth with a heliocentric radial velocity of −3 km/s.This is a suspected chemically peculiar star star with a stellar classification of B9 III, although Adelman et al. (2004) consider it to be a normal star with near-solar elemental abundances. It is relatively sharp-lined with a projected rotational velocity of 25 km/s. The star is radiating 318 times the luminosity of the Sun from its photosphere at an effective temperature of 11,200 K.

7 Sagittarii

7 Sagittarii is a massive star in the southern zodiac constellation of Sagittarius which is located in the Lagoon Nebula (NGC 6530), although multiple sources have considered it a foreground star. It is a dim star but visible to the naked eye with an apparent visual magnitude of 5.37. The distance to this star can be determined from the annual parallax shift of 3.02±0.28 mas, yielding a value of roughly 1,100 light years. It is moving closer to the Sun with a heliocentric radial velocity of −11 km/s.Gray and Garrison (1989) listed a stellar classification of F2 II/III for this star, suggesting it is a K-type star with a spectrum showing mixed traits of a giant/bright giant. Houk and Smith-Moore (1978) had a similar classification of F2/3 II/III. This may indicate it is not a member of NGC 6530, since it shouldn't have evolved to this class from the O-type stars that still populate this cluster, and hasn't had time to evolve from a less massive cluster star.It is a suspected chemically peculiar star. The spectral class from the calcium K line has been given as A8 while the class determined from other metallic lines was F4, making it an Am star. This peculiarity is now considered doubtful.7 Sagittarii has an estimated 18 times the Sun's radius and is radiating 658 times the Sun's luminosity from its photosphere at an effective temperature of around 6,800 K.

HD 105382

HD 105382 (also known as V863 Centauri) is a star in the constellation Centaurus. Its apparent magnitude is 4.47. From parallax measurements, it is located 130 parsecs (440 light years) from the sun.

HD 105382 is a variable star whose apparent magnitude varies with an amplitude of 0.012 over a period of 1.295 days. It has been previously classified as a Be star, which would explain the variability as stellar pulsations, but this classification was probably due to accidental observation of the nearby Be star δ Centauri. A 2004 study showed that the 1.295 day period is actually the rotation period of the star, and that the variability is caused by non-homogeneous distribution of elements in the stellar surface. In particular, HD 105382 is a helium-weak chemically peculiar star with a helium abundance varying between 0.5% and 15% of the solar abundance, and a silicon abundance varying between 0.00044% and 0.0069% the solar value. Regions with more helium appear to coincide with the regions with less silicon, and vice versa. This peculiar abundance pattern is probably related to HD 105382's magnetic field, which has a polar strength of 2.3 kG.From astrometric measurements by the Hipparcos spacecraft, HD 105382 is identified as a probable astrometric binary. It is only 267" away from δ Centauri, and both stars appear to be at the same distance from Earth and have the same motion through space, so they may be related. In total, this may be a five star system. It is a member of the Lower Centaurus-Crux (LCC) subgroup of the Scorpius–Centaurus Association.

HD 137509

HD 137509, also known as NN Apodis, is a class B9p star in the constellation Apus. Its apparent magnitude is 6.87 and it is approximately 640 light years away based on parallax. It shines with a luminosity approximately 111 times that of the Sun and has a surface temperature of 12804 K. It has one of the strongest magnetic fields recorded for a chemically peculiar star.

HD 153201

HD 153201 is a Bp star in the southern constellation of Ara. It is chemically peculiar star that displays an anomalous abundance of the element silicon in its spectrum. This is a suspected variable star of the type known as Alpha² Canum Venaticorum. There is a magnitude 9.86 companion star at an angular separation of 2.30″ along a position angle of 131°.

HD 179791

HD 179791 is suspected variable star in the equatorial constellation of Aquila. It is a challenge to see with the naked eye even under good viewing conditions, having an apparent visual magnitude of 6.48. The distance to HD 179791 can be estimated from its annual parallax shift of 5.3 mas, which yields a value of 616 light years. It is moving further from the Earth with a heliocentric radial velocity of +16 km/s. Astrometric measurements of the star show changes in motion that may indicate it is a member of a close binary system.This is an A-type main-sequence star with a stellar classification of A3 V. It is a suspected chemically peculiar star and formerly a candidate Lambda Boötis star. The status as a Lambda Boötis star was reviewed and changed to non-member in 2015. It is spinning rapidly with a projected rotational velocity of 196 km/s. The star has 2.55 times the mass of the Sun and about 2.5 times the Sun's radius. It is radiating 66 times the Sun's luminosity from its photosphere at an effective temperature of 8,912 K.

HD 29573

HD 29573 is a binary star system in the constellation Eridanus. It has a combined apparent visual magnitude of 4.99, making it visible to the naked eye. Based upon an annual parallax shift of 15 mass, it is located 217 light years from the Sun. The system is moving further away from Earth with a heliocentric radial velocity of +3 km/s.The binary nature of this system was discovered through observations made with the Hipparcos spacecraft. The pair orbit each other with a period of 41 years and an eccentricity of 0.8. The magnitude 5.19 primary component has a class of A1, 2.28 times the mass of the Sun, and is a suspected chemically peculiar star. The secondary has magnitude 7.22, 1.56 times the Sun's mass, and a class of F2. The system has a possible infrared excess due to circumstellar dust.

HD 49976

HD 49976, also known as V592 Monocerotis, is a variable star in the constellation of Monoceros (the Unicorn). It is a chemically peculiar star showing excesses in strontium and the rare earth elements in the photosphere, among others. The magnetic field is complex; not corresponding to a simple dipole.

Mercury-manganese star

A mercury-manganese star is a type of chemically peculiar star with a prominent spectral line at 398.4 nm, due to absorption from ionized mercury. These stars are of spectral type B8, B9, or A0, corresponding to surface temperatures between about 10,000 and 15,000 K, with two distinctive characteristics:

An atmospheric excess of elements like phosphorus, manganese, gallium, strontium, yttrium, zirconium, platinum and mercury.

A lack of a strong dipole magnetic field.Their rotation is relatively slow, and as a consequence their atmosphere is relatively calm. It is thought, but has not been proven, that some types of atoms sink under the force of gravity, while others are lifted towards the exterior of the star by radiation pressure, making a heterogeneous atmosphere.

Tau3 Gruis

Tau3 Gruis is a solitary, white-hued star in the southern constellation of Grus. Its apparent magnitude is 5.71, which is bright enough to be faintly visible to the naked eye. Located around 243 light-years (75 pc) distant, it is a A-type chemically peculiar star of spectral type kA5hA7mF2. This notation indicates the spectrum displays the calcium K-line of an A5 star, the hydrogen lines of an A7 star, and the metal lines of an F2 star.

VZ Arietis

VZ Arietis is single, white-hued star in the northern zodiac constellation of Aries. Varying between magnitudes 5.82 and 5.89, the star can be seen with the naked eye in dark, unpolluted areas. Based upon an annual parallax shift of 5.8 mas, it is located 560 light years from the Sun. It is moving further away with a heliocentric radial velocity of +14 km/s. The star was formerly known as 16 Trianguli, but as the star is no longer in the constellation Triangulum, this designation has fallen out of use.This is a chemically peculiar star of type CP2 (Ap star), showing an anomalous abundance of silicon in its spectrum. It has a stellar classification of A0 V, which indicates this is an A-type main-sequence star that currently fusing hydrogen into helium in its core. This is an Alpha2 Canum Venaticorum variable with 2.7 times the mass of the Sun and about 3.1 times the Sun's radius. It is radiating 79 times the Sun's luminosity from its photosphere at an effective temperature of 10,304 K.

Formation
Evolution
Spectral
classification
Remnants
Hypothetical
Nucleosynthesis
Structure
Properties
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