Am star

An Am star or metallic-line star is a type of chemically peculiar star of spectral type A whose spectrum has strong and often variable absorption lines of metals such as zinc, strontium, zirconium, and barium, and deficiencies of others, such as calcium and scandium. The original definition of an Am star was one in which the star shows "an apparent surface underabundance of Ca (and/or Sc) and/or an apparent overabundance of the Fe group and heavier elements".[1]

The unusual relative abundances cause the spectral type assessed from the Calcium K lines to be systematically earlier than one assessed from other metallic lines. Typically, a spectral type judged solely from hydrogen lines is intermediate. This leads to two or three spectral types being given. For example, Sirius has been given a spectral type of kA0hA0VmA1, indicating that it is A0 when judged by the Calcium k line, A0V when judge by its hydrogen lines, and A1 when judged by the lines of heavy metals.[2] There are other formats, such as A0mA1Va, again for Sirius.[3][4]

The chemical abnormalities are due to some elements which absorb more light being pushed towards the surface, while others sink under the force of gravity. This effect takes place only if the star has low rotational velocity.[5] Normally, A-type stars rotate quickly. Most Am stars form part of a binary system in which the rotation of the stars has been slowed by tidal braking.[5]

The best-known metallic-line star is Sirius (α Canis Majoris). The following table lists some metallic-line stars in order of descending apparent visual magnitude.

List

Name[6] Bayer or other designation Apparent visual magnitude[6]
Sirius A α Canis Majoris A −1.47
Castor Ba α Geminorum Ba 2.96
α Volantis 4.00
Acubens A[7] α Cancri A 4.26
Kurhah[8] ξ Cephei 4.29
θ1 Crucis 4.30
π Virginis[9] 4.64
2 Ursae Majoris 5.46
τ3 Gruis 5.72
WW Aurigae[10] 5.82

δ Delphini and ρ Puppis

A small number of Am stars show unusually late spectral types and particularly strong luminosity effects. Although Am stars in general show abnormal luminosity effects, stars such as ρ Puppis are believed to be more evolved and more luminous than most Am stars, lying above the main sequence. Am stars and δ Scuti variables lie in approximately the same location on the H–R diagram, but it is rare for a star to be both an Am star and a δ Scuti variable. ρ Puppis is one example and δ Delphini is another.[2]

Several authors have referred to a class of stars known as δ Delphini star, Am stars but with relatively little difference between the calcium and other metallic lines. They have also been compared to the δ Scuti stars. Later studies showed that the group was somewhat inhomogeneous, possibly coincidental, and recommended dropping use of the δ Delphini class in favour of a narrower class of ρ Puppis stars with relatively high luminosity and late spectral types.[2][11] However, there is still sometimes confusion, for example with ρ Puppis stars being considered to all be δ Scuti variables.[12]

Notes and references

  1. ^ Conti, Peter S (1970). "The Metallic-Line Stars". Publications of the Astronomical Society of the Pacific. 82: 781. Bibcode:1970PASP...82..781C. doi:10.1086/128965.
  2. ^ a b c Gray, R. O; Garrison, R. F (1989). "The early F-type stars - Refined classification, confrontation with Stromgren photometry, and the effects of rotation". Astrophysical Journal Supplement Series. 69: 301. Bibcode:1989ApJS...69..301G. doi:10.1086/191315.
  3. ^ Conti, P. S; Barker, P. K (1973). "Are all metallic-line stars binaries? Observations of three stars in Coma". Astrophysical Journal. 186: 185. Bibcode:1973ApJ...186..185C. doi:10.1086/152487.
  4. ^ Skiff, B. A (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009-2016)". VizieR On-line Data Catalog: B/mk. Originally published in: Lowell Observatory (October 2014). 1. Bibcode:2014yCat....1.2023S.
  5. ^ a b Am star, The Internet Encyclopedia of Science, David Darling. Accessed on line August 14, 2008.
  6. ^ a b Names and apparent visual magnitudes taken from SIMBAD, unless otherwise noted.
  7. ^ Acubens, Stars, Jim Kaler. Accessed on line August 14, 2008.
  8. ^ Kurhah, Stars, Jim Kaler. Accessed on line August 14, 2008.
  9. ^ Paunzen, E.; et al. (February 2013), "A photometric study of chemically peculiar stars with the STEREO satellites - II. Non-magnetic chemically peculiar stars", Monthly Notices of the Royal Astronomical Society, 429 (1): 119–125, arXiv:1211.1535, Bibcode:2013MNRAS.429..119P, doi:10.1093/mnras/sts318.
  10. ^ WW Aurigae is a binary star both of whose components are Am stars.
  11. ^ Neiner, C; Wade, G. A; Sikora, J (2017). "Discovery of a magnetic field in the δ Scuti F2m star ρ Pup". Monthly Notices of the Royal Astronomical Society: Letters. 468: L46. arXiv:1702.01621. Bibcode:2017MNRAS.468L..46N. doi:10.1093/mnrasl/slx023.
  12. ^ Kochukhov, O. (March 2009). "Asteroseismology of chemically peculiar stars". Communications in Asteroseismology. 159: 61–70. arXiv:0812.0374. Bibcode:2009CoAst.159...61K. doi:10.1553/cia159s61.
20 Aquarii

20 Aquarii, abbreviated 20 Aqr, is a star in the constellation Aquarius. 20 Aquarii is the Flamsteed designation. It is a dim star with an apparent visual magnitude of 6.38. Based upon an annual parallax shift of 15.34 mas, it is located 213 light years away but is moving closer to the Earth with a heliocentric radial velocity of −23 km/s. The star is predicted to come to within 110 light-years in around 1.9 million years.This is an F-type main-sequence star with a stellar classification of F0 V. (Cowley and Fraquelli [1974] had given it a class of F0 III.) It is a suspected chemically peculiar Am star showing metallic lines. It is 761 million years old with a high projected rotational velocity of 92 km/s. The star has 1.52 times the mass of the Sun and is radiating 9 times the Sun's luminosity from its photosphere at an effective temperature of about 7,314 K.

22 Boötis

22 Boötis is a single star in the northern constellation of Boötes, located 319 light years away from the Sun. It has the Bayer designation f Boötis; 22 Boötis is the Flamsteed designation. This object is visible to the naked eye as a dim, white-hued star with an apparent visual magnitude of 5.40. It is moving closer to the Earth with a heliocentric radial velocity of −27 km/s.This is an Am star with a stellar classification of kA7 hA8 mF2 (III) ((Sr II)), showing the calcium K line of a A7 star, the hydrogen lines of an A8 star, and the metal lines of an F2 star. It has the luminosity class of a giant star and does not appear to be variable. The star has twice the mass of the Sun and four times the Sun's radius. It is spinning with a projected rotational velocity of 38 km/s. 22 Boötis is radiating 52 times the Sun's luminosity from its photosphere at an effective temperature of 7,528 K.

25 Cancri

25 Cancri is a common proper motion star system in the zodiac constellation of Cancer, located around 348 light years away from the Sun. It has the Bayer designation d2 Cancri (d2 Cnc); 25 Cancri (25 Cnc) is the Flamsteed designation. It is near the lower limit of visibility to the naked eye in good viewing conditions, appearing as a dim, yellow-white hued star with a combined apparent visual magnitude of 6.11. The pair have a relatively high proper motion, traversing the celestial sphere at an angular rate of 0.245″ per year. It is moving further from the Earth with a heliocentric radial velocity of +37 km/s.Based upon a stellar classification of F6 V, the brighter component is an F-type main-sequence star that is generating energy through hydrogen fusion at its core. Cowley (1976) listed a class of F5 IIIm?, which suggests it may be an Am star. However, this has not been confirmed. It is about 2.5 billion years old with 1.51 times the mass of the Sun. The star is radiating 6.6 times the Sun's luminosity from its photosphere at an effective temperature of 6,487 K.The companion is 4.19 magnitudes fainter than the primary, and lies at an angular separation of 16.798″ along a position angle of 310°, as of 2013. If the pair are gravitationally bound, then they orbit each other with a period of around 4.05 million years.

32 Aquarii

32 Aquarii is a binary star system in the zodiac constellation of Aquarius. 32 Aquarii is its Flamsteed designation. It is visible to the naked eye as a dim, white-hued star with an apparent visual magnitude of 5.29. This system is moving away from the Earth with a heliocentric radial velocity of +19 km/s, and is a possible member of the corona of the Ursa Major flow.This is a single-lined spectroscopic binary with an (assumed) circular orbit having a period of only 7.8 days. It has an a sin i value of 0.777 Gm (0.00519 AU), where a is the semimajor axis and i is the orbital inclination. Since the sine function can be no larger than one, this provides a lower bound on the true semimajor axis of their orbit.

The primary component is an metallic-line (Am) star with the calcium K line of an A3 star, the hydrogen lines of an F1 star, and the metal lines of an F2 star. It is a sharp-lined, slowly rotating star with a projected rotational velocity of 9.6 km/s and is about 465 million years old. The star has 1.69 times the mass of the Sun and three times the Sun's radius. It is radiating 29 times the luminosity of the Sun from its photosphere at an effective temperature of 7,976 K.

39 Andromedae

39 Andromedae, abbreviated 39 And, is a double star in the northern constellation Andromeda. 39 Andromedae is the Flamsteed designation. Its apparent visual magnitude is 5.95, which indicates it is near the lower limit on visibility to the naked eye. The distance to this star, as estimated from its annual parallax shift of 9.57 mas, is 341 light years. It is a suspected member of the Ursa Major Moving Group, although King et al. (2003) list it as a probable non-member.The brighter component is a confirmed Am star with a stellar classification of kA3hA7VmA9. This notation indicates its spectrum displays the calcium K line of an A3 star, the hydrogen lines of an A7 V, or A-type main-sequence star, and the metal lines of an A9 star. It is radiating 40 times the Sun's luminosity from its photosphere at an effective temperature of 8,073 K. As of 2015, the magnitude 12.48 companion star is located at an angular separation of 20.5″ along a position angle of 3° from the primary.

40 Aurigae

40 Aurigae is a binary star in the constellation Auriga. Its apparent magnitude is 5.345, meaning it can just barely be seen with the naked eye. Based on parallax estimates made by the Hipparcos spacecraft, the system is located some 340 light-years (104 parsecs) away.40 Aurigae is a spectroscopic binary, meaning the two stars are too close to be individually resolved, but periodic Doppler shifts in their spectra indicate there must be orbital motion. In this case, light from both stars can be detected and it is a double-lined spectroscopic binary. The two have an orbital period of 28.28 days and a fairly high eccentricity of 0.56. The primary star is an A-type main-sequence star and shows unusual absorption lines in its spectrum, so it is an Am star with an effective temperature of 7,838 K.

41 Aurigae

41 Aurigae is a binary star system located around 310–316 light years away from the Sun in the northern constellation of Auriga. It is visible to the naked eye as a dim, white-hued star with a combined apparent visual magnitude of 5.83. This system is moving further from the Earth with a heliocentric radial velocity of 31 km/s. It is a probable member of the Hyades Supercluster.As of 2012, the pair had an angular separation of 7.39″ along a position angle of 357.7°. The primary component is an A-type main-sequence star with a stellar classification of A2Va+ and a visual magnitude of 6.15. The magnitude 6.84 secondary companion is a possible Am star with a stellar classification of kA5hA5mF0(IV-V), showing the calcium K line and hydrogen lines of an A5 star and the metal lines of an F0 star.

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.

60 Leonis

60 Leonis is a star in the zodiac constellation of Leo, located 127 light years from the Sun. It is visible to the naked eye as a faint, white-hued star with an apparent visual magnitude of 4.4. The star is moving closer to the Earth with a heliocentric radial velocity of −11 km/s.This is an Am star with a stellar classification of A1 Vm, although LeBlanc et al. (2015) consider it an Ap star. The atmosphere displays clear indications of stratification of iron with no significant magnetic field detected. It is 195 million years old with a relatively low projected rotational velocity of 17 km/s. The star has 2.11 times the mass of the Sun and 1.80 times the Sun's radius. It is radiating 24.1 times the Sun's luminosity from its photosphere at an effective temperature of 9,540 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 106112

HD 106112, also known as CO Camelopardalis, is a star in the constellation Camelopardalis. It has an apparent magnitude of about 5.1, meaning that it is just barely visible to the naked eye. Based upon parallax measurements made by the Hipparcos spacecraft, this star is around 177 light years away from the Sun.HD 106112's spectral type shows that it is an A-type giant star. HD 106112 is also an Am star, also known as a metallic-line star. These types of stars have spectra indicating varying amounts of metals, like iron.Observations of the stars spectrum reveal a periodic Doppler shift. This means that Iota Delphini is a spectroscopic binary with a period of 1.271 days and an eccentricity of 0.01. In fact, the two stars orbit so closely that they distort each other into an ellipsoidal shape through gravity, thereby forming a rotating ellipsoidal variable system. However, almost no information is known about the companion star.

HD 30453

HD 30453 (HR 1528) is a double-lined spectroscopic binary in the northern constellation of Auriga. The spectrum is that of metal-lined Am star. There is a third component at an angular separation of 0.04″.

HD 7853

HD 7853 is a double star in the constellation Andromeda. With an apparent magnitude of 6.46, the stars can barely be seen with the naked eye even on the best of nights. The system is located approximately 148 parsecs (480 ly) distant, and the brighter star is an Am star, meaning that it has unusual metallic absorption lines. The spectral type estimates of the primary range between A4 and A9.

HR 178

HR 178 is a probable binary star in the constellation Andromeda. Located approximately 124 parsecs (400 ly) distant, it is an evolved Am star with a combined apparent magnitude of 6.06, meaning that it can only be seen with the naked eye on dark, clear nights. The star is suspected of variability, possibly varying between magnitudes 6.04 and 6.06.The spectrum of HR 178 has been extensively studied for establishing element abundances in the evolved Am stars. It is given a spectral class of kA5hF1mF2, meaning its spectral type is A5, F1, or F2, depending on the particular spectral lines examined. The evolutionary stage of the star, and its mass determined from comparison with theoretical evolutionary tracks, is not precisely determined. However, it is very close to the end of its main sequence evolution and the margin of error in its mass is only about 0.1 M☉.HR 178 has been suspected of being a binary star since 1938 when its spectrum was interpreted as being composite. The pair were resolved using speckle interferometry in 1983. The companion is modelled to be between 1.5 and 3 magnitudes fainter than the primary star. Although there have since been several failed attempts to resolve the pair, a tentative orbit has been calculated with a period of 21.26 years and an eccentricity of 0.5.

Nu Draconis

Nu Draconis (also known as ν Dra, ν Draconis, or traditionally as Kuma ) is a double star in the constellation Draco. The respective components are designated ν1 Draconis and ν2 Draconis. The second component is a spectroscopic binary star system.This star, along with β Dra (Rastaban), γ Dra (Eltanin), μ Dra (Erakis) and ξ Dra (Grumium) were Al ʽAwāïd, "the Mother Camels", which was later known as the Quinque Dromedarii.In Chinese, 天棓 (Tiān Bàng), meaning Celestial Flail, refers to an asterism consisting of ν Draconis, ξ Draconis, β Draconis, γ Draconis and ι Herculis. Consequently, the Chinese name for ν Draconis itself is 天棓二 (Tiān Bàng èr, English: the Second Star of Celestial Flail.)The two stars of the visual binary are considered to be a common proper motion pair on the basis of their very similar parallaxes, radial velocities, and proper motions, although no orbital motion can be observed.ν1 Draconis is an Am star, a slowly-rotating chemically peculiar star with abnormally strong metallic absorption lines in its spectrum. Its spectral type of kA3hF0mF0 means that it would have a spectral class of A3 if determined solely from its calcium K lines, F0 if determined from its hydrogen lines, and F0 if determined from other metallic spectral lines.ν2 Draconis is a spectroscopic binary with a period of 38 days. The two stars are separated by 0.267 au on average, and they have an almost circular orbit with an eccentricity of 0.03. The primary is also an Am star, while the secondary has a low mass and luminosity and is only inferred from the orbital movement of the more massive star.

Omega2 Tauri

Omega2 Tauri is a solitary, white hued star in the zodiac constellation of Taurus. It has an apparent visual magnitude of +4.9, which is bright enough to be seen with the naked eye at night. The distance to this system, as determined using an annual parallax shift of 34.55 mas as seen from the Earth, is about 94 light years.

This is a young Am star with an age of around 100 million years and a stellar classification of A3m. It displays an infrared excess emission, indicating the presence of an orbiting debris disk with a mean temperature of 99 K. This star is a probable member of the Octans Near association, a nearby moving group of stars that share a common motion through space.

Sigma Aquarii

Sigma Aquarii (σ Aqr, σ Aquarii) is the Bayer designation for a double star in the equatorial constellation of Aquarius. It is visible to the naked eye with an apparent visual magnitude of 4.81. Based upon parallax measurements, the distance to this star is roughly 290 light-years (89 parsecs).Sigma Aquarii has a stellar classification of A0 IVs, indicating that it is a subgiant star. The s qualifier indicates that its absorption lines are sharp (narrow) in comparison with standard stars, caused by relatively slow rotation. It has been categorized as a hot Am star, meaning that it is a chemically peculiar, although this is now considered doubtful. The spectrum displays at least double the normal abundances of elements like magnesium, aluminum and silicon, while helium and scandium are under-abundant. Calcium, normally deficient in Am stars, is has near-normal abundance.The Hipparcos catalogue identified Sigma Aquarii as a possible astrometric binary with an orbital period of 654 days.

Theta2 Sagittarii

Theta2 Sagittarii (θ2 Sagittarii) is a solitary star in the zodiac constellation of Sagittarius. It is faintly visible to the naked eye with an apparent visual magnitude of +5.30. The star is progressing in the general direction of the Sun with a radial velocity of −17.60 km/s. Based upon an annual parallax shift of 20.62 mas as seen from Earth, it is located around 158 light years from the Sun.

The spectrum of Theta2 Sagittarii matches a stellar classification of A4/A5 IV, indicating that, at the estimated age of 809 million years, this is an evolving A-type subgiant star. It is a suspected Am star and may display photometric variability, at least during a 1992 observation period. The star has an estimated 1.93 times the mass of the Sun and is radiating 14 times the Sun's luminosity from its photoshere at an effective temperature of 8,113 K. It is spinning with a projected rotational velocity of 45.2 km/s.Theta2 Sagittarii has a pair of visual companions. Component B is a magnitude 11.3 star at an angular separation of 32.8 arc seconds along a position angle of 165°, as of 2000. Component C lies at an angular separation of 1.5 arc seconds along a position angle of 104° from component B, as of 1965. Neither is physically associated with Theta2 Sagittarii itself.

V529 Andromedae

V529 Andromedae, also known as HD 8801, is a variable star in the constellation of Andromeda. It has a 13th magnitude visual companion star 15" away, which is just a distant star on the same line of sight.

It is also an Am star with a spectral classification Am(kA5/hF1/mF2), meaning that it has the calcium K line of a star with spectral type A5, the Balmer series of a F1 star, and metallic lines of an F2 star.

Formation
Evolution
Spectral
classification
Remnants
Hypothetical
Nucleosynthesis
Structure
Properties
Star systems
Earth-centric
observations
Lists
Related articles

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.