Fomalhaut

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This article is about the star. For the extrasolar planet, see Fomalhaut b.
Fomalhaut
Heic0821f.jpg
DSS image of Fomalhaut, field of view 2.7×2.9 degrees.
Credit NASA, ESA, and the Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)
Observation data
Epoch J2000      Equinox J2000
Constellation Piscis Austrinus (Fomalhaut A+B), Aquarius (Fomalhaut C)
Fomalhaut
Right ascension 22h 57m 39.0465s[1]
Declination −29° 37′ 20.050″[1]
Apparent magnitude (V) 1.16[2]
TW Piscis Austrini
Right ascension 22h 56m 24.05327s[1]
Declination −31° 33′ 56.0351″[1]
Apparent magnitude (V) 6.48[3]
LP 876-10
Right ascension 22h 48m 04.47s[4]
Declination −24° 22′ 07.5″[4]
Apparent magnitude (V) 12.618[4]
Characteristics
Spectral type A3 V / K5Vp / M4V[4][5]
U−B color index 0.08 / 1.02 / ?[6]
B−V color index 0.09 / 1.10 / 1.683[4][6]
Variable type None / BY Draconis / ?
Astrometry
Fomalhaut
Radial velocity (Rv) +6.5 km/s
Proper motion (μ) RA: +328.95[1] mas/yr
Dec.: −164.67[1] mas/yr
Parallax (π) 129.81 ± 0.47[1] mas
Distance 25.13 ± 0.09 ly
(7.70 ± 0.03 pc)
Absolute magnitude (MV) 1.72[7]
TW Piscis Austrini
Radial velocity (Rv) +6[3] km/s
Proper motion (μ) RA: −331.11[1] mas/yr
Dec.: −158.98[1] mas/yr
Parallax (π) 131.42 ± 0.62[1] mas
Distance 24.8 ± 0.1 ly
(7.61 ± 0.04 pc)
Absolute magnitude (MV) 7.08[7]
Details
Fomalhaut
Mass 1.92±0.02[7] M☉
Radius 1.842±0.019[7] R☉
Luminosity 16.63±0.48[7] L☉
Surface gravity (log g) 4.21[8] cgs
Temperature 8,590[7] K
Metallicity [Fe/H] −0.03[9] to −0.34[10] dex
Rotational velocity (v sin i) 93[8] km/s
Age (4.4±0.4)×108[7] years
TW Piscis Austrini
Mass 0.725 ± 0.036[5] M☉
Radius 0.629 ± 0.051[5] R☉
Luminosity 0.19[7] L☉
Temperature 4,711 ± 134[5] K
Rotational velocity (v sin i) 2.93[5] km/s
Age 4.4 × 108[7] years
Other designations
Fomalhaut: α Piscis Austrini, α PsA, Alpha PsA, 24 Piscis Austrini, CPD −30° 6685, FK5 867, Gl 881, HD 216956, HIP 113368, HR 8728, SAO 191524
TW Piscis Austrini: Fomalhaut B, TW PsA, Gl 879, HR 8721, CD -32°17321, HD 216803, LTT 9283, GCTP 5562.00, SAO 214197, CP(D)-32 6550, HIP 113283
LP 876-10: Fomalhaut C, 2MASS J22480446-2422075, NLTT 54872, GSC 06964-01226
Database references
SIMBAD AC
AB
A (Fomalhaut)
B (TW PsA)
C (LP 876-10)
planet b
Exoplanet Archive data
ARICNS A (Fomalhaut)
B (TW PsA)
Extrasolar Planets
Encyclopaedia		 data

Fomalhaut (Alpha Piscis Austrini, Alpha PsA, α Piscis Austrini, α PsA) is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. It is a class A star on the main sequence approximately 25 light-years (7.7 pc) from Earth as measured by the Hipparcos astrometry satellite.[11] Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[12] It is classified as a Vega-like star that emits excess infrared radiation, indicating it is surrounded by a circumstellar disk.[13] Fomalhaut, K-type star TW Piscis Austrini and M-type star LP 876-10 constitute a triple system even though the companions are separated by several degrees.[14]

Fomalhaut holds a special significance in extrasolar planet research, as it is the center of the first stellar system with an extrasolar planet candidate (Fomalhaut b) imaged at visible wavelengths. The image was published in Science in November 2008.[15] Fomalhaut is the third brightest star (as viewed from Earth) known to have a planetary system, after the Sun and Pollux.

Fomalhaut A

Dust ring around Fomalhaut from the Atacama Large Millimeter/submillimeter Array (ALMA)[16]

At a declination of −29.6°, Fomalhaut is located south of the celestial equator, and hence is best viewed from the Southern Hemisphere. However, its southerly declination is not as great as that of stars such as Acrux, Alpha Centauri and Canopus, meaning that, unlike them, Fomalhaut is visible from a large part of the Northern Hemisphere as well. Its declination is greater than that of Sirius and similar to that of Antares. At 40°N, Fomalhaut rises above the horizon for eight hours and reaches only 20° above the horizon, while Capella which rises at approximately the same time will stay above the horizon for twenty hours. From England the star never appears much brighter than magnitude 2.2 due to it being so close to the horizon, and from southern Alaska or Scandinavia it never rises above the horizon at all.[17] Fomalhaut can be located in these northern latitudes by the fact that the western (right-hand) side of the Square of Pegasus points to it. Continue the line from Beta to Alpha Pegasi towards the southern horizon: Fomalhaut is about 45˚ south of Alpha Pegasi, with no bright stars in between.[18]

Properties

Fomalhaut is a young star, for many years thought to be only 100 to 300 million years old, with a potential lifespan of a billion years.[19][20] A 2012 study gave a slightly higher age of 440±40 million years.[7] The surface temperature of the star is around 8,590 K (8,320 °C). Fomalhaut's mass is about 1.92 times that of the Sun, its luminosity is about 16.6 times greater, and its diameter is roughly 1.84 times as large.[7]

Fomalhaut is slightly metal-deficient as compared to the Sun, which means it is composed of a smaller percentage of elements other than hydrogen and helium.[8] The metallicity is typically determined by measuring the abundance of iron in the photosphere relative to the abundance of hydrogen. A 1997 spectroscopic study measured a value equal to 93% of the Sun's abundance of iron.[9][nb 1] A second 1997 study deduced a value of 78% by assuming Fomalhaut has the same metallicity as the neighboring star TW Piscis Austrini, which has since been argued to be a physical companion.[7][21] In 2004, a stellar evolutionary model of Fomalhaut yielded a metallicity of 79%.[8] Finally, in 2008, a spectroscopic measurement gave a significantly lower value of 46%.[10]

Fomalhaut has been claimed to be one of approximately 16 stars belonging to the Castor Moving Group. This is an association of stars that share a common motion through space and have been claimed to be physically associated. Other members of this group include Castor and Vega. The moving group has an estimated age of 200±100 million years and originated from the same location.[19] Unfortunately more recent work that has found that purported members of the Castor Moving Group appear to not only have a wide range of ages, but their velocities are too different to have been possibly associated with one another in the distant past.[14] Hence, "membership" to this dynamical group has no bearing on the age of the Fomalhaut system.[14]

Debris disks and planet

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The debris disk around the star
Debris ring around Fomalhaut showing location of planet Fomalhaut b—imaged by
Hubble Space Telescope's coronagraph.
(January 8, 2013) (NASA).

Fomalhaut is surrounded by several debris disks.

The inner disk is a high-carbon small-grain (10-300 nm) ash disk clustering at 0.1 AU from the star. Next is a disk of larger particles with inner edge 0.4-1 AU of the star. The innermost disk is unexplained as yet.[13]

The outermost disk is at a radial distance of 133 AU (1.99×1010 km; 1.24×1010 mi), in a toroidal shape with a very sharp inner edge, all inclined 24 degrees from edge-on.[22][23] The dust is distributed in a belt about 25 AU wide. The geometric center of the disk is offset by about 15 AU (2.2×109 km; 1.4×109 mi) from Fomalhaut.[24] The disk is sometimes referred to as "Fomalhaut's Kuiper belt". Fomalhaut's dusty disk is believed to be protoplanetary,[25] and emits considerable infrared radiation. Measurements of Fomalhaut's rotation indicate that the disk is located in the star's equatorial plane, as expected from theories of star and planet formation.[26]

On November 13, 2008, astronomers announced an object, which they assumed to be an extrasolar planet, orbiting just inside the outer debris ring. This was the first extrasolar orbiting object to be seen with visible light, captured by the Hubble Space Telescope.[27] A planet's existence had been previously suspected from the sharp, elliptical inner edge of that disk.[28] The mass of the planet, Fomalhaut b, was estimated to be no more than three times the mass of Jupiter but at least the mass of Neptune.[29] There are indications that the orbit is not apsidally aligned with the dust disk, which may indicate that additional planets may be responsible for the dust disk's structure.[30]

However M-band images taken from the MMT Observatory put strong limits on the existence of gas giants within 40 AU of the star[31] and Spitzer Space Telescope imaging suggested that the object Fomalhaut b was more likely to be a dust cloud.[32] In 2012, two independent studies confirmed that Fomalhaut b does exist; but it is shrouded by debris, so it may be a gravitationally-bound accumulation of rubble rather than a whole planet.[33][34]

Herschel Space Observatory images of Fomalhaut reveal a large amount of fluffy micrometer-sized dust is present in the outer dust belt. Because such dust is expected to be blown out of the system by stellar radiation pressure on short timescales, its presence indicates a constant replenishment by collisions of planetesimals. The fluffy morphology of the grains suggests a cometary origin. The collision rate is estimated to be approximately 2000 kilometre-sized comets per day.[35]

Observations of the star's outer dust ring by the Atacama Large Millimeter Array point to the existence of two planets in the system, neither one at the orbital radius proposed for the HST-discovered Fomalhaut b.[36]

If there are additional planets from 4 to 10 AU, they must be under 20 MJ; if from 2.5 outward, then 30 MJ.[37]

The Fomalhaut planetary system[13][38]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(years)		 Eccentricity Inclination Radius
Inner hot disk 0.08–0.11 AU — —
Outer hot disk 0.21–0.62 AU or 0.88–1.08 AU — —
10 AU belt 8–12 AU — —
Interbelt dust disk 35–133 AU — —
b (Dagon)  ? MJ 177±68 ~1700 0.8±0.1 −55° —
Main belt 133–158 AU −66.1° —
Main belt outer halo 158–209 AU — —

Fomalhaut b is one of the planets selected by the International Astronomical Union as part of their public process for giving proper names to exoplanets.[39][40] The process involves public nomination and voting for the new name[41] In December 2015, the IAU announced the winning name was Dagon.[42]The name Dagon was proposed by Dr. Todd Vaccaro and forwarded by the St. Cloud State University Planetarium to the IAU for consideration.[43]

Fomalhaut B (TW Piscis Austrini)

Fomalhaut forms a binary star with the K4-type star TW Piscis Austrini (TW PsA, Fomalhaut B). TW Piscis Austrini lies 0.28 parsecs (0.91 light years) away from Fomalhaut, and its space velocity agrees with that of Fomalhaut within 0.1±0.5 km/s, consistent with being a bound companion. A recent age estimate for TW PsA (400±70 million years), agrees very well with the isochronal age for Fomalhaut (450±40 million years), further arguing for the two stars forming a physical binary.[7]

The designation TW Piscis Austrini is astronomical nomenclature for a variable star. Fomalhaut B is a flare star of the type known as a BY Draconis variable. It varies slightly in apparent magnitude, ranging from 6.44 to 6.49 over a 10.3 day period. While smaller than the Sun, it is relatively large for a flare star. Most flare stars are red M-type dwarfs.

Fomalhaut C (LP 876-10)

LP 876-10 (Fomalhaut C) is also associated with the Fomalhaut system, making it a trinary star. In October 2013, Eric Mamajek and collaborators from the RECONS consortium announced that the previously known high-proper-motion star LP 876-10 had a distance, velocity, and color-magnitude position consistent with being another member of the Fomalhaut system.[14] LP 876-10 was originally catalogued as a high-proper-motion star by Willem Luyten in his 1979 NLTT catalogue, however a precise trigonometric parallax and radial velocity was only measured quite recently. LP 876-10 is a red dwarf of spectral type M4V, and located even further from Fomalhaut A than TW PsA—about 5.7° away from Fomalhaut A in the sky in the neighbouring constellation Aquarius, whereas both Fomalhaut A and TW PsA are located in constellation Piscis Austrinus. Its current separation from Fomalhaut A is about 0.77 parsecs (2.5 light years), and it is currently located 0.987 parsecs (3.2 light years) away from TW PsA (Fomalhaut B). LP 876-10 is located well within the tidal radius of the Fomalhaut system, which is 1.9 parsecs (6.2 light years).[14] Although LP 876-10 is itself catalogued as a binary star in the Washington Double Star Catalog (called "WSI 138"), there was no sign of a close-in stellar companion in the imaging, spectral, or astrometric data in the Mamajek et al. study.[14] In December 2013, Kennedy et al. reported the discovery of a cold dusty debris disks associated with Fomalhaut C, using infrared images from the Herschel Space Observatory. Multiple-star systems hosting multiple debris disks are exceedingly rare.[44]

The Fomalhaut C planetary system[44]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
Debris disk ~10–<40 AU — —

Etymology and cultural significance

Fomalhaut has had various names ascribed to it through time. Its modern name derives from Arabic fum al-ḥawt, meaning "mouth of the [Southern] Fish" (فُمْ اَلْحَوْتْ), a translation of how the classical astronomer Ptolemy labeled it.[45]

The star has been recognized by many cultures of the northern hemisphere, including the Arabs, Persians and Chinese. It marked the solstice in 2500 BC.[46] It was also a marker for the worship of Demeter in Eleusis.[47]

Ptolemy put it in Aquarius as well as Piscis Austrinus. In the 1600s Bayer firmly planted it in the primary position of Piscis Austrinus. Following Ptolemy, Flamsteed in 1725 denoted it 79 Aquarii as well as 24 Piscis Austrini. The current name reflects modern consensus on Bayer's decision, that the star belongs in Piscis Austrinus.[48]

  • It was called Hastorang by the Persians, one of the four "royal stars".[49]
  • The name Fom al-Haut comes from scientific Arabic فم الحوت fam al-ħūt (al-janūbī) "the mouth of the [Southern] Fish".[49]
  • The Latin names are ōs piscis merīdiāni, ōs piscis merīdionālis, ōs piscis notii "the mouth of the Southern Fish".[49]
  • The name Difda al Auwel comes from the colloquial Arabic الضفدع الأول aḍ-ḍifdiˤ al-’awwal "the first frog" (the second frog is Beta Ceti).[49]
  • The Chinese name 北落師門/北落师门 (Mandarin: Běiluòshīmén) meaning North Gate of the Military Camp, because this star is marking itself and stand alone in North Gate of the Military Camp asterism, Encampment mansion (see : Chinese constellation).[50] 北落师门 (Běiluòshīmén) westernized into Pi Lo Sze Mun in R.H. Allen's work.[49]
  • To the Moporr Aboriginal people of South Australia, it is a masculine being called Buunjill.[51] The Wardaman people of the Northern Territory called Fomalhaut Menggen —white cockatoo.[52]

Fomalhaut/Earthwork B in Mounds State Park near Anderson, Indiana, lines up with the rising of the star Fomalhaut in the fall months, according to the Indiana Department of Natural Resources. In 1980, astronomer Jack Robinson proposed that the rising azimuth of Fomalhaut was marked by cairn placements at both the Bighorn and Moose Mountain Medicine Wheels in Wyoming, USA and Saskatchewan, Canada, respectively.[53]

The New Scientist magazine termed it the "Great Eye of Sauron" due to its shape and debris ring, when viewed from a distance, bearing similarity to the aforementioned "Eye" in the Peter Jackson Lord of the Rings films.[54]

In Walter Tevis' novel Steps of the Sun, Fomalhaut is visited by the protagonist and two potentially inhabitable planets are found (and described). Parts of Philip K Dick's novel Lies, Inc (originally titled The Unteleported Man) are set on the fictional planet Fomalhaut IX.

USS Fomalhaut (AK-22) was a United States navy amphibious cargo ship.

See also

Notes

  1. ↑ Calculation of metallicity: if m = [Fe/H], then the ratio of iron to hydrogen for Fomalhaut divided by the ratio of iron to hydrogen for the Sun is given by 10m.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ 3.0 3.1 Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ 6.0 6.1 Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ 8.0 8.1 8.2 8.3 Lua error in package.lua at line 80: module 'strict' not found. This paper lists [Fe/H] = −0.10 dex.
  9. ↑ 9.0 9.1 Lua error in package.lua at line 80: module 'strict' not found. This paper lists [Fe/H] = −0.03 dex.
  10. ↑ 10.0 10.1 Lua error in package.lua at line 80: module 'strict' not found. This paper lists [Fe/H] = −0.34 dex.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ 13.0 13.1 13.2 Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  16. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  17. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  18. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  19. ↑ 19.0 19.1 Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ Lua error in package.lua at line 80: module 'strict' not found. This paper lists [Fe/H] = −0.11 dex.
  22. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  23. ↑ The disc was reported by Lua error in package.lua at line 80: module 'strict' not found. They noted that the disc was centered on a cavity, which they suggested might have been swept out by planets.
  24. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  25. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  26. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  27. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  28. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  29. ↑ Lua error in package.lua at line 80: module 'strict' not found. (at 3 minutes 45 seconds: "... has to be less than three Jupiter masses. In fact our lower limit to Fomalhaut b is Neptune")
  30. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  31. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  32. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  33. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  34. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  35. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  36. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  37. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  38. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  39. ↑ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  40. ↑ NameExoWorlds.
  41. ↑ NameExoWorlds.
  42. ↑ Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  43. ↑ http://www.sctimes.com/story/news/local/2015/12/25/scsu-planetarium-names-exoplanet/77875858/
  44. ↑ 44.0 44.1 Lua error in package.lua at line 80: module 'strict' not found.
  45. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  46. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  47. ↑ Lua error in package.lua at line 80: module 'strict' not found., citing Richard Hinckley Allen
  48. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  49. ↑ 49.0 49.1 49.2 49.3 49.4 Richard Hinckley Allen:Star Names — Their Lore and Meaning (Piscis Australis, the Southern Fish)
  50. ↑ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 7 日
  51. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  52. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  53. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  54. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

Coordinates: Sky map 22h 57m 39.1s, −29° 37′ 20″

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