Fomalhaut was the first stellar system with an extrasolar planet candidate imaged at visible wavelengths, designatedFomalhaut b. However, analyses in 2019 and 2023 of existing and new observations indicate that Fomalhaut b is not a planet, but rather an expanding region of debris from a massive planetesimal collision.[19][20][15]
Fomalhaut is the brightest star in the constellation of Piscis Austrinus (center).
α Piscis Austrini, orAlpha Piscis Austrini, is the system'sBayer designation. It also bears theFlamsteed designation of24 Piscis Austrini. The classical astronomerPtolemy included it in the constellation ofAquarius, along with the rest of Piscis Austrinus. In the 17th century,Johann Bayer firmly planted it in the primary position of Piscis Austrinus. Following Ptolemy,John Flamsteed in 1725 additionally denoted it79 Aquarii. The current designation reflects modern consensus on Bayer's decision, that the star belongs in Piscis Austrinus.[21] Under the rules for naming objects in multiple-star systems, the three components – Fomalhaut, TW Piscis Austrini and LP 876-10 – are designated A, B and C, respectively.[22]
The star's traditional name derives fromFom al-Haut from scientific Arabicفم الحوتfam al-ḥūt (al-janūbī) "the mouth of the [Southern] Fish" (literally, "mouth of the whale"), a translation of how Ptolemy labeled it.[23][24][25]Fam in Arabic means "mouth",al "the", andḥūt "fish"[26]or "whale".[27]In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[28] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[29] included a table of the first two batches of names approved by the WGSN, which included the name "Fomalhaut" for this star.
At adeclination of −29.6°, Fomalhaut is located south of thecelestial equator, and hence is best viewed from theSouthern Hemisphere. However, its southerly declination is not as great as that of stars such asAcrux,Alpha Centauri andCanopus, meaning that, unlike them, Fomalhaut is visible from a large part of theNorthern Hemisphere as well, being best seen in autumn. Its declination is greater than that ofSirius and similar to that ofAntares. At 40°N, Fomalhaut rises above the horizon for eight hours and reaches only 20° above the horizon, whileCapella, which rises at approximately the same time, will stay above the horizon for twenty hours. Fomalhaut can be located in northern latitudes by the fact that the western (right-hand) side of theSquare of Pegasus points to it. Continuing the line fromBeta toAlpha Pegasi towards the southern horizon, Fomalhaut is about 45˚[clarification needed] south of Alpha Pegasi, with no bright stars in between.[36]
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.[37][38] A 2012 study gave a slightly higher age of440±40 million years.[6] The surface temperature of the star is around 8,590 K (8,320 °C). Fomalhaut's mass is about 1.92 times that of theSun, itsluminosity is about 16.6 times greater, and its diameter is roughly 1.84 times as large.[6]
Fomalhaut is slightlymetal-deficient compared to the Sun, which means it is composed of a smaller percentage of elements other than hydrogen and helium.[7] The metallicity is typically determined by measuring the abundance of iron in thephotosphere relative to the abundance of hydrogen. A 1997spectroscopic study measured a value equal to 93% of the Sun's abundance of iron.[8][nb 1] A second 1997 study deduced a value of 78%, by assuming Fomalhaut has the same metallicity as the neighboring starTW Piscis Austrini, which has since been argued to be a physical companion.[6][39] In 2004, a stellar evolutionary model of Fomalhaut yielded a metallicity of 79%.[7] Finally, in 2008, a spectroscopic measurement gave a significantly lower value of 46%.[9]
Fomalhaut has been claimed to be one of approximately 16 stars belonging to theCastor Moving Group. This is an association of stars which share a common motion through space, and have been claimed to be physically associated. Other members of this group includeCastor andVega. Themoving group has an estimated age of200±100 million years and originated from the same location.[37] More recent work 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.[17] Hence, "membership" in this dynamical group has no bearing on the age of the Fomalhaut system.[17]
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.[16]
The outermost disk is at a radial distance of 133 AU (1.99×1010 km; 1.24×1010 mi), in atoroidal shape with a very sharp inner edge, all inclined 24 degrees from edge-on.[41][42] 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.[43] The disk is sometimes referred to as "Fomalhaut'sKuiper belt". Fomalhaut's dusty disk is believed to beprotoplanetary,[44] and emits considerableinfrared 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.[45]
Herschel Space Observatory images of Fomalhaut, analysed in 2012, reveal that 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.[46] Observations of this outer dust ring by theAtacama Large Millimeter Array also suggested the possible existence of two planets in the system.[47] If there are additional planets from 4 to 10 AU, they must be under20MJ; if from 2.5 outward, then20MJ.[48]
On November 13, 2008, astronomers announced anextrasolar planet candidate, orbiting just inside the outer debris ring. This was the first extrasolar orbiting object candidate to be directly imaged in visible light, captured by theHubble Space Telescope.[49][50] The mass of the tentative planet,Fomalhaut b, was estimated to be less than three times the mass ofJupiter, and at least the mass ofNeptune. However, M-band images taken from theMMT Observatory put strong limits on the existence of gas giants within 40 AU of the star,[51] andSpitzer Space Telescope imaging suggested that the object Fomalhaut b was more likely to be a dust cloud.[52] A later 2019 synthesis of new and existing direct observations of the object confirmed that it is expanding, losing brightness, has not enough mass to detectably perturb the outer ring while crossing it, and is probably a dispersing cloud of debris from a massive planetesimal collision on a hyperbolic orbit destined to leave the Fomalhaut A system.[19] Further 2022 observations with theJames Webb Space Telescope in mid-infrared failed to resolve the object in the25.5 μmMIRI wideband filter wavelength range, reported by the same team to be consistent with the previous result.[15]
The same 2022 JWST imaging data discovered another apparent feature in the outer disk, dubbed the "Great Dust Cloud".[15] However, another team's analysis, which included other existing data, preferred its interpretation as a coincident background object, not part of the outer ring.[53] Another 2023 study detected 10 point sources around Fomalhaut; all but one of these are background objects, including the "Great Dust Cloud", but the nature of the last is unclear. It may be a background object, or a planetary companion to Fomalhaut.[54]
Fomalhaut forms a binary star with theK4-type starTW Piscis Austrini (TW PsA), which lies 0.28 parsecs (0.91 light-years) away from Fomalhaut, and its space velocity agrees with that of Fomalhaut within0.1±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.[6]
The designation TW Piscis Austrini is astronomical nomenclature for avariable star. Fomalhaut B is aflare star of the type known as aBY Draconis variable. It varies slightly inapparent magnitude, ranging from 6.44 to 6.49 over a 10.3 day period. While smaller than theSun, it is relatively large for a flare star. Most flare stars are redM-type dwarfs.
In 2019, a team of researchers analyzing the astrometry, radial velocity measurements, and images of Fomalhaut B suggested the existence of a planet orbiting the star with a mass of1.2+0.7 −0.6 Jupiter masses, and a poorly defined orbital period with an estimate loosely centering around 25 years.[56]
LP 876-10 is also associated with the Fomalhaut system, making it atrinary star. In October 2013, Eric Mamajek and collaborators from theRECONS consortium announced that the previously known high-proper-motion starLP 876-10 had a distance, velocity, and color-magnitude position consistent with being another member of the Fomalhaut system.[17] LP 876-10 was originally catalogued as a high-proper-motion star byWillem Luyten in his 1979NLTT catalogue; however, a precise trigonometric parallax and radial velocity was only measured quite recently.LP 876-10 is ared dwarf of spectral type M4V, and located even farther from Fomalhaut A than TW PsA—about 5.7° away from Fomalhaut A in the sky, in the neighbouring constellationAquarius, whereas both Fomalhaut A and TW PsA are located in constellationPiscis Austrinus. Its current separation from Fomalhaut A is about 0.77 parsecs (2.5 light-years), and it is currently located 0.987 parsecs (3.22 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).[17] Although LP 876-10 is itself catalogued as a binary star in theWashington 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.[17] In December 2013, Kennedy et al. reported the discovery of a cold dusty debris disk associated with Fomalhaut C, using infrared images from theHerschel Space Observatory. Multiple-star systems hosting multiple debris disks are exceedingly rare.[57]
Fomalhaut has had various names ascribed to it through time, and has been recognized by many cultures of thenorthern hemisphere, including theArabs,Persians, andChinese. It marked the solstice in 2500 BC. It was also a marker for the worship ofDemeter inEleusis.[58]
The Latin names areōs piscis merīdiāni, ōs piscis merīdionālis, ōs piscis notii "the mouth of the Southern Fish".[24]
A folk name among the early Arabs wasDifdi' al Awwal (الضفدع الأولal-ḍifdiʿ al-awwal) "the first frog" (the second frog isBeta Ceti).[24]
TheChinese name北落師門/北落师门 (Mandarin: Běiluòshīmén), meaningNorth Gate of the Military Camp, because this star is marking itself and stands alone inNorth Gate of the Military Campasterism,Encampment mansion (see:Chinese constellations).[59]北落师门 (Běiluòshīmén), westernized intoPi Lo Sze Mun by R.H. Allen.[24]
To the Moporr Aboriginal people of South Australia, it is a male being calledBuunjill.[60] TheWardaman people of the Northern Territory called FomalhautMenggen —white cockatoo.[61]
^Calculation ofmetallicity: ifm = [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.
^Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system".CDS/ADC Collection of Electronic Catalogues.2237: 0.Bibcode:2002yCat.2237....0D.
^abJohnson, H. L.; Iriarte, B.; Mitchell, R. I.; Wisniewskj, W. Z. (1966). "UBVRIJKL photometry of the bright stars".Communications of the Lunar and Planetary Laboratory.4 (99): 99.Bibcode:1966CoLPL...4...99J.
^abcMennesson, B.; Absil, O.; Lebreton, J.; Augereau, J.-C.; Serabyn, E.; Colavita, M. M.; Millan-Gabet, R.; Liu, W.; Hinz, P.; Thébault, P. (2012). "An interferometric study of the Fomalhaut inner debris disk II. Keck Nuller mid-infrared observations".Astrophysical Journal.763 (2): 119.arXiv:1211.7143.Bibcode:2013ApJ...763..119M.doi:10.1088/0004-637X/763/2/119.S2CID102339596.
^Bob King (1 October 2014)."Fomalhaut: A crazy-wide triple stat".Sky & Telescope. Retrieved13 June 2020.When you next look at Fomalhaut twinkling above the fall leaves, put four fingers together and hold them up against the sky. They'll cover about 8°, or the amount of real estate spanned by the triple system.
^Couper, Heather;Henbest, Nigel (5 December 2011) [2007].The Story of Astronomy: How the universe revealed its secrets. Hachette UK.ISBN978-1-84403-726-1. Retrieved25 May 2024.Ptolemy [...] generally described stars by their positions in the constellation patterns: but this could get quite cumbersome, such as 'the northernmost of two stars close together over the little shield in the stern.' [...] The jewel in the constellation of Piscis Austrinus [...] is dubbed by Ptolemy as 'the mouth of the southern fish.' In Arabic, this became fam al-hut al-janub – our star Fomalhaut.
^The disc was reported byHolland, Wayne S.; et al. (1998). "Submillimetre images of dusty debris around nearby stars".Nature.392 (6678):788–791.Bibcode:1998Natur.392..788H.doi:10.1038/33874.S2CID4373502. They noted that the disc was centered on a cavity, which they suggested might have been swept out by planets.
^天文教育資訊網 [Astronomy Education Information Network]. Activities of Exhibition and Education in Astronomy (AEEA) (in Chinese). 7 July 2006. Archived fromthe original on 21 May 2011. Retrieved24 August 2012.
^Harney, Bill Yidumduma; Cairns, Hugh C. (2004) [2003].Dark Sparklers (revised ed.). Merimbula, New South Wales: Hugh C. Cairns. p. 204.ISBN0-9750908-0-1.
^Robinson, J.H. (September 1980). "Fomalhaut and cairn D at the Big Horn and Moose Mountain medicine wheels".Bulletin of the Astronomical Society.12: 887.Bibcode:1980BAAS...12..887R.
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