Fomalhaut

Fomalhaut (Alfa Piscis Austrini / α PsA / 24 Piscis Austrini) is a star of apparent magnitude +1.16, the brightest in the constellation Piscis Austrinus ("The Southern Fish") and one of the brightest stars in the night sky.

It has the Bayer designation Alpha Piscis Austrini', which is Latinized for α Piscis Austrini', and is abbreviated Alpha PsA or α PsA'. It is a class A main sequence star approximately 25 light-years from the Sun as measured by the Hipparcos astrometry satellite. Since 1943, the spectrum of this star has served as one of the stable anchor points by which classify other stars.

It is classified as a Vega-like star that emits excess infrared radiation, indicating that it is surrounded by a circumstellar disk. Fomalhaut, a K-type main sequence star. TW Piscis Austrini, and M-type, red dwarf star LP 876-10 constitute a triple system, although the companions are separated by approximately 8 degrees.

Fomalhaut was the first star system with an extrasolar planet candidate imaged at visible wavelengths, designated Fomalhaut b. However, analyzes from 2019 and 2023 of existing and new observations indicate that Fomalhaut b is not a planet, but rather an expanding blob of debris from a massive planetesimal collision.

Nomenclature

Fomalhaut is the brightest star of the constellation of Piscis Austrinus (centro)

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α Piscis Austrini (Latinized to Alpha Piscis Austrini) is the Bayer designation for the system. It also bears the Flamsteed designation of 24 Piscis Austrini. The classical astronomer Ptolemy placed it in Aquarius, as well as Piscis Austrinus. In the 17th century, Johann Bayer placed it firmly in the primary position of Piscis Austrinus. Following Ptolemy, John Flamsteed in 1725 also named it 79 Aquarii. The current designation reflects the modern consensus on Bayer's decision that the star belongs to Piscis Austrinus. According to the rules for naming objects in multiple star systems, the three components—Fomalhaut, TW Piscis Austrini, and LP 876-10—are they designate A, B and C, respectively.

The traditional name of the star derives from Fom al-Haut from scientific Arabic فم الحوت fam al-ḥūt (al-janūbī) "the mouth of the Fish [of the South]" (lit. "mouth of the whale"), translation of how Ptolemy labeled it. In 2016, the International Astronomical Union organized an International Astronomical Union Working Group on Star Naming (WGSN) to catalog and standardize the proper names of the stars. The first WGSN newsletter from July 2016 included a table listing the first two batches of names approved by the WGSN, which included the name Fomalhaut for this star.

In July 2014, the International Astronomical Union (IAU) launched NameExoWorlds, a process for giving proper names to certain exoplanets. The process involved a public nomination and vote on the new names. In December 2015, the IAU announced that the winning name was Dagon for Fomalhaut b. The winning name was proposed by Todd Vaccaro and submitted by edu/planetarium/ St. Cloud State University Planetarium, St. Cloud, Minnesota, United States of America, to the IAU for consideration. Dagon was a Semitic deity, often depicted as half-man, half-fish.

History

The name Fomalhaut comes from the Arabic فم الحوت, fum al-ħūt, meaning "whale's (or fish's) mouth". Throughout history this star has had various variations of its name, from Fomahant —as it appears in the Alphonse Tables— to Fomalhout. It was already identified in prehistoric times and there is archaeological evidence that it was part of rituals in ancient Persia, where it could have been one of the four Persian "royal stars", receiving the name Hastorang.

Physical characteristics

Fomalhaut is a white main-sequence star of spectral type A4V that lies approximately 25 light-years—7.7 parsecs—from Earth. It is thought to be a young star, only about 200 million years old. Its surface temperature is 8750 K. Compared to that of the Sun, its mass is 2.3 times greater, and its diameter is about 1.7 times greater. It shines with a luminosity 17.7 times greater than the solar luminosity.

Fomalhaut is part of the Castor stellar association, a group of stars that share a common motion through space. Other members of this association are Castor (α Geminorum) and Vega (α Lyrae). The flare variable star TW Piscis Austrini, a member of this group and only 0.9 light-years distant from Fomalhaut, could be gravitationally bound to it.

Circumstellar disk and planetary system

Circular disk of Fomalhaut and location of the planet b.

Comparison between the solar system and the Fomalhaut system.

Fomalhaut is surrounded by a torus-shaped dust disk with a very sharp inner edge at a radial distance of 133 astronomical units (au); the disk is tilted 24 degrees relative to the plane of the sky. The dust is distributed in a belt 25 AU wide, and the geometric center of the disk is displaced about 15 AU from Fomalhaut. The disk is also known as the "Fomalhaut Kuiper belt".

The disk surrounding Fomalhaut is thought to be a protoplanetary disk that emits a considerable amount of infrared radiation. Similar discs have been discovered in Vega, β Pictoris and Denébola (β Leonis).

On November 13, 2008, it was announced the discovery of an extrasolar planet, named Dagon, orbiting the inner edge of the circumstellar ring. It was the first extrasolar planet to be observed in the visible spectrum, which was done with the Hubble Space Telescope. The planet's existence had previously been intuited because of the sharp, elliptical inner edge of the disk.

The planet's mass is estimated to be no more than three times the mass of Jupiter, and is at least as massive as Neptune. The average separation from the star is 115 au—12 times the distance between Saturn and the Sun—but the eccentricity of Dagon's orbit implies a range variation between 102 and 127 au. Its orbital period is approximately 872 years.

There is evidence that the planet's orbit is not apsidally aligned with the dusty disk, which may indicate that additional planets may be responsible for the disk's current structure. However, M-band images captured from the MMT Observatory strongly constrain the possible existence of gas giants within the nearest 40 au of the star.

In October 2013 it was discovered that the star LP 876-10 is part of the system, so Fomalhaut is a triple system.