Apep (star system)
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Apep is a
triple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking, ...
containing a Wolf–Rayet
binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that t ...
and a hot
supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...
, located in the
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...
of
Norma Norma may refer to: * Norma (given name), a given name (including a list of people with the name) Astronomy *Norma (constellation) * 555 Norma, a minor asteroid *Cygnus Arm or Norma Arm, a spiral arm in the Milky Way galaxy Geography *Norma, Lazi ...
. Named after the serpent deity from
Egyptian mythology Egyptian mythology is the collection of myths from ancient Egypt, which describe the actions of the Egyptian gods as a means of understanding the world around them. The beliefs that these myths express are an important part of ancient Egyptia ...
, the star system is surrounded by a vast complex of
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. D ...
and
cosmic dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...
thrown into space by the high rotation speed of the binary's primary star and formed into a "pinwheel" shape by the secondary star's influence. Ground-based studies of the system in the 2010s concluded that the system was the best known
gamma-ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten milli ...
progenitor candidate in the
Milky Way galaxy The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
.


Nomenclature

Apep, pronounced , was named by a team of astronomers led by Joseph Callingham of
ASTRON Astron may refer to: * Mitsubishi Astron engine * ASTRON, the Dutch foundation for astronomy research, operating the Westerbork Synthesis Radio Telescope and LOFAR * Astron (comics), a fictional character, a member of the Marvel Comics group The Et ...
, who studied the system between 2016 and 2018 and published a scientific paper on their observations. It was named after the eponymous mortal enemy of the deity of the sun Ra in
Egyptian mythology Egyptian mythology is the collection of myths from ancient Egypt, which describe the actions of the Egyptian gods as a means of understanding the world around them. The beliefs that these myths express are an important part of ancient Egyptia ...
, who was often illustrated as a giant serpent; their rivalry was described as "an apt allusion" to the appearance of the system and its
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. D ...
in infrared as "a star embattled within a dragon’s coils". In the XMM-''Newton'' Serendipitous Source Catalogue (2XMM), a
star catalog A star catalogue is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, ...
of X-ray sources observed by the XMM-''Newton'' space telescope, the system is catalogued as 2XMM J160050.7–514245. It is also known as WR 70-16.


Characteristics

Apep is a
triple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking, ...
containing a Wolf–Rayet
binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that t ...
described as the "central engine", orbiting with a period of ~100 years, and a third hot
supergiant star Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars ...
described as the "northern companion", orbiting the central engine at a distance of ~1,700
astronomical units The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
and a period of >10,000 years. The binary at the centre of Apep is composed of two classical Wolf-Rayet stars of carbon- (WC8) and nitrogen-sequence (WN4-6b) subtypes, making Apep the strongest case of a classical WR+WR binary system in the Milky Way. Carbon-sequence Wolf-Rayet stars are often dust-making factories. A vast complex of
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. D ...
and
cosmic dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...
surrounds the system, resembling
WR 104 WR 104 is a triple star system located about from Earth. The primary star is a Wolf–Rayet star (abbreviated as WR), which has a B0.5 main sequence star in close orbit and another more distant fainter companion. The WR star is surr ...
, another Wolf-Rayet star system producing a pinwheel nebula. The wind, travelling at a velocity of , and dust travelling at at the edge of the system, suggest that at least one component of the central engine is rapidly rotating, in which its
surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experien ...
is close to being balanced by its
centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...
outwards. This component produces faster stellar winds from its poles and slower winds from its equator, and the equatorial wind's interaction with the wind of its secondary produces the system's "pinwheel" shape. Rapidly-rotating Wolf–Rayet stars are theoretically capable of producing a gamma-ray burst during a
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
, and the system has been identified as a progenitor for a gamma-ray burst. Apep is estimated to be at a distance of ~2.4
kiloparsecs The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...
, or ~8,000
light year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s, from
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, with a potential discrepancy of +0.2 and −0.5 kiloparsecs at its estimated
visual extinction Visual extinction is a neurological disorder which occurs following damage to the parietal lobe of the brain. It is similar to, but distinct from, hemispatial neglect. Visual extinction has the characteristic symptom of difficulty to perceive cont ...
of 11.4.


Observation

Apep is located in the constellation of
Norma Norma may refer to: * Norma (given name), a given name (including a list of people with the name) Astronomy *Norma (constellation) * 555 Norma, a minor asteroid *Cygnus Arm or Norma Arm, a spiral arm in the Milky Way galaxy Geography *Norma, Lazi ...
, at a
right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the earth. When paired w ...
of and
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
of , The system can be resolved into two components, the "central engine" Wolf–Rayet binary, and the "northern companion" supergiant. The total
apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
of the system is 17.5, with the apparent magnitude of a resolved central engine and northern companion being 19.0 and 17.8 respectively. Its infrared spectral energy distribution (SED) is unique, with brightness ranging from an apparent magnitude of 6.4 at 2.2 µm to −2.4 at 22 µm. Surveys conducted with the
European Southern Observatory The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental organization, intergovernmental research organisation made up of 16 mem ...
(ESO)'s SINFONI instrument on the
Very Large Telescope The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across, ...
measured the apparent magnitude in the infrared J band for the central engine as , and for the northern companion as . SINFONI also measured the apparent magnitude of the system in the K band as for the central engine and for C, in the
L band The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz). This is at the top end of the ultra high frequency (UHF) band, at the lower en ...
as for the central engine and for the northern companion, and in the
M band The NATO M band is the obsolete designation given to the radio frequencies from 60 to 100 GHz (equivalent to wavelengths between 5 and 3 mm) during the cold war period. Since 1992 frequency allocations, allotment and assignments are in line to ...
as for the central engine and for the northern companion. SINFONI observations further detailed that the northern companion is possibly a conventional B1Ia+ high luminosity star. A and B show a typical spectrum from a WC7 star, but with additional WN4 or WN5 star features theorised to be from one of the stars of the central engine; if confirmed, this would make Apep a rare binary system of WR stars. An alternative hypothesis also based on SINFONI data proposes that the spectra could all be from an unusual transitional WN/WC star, and that the northern companion would then be a conventional OB star. Combining the spectra of the WR stars
EZ Canis Majoris EZ Canis Majoris (abbreviated to EZ CMa, also designated as WR 6) is binary system in the constellation of Canis Major. The primary is a Wolf-Rayet star and it is one of the ten brightest Wolf-Rayet stars, brighter than ...
and WR 90 would produce a spectrum almost identical to the one observed of the WR binary. The system was the first gamma-ray burst progenitor candidate to be discovered in the
Milky Way galaxy The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
, although it had not been known as such in early observations, such as those with the XMM-''Newton'' and ''
Chandra Chandra ( sa, चन्द्र, Candra, shining' or 'moon), also known as Soma ( sa, सोम), is the Hindu god of the Moon, and is associated with the night, plants and vegetation. He is one of the Navagraha (nine planets of Hinduism) and ...
''
space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launched ...
s, where it had been identified simply as an X-ray source as early as August 2004. Astronomer Joe Callingham first observed Apep during undergraduate studies at the
University of Sydney The University of Sydney (USYD), also known as Sydney University, or informally Sydney Uni, is a public research university located in Sydney, Australia. Founded in 1850, it is the oldest university in Australia and is one of the country's si ...
with the
Molonglo Observatory Synthesis Telescope The Molonglo Observatory Synthesis Telescope (MOST) is a radio telescope operating at 843 MHz. It is operated by the School of Physics of the University of Sydney. The telescope is located in Hoskinstown, near the Molonglo River and Canberra, ...
, and was noted as a potential colliding-wind binary, with a radio source as bright as
Eta Carinae Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around distant in t ...
. Callingham and Peter Tuthill, who assisted in the discovery of
WR 104 WR 104 is a triple star system located about from Earth. The primary star is a Wolf–Rayet star (abbreviated as WR), which has a B0.5 main sequence star in close orbit and another more distant fainter companion. The WR star is surr ...
in 1998 and sought interest in Apep after observing its extreme infrared properties, used the ESO's Very Large Telescope for observations in August 2016. Further observations with the
Anglo-Australian Telescope The Anglo-Australian Telescope (AAT) is a 3.9-metre equatorially mounted telescope operated by the Australian Astronomical Observatory and situated at the Siding Spring Observatory, Australia, at an altitude of a little over 1,100 m. In 20 ...
and the
Australia Telescope Compact Array The Australia Telescope Compact Array (ATCA) is a radio telescope operated by CSIRO at the Paul Wild Observatory, twenty five kilometres (16 mi) west of the town of Narrabri in New South Wales, Australia. Its opening ceremony took place on ...
, along with contributions from various international institutions, led to the publication of a
scientific paper : ''For a broader class of literature, see Academic publishing.'' Scientific literature comprises scholarly publications that report original empirical and theoretical work in the natural and social sciences. Within an academic field, scienti ...
in ''
Nature Astronomy ''Nature Astronomy'' is a peer reviewed scientific journal published by Nature Portfolio. It was first published in January 2017 (volume 1, issue 1), although the first content appeared online in December 2016. The editor-in-chief is May Chiao, wh ...
'' on 19 November 2018. It concluded that the system was a Wolf–Rayet binary and a progenitor for a gamma-ray burst. It had been previously assumed that such systems were only found in galaxies younger than the Milky Way.


See also

*
List of gamma-ray bursts The following is a list of significant gamma-ray bursts (GRBs) listed in chronological order. GRBs are named after the date on which they were detected: the first two numbers correspond to the year, the second two numbers to the month, and the last ...


References

Notes Sources # # # Citations


External links


Anisotropic winds in a Wolf–Rayet binary identify a potential gamma-ray burst progenitor
at ''
Nature Astronomy ''Nature Astronomy'' is a peer reviewed scientific journal published by Nature Portfolio. It was first published in January 2017 (volume 1, issue 1), although the first content appeared online in December 2016. The editor-in-chief is May Chiao, wh ...
'' {{Stars of Norma, state=collapsed Astronomical objects discovered in 2016 Binary stars Norma (constellation) Pinwheel nebulae Stars with proper names Wolf–Rayet stars