Apep (star system) explained

Apep is a triple star system containing a Wolf–Rayet binary and a hot supergiant, located in the constellation of Norma. Named after the serpent deity from Egyptian mythology, the star system is surrounded by a vast complex of stellar wind and cosmic dust 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 progenitor candidate in the Milky Way galaxy.

Nomenclature

Apep, pronounced, was named by a team of astronomers led by Joseph Callingham of ASTRON, who studied the system between 2016 and 2018 and published a scientific paper on their observations.[1] It was named after the mortal enemy of the sun god Ra in Egyptian mythology, 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 in infrared as "a star embattled within a dragon's coils".[2] In the XMM-Newton Serendipitous Source Catalogue (2XMM), a star catalog of X-ray sources observed by the XMM-Newton space telescope, the system is catalogued as 2XMM J160050.7–514245.[3] It is also known as WR 70-16.

Characteristics

Apep is a triple star system[1] [4] containing a Wolf–Rayet binary described as the "central engine", orbiting with a period of ~100 years,[5] and a third hot supergiant star described as the "northern companion", orbiting the central engine at a distance of ~1,700 astronomical units 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.[6] Carbon-sequence Wolf–Rayet stars are often dust-making factories. A vast complex of stellar wind and cosmic dust surrounds the system,[2] [4] [7] resembling WR 104, another Wolf–Rayet star system producing a pinwheel nebula. The wind, travelling at a velocity of 12e6kph,[1] [7] and dust travelling at 2e6kph at the edge of the system, suggest that at least one component of the central engine is rapidly rotating, so that its surface gravity is close to being balanced by its centrifugal force outwards.[4] 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.[8] [9] Rapidly rotating Wolf–Rayet stars are theoretically capable of producing a gamma-ray burst during a supernova, and the system has been identified as a progenitor for a gamma-ray burst.[10] Apep is estimated to be at a distance of ~2.4 kiloparsecs, or ~8,000 light-years,[5] [11] from Earth, with a potential discrepancy of +0.2 and −0.5 kiloparsecs at its estimated visual extinction of 11.4.

Observation

Apep is located in the constellation of Norma, at a right ascension of and declination of, The system can be resolved into two components, the "central engine" Wolf–Rayet binary, and the "northern companion" supergiant. The total apparent magnitude 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 (ESO)'s SINFONI instrument on the Very Large Telescope 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 as for the central engine and for the northern companion, and in the M band 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 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, although it had not been known as such in early observations, such as those with the XMM-Newton and Chandra space telescopes, 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 with the Molonglo Observatory Synthesis Telescope,[7] and was noted as a potential colliding-wind binary, with a radio source as bright as Eta Carinae. Callingham and Peter Tuthill, who led the discovery of WR 104 in 1998[12] and sought interest in Apep after observing its extreme infrared properties,[13] used the ESO's Very Large Telescope for observations in August 2016. Further observations with the Anglo-Australian Telescope and the Australia Telescope Compact Array,[11] along with contributions from various international institutions, led to the publication of a scientific paper in Nature Astronomy on 19 November 2018.[8] It concluded that the system was a Wolf–Rayet binary and a progenitor for a gamma-ray burst.[4] It had been previously assumed that such systems were only found in galaxies younger than the Milky Way.[9]

See also

References

NotesSources

  1. Web site: Callingham. Joseph. Riding the serpent: The discovery and study of Apep. Nature. 26 November 2018. https://web.archive.org/web/20181126145443/https://astronomycommunity.nature.com/users/181582-joseph-callingham/posts/40890-riding-the-serpent-the-discovery-and-study-of-apep. 26 November 2018. 20 November 2018. live.
  2. Web site: Callingham. J. R.. Tuthill. P. G.. Pope. B. J. S.. Williams. P. M.. Crowther. P. A.. Edwards. M.. Norris. B.. Kedziora-Chudczer. L.. Anisotropic winds in a Wolf–Rayet binary identify a potential gamma-ray burst progenitor. University of Sydney School of Physics. 20 November 2018. https://web.archive.org/web/20181120024637/http://www.physics.usyd.edu.au/~gekko/Apep/revision-3-anisotropic-2.pdf. 20 November 2018. 24 September 2018. . live.
  3. Web site: Plait. Phil. Phil Plait. Bad Astronomy: Is this cosmic sprinkler surrounding galaxy's next gamma-ray burst?. Syfy Wire. 22 November 2018. https://web.archive.org/web/20181122011234/https://www.syfy.com/syfywire/is-this-cosmic-sprinkler-surrounding-galaxys-next-gamma-ray-burst. 22 November 2018. 19 November 2018. live.

Citations

External links

Notes and References

  1. Web site: Dvorsky. George. Stunning Pinwheel Nebula Is a Cosmic Cataclysm in the Making. Gizmodo. 20 November 2018. https://web.archive.org/web/20181120032137/https://gizmodo.com/stunning-pinwheel-nebula-is-a-cosmic-cataclysm-in-the-m-1830544916?IR=T. 20 November 2018. 19 November 2018. ...but to the researchers who recently investigated this enigmatic object, it's simply "Apep" [...] The speed of gas within the nebula was clocked at 12 million kilometers per hour [...] featuring a massive triple star system at its core—a binary pair and a lone star.... live.
  2. Web site: Letzter. Rafi. This Spinning, Snakelike Star System Might Blast Gamma Rays into the Milky Way When It Dies. Live Science. 20 November 2018. https://web.archive.org/web/20181120095640/https://www.livescience.com/64120-gamma-ray-burst-apep.html. 20 November 2018. 19 November 2018. For the first time, astronomers have found a star system in our galaxy that could produce a gamma-ray burst [...] the researchers nicknamed it "Apep" after the Egyptian snake-deity of chaos. [...] The name works nicely for the system, which is surrounded by long, fiery pinwheels of matter cast out into space.... live.
  3. Web site: XMM-Newton Survey Science Centre. The XMM-Newton Serendipitous Source Catalogue: 2XMM User Guide. University of Leicester Department of Physics and Astronomy. 20 November 2018. https://web.archive.org/web/20181120034116/https://xmmssc-www.star.le.ac.uk/Catalogue/2XMM/UserGuide_xmmcat.html. 20 November 2018. 20 August 2008. 2XMM is a catalogue of serendipitous X-ray sources from the European Space Agency's (ESA) XMM-Newton observatory. live. European Space Astronomy Centre.
  4. Web site: Carpineti. Alfredo. This 'Cosmic Serpent' Is The First System Of Its Kind To Be Discovered In Our Galaxy. IFL Science!. 20 November 2018. https://web.archive.org/web/20181120050517/https://www.iflscience.com/space/this-cosmic-serpent-is-the-first-system-of-its-kind-to-be-discovered-in-our-galaxy/. 20 November 2018. 19 November 2018. Three stars are in this picture, although the two Wolf-Rayet stars look like a single one in the center [...] the winds are moving at 12 million kilometers (7.5 million miles) per hour. [...] The observations were possible thanks to the Very Large Telescope [...] the dust at the edge of the system is moving at the slower pace of 2 million kilometers (1.2 million miles) per hour.. live.
  5. Web site: Griffin. Andrew. Huge star system near Earth could produce one of the most spectacular explosions in the universe. The Independent. 20 November 2018. https://web.archive.org/web/20181120095351/https://www.independent.co.uk/news/science/nasa-star-system-explosion-space-apep-galaxy-image-very-large-telescope-a8641596.html. 20 November 2018. 19 November 2018. The swirling cloud of dust is a mere 8,000 light years from Earth is a vast system made up of two shockingly bright stars. [...] The two bright stars orbit each other every hundred years or so, according to the researchers.. live.
  6. 2005.00531 . Callingham . J. R. . Crowther . P. A. . Williams . P. M. . Tuthill . P. G. . Han . Y. . Pope . B. J. S. . Marcote . B. . Two Wolf-Rayet stars at the heart of colliding-wind binary Apep . Monthly Notices of the Royal Astronomical Society . 2020 . 495 . 3 . 3323–3331 . 10.1093/mnras/staa1244 . free . 2020MNRAS.495.3323C . 218470247 .
  7. Web site: Strom. Marcus. Doomed star in Milky Way threatens rare gamma-ray burst. University of Sydney. 20 November 2018. https://web.archive.org/web/20181120094830/https://sydney.edu.au/news-opinion/news/2018/11/20/doomed-star-in-milky-way-threatens-rare-gamma-ray-burst.html. 20 November 2018. 20 November 2018. ...the astronomers have measured the velocity of the stellar winds as fast as 12 million kilometres an hour, about 1 percent the speed of light. [...] We discovered this star as an outlier in a survey with a radio telescope operated by the University of Sydney.. live.
  8. Web site: Weule. Genelle. Spectacular cosmic pinwheel is a 'ticking bomb' set to blast gamma rays across the Milky Way. ABC News Australia. 20 November 2018. https://web.archive.org/web/20181120104408/https://www.abc.net.au/news/science/2018-11-20/wolf-rayet-star-may-blast-milky-way-with-gamma-ray-burst/10506316. 20 November 2018. 20 November 2018. Writing in the journal Nature Astronomy [...] the most violent star is creating stellar winds at two speeds — fast at the poles, slow at the equator [...] the beautiful pinwheel of blazing dust is created not by the fast polar winds, but by the turbulence that arises when the second star in the central engine passes through that first star's slow-moving equatorial wind.. live.
  9. Web site: Devitt. James. Scientists Discover New "Pinwheel" Star System. New York University. 20 November 2018. https://archive.today/20181120111942/http://www.nyu.edu/about/news-publications/news/2018/november/scientists-discover-new--pinwheel--star-system.html. 20 November 2018. 19 November 2018. "It was not expected such a system would be found in our galaxy—only in younger galaxies much further away," [...] The discovery of the system [...] also included scientists from the Netherlands Institute for Radio Astronomy, the University of Sydney, the University of Edinburgh, the University of Sheffield, and the University of New South Wales. [...] is adorned with a dust "pinwheel"— whose strangely slow motion suggests current theories on star deaths may be incomplete.. live.
  10. Web site: Clery. Daniel. Massive star system primed for intense explosion. Science. 20 November 2018. https://web.archive.org/web/20181120105241/https://www.sciencemag.org/news/2018/11/massive-star-system-primed-intense-explosion. 20 November 2018. 20 November 2018. One of stars is an unusually massive sun known as a Wolf-Rayet star. When such stars run out of fuel, they collapse, causing a supernova explosion. Theorists believe that if the Wolf-Rayet star is also spinning fast, the explosion will produce intense jets of gamma rays out of either pole.... live.
  11. Web site: Mannix. Liam. Super-powerful interstellar 'ticking time bomb' found not far from Earth. The Sydney Morning Herald. 20 November 2018. https://web.archive.org/web/20181120042126/https://www.smh.com.au/national/super-powerful-interstellar-ticking-time-bomb-found-not-far-from-earth-20181119-p50gyy.html. 20 November 2018. 20 November 2018. In a part of the Milky Way 8000-odd light-years away [...] The system was spotted by PhD student Dr Joe Callingham while he was sorting through data, and later confirmed using the Anglo-Australian Telescope at Coonabarabran in regional NSW.. live.
  12. Tuthill . Peter G. . Monnier . John D. . Danchi . William C. . A dusty pinwheel nebula around the massive star WR104 . Nature . 1 April 1999 . 398 . 6727 . 487–489 . 10.1038/19033 . astro-ph/9904092 . 1999Natur.398..487T . 4373103 . 0028-0836.
  13. Web site: Tuthill. Peter. The Twisted Tale of Wolf-Rayet 104 First of the Pinwheel Nebulae. University of Sydney School of Physics. 26 November 2018. https://web.archive.org/web/20181126155407/http://www.physics.usyd.edu.au/~gekko/wr104.html. 26 November 2018. 1999. These results are further described in our letter in Nature "A dusty pinwheel nebula around the massive star wr 104" by Peter Tuthill, John Monnier and William Danchi Volume 398, pp. 487–489, April 8, 1999..

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