BX Circini

BX Circini is a star in the constellation Circinus. Its variability was discovered in 1995,[2] with its apparent magnitude ranging from 12.57 to 12.62 over a period of 2 hours 33 minutes.[3] It is currently classified as a PV Telescopii variable star,[4] but has been put forward as the prototype of a new class of pulsating star—the BX Circini variables—along with the only other known example, V652 Herculis.[5] This class of star is rare, possibly because this is a brief stage of stellar evolution.[6] Its mass has been calculated to be around 40 percent that of the Sun and the average surface temperature has been measured at 23,390 ±90 K using optical spectra, but 1750 K cooler if analysing it in both the visual and ultraviolet. The temperature appears to vary by 3450 K.[6] It has an extremely low proportion of hydrogen, which was first noticed in 1980.[7] In fact, over 99% of its composition appears to be helium, qualifying it as an extreme helium star. Its origin is unclear, but thought to be a result of the merger of a helium white dwarf with a carbon/oxygen one.[6] The two merge violently, with material from the lighter helium white dwarf forming the outer envelope. The resulting star expands and shines as a yellow giant, its outer helium shell igniting and undergoing fusion as material continues to be accreted from the lighter star. The size of the star is maintained by the weight upon the helium shell, and once that has become light enough and the helium is exhausted, the star begins heating and shrinking, becoming the smaller blue star now observed.[8]

BX Circini
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Circinus
Right ascension  14h 01m 36.468s[1]
Declination −66° 09′ 56.30″[1]
Spectral typeOB+ [1]

References
  1. "V* BX Cir". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 4 August 2019.
  2. Kilkenny, D.; Koen, C. (1995). "The detection of small-amplitude variations in the extreme helium star LSS 3184" (PDF). Monthly Notices of the Royal Astronomical Society. 275 (2): 327–30. Bibcode:1995MNRAS.275..327K. doi:10.1093/mnras/275.2.327.
  3. Otero, Sebastian Alberto (30 October 2011). "BX Circini". AAVSO Website. American Association of Variable Star Observers. Retrieved 24 January 2014.
  4. "GCVS Query Forms: Query=BX Cir". General Catalogue of Variable Stars. Lomonosov Moscow State University. Retrieved 24 January 2014.
  5. Jeffery, Simon C. (2008). "Variable star designations for extreme helium stars". Information Bulletin on Variable Stars (5817): 1–7. ISSN 0374-0676.
  6. Woolf, V. M.; Jeffery, C. S. (2002). "Temperature and gravity of the pulsating extreme helium star LSS 3184 (BX Cir) through its pulsation cycle". Astronomy and Astrophysics. 395: 535–40. arXiv:astro-ph/0208269. Bibcode:2002A&A...395..535W. doi:10.1051/0004-6361:20021113.
  7. Drilling, J.S.; Jeffery, C.S.; Heber, U. (1998). "Spectral analysis of the extreme helium star LSS 3184" (PDF). Astronomy & Astrophysics. 329 (3): 1019–27. Bibcode:1998A&A...329.1019D.
  8. Saio, Hideyuki; Jeffery, C. Simon (2002). "Merged binary white dwarf evolution: rapidly accreting carbon-oxygen white dwarfs and the progeny of extreme helium stars" (PDF). Mon. Not. R. Astron. Soc. 333: 121–32. Bibcode:2002MNRAS.333..121S. doi:10.1046/j.1365-8711.2002.05384.x.
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