Helium star

Ahelium staris aclassO or Bstar(blue), which has extraordinarily stronghelium linesand weaker than normalhydrogen lines,indicating strong stellar winds and a mass loss of the outer envelope.Extreme helium stars(EHe) entirely lack hydrogen in their spectra. Pure helium stars lie on or near ahelium main sequence,analogous to themain sequenceformed by the more common hydrogen stars.^[1]

Terminology

Previously, ahelium starwas a synonym for aB-type star,but this use of for the term is considered obsolete.^[2]

Ahelium staris also a term for ahypothetical starthat could occur if two heliumwhite dwarfswith a combined mass of at least 0.5solar massesmerge and subsequently start nuclear fusion of helium, with a lifetime of a few hundred million years. This may only happen if these two binary masses share the same type of envelope phase. It is believed this is the origin of the extreme helium stars.

Description

The helium main sequence is a line in theHR diagramwhere unevolved helium stars lie. It lies mostly parallel and to the left (i.e. higher temperatures) of the better-known hydrogenmain sequence,although at high masses and luminosities it bends to the right and even crosses the hydrogen main sequence. Therefore, pure helium stars have a maximum temperature, between about100,000Kand150,000 Kdepending onmetallicity,because high luminosity causes dramatic inflation of the stellar envelope.^[3]

Helium stars' great capability of transforming into other stellar objects has been observed over recent years since they were first identified. The blue progenitor system of thesupernova type Iax SN 2012Zin the spiral galaxyNGC 1309is similar to the progenitor of theGalactichelium novaV445 Puppis,suggesting thatSN 2012Zwas the explosion of a white dwarf accreting from a helium-star companion. It is observed to have caused a growing helium star that has the potential to transform into a red giant after losing its hydrogen envelope in the future.^[4]

References

^Yoon, S.-C.; Langer, N. (2004). "Helium accreting CO white dwarfs with rotation: Helium novae instead of double detonation".Astronomy and Astrophysics.419(2): 645–652.arXiv:astro-ph/0402288.Bibcode:2004A&A...419..645Y.doi:10.1051/0004-6361:20035823.S2CID 7367981.
^Frost, E. B.; Barrett, S. B.; Struve, O. (1926)."Radial velocities of 368 helium stars".The Astrophysical Journal.64:1.Bibcode:1926ApJ....64....1F.doi:10.1086/142986.PMC1084541.
^Köhler, K.; Langer, N.; de Koter, A.; De Mink, S.E.; Crowther, P.A.; Evans, C.J.; et al. (2015). "The evolution of rotating very massive stars with LMC composition".Astronomy & Astrophysics.573:A71.arXiv:1501.03794.Bibcode:2015A&A...573A..71K.doi:10.1051/0004-6361/201424356.S2CID 28962151.
^McCully, Curtis (2014). "A luminous, blue progenitor system for the type Iax supernova 2012Z".Nature.512(7512): 54–56.arXiv:1408.1089.Bibcode:2014Natur.512...54M.doi:10.1038/nature13615.PMID 25100479.S2CID 4464556.

[yoon-1] Yoon, S.-C.; Langer, N. (2004). "Helium accreting CO white dwarfs with rotation: Helium novae instead of double detonation".Astronomy and Astrophysics.419(2): 645–652.arXiv:astro-ph/0402288.Bibcode:2004A&A...419..645Y.doi:10.1051/0004-6361:20035823.S2CID 7367981.

[2] Frost, E. B.; Barrett, S. B.; Struve, O. (1926)."Radial velocities of 368 helium stars".The Astrophysical Journal.64:1.Bibcode:1926ApJ....64....1F.doi:10.1086/142986.PMC1084541.

[3] Köhler, K.; Langer, N.; de Koter, A.; De Mink, S.E.; Crowther, P.A.; Evans, C.J.; et al. (2015). "The evolution of rotating very massive stars with LMC composition".Astronomy & Astrophysics.573:A71.arXiv:1501.03794.Bibcode:2015A&A...573A..71K.doi:10.1051/0004-6361/201424356.S2CID 28962151.

[4] McCully, Curtis (2014). "A luminous, blue progenitor system for the type Iax supernova 2012Z".Nature.512(7512): 54–56.arXiv:1408.1089.Bibcode:2014Natur.512...54M.doi:10.1038/nature13615.PMID 25100479.S2CID 4464556.

[1]

[2]

[3]

[4]

Terminology

Description

See also

References