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An extreme helium star (abbreviated EHe), or a PV Telescopii Variable, is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe. Since there are no known conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs.

Properties

Extreme helium stars form a sub-group within the broader category of hydrogen-deficient stars. The latter includes cool carbon stars like R Coronae Borealis variables, helium-rich spectral class O or B stars, population I Wolf–Rayet stars, AM CVn stars, white dwarfs of spectral type WC, and transition stars like PG 1159.[1]

The first known extreme helium star, HD 124448, was discovered in 1942 by Daniel M. Popper at the McDonald Observatory near Fort Davis, Texas, United States. This star displayed no lines of hydrogen in its spectrum, but strong helium lines as well as the presence of carbon and oxygen.[2] The second, PV Telescopii, was discovered in 1952, and by 1996 a total of 25 candidates had been found. (This list was narrowed to 21 by 2006.)[3] A common characteristic of these stars is that the abundance ratio of carbon to helium is always in the range of 0.3 to 1%. This is despite wide variation of other abundance ratios in EHe stars.[4]

The known extreme helium stars are supergiants where hydrogen is underabundant by a factor of 10,000 or more. The surface temperatures of these stars range from 9,000–35,000 K. They are primarily composed of helium, with the second most abundant element, carbon, forming about one atom per 100 atoms of helium. The chemical composition of these stars implies that they have undergone both hydrogen and helium burning at some stage of their evolution.[3]
Theoretical models

Two possible scenarios were proposed to explain the composition of extreme helium stars.[3]

The double-degenerate (DD) model explained the stars as forming in a binary system consisting of a smaller helium white dwarf and a more massive carbon-oxygen white dwarf. Both stars had ceased to produce energy through nuclear fusion and were now compact objects. The emission of gravitational radiation caused their orbit to decay until they merged. If the combined mass does not exceed the Chandrasekhar limit, the helium will accrete onto the C-O dwarf and ignite to form a supergiant. Later this will become an EHe star before cooling to become a white dwarf.[3]
The final-flash (FF) model suggested that an EHe star could form as a late evolutionary stage of a star after it had left the asymptotic giant branch. As the star is cooling to form a white dwarf, helium ignites in a shell around the core, causing the outer layers to expand rapidly. If the hydrogen in this envelope is consumed, the star becomes hydrogen deficient and it contracts to form an EHe.[3]

Examination of element abundances from seven EHe stars agreed with those predicted by the DD model.[3]
References

Jeffery, C. S.; Heber, U.; Hill, P. W.; Dreizler, S.; Drilling, J. S.; Lawson, W. A.; Leuenhagen, U.; Werner, K. (August 28 – September 1, 1995). "A catalogue of hydrogen-deficient stars". In Jeffery, C. S.; Heber, U. (eds.). Hydrogen deficient stars, Proceedings. 96. Bamberg, Germany: Astronomical Society of the Pacific Conference Series (published 1996). Bibcode:1996ASPC...96..471J.
Popper, Daniel M. (June 1942). "A Peculiar B-Type Spectrum". Publications of the Astronomical Society of the Pacific. 54 (319): 160–161. Bibcode:1942PASP...54..160P. doi:10.1086/125431.
Pandey, Gajendra; Lambert, David L.; Jeffery, C. Simon; Rao, N. Kameswara (February 2006). "An Analysis of Ultraviolet Spectra of Extreme Helium Stars and New Clues to Their Origins". The Astrophysical Journal. 638 (1): 454–471.arXiv:astro-ph/0510161. Bibcode:2006ApJ...638..454P. doi:10.1086/498674. S2CID 119359673.

Pandey, Gajendra; Kameswara Rao, N.; Lambert, David L.; Jeffery, C. Simon; Asplund, Martin (July 2001). "Abundance analyses of cool extreme helium stars". Monthly Notices of the Royal Astronomical Society. 324 (4): 937–959.arXiv:astro-ph/0101518. Bibcode:2001MNRAS.324..937P. doi:10.1046/j.1365-8711.2001.04371.x. S2CID 13468557.

External links

faulkes-telescope.com: "Extreme helium stars"
"Astronomers Find Origin Of Extreme-Helium Stars"
C. Simon Jeffery: "Extreme Helium Stars: Pulsation and Evolution" suspected primary source to use.

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Stars
Formation

Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]

Remnants

White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster

Hypothetical

Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova

Structure

Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
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Properties

Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

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Earth-centric
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Sun
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Lists

Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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White dwarf
Formation

Chandrasekhar limit PG 1159 star Stellar evolution Hertzsprung–Russell diagram Mira variable

Fate

Black dwarf Type Ia supernova
Candidates Neutron star
Pulsar Magnetar Related links Stellar black hole
Related links Compact star
Quark star Exotic star Extreme helium star Subdwarf B star Helium planet

In binary
systems

Nova
Remnant List Dwarf nova Symbiotic nova Cataclysmic variable star
AM CVn star Polar Intermediate polar X-ray binary
Super soft X-ray source Binary pulsar
X-ray pulsar List Helium flash Carbon detonation

Properties

Pulsating Urca process Electron-degenerate matter Quasi-periodic oscillations

Related

Planetary nebula
List RAMBOs White dwarf luminosity function Timeline of white dwarfs, neutron stars, and supernovae

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