Extreme helium star
Encyclopedia
An extreme helium star is a low-mass supergiant
that is almost devoid of hydrogen
, the most common chemical element
of the Universe
. Since there are no conditions where stars devoid of hydrogen can be formed from molecular cloud
s, it is theorized that they are the product of mergers of white dwarf
s, one of the helium type (DB or DO), and another of the carbon
-oxygen
rich type (DQ).
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
, helium-rich spectral class O or B stars, population I Wolf-Rayet star
s, AM CVn star
s, white dwarf
s of spectral type WC, and transition stars like PG 1159
.
The first known extreme helium star, HD 124448, was discovered in 1942 by Daniel M. Popper at the McDonald Observatory
in Austin
, USA
. This star displayed no lines of hydrogen in its spectrum, but strong helium lines as well as the presence of carbon and oxygen. 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.) 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.
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.
Two possible scenarios were proposed to explain the composition of extreme helium stars.
Examination of element abundances from seven EHe stars agreed with those predicted by the DD model.
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...
that is almost devoid of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, the most common chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
of the Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
. Since there are no conditions where stars devoid of hydrogen can be formed from molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
s, it is theorized that they are the product of mergers of white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
s, one of the helium type (DB or DO), and another of the carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
-oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
rich type (DQ).
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
R Coronae Borealis
R Coronae Borealis is a yellow supergiant star, and is the prototype of the RCB class of variable stars, which fade by several magnitudes at irregular intervals...
, helium-rich spectral class O or B stars, population I Wolf-Rayet star
Wolf-Rayet star
Wolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
s, AM CVn star
AM CVn star
An AM CVn star, or AM Canum Venaticorum star, is a type of cataclysmic variable star. These binary stars are named after their type star, AM Canum Venaticorum. In these variables, a white dwarf accretes hydrogen-poor matter from a compact companion star...
s, white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
s of spectral type WC, and transition stars like PG 1159
PG 1159 star
A PG 1159 star, often also called a pre-degenerate, is a star with a hydrogen-deficient atmosphere which is in transition between being the central star of a planetary nebula and being a hot white dwarf...
.
The first known extreme helium star, HD 124448, was discovered in 1942 by Daniel M. Popper at the McDonald Observatory
McDonald Observatory
The McDonald Observatory is an astronomical observatory located near the unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Fowlkes and Mount Locke in the Davis Mountains of West Texas...
in Austin
Austin
Austin is the capital of the U.S. state of Texas.Austin may also refer to:-In the United States:*Austin, Arkansas*Austin, Colorado*Austin, Chicago, Illinois*Austin, Indiana*Austin, Minnesota*Austin, Nevada*Austin, Oregon...
, USA
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. This star displayed no lines of hydrogen in its spectrum, but strong helium lines as well as the presence of carbon and oxygen. The second, PV Telescopii
PV Telescopii
PV Telescopii is a class B-type supergiant in the constelation Telescopium. It is also the prototype of variable stars called PV Telescopii variables. These are blue supergiants with weak Hydrogen lines and enhanced Helium and Carbon lines. Their variable periods are from a few hours to a few...
, was discovered in 1952, and by 1996 a total of 25 candidates had been found. (This list was narrowed to 21 by 2006.) 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.
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.
Two possible scenarios were proposed to explain the composition of extreme helium stars.
- 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 limitChandrasekhar limitWhen a star starts running out of fuel, it usually cools off and collapses into one of three compact forms, depending on its total mass:* a White Dwarf, a big lump of Carbon and Oxygen atoms, almost like one huge molecule...
, the helium will accrete onto the C-O dwarf and ignite to form a supergiant. Later this will become a EHe star before cooling to become a white dwarf. - The final-flash (FF) model suggested that an EHe star could form as a late evolutionary stage of a star after if had left the asymptotic giant branchAsymptotic Giant BranchThe asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
. As the star is cooling to form a white dwarf, helium ignites in a shell around the core, causing the outer laters to expand rapidly. If the hydrogen in this envelope is consumed, the star becomes hydrogen deficient and it contracts to form an EHe.
Examination of element abundances from seven EHe stars agreed with those predicted by the DD model.