Extreme helium star

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, 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 in Austin, Texas, USA. 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]

  1. 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]
  2. 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 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

  1. ^ 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.
  2. ^ 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.
  3. ^ a b c d e f 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.
  4. ^ 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.

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BX Circini

BX Circini is a star in the constellation Circinus. Its variability was discovered in 1995, with its apparent magnitude ranging from 12.57 to 12.62 over a period of 2 hours 33 minutes. It is currently classified as a PV Telescopii variable star, 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. This class of star is rare, possibly because this is a brief stage of stellar evolution. 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. It has an extremely low proportion of hydrogen, which was first noticed in 1980. 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. 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.

Circinus

Circinus is a small, faint constellation in the southern sky, first defined in 1756 by the French astronomer Nicolas-Louis de Lacaille. Its name is Latin for compass, referring to the drafting tool used for drawing circles (it should not be confused with Pyxis, a constellation that represents a mariner's compass which points north). Its brightest star is Alpha Circini, with an apparent magnitude of 3.19. Slightly variable, it is the brightest rapidly oscillating Ap star in the night sky. AX Circini is a Cepheid variable visible with the unaided eye, and BX Circini is a faint star thought to have been formed from the merger of two white dwarfs. Two sun-like stars have planetary systems: HD 134060 has two small planets, and HD 129445 has a Jupiter-like planet. Supernova SN 185 appeared in Circinus in 185 AD and was recorded by Chinese observers. Two novae have been observed more recently, in the 20th century.

The Milky Way runs through the constellation, featuring prominent objects such as the open cluster NGC 5823 and the planetary nebula NGC 5315. Circinus hosts a spiral galaxy, the Circinus Galaxy, which was discovered in 1977 and is the closest Seyfert galaxy to the Milky Way. The Alpha Circinids (ACI), a meteor shower also discovered in 1977, radiate from this constellation.

Compact star

In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects have a high mass relative to their radius, giving them a very high density, compared to ordinary atomic matter.

Compact stars are often the endpoints of stellar evolution, and are in this respect also called stellar remnants. The state and type of a stellar remnant depends primarily on the mass of the star that it formed from. The ambiguous term compact star is often used when the exact nature of the star is not known, but evidence suggests that it has a very small radius compared to ordinary stars. A compact star that is not a black hole may be called a degenerate star.

DY Centauri

DY Centauri is a variable star in the constellation Centaurus. From its brightness, it is estimated to be 7000 parsecs (23000 light-years) away from Earth.DY Centauri is classified as a R Coronae Borealis variable (RCB), a rare class of supergiant stars which show rapid and irregular decreases in brightness due to the formation of dust clouds on the stellar surface. However, DY Centauri is not an active RCB star anymore, and the last registered obscuration event was in 1934. This seems to be related to evolutionary changes in the star, represented by a very fast horizontal movement across the top of the HR diagram. Spectroscopic and photometric evidence show DY Centuari has increased its effective temperature from 5800 K in 1906 to 24800 K in 2010, while maintaining constant luminosity. As consequence, its visual apparent magnitude has faded from about 11.75 in the beginning of the 20th century to 13.2 in 2010 (due to changes in the bolometric correction), while its radius is calculated to have decreased from 100 R☉ to 8 R☉. There are only three other known stars with this behavior, called hot RCB stars.Periodic changes in the radial velocity of DY Centauri have been detected, indicating that the star in a single-lined spectroscopic binary in an eccentric orbit (e = 0.44) with a period of 39.67 days. The companion star has an estimated minimum mass of 0.2 M☉, so it can be a low mass white dwarf or main sequence star. With an estimated separation of only 10 R☉ at periastron, the system must have interacted in the past when the primary had larger dimensions, forming a common envelope.DY Centauri has a peculiar chemical composition and is poor in hydrogen and rich in helium and carbon, being identified as an extreme helium star (EHe). In comparison to other RCB and EHe stars, however, its hydrogen content is relatively high. Stars of this type are believed to be the product of the merger of two white dwarfs, therefore being single stars, which is inconsistent with the identification of DY Centauri as a close binary. Thus, the origin and evolutionary state of the DY Centauri system remain uncertain. In the future, it is likely that the primary will evolve to a B subdwarf, a class of stars frequently found in binary systems.The spectrum of DY Centauri indicates the presence of a low density expanding nebula around it, formed by gas ionized by ultraviolet radiation from the star. The nebula has an estimated dimension of 1.2 arcseconds and, from its expansion velocity, was probably created about a thousand years ago.

EHE

EHE may refer to:

Epithelioid hemangioendothelioma

Extreme helium star

Marriage (1929 film) (German: Die Ehe)

Helium-weak star

Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type. Their helium lines place them in a later (ie. cooler) spectral type then their hydrogen lines.

Outline of astronomy

The following outline is provided as an overview of and topical guide to astronomy:

Astronomy – studies the universe beyond Earth, including its formation and development, and the evolution, physics, chemistry, meteorology, and motion of celestial objects (such as galaxies, planets, etc.) and phenomena that originate outside the atmosphere of Earth (such as the cosmic background radiation).

PV Telescopii

PV Telescopii (also known as HD 168476) is a class B-type (blue) supergiant extreme helium star in the constellation Telescopium. It is also the prototype of variable stars called PV Telescopii variables.

PV Telescopii variable

PV Telescopii variable is a type of variable star that is established in the General Catalogue of Variable Stars with the acronym PVTEL. This class of variables are defined as "helium supergiant Bp stars with weak hydrogen lines and enhanced lines of He and C". That is, the hydrogen spectral lines of these stars are weaker than normal for a star of stellar class B, while the lines of helium and carbon are stronger. The prototype for this category is HD 168476, also known as PV Telescopii, which undergoes small but complex luminosity variations and radial velocity fluctuations. The PV Tel stars are extremely hydrogen-deficient compared to other B-class stars and vary in luminosity on time scales ranging from a few hours to several years. As of 2008, there are twelve confirmed PV Tel variables in the General Catalogue of Variable Stars.PV Telescopii variables are subdivided into three distinct types on the basis of spectral type: type I represents late B and A stars, type II represents O and early B, and type III represents F and G stars. The type III stars are always carbon-rich and hydrogen-deficient, while the type I and II stars do not necessarily have an excess of carbon. The hotter types pulsate more quickly than the cooler types.

R Coronae Borealis

R Coronae Borealis is a peculiar low-mass yellow supergiant star in the constellation of Corona Borealis. It is the prototype of the rare RCB class of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Borealis itself normally shines at approximately magnitude 6, just about visible to the naked eye, but at intervals of several months to many years fades to as faint as 15th magnitude. Over successive months it then gradually returns to its normal brightness, giving it the nickname "reverse nova".

Telescopium

Telescopium is a minor constellation in the southern celestial hemisphere, one of twelve named in the 18th century by French astronomer Nicolas-Louis de Lacaille and one of several depicting scientific instruments. Its name is a Latinized form of the Greek word for telescope. Telescopium was later much reduced in size by Francis Baily and Benjamin Gould.

The brightest star in the constellation is Alpha Telescopii, a blue-white subgiant with an apparent magnitude of 3.5, followed by the orange giant star Zeta Telescopii at magnitude 4.1. Eta and PZ Telescopii are two young star systems with debris disks and brown dwarf companions. Telescopium hosts two unusual stars with very little hydrogen that are likely to be the result of two merged white dwarfs: PV Telescopii, also known as HD 168476, is a hot blue extreme helium star, while RS Telescopii is an R Coronae Borealis variable. RR Telescopii is a cataclysmic variable that brightened as a nova to magnitude 6 in 1948.

Variable star

A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.

This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.

Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

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