Subdwarf O star

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Schematic cross-section of an O-type subdwarf. Subdwarf O star schematic cross section.png
Schematic cross-section of an O-type subdwarf.

A subdwarf O star (sdO) is a type of hot, but low-mass star. O-type subdwarfs are much dimmer than regular O-type main-sequence stars, but with a brightness about 10 to 100 times that of the Sun, [1] and have a mass approximately half that of the Sun. Their temperature ranges from 40,000 to 100,000 K. Ionized helium is prominent in their spectra. Gravity acceleration is expressed by log g between 4.0 and 6.5. [2] Many sdO stars are moving at high velocity through the Milky Way and are found at high galactic latitudes. [3]

Contents

Structure

The structure of a subdwarf O star is believed to be a carbon and oxygen core surrounded by a helium burning shell. The spectrum shows that the content is from 50 to 100% helium. [2]

History

In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung-Russell diagram. The Palomar-Green survey, Hamburg surveys, Sloan Digital Sky Survey and Supernova Ia Progenitor Survey (ESO-SPY) have documented many of these stars. [4]

Occurrence

Subdwarf O stars are only a third as common as subdwarf B stars. [4]

Spectrum

There is actually a variety of spectra from the sdO stars. They can be grouped into those with strong helium lines, termed He-sdO, and those with stronger hydrogen lines, H strong sdO. The He-sdO are fairly rare. [4] Usually nitrogen is enriched and carbon depleted. However, there are variations with enhancement in concentration of even numbered elements such as carbon, oxygen, neon, silicon, magnesium or iron. [2]

Examples

Life cycle

Planetary nebula remains of a dead giant star leaving behind a Subdwarf O star. A Fleeting Moment in Time - Planetary Nebula ESO 577-24.jpg
Planetary nebula remains of a dead giant star leaving behind a Subdwarf O star.

They can be plotted on the Hertzsprung–Russell diagram. They are from two stages in the stellar lifecycle, post–asymptotic giant branch (the luminous sdO), and post–extended horizontal branch compact sdO. The post-AGB stars are expected to be found in planetary nebulas, but only four of the sdO stars are known to be like this. The compact sdOs would be descendants of the B-type subdwarfs. However, statistics do not match sdB. An alternate theory is that sdOs have been formed by coalescing two white dwarfs. This could happen from a close binary that decays due to gravitational waves. [2]

Related Research Articles

Main sequence Continuous band of stars that appears on plots of stellar color versus brightness

In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. Stars on this band are known as main-sequence stars or dwarf stars. These are the most numerous true stars in the universe, and include the Sun.

Stellar evolution Changes to a star over its lifespan

Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

Stellar classification Classification of stars based on their spectral characteristics

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.

Supergiant Type of star that is massive and luminous

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.

Wolf–Rayet star Heterogeneous stars with unusual spectra

Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. The surface temperatures of known Wolf–Rayet stars range from 20,000 K to around 210,000 K, hotter than almost all other kinds of stars. They were previously called W-type stars referring to their spectral classification.

Blue giant Giant star of early spectral type

In astronomy, a blue giant is a hot star with a luminosity class of III (giant) or II. In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence.

Giant star Type of star, larger and brighter than the Sun

A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.

A carbon star is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance. There are also some dwarf and supergiant carbon stars, with the more common giant stars sometimes being called classical carbon stars to distinguish them.

Subdwarf Star of luminosity class VI under the Yerkes spectral classification system

A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On a Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence.

B-type main-sequence star Stellar classification distinguished by bright blue luminosity

A B-type main-sequence star is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars are extremely luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Examples include Regulus and Algol A.

IK Pegasi Variable star in the constellation Pegasus

IK Pegasi is a binary star system in the constellation Pegasus. It is just luminous enough to be seen with the unaided eye, at a distance of about 154 light years from the Solar System.

O-type main-sequence star

An O-type main-sequence star is a main-sequence star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 50,000 K. They are between 40,000 and 1,000,000 times as luminous as the Sun.

Subdwarf B star Subdwarf star with spectral type B - extremely hot small star

A B-type subdwarf (sdB) is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf by being much hotter and brighter. They are situated at the "extreme horizontal branch" of the Hertzsprung–Russell diagram. Masses of these stars are around 0.5 solar masses, and they contain only about 1% hydrogen, with the rest being helium. Their radius is from 0.15 to 0.25 solar radii, and their temperature is from 20,000 to 40,000K.

Red giant Type of large cool star that has exhausted its core hydrogen

A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

KPD 1930+2752

KPD 1930+2752 is a binary star system including a subdwarf B star and a probable white dwarf with relatively high mass. Due to the nature of this astronomical system, it seems like a likely candidate for a potential type Ia supernova, a type of supernova which occurs when a white dwarf star takes on enough matter to approach the Chandrasekhar limit, the point at which electron degeneracy pressure would not be enough to support its mass. However, carbon fusion would occur before this limit was reached, releasing enough energy to overcome the force of gravity holding the star together and resulting in a supernova.

O-type star Stellar classification

An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 kelvin (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B.

US 708 Hypervelocity O-type subdwarf in the halo of the Milky Way Galaxy

US 708 is a hyper-velocity O class subdwarf in Ursa Major in the halo of the Milky Way Galaxy. One of the fastest-moving stars in the galaxy, the star was first surveyed in 1982.

HVS 7 -- hyper-velocity star 7, otherwise known as SDSS J113312.12+010824.9 is a rare star that has been accelerated to faster than our Milky Way Galaxy's escape velocity. In 2013 a team under N. Przybilla wrote that the star had a chemically peculiar photosphere, which masked its origins. The star was first cataloged during the Sloan Digital Sky Survey. It was identified as a hyper-velocity star in 2006.

Hydrogen-deficient star Star that has little or no hydrogen in its atmosphere

A hydrogen-deficient star is a type of star that has little or no hydrogen in its atmosphere. Hydrogen deficiency is unusual in a star, as hydrogen is typically the most common element in a stellar atmosphere. Despite being rare, there are a variety of star types that display a hydrogen deficiency.

HD 219617 is a binary star system some 220 light-years away from our solar system in the constellation Aquarius. It is composed of two metal-poor F-type subdwarf stars orbiting each other in a 388-year orbit. Another theory suggests that the binary star is composed of subgiant stars. Unlike many halo stars, which exhibit an excess of alpha elements relative to iron, HD 219617 is depleted in iron peak and alpha elements, although alpha elements concentrations are poorly constrained. The stellar chemical composition is peculiar, being relatively oxygen-enriched and extremely depleted in neutron capture elements. The helium fraction of the binary star at present cannot be reliably determined, and appears to be near the primordial helium abundance.

References

  1. Napiwotski, Ralf. "The Origin of Helium Rich Subdwarf O Stars" (PDF). Retrieved 9 June 2011.
  2. 1 2 3 4 5 6 Rey, Raquel Obeiro. "Asterosismology of Hot Subdwarf Stars" (PDF). Retrieved 9 June 2011.
  3. 1 2 Viotti, R.; D. Cardini; A. Emanuele; M. Badiali. "The Luminosity and Kinematics of a Sample of Hot Subdwarfs" (PDF). pp. 395–396. Retrieved 9 June 2011.
  4. 1 2 3 Heber, Ulrich (September 2009). "Hot Subdwarf Stars" (PDF). Annual Review of Astronomy and Astrophysics. 47 (1): 211–251. Bibcode:2009ARA&A..47..211H. doi:10.1146/annurev-astro-082708-101836. Archived from the original (PDF) on 21 July 2011. Retrieved 10 June 2011.
  5. Heber, Uli; Hirsch, Heiko; Edelmann, Heinz; Napiwotzki, Ralf; O'Toole, Simon; Brown, Warren; Altmann, Martin (2008). "Hypervelocity Stars: Young and Heavy or Old and Light?". Hot Subdwarf Stars and Related Objects. 392: 167. arXiv: 0805.1050 . Bibcode:2008ASPC..392..167H.
  6. "A Fleeting Moment in Time - European Southern Observatory's Cosmic Gems Programme captures last breath of a dying star". www.eso.org. Retrieved 22 January 2019.