A subdwarf O star (sdO) is a type of hot, but low-mass star. O-type

subdwarf 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 ...

s are much dimmer than regular

O-type main-sequence star An O-type main-sequence star (O V) is a main-sequence (core hydrogen-burning) 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. T ...

s, but with a brightness about 10 to 100 times that of the Sun, 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. Many sdO stars are moving at high velocity through the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

and are found at high

galactic latitude The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane parallel to an a ...

Structure

The structure of a subdwarf O star is believed to be a carbon and oxygen core surrounded by a

helium burning The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon. Triple-alpha process in stars Helium accumulates in the cores of stars as a result of the proton–pr ...

shell. The spectrum shows that the content is from 50 to 100% helium.

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.

Occurrence

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

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. 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.

Examples

* HD 128220 was studied by Corrado Bartolini *

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pulsates with a frequency of 1.04 milliHertz. *

HD 49798 HD 49798 is a binary star in the constellation Puppis about 650 parsecs from Earth. It has an apparent magnitude of 8.3, making it one of the brightest known O class subdwarf stars. HD 49798 was discovered in 1964 to be a rare hydroge ...

is a carbon poor X-ray binary at 830pc. *

US 708 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. Discovery US 708 was first discovered in 1982 by Peter ...

is a hypervelocity star that exceeds the escape velocity of the Milky Way.

Life cycle

A Fleeting Moment in Time - Planetary Nebula ESO 577-24

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 Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * E ...

compact sdO. The post-AGB stars are expected to be found in

planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelate ...

s, but only four of the sdO stars are known to be like this. The compact sdOs would be descendants of the

B-type subdwarf 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. Masse ...

s. However, statistics do not match sdB. An alternate theory is that sdOs have been formed by coalescing two

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

s. This could happen from a close binary that decays due to gravitational waves.

References

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