Subdwarf O star

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]

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

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

  • HD 128220 was studied by Corrado Bartolini[2]
  • HIP 52181 pulsates with a frequency of 1.04 milliHertz.[2]
  • HD 49798 is a carbon poor X-ray binary at 830pc.[3]
  • US 708 is a hypervelocity star that exceeds the escape velocity of the Milky Way.[5]

Life cycle

A Fleeting Moment in Time - Planetary Nebula ESO 577-24
Planetary nebula remains of a dead giant star leaving behind a Subdwarf O star.[6]

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]

References

  1. ^ Napiwotski, Ralf. "The Origin of Helium Rich Subdwarf O Stars" (PDF). Retrieved 9 June 2011.
  2. ^ a b c d e f Rey, Raquel Obeiro. "Asterosismology of Hot Subdwarf Stars" (PDF). Retrieved 9 June 2011.
  3. ^ a b 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. ^ a b c Heber, Ulrich (September 2009). "Hot Subdwarf Stars" (PDF). Annual Review of Astronomy and Astrophysics. 47: 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. ^ arXiv:0805.1050
  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.
56 Pegasi

56 Pegasi (56 Peg) is a spectroscopic binary star in the constellation Pegasus. Its apparent magnitude is 4.74 and it is approximately 590 light years away based on parallax.The companion, a subdwarf O star, has a period of about 111 days.

Abell 39

Abell 39 is a low surface brightness planetary nebula in the constellation of Hercules. It is the 39th entry in George Abell's 1966 Abell Catalog of Planetary Nebulae (and 27th in his 1955 catalog) of 86 old planetary nebulae which either Abell or Albert George Wilson discovered before August 1955 as part of the National Geographic Society - Palomar Observatory Sky Survey. It is estimated to be about 6,800 light-years from earth and 4,600 light-years above the Galactic plane. It is almost perfectly spherical and also one of the largest known spheres with a radius of about 2.5 light-years.

Beta Canis Minoris

Beta Canis Minoris (β Canis Minoris, abbreviated Beta CMi, β CMi), also named Gomeisa , is a star in the constellation of Canis Minor. In the night sky it is notable for its proximity to the prominent star Procyon.

O-Type

O-Type may refer to:

O-type asteroid

O type blood

O-type star

O-type giant

O-type main sequence star

Subdwarf O star

Formation
Evolution
Spectral
classification
Remnants
Hypothetical
Nucleosynthesis
Structure
Properties
Star systems
Earth-centric
observations
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