A sub-dwarf of spectral class O ( English subdwarf O star , sdO ) is a subclass of hot low-mass stars. Sub-dwarfs of spectral class O are weaker than ordinary main sequence O-stars, but still their luminosity exceeds the solar 10-100 times [1] and the mass is about half the mass of the sun. Temperatures range from 40,000 K to 100,000 K. The spectrum shows ionized helium. log g is from 4.0 to 6.5. [2] Many sdO stars move in the Milky Way at high speeds and are found at high galactic latitudes . [3]
Content
Building
It is believed that the nucleus of a sub-dwarf of spectral class O consists of carbon and oxygen and is surrounded by a shell in which helium burns . The spectrum shows a helium content of 50 to 100%. [2]
History
In the early 1970s, Greenstein and Sargent measured temperatures and the magnitude of gravity, which made it possible to determine the correct position of these objects on the Hertzsprung-Russell diagram . Palomar Green, Hamburg Review, SDSS and ESO-SPY ( Supernova Ia Progenitor Survey ) contain many of these stars. [four]
Prevalence
Sub-dwarfs of spectral class O comprise one third of the number of sub-dwarfs of spectral class B. [four]
Spectrum
There are a number of varieties of the spectra of sdO stars. Among them, one can distinguish a class with strong helium lines (He-sdO) and a class with strong hydrogen lines. He-sdO stars are relatively rare. [4] Usually, sdO stars have a high nitrogen content and a low carbon content. However, there are variations in the concentrations of carbon, oxygen, neon, silicon, magnesium, or iron. [2]
Examples
- HD 128220 [2]
- HIP 52181 pulsates at a frequency of 1.04 MHz. [2]
- HD 49798 is a carbon - poor x-ray double at a distance of 650 parsecs (about 2000 light years) from the Earth [5] [3] .
- US 708 is a superfast star whose speed exceeds the runaway speed for the Milky Way. [6]
Life Cycle
Such stars can be indicated on the Hertzsprung-Russell diagram. They represent two stages in the life of stars: stars after the asymptotic branch of giants (bright sdO) and stars after the horizontal branch (compact sdO). It is believed that stars after AWG can be found in planetary nebulae , but only four of the known sdO stars are. Compact sdO stars are considered to be descendants of spectral class B dwarfs. However, the statistics do not correspond to the observations of sdB stars. An alternative theory is that sdO stars form when two white dwarfs merge. This can happen in a close binary system, the distance between which decreases due to the radiation of gravitational waves . [2]
Notes
- ↑ Napiwotski, Ralf The Origin of Helium Rich Subdwarf O Stars . Date of treatment June 9, 2011.
- ↑ 1 2 3 4 5 6 Rey, Raquel Obeiro Asterosismology of Hot Subdwarf Stars . Date of treatment June 9, 2011.
- ↑ 1 2 Viotti, R. The Luminosity and Kinematics of a Sample of Hot Subdwarfs Neopr 395–396. Date of treatment June 9, 2011.
- ↑ 1 2 3 Heber, Ulrich. Hot Subdwarf Stars (Eng.) // Annual Review of Astronomy and Astrophysics : journal. - 2009 .-- September ( vol. 47 ). - P. 211-251 . - DOI : 10.1146 / annurev-astro-082708-101836 . - . Archived July 21, 2011.
- ↑ S .; Mereghetti; La Palombara, N .; Tiengo, A .; Sartore, N .; Esposito, P .; Israel, GL; Stella, L. X-ray emission from the luminous O-type subdwarf HD 49798 and its compact companion // Astronomy and Astrophysics : journal. - 2013 .-- Vol. 553 . - P. A46 . - DOI : 10.1051 / 0004-6361 / 201321271 . - . - arXiv : 1304.1653 .
- ↑ arXiv : 0805.1050