A sub-dwarf of spectral class B ( English Subdwarf B star , sdB) is a type of sub-dwarf star belonging to spectral class B. They differ from ordinary sub-dwarfs because they are brighter and hotter. [1] Such stars are located on the extreme horizontal branch of the Hertzsprung-Russell diagram . The masses of such objects make up about 0.5 of the mass of the Sun , only about 1% of hydrogen is present in the composition, the rest is helium. The radii of the sub-dwarfs of spectral class B lie in the range from 0.15 to 0.25 of the radii of the Sun , temperatures range from 20,000 to 40,000 K.
These stars represent a late stage in the evolution of some stars that occurs when the red giant loses its outer hydrogen layers until helium begins to burn in the nucleus. The reasons for such a preliminary loss of mass are unclear, but the interaction of stars in a binary system is considered one of the main mechanisms. Single sub-dwarfs may result from the merger of two white dwarfs. It is believed that sdB stars become white dwarfs without going through other stages of giants.
Sub-dwarfs of spectral class B are brighter than white dwarfs and represent a significant proportion of the population of hot stars in old star systems such as globular clusters , bulges of spiral galaxies and elliptical galaxies . [2] Such objects stand out in ultraviolet images. It is assumed that hot sub-dwarfs are responsible for the increased ultraviolet flux in the total radiation flux of elliptical galaxies. [one]
Content
History
Spectral-class sub-dwarfs were discovered by F. Zwicky and M. Humason around 1947 when they discovered super-bright blue stars near the north pole of the Galaxy. As part of the Palomar Green survey, it was found that sdB stars are typical representatives of faint blue stars with a magnitude greater than 18. During the 1960s, it was found from spectroscopy that many sdB stars lack hydrogen. In the early 1970s, D. Greenstein and A. Sargent measured the temperature and magnitude of gravity, and then determined the correct position of such stars on the Hertzsprung-Russell diagram. [one]
Variables
In this category of stars, there are three types of variable stars .
Firstly, there are variable sdB stars with periods of brightness change from 90 to 600 seconds. They are also called EC14026 or V361 Hya stars. For such objects, the notation sdBV r is proposed, where r stands for fast ( English rapid ) variability. [3] The Sharpine theory of vibrations in these stars implies that brightness changes occur due to the acoustic mode of vibrations with a low degree (l) and low order (n). The mode arises due to the ionization of atoms of the iron group, which leads to opacity. The velocity curve differs in phase by 90 degrees from the light curve , the curves of effective temperature and surface gravity seem to coincide in phase with the curve of the flow. On the graph of temperature dependence on surface gravity, stars with short-period pulsations are grouped together in the so-called empirical instability band, which occupies the region T = 28000–35000 K and log g = 5.2–6.0. Only 10% of sdB stars falling into the empirical band of instability really pulsate.
Secondly, there are variables with large periods, from 45 to 180 minutes. The suggested notation for them is sdBV s , where s means slow periodicity. [3] The variability of such objects is 0.1%. Such stars are also called PG1716 or V1093 Her, sometimes they use the abbreviation LPsdBV. Another common name is Betsy stars . [4] Long-period pulsating sdB stars are usually colder than their short-period counterparts, the temperature of the former is about 23000-30000 K.
The stars oscillating in both modes are hybrid , the standard designation is sdBV rs . The prototype is DW Lyn , also referred to as HS 0702 + 6043. [3]
| Variable star | Other name | Constellation | Distance ( St. years ) |
|---|---|---|---|
| V361 Hydrae | EC 14026-2647 | Hydra | ? |
| V1093 Hercules | GSC 03081-00631 | Hercules | ? |
| HW Virgin * | HIP 62157 | Virgo | 590 |
| NY Virgin * | GSC 04966-00491 | Virgo | ? |
| V391 Pegasus | HS 2201 + 2610 | Pegasus | 4570 |
* eclipsing double star
Planet Systems
At least two sdB stars are known to have planets. V391 Pegasus was the first sdB star to own a planet, and KOI-55 has a system of orbiting planets that are probably remnants of a giant planet destroyed while the star was in the red giant stage. [five]
Notes
- ↑ 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.
- ↑ Jeffery, CS Pulsations in Subdwarf B Stars (Eng.) // Journal of Astrophysics and Astronomy : journal. - 2005. - Vol. 26 , no. 2-3 . - P. 261 . - DOI : 10.1007 / BF02702334 . - .
- ↑ 1 2 3 D. Kilkenny; Fontaine, G .; Green, EM; Schuh, S. A Proposed Uniform Nomenclature for Pulsating Hot Subdwarf Stars // Commissions 27 and 42 of the IAU: Information Bulletin on Variable Stars: journal. - 2010 .-- March 8 ( vol. 5927 , no. 5927 ). - P. 1 . - .
- ↑ Rey, Raquel Obeiro Asterosismology of Hot Subdwarf Stars . Date of treatment June 9, 2011.
- ↑ Charpinet, S .; Fontaine, G .; Brassard, P. & Green, EM (December 21, 2011), " A compact system of small planets around a former red-giant star ", Nature T. 480 (7378): 496–499, PMID 22193103 , DOI 10.1038 / nature10631