Subdwarf B star
The subdwarf B star is a kind of subdwarf star with spectral type B. They are from the "extreme horizontal branch stars" of the Hertzsprung-Russell diagram. These stars represent a late stage in the evolution of some stars, caused when a red giant star loses its outer hydrogen layers before the core begins to fuse helium. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a binary star system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two white dwarf stars. Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters. spiral galaxy bulges and elliptical galaxies.[1] They are prominent on ultraviolet images.
The sdB stars are expected to become white dwarfs without going through any more giant stages.
Masses of these stars are around 0.5 solar masses, and they have only about 1% hydrogen, with the rest helium. Their radius is from 0.15 to 0.25 solar radii, and their temperature is from 20000 to 40000K.
Variables
There are three kinds of variable stars in this category:
Firstly there are the sdBV with periods from 90 to 600 seconds. They are also called EC14026 or V361 Hya stars. A proposed new nomenclature is sdBVr, with r standing for rapid.[2] The Charpinet theory of the oscillations of these stars is that the variations is brightness are due to acoustic mode oscillations with low degree (l) and low order (n). It is driven by ionisation of iron group atoms causing opacity. The velocity curve is 90 degrees out of phase with the brightness curve. The gravity acceleration g has also been measured and is at a minimum when the brightness is a maximum. This is mostly due to the change in velocity rather than a change in gravity due to size differences.
Secondly there are the long period variables with periods from 45 to 180 minutes. A proposed new nomenclature is sdBVs, with s standing for slow.[2] These only have a very small variation of 0.1%. They have also been called PG1716 or V1093 Her or abbreviated as LPsdBV. The long period variables have a narrower range of temperature from 29000K to 35000K
Stars that oscillate with both kinds of periods are called hybrid. with a standard nomenclature of sdBVrs. A prototype is DW Lyn also identified as HS 0702+6043.[2]
| variable star | Other name | Constellation | Distance (ly) |
|---|---|---|---|
| V361 Hydrae | EC 14026-2647 | Hydra | ? |
| V1093 Herculis | GSC 03081-00631 | Hercules | ? |
| HW Virginis* | HIP 62157 | Virgo | 590 |
| NY Virginis* | GSC 04966-00491 | Virgo | ? |
| V391 Pegasi | HS 2201+2610 | Pegasus | 4570 |
References
- ^ Jeffery, C. S. (2005). "Pulsations in Subdwarf B Stars". Journal of Astrophysics and Astronomy. 26: 261. doi:10.1007/BF02702334.
- ^ a b c D. Kilkenny (8 March 2010). "A Proposed Uniform Nomenclature for Pulsating Hot Subdwarf Stars". Commissions 27 and 42 of the IAU: Information Bulletin on Variable Stars (5927).