Hot sub-dwarf

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Section through a hot blue sub-dwarf subtype O (rich in helium), not to scale
Section through a hot blue sub-dwarf subtype B (low in helium), not to scale

Hot Subdwarfs (also Blue Subdwarfs ) are - like the cooling sub dwarfs - with the prefix  (sd) classified . They are hotter than 10,000 K on their surface  . They are helium-burning stars that only have a very thin hydrogen shell. Normally , a star in the red giant helium stage fuses under a massive hydrogen shell. According to the current state of research, the hot sub-dwarfs are the nuclei of such stars that have almost completely lost their hydrogen-rich shell. The masses of the hot sub-dwarfs are 0.46 solar masses with a small scatter  , and they have radii of a few tenths of the sun . This is in stark contrast to the giant stars , which are in a similar phase of stellar evolution. The sdB stars are referred to as low in helium and the even hotter sdO stars as high in helium . These stars have a completely different structure than the main sequence stars and are therefore in a different place in the Hertzsprung-Russell diagram ; in this case left below the main row.

According to the current state of research, the hot and the cool sub-dwarfs are in completely different phases of their stellar evolution and therefore only have one thing in common: a similar position in the HRD. sdB stars, which are located between the upper main series and the white dwarfs in the Hertzsprung-Russell diagram , represent a significant proportion of hot stars in ancient star systems such as globular clusters and elliptical galaxies . They develop directly into white dwarfs.

Channels of origin

The cover can be lost:

  • as a result of a late helium flash .
  • In binary star systems , the envelope of a developed star can flow away through one or two common envelope phases or through a flow of matter over the Roche boundary to a companion. This formation channel has been confirmed by the discovery of faint companions of hot sub-dwarfs, which can be observed in around 50 percent of all sdB and sdO stars.
  • Individual hot sub -dwarfs could be the product of the merging of two white helium dwarfs (less than 0.5 solar masses each, so that no helium fusion could take place). However, these must also have lost a large part of their shell beforehand, as individual white helium dwarfs could not yet arise due to the fact that the universe was too young for this . This formation channel leads to rapidly rotating blue sub-dwarfs such as SB 290 and EC22081−1916 with rotation speeds of over 160 km / s.
  • Also planet in the form of hot Jupiters or brown dwarfs could lead to the emergence of Blue sub dwarfs. As soon as the original star swells into a red giant, the substellar companion runs within the star's atmosphere and transfers part of its kinetic energy to the outer layers of the star. As a result, the hydrogen-rich atmosphere is thrown off, and what remains is an sdB star with a companion that also lost part of its mass during the Common Envelope phase as in J0820 + 0008.

Pulsation-changeable hot sub-dwarfs

Some of the hot sub-dwarfs belong to the pulsation-variable stars . According to the period of the fundamental, they are divided into

  • the short-period V361 Hya stars with values ​​of two to ten minutes and surface temperatures above 28,000 K.
  • the long-period V1093-Her stars with values ​​between 45 and 120 minutes and surface temperatures below 28,000 K.
  • There is also a small group of hybrid stars that show both the g-vibrations of the V361-Hya group and the p-vibrations of the V1093-Her group.

All pulsation-variable hot sub-dwarfs vibrate in a variety of vibrational modes and can therefore be analyzed using the methods of asteroseismology . These analyzes have improved our understanding of the structure and evolution of this group of stars.

The vibrations in pulsation-variable blue sub-dwarfs are very stable; small periodic deviations in the arrival time of the minima or maxima are attributed to the gravitational influence of planets around the stars due to the light travel time effect and could thus confirm the hypotheses about the formation of these extreme horizontal branch stars .

Planets in hot sub-dwarfs

Some hot sub-dwarfs with planets have already been discovered. For example, sub-dwarf Kepler-70 has two exoplanets that orbit it in 5 hours and 46 minutes and in 8 hours and 14 minutes, respectively.

The discoverers suspect that these are the remaining nuclei of gas giants . Their outer layers were lost when they flew through the star's atmosphere while it was in the red giant stage. The passage of the planets could also have led to the loss of the bloated star shell, so that the blue subdwarf could have formed.

Examples

See also

Individual evidence

  1. ↑ Jeffery, CS: Pulsations in Subdwarf B Stars . In: Journal of Astrophysics and Astronomy . 26, 2005, p. 261. doi : 10.1007 / BF02702334 .
  2. ↑ J. Girven, D. Steeghs, U. Heber, et al .: The Unseen Population of F to K-type Companions to Hot Subdwarf Stars . In: Monthly Notices of the Royal Astronomical Society . tape 425 , 2012, p. 1013-1041 , doi : 10.1111 / j.1365-2966.2012.21415.x , arxiv : 1205.6803 .
  3. ↑ S. Geieret et al .: The subdwarf B star SB290 - A fast rotator on the extreme horizontal branch . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.4129 .
  4. ↑ S. Geier: Hot Subdwarf chosen: Confronting Theory with observation . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1212.0418 .
  5. ↑ Ulrich Heber, Stephan Geier, Boris Gaensicke: Hot subluminous Stars: Highlights from the MUCHFUSS and Kepler missions . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.5315 .
  6. ^ R. Lutz, S. Schuh, and R. Silvotti: EXOTIME: searching for planets and measuring Pdot in sdB pulsators . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.2048 .
  7. ↑ S. Charpinet, G. Fontaine, P. Brassard, et al .: A compact system of small planets around a former red-giant star . In: Nature . tape 480 , 2011, pp. 496-499 , doi : 10.1038 / nature10631 .
  8. ↑ chs / dpa: Cosmic Fire Hell, senior star leaves roasted planets behind , in Spiegel Online, date: December 22, 2011, accessed: December 22, 2011