Subdwarf

Subdwarfs (abbreviated sd of English subdwarf ) are stars that are much fainter compared to " normal" main sequence stars with the same surface temperature and luminosity class VI are assigned. In the Hertzsprung -Russell diagram, the sub- dwarfs are 1.5 to 2 magnitudes below the main sequence. The term was coined in 1939 by subdwarf Gerard Peter Kuiper, which allow a number of stars described previously considered " intermediate white dwarfs ". A distinction is cool and hot sub- dwarfs, which form two different classes and their members are in completely different stages of development. Both classes must not be concluded that there is actually existing light weakness of these stars ( in proportion to their mass ) from the position in the Hertzsprung -Russell diagram; they only inhabit an area next to the main sequence.

Cool subdwarfs

These stars are of spectral type G to M, or SDG to SDM together with their luminosity class and thus have a surface temperature of about 2000 to 6000 K. They are metal-poor, which means they have a smaller proportion of elements heavier than usual as helium, but are otherwise main-sequence stars and derive their energy from the hydrogen burning. Due to the lower metallicity the opacity of the stellar interior is reduced (it is translucent ) and thus the outward radiation pressure decreases in the star. This in turn means that the star is smaller and hotter than an ordinary Population I main-sequence star of the same mass. Due to their lower opacity cool subdwarfs radiate strongly in the ultraviolet compared to main-sequence stars of the same spectral class.

Comparing in the Hertzsprung -Russell diagram for a given mass "normal" dwarf star ( such as the sun) with sub- dwarfs, so "walk" the cool subdwarfs due to their hotter surface of the main sequence to the left, and because they are lighter and slightly upward back to the main sequence. However, they are not bright enough to get back to the normal main sequence. In this way cool subdwarfs form a separate main sequence below the usual and are therefore misleadingly referred to as fainter.

Cool subdwarfs are generally very old so-called Population II stars, which mainly belong to the galactic halo of the Milky Way and have high velocities relative to the Sun. An example of this type is Kapteyn's star.

Hot or blue subdwarfs

Hot or blue sub- dwarfs are of spectral type O or B and are classified by analogy to the cool sub- dwarfs as sdO, sdB or sdOB. They are according to the spectral type hotter than 10000 K. It is helium- burning stars with only a very thin hydrogen envelope. The sdB stars are as heliumarm and refers to the sdO star as helium- rich. Compared to stars of the main sequence helium- burning stars have a completely different stellar structure and are therefore located at a different location in the Hertzsprung -Russell diagram; in this case, its position is on the left below the main sequence.

Normally fused a star in the red giant stage helium under a massive hydrogen shell. Hot subdwarfs are according to the current state of research, the cores of such stars that have lost most of their hydrogen-rich envelope. The masses of hot subdwarfs are having small dispersion at 0.46 solar masses and they have radii of a few tenths of the sun. The envelope can be lost as a result of a late helium flash. In binary systems, the shell of a star -developed flow out through one or two common- envelope phases or by a Materiefluß about Roche limit for a companion. This channel formation has been confirmed by the discovery of faint companions of hot sub- dwarfs, which can be observed in about 50 percent of all sdB and sdO stars. Individual hot subdwarfs could be the product of the merger of two white -helium dwarfs be (each less than 0.5 solar masses, so that could take place no helium fusion), which would likewise have previously lost much of its case because single white -helium dwarfs could not arise because of this insufficient years of the universe. This channel formation leads to rapidly rotating blue sub- dwarfs such as SB 290 and EC22081 - 1916 at rotational speeds of more than 160 km / s Also planet in the form of hot Jupiters and brown dwarfs could lead to the creation of the Blue sub- dwarfs. Once the parent star into a red giant swells running the substellar companion within the star's atmosphere and transfers part of its kinetic energy to the outer layers of the star. Thus, the hydrogen-rich atmosphere is discarded and remains an sdB star with a companion who has also lost some of its mass during the common envelope phase as for J0820 0008.

A portion of the hot sub- dwarfs among the stars pulsating variables. They are after the period of the fundamental oscillation divided into short-period V361 Hya - stars with values ​​between two and ten minutes and the long-period V1093 -Her - stars with values ​​of 45 and 120 minutes. All - V361 Hya stars have surface temperatures, while the V1093 -Her - stars are above 28,000 K below this limit. There is also a small group of hybrid stars which show both the g - oscillations of V361 Hya - group and the p - oscillations of the V1093 -Her - group. All hot subdwarfs pulsating variables oscillate in a variety of vibrational modes and can therefore be analyzed using the methods of asteroseismology. These analyzes have improved the understanding of the structure and the evolution of this group of stars. The oscillations in pulsating variables blue sub- dwarfs are very stable. Small periodic variations in the arrival time of the minima or maxima are attributed to the gravitational influence of planets around the star due to the light travel time effect and could confirm the hypotheses regarding the origin of this extreme horizontal branch stars with it.

SdB stars that are located in the Hertzsprung -Russell diagram between the upper main sequence and the white dwarfs, represent a significant proportion of hot stars in old stellar systems such as globular clusters and elliptical galaxies Represent develop directly on to white dwarfs.

Planet with a hot subdwarf

The hot sub- dwarf KIC 05807616 could have two planets revolving around it in 5 hours and 46 minutes and 8 hours and 14 minutes. The discoverers suggest that it is the leftover cores of gas giants. The outer layers were made to the flight through the star atmosphere during the red giant stage lost. The passage of the planets through the inflated stellar envelope of the red giant could have resulted in the loss of the latter, so that as the blue subdwarf could form.