Extreme helium star

Extreme helium stars ( shortly before stars) are giant stars with spectral type A or B and a strong underabundance of hydrogen in their atmospheres. They are very rare and closely related to the R Coronae Borealis stars.

Properties

The main energy source of the stars is the hydrogen burning and the atmosphere of stars is predominantly at the birth of hydrogen with a mass fraction of 70 percent. As part of its development stars may convert their hydrogen-rich atmosphere or lose by various mechanisms, so that in the extreme case comes only a hydrogen atom on 10,000 other atoms. This group of hydrogen- poor stars are the extreme helium stars, which are predominantly supergiant with a spectral class A or B. They are extremely rare with about 15 stars in the visible part of the Milky Way. Due to their kinematic properties, their metallicity and their galactic distribution they belong to the bulge of the Milky Way. You can reach luminosities to 10,000 times the sun. Its atmosphere is in addition to a lower incidence of hydrogen an overabundance of nitrogen, carbon, and sometimes oxygen. Extreme helium stars are closely related to the R Coronae Borealis stars. They feature a slightly higher surface temperature and show no caused by dust condensations deep minima.

Interpretation of their spectra

The source of the chemical elements in the spectra of the extreme helium star is interpreted in the literature as follows:

• The small amount of hydrogen is a relic from the time of star formation
• Calcium, titanium, sodium, manganese and nickel have not been processed in the interior of stars, what an upper limit on the mass of the progenitor star puts on about eight solar masses
• The ratio of ( N / Fe ) is proportional to (C O N) / Fe. This observation is typical for stars who have gone through a phase of helium burning as a red giant
• In some extreme helium stars is the s-process elements such as yttrium and zirconium show that are likely caused by a thermal pulse on the asymptotic giant branch

Origin

First hypotheses regarding the origin of the origin of the extreme helium star tried the unusual chemical composition of the atmosphere of the early supergiants explained as follows:

• As red giants that have lost their entire hydrogen-rich atmosphere
• As a result of Hot Bottom Burning on the asymptotic giant branch
• As a pure helium star, which has already been done with an extremely low frequency of hydrogen

None of these hypotheses can the composition of the atmosphere of extreme helium stars over all the elements of time satisfactory to explain. The current discussion is now rather out of a late thermal pulse or a merger of two white dwarfs.

In a late thermal pulse has a red giant is already the asymptotic giant moves towards higher temperatures when hydrogen-and helium- rich material is transported from the atmosphere to the core of the star and re- ignite a nuclear fusion. At a very late thermal pulse, the star has already happened, the knee in the blue region of the Hertzsprung -Russell diagram and is located on the cooling sequence of white dwarfs, when again ignite nuclear fusion inside. During simulations at late thermal pulses produce chemical compounds that are similar to those of extreme helium stars show the observations of stars that just a thermal pulse through as FG Sge, V605 Aql and V4334 Sgr a significantly greater incidence of carbon and hydrogen as marriage star.

Two white dwarfs in a close binary system can lose as much orbital energy that they merge on the mechanism of emission of gravitational waves. If it is in the white dwarfs to one that consists mostly of helium and a carbon-oxygen white dwarf with a mass of 0.7 solar masses, then the CO white dwarf accrete up to about 0.3 solar masses of helium. A subsequent helium burning produces a supergiant with a mass, luminosity and chemical composition as that of the extreme helium stars. The frequency of such a merger is 3 * 10-3 per year estimated from the frequency of the AM Canum - Venaticorum -Stars and a contraction time of a few hundred years gives an estimated frequency of marriage star, which is consistent with the observations.

Variability

Some extreme helium stars show a variability of a few tenths mag and periods of several hours. The variability is caused by radial pulsations, which can reach several kilometers per second. After its prototype, the stars are referred to as PV Telescopii stars. Most PV Tel stars are multi- periodic, and can also additionally long periods of tens of days show. All PV Tel stars lie in a instability strip and the oscillations are probably caused by the kappa mechanism.

Examples

• HD 124 448
• HD 160641
• BD 10 ° 2179
• BD 13 ° 3224

Cite

• C. Simon Jeffery: The origin and pulsation of extreme helium stars. In: Astrophysics. Solar and Stellar Astrophysics. 2013, arXiv: 1311.1635v1.
• P. Tisserand: Tracking down R Coronae Borealis stars From Their mid -infrared WISE colors. In: Astrophysics. Solar and Stellar Astrophysics. 2011, arXiv: 1110.6579v1.

• Star class