Extreme helium star

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Extreme helium stars ( EHe stars for short ) are very rare giant stars of spectral class  A or B with a very low frequency of hydrogen in their atmospheres .

They are closely related to the R-Coronae-Borealis stars , but have a slightly higher surface temperature and do not show any deep minima caused by dust condensation.

properties

The main energy source of the stars is the burning of hydrogen , the atmosphere of stars at birth consists predominantly of hydrogen with a mass fraction of 70 percent. In the course of their development , stars can transform their hydrogen-rich atmosphere or lose it through various mechanisms, so that in extreme cases there is only one hydrogen atom for every 10,000 other atoms.

The extreme helium stars, which are predominantly supergiants with a spectral class A or B, belong to this group of hydrogen-poor stars . With around 15 stars in the visible part of the Milky Way , they are extremely rare. Due to their kinematic properties, metallicity, and galactic distribution, they belong to the bulge of the Milky Way. They achieve luminosity around 10,000 times that of the sun . Their atmospheres show an under-abundance of hydrogen, an abundance of nitrogen , carbon and sometimes oxygen .

Interpretation of the spectra

The origin of the chemical elements in the spectra of the extreme helium stars is interpreted in the literature as follows:

  • the small amount of hydrogen is a relic from the time the stars were formed
  • Calcium , titanium , sodium , manganese and nickel were not processed inside the stars, which sets an upper limit of about eight solar masses for the mass of the precursor stars .
  • The ratio (N / Fe) is proportional to (C + O + N) / Fe. This observation is typical of stars that have gone through a phase of helium burning as red giants
  • In some extreme helium stars to show elements of the s-process such as yttrium and zirconium , probably by a thermal pulse on the asymptotic giant arose

origin

First hypotheses on the formation of the extreme helium stars attempted to explain the unusual chemical composition of the atmosphere of the early supergiants as follows:

  • as red giants who have lost their entire hydrogen-rich atmosphere
  • as a result of hot bottom burning on the asymptotic giant branch
  • as a pure helium star that has already formed with an extremely low frequency of hydrogen.

None of these hypotheses can satisfactorily explain the composition of the atmosphere of the extreme helium stars across all elements. The current discussion today rather assumes a late thermal pulse or a merger of two white dwarfs .

Later thermal pulse

With a late thermal pulse , a red giant has already moved away from the asymptotic giant branch in the direction of higher temperatures when hydrogen and helium-rich material is transported from the atmosphere into the core of the star and a nuclear fusion ignites again. With a very late thermal pulse, the star has already passed the knee in the blue area of ​​the Hertzsprung-Russell diagram and is on the cooling sequence of the white dwarfs when nuclear fusions inside again ignite.

While simulation calculations for late thermal pulses produce chemical compositions that are similar to those of extreme helium stars, the observations of stars that are currently passing through a thermal pulse, such as FG Sge, V605 Aql and V4334 Sgr, show a significantly higher abundance of carbon and hydrogen than EHe stars.

Fusion of white dwarfs

Two white dwarfs in a close binary star system can lose so much orbital energy through the mechanism of radiation of gravitational waves that they merge. If it is at the white dwarfs one, which consists predominantly of helium, and an oxygen-carbon white dwarf 0.7 solar masses, so the CO can White dwarf up to about 0.3 solar masses of helium accrete . A subsequent helium burn creates a supergiant with a mass, luminosity and chemical composition like those 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 period of a few hundred years gives an estimated frequency of the EHe stars that is consistent with the observations stands.

PV Telescopii stars

Some extreme helium stars show a variability of a few tenths of a  mag and periods of a few hours. The variability is caused by radial pulsations that can reach a few kilometers per second. After their prototype PV Telescopii , the stars are called PV-Telescopii stars . Most of the PV-Tel stars are multiperiod; H. in addition, they can show long periods of tens of days. All PV-Tel stars lie in the instability strip , their vibrations are probably caused by the kappa mechanism .

Occurrence in star catalogs

The PV Telescopii stars are very rare and so the General Catalog of Variable Stars currently lists just over 10 stars with the abbreviation PVTEL , which means that only 0.02% of all stars in this catalog belong to this class.

Examples

See also

literature

  • C. Simon Jeffery: The origin and pulsations 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 .

Individual evidence

  1. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved May 12, 2019 .