Asymptotic giant branch

An AGB star (English Asymptotic giant branch ) is the developed star with about 0.6 to 10 solar masses at a late stage of development. The internal structure of stars on the asymptotic giant branch is characterized by the helium burning and hydrogen burning in shells around a core of carbon and oxygen, which occurred during the three- alpha process of helium burning. The star appears as a red giant with strong mass loss by stellar wind with variable brightness.

Development

The AGB stage is traversed by stars with an average mass, the exact mass limits depend on the metal abundance. On the main sequence of the Hertzsprung -Russell diagram, the energy takes place by hydrogen burning in the core region. If the hydrogen in the core region fused to helium, the hydrogen burning shifted in a shell around the core. In the progression of the hydrogen burning of the star is both cooler and more luminous and migrates as a red giant red giant branch of the Hertzsprung -Russell diagram up.

For sufficiently massive stars the core reaches a temperature and density, which allows the insertion of helium burning. To restore the hydrostatic equilibrium, the star in the Hertzsprung -Russell diagram to higher temperatures and lower luminosity shifts. In further development begins after the exhaustion of the helium in the core a shell firing of helium. In this case, the star is more luminous and shows on the surface lower temperatures. In the HR diagram, the AGB star approaches asymptotically the course of development the Red Giant Branch to where the name of asymptotic giant branch (English Asymptotic Giant Branch) is derived.

In contrast to the early phase on the asymptotic giant void in the thermal pulse phase (TP- AGB), the helium burning zone. Only every 10,000 to 100,000 years, there is a helium - flash, an explosive ignition of the helium firing. The thermal pulse leads to extinction of the hydrogen firing, the outer shell and a thorough mixing of the atmosphere of the red Riesens with elements that have been generated in the S- process. Furthermore, the diameter of the AGB star expanded for a period of several thousand years.

Spectrum

Red giants on the Asymptotic Giant Branch are assigned to three spectral classes:

• In the spectral class M, the bands of titanium oxide dominate
• In the spectral class C, the Swanbanden of C2 are detected. These stars are also known as carbon stars.
• In the spectral class S, the bands of the zirconia dominate

The differences in the spectra are controlled by the ratio of carbon to oxygen C O. Due to the high chemical affinity of the two elements go preferably a bond as a carbon monoxide, which is not visible in the visible spectrum. Where in the star's atmosphere, an excess of carbon, so Swanbanden form of carbon stars. The ratio, C / O <1, the bound oxygen is not included in the carbon monoxide with a compound of titanium as a titanium oxide. If C / O about 1 dominate the Zirkonoxidbanden at the S- stars, since zirconium has a stronger affinity for oxygen than titanium.

Red giants on the asymptotic giant branch show in their spectra both lithium 99Technetium. Both isotopes can be developed only recently by nucleosynthesis. 99Tc has a half-life 200,000 years and lithium is destroyed by fusion at low temperatures. Both the high carbon content as well as the detection of 99Technetium and lithium in the atmospheres of AGB stars is regarded as evidence of a dredge -up (English dredge up ) called phase. During the late helium flashes of energy transport in the atmosphere of the red giant's predominantly by convection to the helium- burning zone and therefore elements produced by s- processes are transported to the surface of the star.

Variability

All AGB stars show a variable brightness. At the beginning of the development on the asymptotic giant branch, the amplitudes are rather small and the brightness changes irregularly. The traditional classification in the course of development as AGB star goes from slow irregular variable star, semi- regular variable star, Mira star and the final OH / IR star. The first two pulse groups in the first and / or higher harmonics while the Mira- OH and / IR star have the greatest amplitude of the fundamental. With the development of the asymptotic giant the diameter of the red giant increases and thus the period of the pulsating variable stars.

At a thermal pulse, the explosive ignition of helium burning zone, reacts a AGB star with a rapid expansion and subsequent contraction after re- extinction of helium burning. The radius changes should be reflected in a rapid period change and the Mira star R Aql, T UMi, R Hya, BH Cru and W Dra are considered as candidates for a recent thermal pulse. This hypothesis is not without controversy, as no correlation between the period changes and the occurrence of secondary indicators of a thermal pulse exists as an increase in the frequency of the elements lithium and 99Technetium in the atmospheres of AGB stars.

The mechanism which sets the atmosphere of AGB stars in vibrations, the kappa mechanism as for the Cepheids. However, the radiation energy into the ionization of the hydrogen is temporarily stored, whereas the ionization of helium is most pulsating variable. The energy stored between runs as a shock wave through the extended atmosphere of the red giant's and accelerated some of the gas out of the gravitational field of the star beyond.

About 30 % of all pulsationsveränderlicher AGB stars show a superimposed modulation of Pulsationslichtwechsels, referred to as a long secondary period ( in German about long second period). This modulation occurs almost always in the form of different depths minima of cycle -to-cycle, and has a length 250 to 1400 days. The ratio of the long secondary period to the primary pulsation period in the range of 8 to 10 observation data includes both as cause a superimposed pulsation as well as elliptical or Bedeckungsveränderlichkeit by a double star nature. It is probably at the minima of the long secondary period at a light absorption in clouds of dust that have been transported by a mass output of the AGB star in a circumstellar orbit around a red giant.

Furthermore, occurs in AGB stars ellipsoidal light variation due to the distortion of the shape of the red giant's by a companion in a binary star system. This can be detected by the phase difference between the radial speed and the brightness curve. The amplitude may be up to 0.3 may be for periods between 50 and 1000 days.

Mass loss

The pulsations transport in density-wave material into the outer atmosphere of the red giant, which condenses predominantly carbides. The carbides are deposited on each other and form macroscopic dust particles that are accelerated by the radiation pressure at speeds of about 10 km / s. Collisions also the atomic constituents of the circumstellar envelope are entrained and is formed during a period of about a million years, a zone with a diameter of some 10 light years from the processed material of the AGB star to. The strongest mass loss occurs at the end of the AGB phase and reaches OH / IR stars values ​​of up to 10-4 solar masses per year. The AGB stars are surrounded by a dense shell and can be detected only in the infrared due to the high absorbance. In developed AGB stars as the OH / IR stars and Mira stars, the conditions are present to let arise a natural grain. It involves non-thermal radiation of OH, water, and silicon oxide with a U-shaped line profile at a radiation temperature of more than 106 degrees. The occupation of the molecular energy levels by absorption of infrared radiation of warm dust and the maser radiation follows the brightness variations in the infrared. With the help of the circumstellar environment of the maser red giants can be examined in detail the use of interferometry. The achievable resolution is in the range of micro- arcseconds. Since the maser radiation is pumped through the variable infrared radiation of the AGB star can be determined for red giants from measurements of the angular diameter over time the distance.

AGB stars are still in front of the novae and supernovae, the most important source for the enrichment of the interstellar medium with heavy elements and thus responsible for a higher metallicity of subsequent generations of stars. The mass loss terminates the AGB phase when the external atmosphere was dropped down to a thin hydrogen-rich layer.

Post-AGB evolution

The star leaves the asymptotic giant branch, when the atmosphere has shrunk to a value of only one hundredth solar masses by the mass loss. Then shrink the radius and the post-AGB object moves in the Hertzsprung -Russell diagram to the left to higher temperatures. The rate of development is dependent on the concentration in the core of the star mass and is 104-105 years. A post-AGB star is a giant to supergiant with a spectral class B to K and a strong infrared excess. The infrared excess is caused by the absorption and re-emission of radiation from the star in the extended circumstellar envelope which is caused by the previous mass loss. The post- AGB stars cross the instability strip on their way to higher temperatures and begin to throb again as a yellow giant. Some authors include the 89- Herculi -Stars and the UU Herculi star to the semi-regular pulsating post-AGB stars. The RV Tauri stars with their characteristic alternating deep and shallow minima are counted as post-AGB objects. The development is accelerated by a higher temperature radiation pressure driven loss of mass by the resulting S- processes due to elements to be exposed in the atmosphere.

Not all post-AGB stars evolve into planetary nebulae. A planetary nebula is an emission nebula with a characteristic diameter of about one light year in which the abgeströmte during the AGB- phase material is excited by a plurality of 100,000 K hot central star to radiation. Only serious post-AGB stars can its atmosphere fast enough using the radiation pressure drop to achieve the high temperatures required before the discarded on the asymptotic giant matter has moved too far away from the central star. An alternative path of development is when the outer atmosphere of a red giant's accelerated flows due to an interaction in a binary system during a common- envelope phase. This hypothesis also explains the frequently observed bipolar structure of many planetary nebulae.

Some post - AGB stars in the infrared show signs of warm dust. The color temperature of the dust is an indication of a close proximity to the central star and the observed energy distributions are interpreted by a binary star system as a ring of large dust particles and oxygen-rich silicates. This double star systems almost always show a large orbital eccentricity. This result is unexpected for a binary star system that has gone through a common envelope phase. Friction during passage through the joint the tracks would have to circularize atmosphere. The dust rings around the binary star system with a post- AGB star have probably formed from remnants of the common envelope, which were not accelerated to escape velocity. The orbital eccentricity could be formed by resonances between the components of the binary system and the dust ring, where energy is pumped back into the ring and acts on the orbits.

Later thermal pulse

Leave stellar evolution calculations expect that about a quarter of all post-AGB stars undergo a final thermal pulse. Since in this phase of development the atmosphere of the star has a mass of only about one hundredth of a solar mass, the explosive ignition of helium burning leads to a rapid expansion of the envelope of the star. The diameter swells again to values ​​comparable to that of a red Riesens and the temperature decreases to a value of 3000 K. In the Hertzsprung -Russell diagram of the post-AGB star from the area of ​​the central star moves planetary nebula to the red giant branch in a period of several years to decades. This rapid development is referred to as a born-again star (English born again star).

In addition to the conversion into a red giant show the evolutionary calculations an increase in the proportion of carbon and other elements from the s- process as a result of helium flash in the atmosphere of the reborn star. At this stage of stellar evolution, the variable V605 Aquilae, and FG Sagittae V4334 Sagittarii ( Sakurai's object) are counted. You have migrated within years or decades once across the Hertzsprung -Russell diagram, have been transformed from a blue object into a red giant and are in a planetary nebula, which has formed during the last phase on the asymptotic giant branch. The high carbon content in their atmospheres leads to deep minima as in the R Coronae Borealis stars. Since the helium burning goes out quickly, moves the star after the conditional by the radiation pressure loss of its atmosphere back into the field of planetary nebula central stars within a few hundred years. The low-hydrogen atmosphere is classified as a Wolf- Rayet star and the 10 % of the planetary nebula central stars with a spectral type WN or WC are considered as the successor of born-again stars considered.

Diffusion -induced nova

While it comes in late thermal pulse to a renewed ignition of helium burning in a helium flash, and the hydrogen burning by the CNO cycle, in the post-AGB phase reignite. On the Abkühlbahn the Conditions to the white dwarf, the chemical elements on separate means of gravitational separation. The result is a hydrogen- rich outer atmosphere, a helium- rich middle class and below a layer with the elements that have arisen in the helium burning. These are in particular carbon (C), nitrogen (N ) and oxygen (O). For a diffusion -induced nova can occur when a later thermal pulse, the thickness of the helium layer is greatly reduced and is mixed in the cooling of white dwarf convection hydrogen from the external atmosphere in the CNO - layer. Due to the high density, the temperatures range to a renewed ignition of hydrogen burning and there is like the late thermal pulse again later giant. Simulations show the migration of the star in the Hertzsprung -Russell diagram within a decade of a white dwarf to a yellow supergiant. A diffusion -induced nova differs from a late thermal pulse by the lack of a planetary nebula and a discharge of hydrogen-rich matter. The strange slow Nova CK Vul is considered as a candidate for a diffusion -induced nova.