AGB star

from Wikipedia, the free encyclopedia
The evolution of stars of different masses in the Hertzsprung-Russell diagram . The asymptotic giant branch, abbreviated as AGB, shows a star with two solar masses.

A AGB star ( English asymptotic giant branch ) is a developed star with about 0.6 to 10 solar masses in a late development phase . The inner structure of the 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 were created during the three-alpha process of helium burning. The star appears as a red giant with a strong loss of mass due to stellar winds with variable brightness .

development

The AGB stage is passed through by stars with an average mass, the exact mass limits depending on the metallicity . On the main series of the Hertzsprung-Russell diagram , energy is generated by burning hydrogen in the core region. If the hydrogen in the core region has fused to form helium, the hydrogen burning shifts into a shell around the core. As the hydrogen burns progresses, the star becomes both cooler and more luminous and moves as a red giant up the giant red branch of the Hertzsprung-Russell diagram.

If the stars are sufficiently massive, the core reaches a temperature and density that enables the helium to start burning . In order to restore the hydrostatic equilibrium , the star in the Hertzsprung-Russell diagram shifts to higher temperatures and lower luminosity. In the further development one starts after the exhaustion of helium in the core shell burning of helium. The star becomes more luminous and shows lower temperatures on the surface. The HR diagram, the asymptotic giant branch asymptotically approaches the course of development the red giant branch to where the name of asymptotic giant branch (Engl. Asymptotic giant branch ) originates.

In contrast to the early phase on the asymptotic giant branch, the helium zone burn goes out in the thermal pulse phase (TP-AGB). A helium flash , an explosive ignition of the helium flame, only occurs every 10,000 to 100,000 years . The thermal pulse leads to the extinction of the hydrogen burning in the outer shell and a mixing of the atmosphere of the red giant with elements that were generated in the s-process . In addition, the diameter of the AGB star will expand for a period of several thousand years.

spectrum

Red giants on the asymptotic giant branch are assigned to three spectral classes :

The differences in the spectra are controlled by the ratio of carbon C to oxygen O. Due to the high chemical affinity , the two elements preferentially bond as carbon monoxide , which is not visible in the visible spectrum. If there is an excess of carbon in the star's atmosphere, swan bands of the carbon stars are formed. If the ratio C / O <1, the oxygen not bound in the carbon monoxide forms a compound with the titanium as titanium oxide. If C / O is approximately 1, the zirconium oxide bands dominate in the S-stars, since zirconium has a stronger affinity for oxygen than titanium.

Red Giants on the asymptotic giant show in their spectra both lithium and 99 technetium . Both isotopes may have been created by nucleosynthesis only recently . 99 Tc has a half-life of 200,000 years and lithium is destroyed by nuclear fusion at low temperatures. Both the high carbon content and the evidence of 99 technetium and lithium in the atmospheres of AGB stars are seen as an indication of a phase called dredge-up. During the late helium flashes , the energy transport in the atmosphere of the red giant takes place mainly by convection up to the helium-burning zone and thus elements generated 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 changes in brightness are irregular. The traditional classification in the course of development as a AGB star runs from slowly, irregularly variable star , semi-regularly variable star , Mira star and finally to OH / IR star . While pulsing the first two groups in the first and / or higher harmonics , while the Mira and OH / IR stars the largest amplitude in the fundamental have. With the development on the asymptotic giant branch, the diameter of the red giants increases and with it the period of the pulsating variable stars .

A AGB star reacts to a thermal pulse, the explosive ignition of the helium zone burning, with a rapid expansion and subsequent 10,000 to 100,000 year contraction after the helium burning has gone out again . The changes in radius should be reflected in a rapid change in period and the Mira stars R Aql, T UMi, R Hya, BH Cru and W Dra are considered candidates for a recent thermal pulse. This hypothesis is not undisputed, because there is no correlation between the period of change and the occurrence of secondary indicators of the thermal pulse exists as an increase in the frequency of the elements lithium and 99 technetium in the atmospheres of AGB stars.

The mechanism that sets the atmosphere of the AGB stars in motion is the kappa mechanism as in the Cepheids . However, the radiation energy is temporarily stored in the ionization zone of the hydrogen , while this is the ionization zone of the helium for most pulsating variables . The stored energy runs as a shock wave through the extensive atmosphere of the red giant and accelerates part of the gas out of the star's gravitational field .

About 30% of all pulsation- variable AGB stars show a superimposed modulation of the pulsation light change, which is referred to as long secondary period. This modulation almost always occurs in the form of minima of varying depth from cycle to cycle and has a length of 250 to 1400 days. The ratio of the long secondary period to the primary pulsation period is in the range from 8 to 10. The observation data rule out both superimposed pulsation and elliptical or eclipsing variability due to binary star nature as the cause . The minima of the long secondary period are probably due to the absorption of light in dust clouds, which were carried into a circumstellar orbit around the red giant by a mass ejection of the AGB star .

Furthermore, with AGB stars, ellipsoidal light changes occur due to the distortion of the shape of the red giant by a companion in a binary star system . This can be proven by the phase shift between the radial speed and the brightness curve. The amplitude can be up to 0.3 mag for periods between 50 and 1000 days.

Mass loss

The pulsations transport material in density waves into the outer atmosphere of the red giant, where it mainly condenses to form carbides . The carbides adhere to one another and form macroscopic dust particles that are accelerated to speeds of around 10 km / s by the radiation pressure . The atomic components of the circumstellar shell are also carried away by collisions and a zone with a diameter of a few ten light years is formed from the material of the AGB star over a period of approximately one million years. The greatest loss of mass occurs at the end of the AGB phase and reaches values ​​of up to 10 −4 solar masses per year for OH / IR stars . The AGB stars are surrounded by a tight cover and can only be detected in the infrared due to the high extinction . In developed AGB stars such as the OH / IR stars and Mira stars , the conditions exist to create a natural grain . It is non-thermal radiation of OH , water and silicon oxide with a U-shaped line profile at a radiation temperature of more than 10 6 degrees Celsius. The occupation of the molecular energy levels takes place through absorption of infrared radiation of the warm dust and the maser radiation follows the brightness variations in the infrared. Thanks to the maser radiation, the circumstellar environment of the red giants can be examined in detail using interferometry . The achievable resolution is in the range of micro arc seconds . Since the maser radiation is pumped by the variable infrared radiation of the AGB star, the distance to the red giant can be determined from measurements of the angular diameter over time.

AGB stars are the most important source for the enrichment of the interstellar medium with heavy elements , even before the novae and supernovae, and are therefore responsible for a higher metallicity of subsequent generations of stars. The loss of mass ends the AGB phase when the outer atmosphere has been thrown off except for a thin, hydrogen-rich layer.

Post terms and conditions development

The star leaves the asymptotic giant branch when, due to the loss of mass, the atmosphere has shrunk to a value of only one hundredth of a solar mass. The radius then shrinks and the Post-AGB object moves to the left in the Hertzsprung-Russell diagram to higher temperatures. The speed of development depends on the mass concentrated in the core of the star and is between 10 4 and 10 5 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 arises from the absorption and re-emission of the star's radiation in the extended circumstellar envelope, which was caused by the previous loss of mass. The Post-AGB-Stars cross the instability strip on their way to higher temperatures and begin to pulsate again as a yellow giant . Some authors count the 89 Herculis stars and the UU Herculis stars among the semi-regular pulsating Post-AGB stars. The RV Tauri stars with their characteristic alternating deep and shallow minima are also counted among the Post AGB objects. The development towards higher temperatures is accelerated by a radiation pressure -driven mass loss , through which elements created by s-processes are exposed in the atmosphere.

The Bipolar Planetary Nebula ( M2-9 )

Not all Post AGB stars develop into planetary nebulae . A planetary nebula is an emission nebula with a characteristic diameter of approximately one light year , in which the matter released during the AGB phase is excited to radiation by a central star with a temperature of several 100,000 K. Only heavy Post-AGB stars can eject their atmosphere quickly enough with the help of the radiation pressure to reach the required high temperatures before the matter shed on the asymptotic giant branch has moved too far away from the central star. An alternative path of development is when the outer atmosphere of a red giant escapes accelerated due to an interaction in a binary star system during a common envelope phase . This hypothesis also explains the frequently observed bipolar structure of many planetary nebulae.

Some Post-AGB-Stars show signs of warm dust in the infrared. The color temperature of the dust is an indication of a close proximity to the central star and the observed energy distributions are interpreted as a ring of large dust particles and oxygen-rich silicates around a binary star system. These binary 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. The friction when passing through the common atmosphere should have circularized the tracks. The dust rings around the binary star systems with a Post-AGB star were probably formed from remnants of the common shell that were not accelerated to escape speed. The orbital eccentricity could be caused by resonances between the components of the binary star system and the dust ring, with energy being pumped into the ring and acting back on the orbits.

Later thermal pulse

Star evolution calculations suggest that around a quarter of all Post-AGB stars will go through a final thermal pulse. Since in this phase of development the star's atmosphere only has a mass of one hundredth solar mass, the explosive ignition of the helium flame leads to a rapid expansion of the star's shell. The diameter swells again to levels comparable to that of a red giant and the temperature drops to a value of 3000 K from. In the Hertzsprung-Russell diagram , the Post AGB star moves from the area of ​​the central stars of the planetary nebula to the giant red branch in a period of a few years to decades. This rapid development is a born-again star (Engl. Bornagain star), respectively.

In addition to the change to a red giant, the evolutionary calculations show an increase in the proportion of carbon and other elements from the s-process as a result of the helium flash in the atmosphere of the reborn stars. The variables V605 Aquilae, FG Sagittae and V4334 Sagittarii (Sakurai's object) are counted at this stage of stellar evolution . They have hiked across the Hertzsprung-Russell diagram once within years or decades, changed from a blue object to a red giant and lie in a planetary nebula that formed on the asymptotic giant branch during the last phase. The high carbon content in their atmospheres leads to deep minima like the R-Coronae-Borealis stars . Since the helium burn quickly goes out again, after the loss of its atmosphere due to the radiation pressure , the star migrates back to the area of ​​the central stars Planetary Nebula within a few hundred years. The hydrogen-poor atmosphere is classified as a Wolf-Rayet star and the 10% of the central stars Planetary Nebulae with a spectral type WN or WC are considered to be the successors of reborn stars.

Diffusion Induced Nova

While the late thermal pulse re-ignites the helium burn in a helium flash , in the post-AGB phase the hydrogen burn can also re-ignite after the CNO cycle . On the cooling path from the AGB to the white dwarf , the chemical elements separate by means of gravitational separation. The result is a hydrogen-rich outer atmosphere, a helium-rich middle layer and, underneath, a layer with the elements that were created during the helium burning. These are in particular carbon (C), nitrogen (N) and oxygen (O). A diffusion-induced nova can occur if a later thermal pulse has greatly reduced the thickness of the helium layer and when the white dwarf cools down, hydrogen from the outer atmosphere is mixed into the CNO layer by means of convection . Due to the high density, the temperatures are sufficient to re-ignite the hydrogen combustion and, as with the late thermal pulse, another late giant is created. Simulation calculations show the wandering of the star in the Hertzsprung-Russell diagram from a white dwarf to a yellow supergiant within a decade. A diffusion-induced nova differs from a late thermal pulse in the absence of a planetary nebula and an ejection of hydrogen-rich matter. The strange slow nova CK Vul is considered a candidate for a diffusion-induced nova.

Individual evidence

  1. ↑ A. Weigert, HJ Wendker, L. Wisotzki: Astronomy and Astrophysics: A basic course . 5th, revised and expanded edition. Wiley-VCH, Weinheim 2009, ISBN 978-3-527-40793-4 .
  2. ↑ A. Unsöld, B. Baschek: The new cosmos: Introduction to astronomy and astrophysics . 7th revised and expanded edition. Springer Verlag, Berlin 2006, ISBN 3-540-42177-7 .
  3. ↑ H. Scheffler, H. Elsässer: Physics of the stars and the sun . 2nd revised and expanded edition. Spektrum Akademischer Verlag, Heidelberg 1990, ISBN 3-86025-637-8 .
  4. ↑ James B. Kaler: Stars and their spectra. Astronomical signals from light . 1st edition. Spektrum Akademischer Verlag, Heidelberg 1994, ISBN 3-86025-089-2 .
  5. ↑ HJ habing, H. Olofsson: asymptotic giant branch stars . 1st edition. Springer Verlag, Berlin 2003, ISBN 0-387-00880-2 .
  6. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  7. ↑ K. Szatmáry, LL Kiss, Zs. Bebesi: The He-shell flash in action: T Ursae Minoris revisited . In: Astronomy & Astrophysics . tape 398 , 2003, p. 277-284 , doi : 10.1051 / 0004-6361: 20021646 .
  8. ↑ Stefan Uttenthaler et al: The evolutionary state of Miras with changing pulsation periods . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1105.2198v1 .
  9. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  10. ^ CP Nicholls, PR Wood, M.-RL Cioni, I. Soszyńsk: Long Secondary Periods in variable red giants . In: Monthly Notices of the Royal Astronomical Society . tape 399 , no. 4 , 2009, p. 2063-2078 , doi : 10.1111 / j.1365-2966.2009.15401.x .
  11. ↑ PR Wood, CP Nicholl: EVIDENCE FOR MASS EJECTION ASSOCIATED WITH LONG SECONDARY PERIODS IN RED GIANTS . In: The Astrophysical Journal . tape 707 , no. 1 , 2009, p. 573 , doi : 10.1088 / 0004-637X / 707/1/573 .
  12. ↑ CP Nicholls, PR Wood: Eccentric Ellipsoidal Red Giant Binaries in the LMC: Complete Orbital Solutions and Comments on Interaction at Periastron . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.1043v1 .
  13. ↑ CP Nicholls, PR Wood, M.-RL Cioni: Ellipsoidal Variability and the Difference between Sequence D and E Red Giants . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1002.3751v1 .
  14. ↑ Hiroshi Imai et al: Pilot VLBI Survey of SiO v = 3 J = 1! 0 Maser Emission around Evolved Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.3879 .
  15. ^ W. Nowotny, B. Aringer, S. Höfner, MT Lederer: Synthetic photometry for carbon-rich giants II. The effects of pulsation and circumstellar dust . In: Astronomy & Astrophysics . tape 529 , A129, 2011, doi : 10.1051 / 0004-6361 / 201016272 .
  16. ↑ T. Lebzelter, PR Wood: Long period variables and mass loss in the globular clusters NGC 362 and NGC 2808 . In: Astronomy & Astrophysics . tape 529 , A137, 2011, doi : 10.1051 / 0004-6361 / 201016319 .
  17. ^ E. Zsoldos: Post-V487 Cassiopeiae (HD 6474): a UU Herculis variable in the galactic plane? In: Astronomy and Astrophysics . tape 280 , 1992, pp. 177-180 .
  18. ↑ Valentina Klochkova, Vladimir Panchuk: High-latitude supergiants: anomalies in the spectrum of LNHya in 2010 . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1112.3732v1 .
  19. ^ Lee Anne Willson, Matthew Templeton: Miras, RV Tauri Stars, and the Formation of Planetary Nebulae . In: STELLAR PULSATION: CHALLENGES FOR THEORY AND OBSERVATION: Proceedings of the International Conference. AIP Conference Proceedings . tape 1170 , 2000, pp. 113-121 .
  20. ↑ Hans Van Winckel: Post-AGB Stars . In: Annual Review of Astronomy and Astrophysics . tape 41 , 2003, p. 391-427 , doi : 10.1146 / annurev.astro.41.071601.170018 .
  21. ^ H. Van Winkel: Why Galaxies Care about Post-AGB stars . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1105.2615v1 .
  22. ^ Sun Kwok: The Origin and Evolution of Planetary Nebulae . In: Cambridge Astrophysics Series . No. 31 . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-03907-9 .
  23. ↑ Tyl Dermine, Robert G. Izzard, Alain Jorissen, and Hans Van Winckel: post-AGB stars with Circumbinary discs . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.6471v1 .
  24. ^ Herbert HB Lau, Orsola De Marco, XW Liu: V605 Aquilae: a born again star, a nova or both? In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1009.3138 .
  25. ↑ Falk Herwig: Modeling the evolution of Sakurai's Object . In: Astrophysics and Space Science . tape 279 , 2002, pp. 103-113 , doi : 10.1023 / A: 1014660325834 .
  26. ↑ M. Asplund, DL Lambert, T. Kipper, D. Pollacco, MD Shetrone: The rapid evolution of the born-again giant Sakurai's object . In: Astronomy and Astrophysics . tape 343 , 1999, p. 507-518 .
  27. ↑ Marcelo M. Miller Bertolami, Leandro G. Althaus, Carlos Olano, Noelia Jimenez: The diffusion-induced nova scenario. CK Vul and PB 8 as possible observational counterparts . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1103.5455 .