Hot bottom burning

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Hot Bottom Burning or Envelope Burning is a phase in stars on the asymptotic giant branch in which the outer convection zone extends into the shell near the core with active core fusions . As a result, elements are transported into the atmosphere of the stars that are otherwise not to be found in red giants .

Energy is transported in stars either by convection or by radiation . Since convection normally only occurs in the core or only in the outer atmosphere, the elements generated in the nuclear reactions in the interior are by default not transported into the photosphere and thus cannot be analyzed by spectroscopy .

In later stages of development of medium-mass stars, shells develop around the core, in which hydrogen or helium burning takes place. In the case of stars on the asymptotic giant branch, the zone in which the energy transport mainly takes place by convection extends temporarily from the photosphere to thermonuclearly active shells. This z. B. freshly synthesized lithium transported into the atmosphere of red giants. Atmospheric lithium is usually an indication of the early age of stars, as lithium is destroyed by thermonuclear reactions at temperatures below the threshold of hydrogen burning and thus before reaching the main sequence .

Hot bottom burning is not the only phase in which material synthesized in the star itself is transported into the photosphere. If the convection zone extends into a red giant shell, which has since been extinguished, this is known as dredge up (i.e. dredging). The spectroscopic detection of technetium in red giants, the most stable isotope of which decays with a half-life of 4 million years , is mostly associated with the third dredge up. The distinction depends heavily on the parameters used in star models such as convective overshooting .

Individual evidence

  1. ↑ DA Garcia-Hernandez, O. Zamora, A. Yagüe, S. Uttenthaler, AI Karakas, M. Lugaro, P. Ventura, DL Lambert: Hot bottom burning and s-process nucleosynthesis in massive AGB stars at the beginning of the thermally -pulsing phase . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.2134v1 .
  2. ↑ Marcella Di Criscienzo, Flavia Dell'Agli, Paolo Ventura, Raffaella Schneider, Rosa Valiante, Fabio La Franca, Corinne Rossi, Simona Gallerani, Roberto Maiolino: Dust formation in the winds of AGBs: the contribution at low metallicities . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.7120v1 .
  3. ^ Stefan Uttenthaler et al .: Lithium destruction and production observed in red giant stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.2759v1 .
  4. ↑ Paola Marigo, Alessandro Bressan, Ambra Nanni, Leo Girardi, Maria Letizia Pumo: Evolution of Thermally Pulsing Asymptotic Giant Branch Stars I. The COLIBRI Code . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.4485v1 .