| home-page von H.-P. Gail | contact | Deutsch |
Location and time: Seminar room ARI, Mönchhofstr 12, Mi 9.15-10.45
Begin: October, 20.
see also:
lectures in astronomy
Stellar evolution in close binary systems is much more complicated then that of single stars because mass can be exchanged between the binary components. As a result the evolution often ends with the explosion of one or both components even in cases where none of them would suffer a supernova explosion as a single star. This is important for the synthesis of heavy elements in the cosmos at one hand. On the other hand, the particular case of supernovae of type Ia supplies important standard candles for distance determinations in the universe.
The seminar talk shall give an overview over the most important types of stellar evolution in binary systems and the specific stellar types encountered in this context.
Observations in the microwave region show that the dark clouds of the interstellar medium contain a big number of often very complex molecules (formed from up to 15 atoms). This is in so far remarkable as the temperature in such clouds often is less than 10 K and the particle density, apart from local density enhancements, usually is less than 100 particles per cm3. Under normal circumstances no chemical reactions are possible under such conditions. The chemistry in such clouds is driven by the ionization due to cosmic rays and the harsh UV-radiation from cosmic sources. The products of this ion-molecule chemistry are incorporated during stellar formation into the freshly formed planetary systems and possibly supplied the earth with a first basic equipment of complex hydrocarbons.
The seminar talk shall briefly show some observational results and shall discuss the particular type of chemistry operating in molecular clouds and the results of model calculations.
During the big bang only H, He and tiny amounts of Li have been produced. All heavier elements in the present universe were synthesized by nuclear burning processes in stellar interiors or by explosive events (like supernovae). The gradual enrichment of the galactic matter is due to a permanent cycling of matter between the interstellar medium and stars. There is a continuous formation of new stars from the interstellar medium, which exist for some time and balance their radiative energy losses by burning light to heavier nuclei. At the end of their lives they return part of their initial masses together with the freshly synthesized heavy nuclei into the interstellar medium. Each stellar generation therefore is somewhat enriched with heavy elements as compared to its parent generation.
The seminar talk shall discuss the the different processes of nucleosynthesis and their role for the origin of the chemical elements.
Stars with initial masses of less than about 8 solar masses develop at the end of their life towards the asymptotic giant branch. In this phase ashes of nuclear burning processes are brought to their surface by a very extended convection zone in their envelope extending from the stellar surface deep into the central region. By this both the element abundances and the relative abundances of the isotopes of some elements change drastically. At the same time the stars loose within a few 105 years their whole outer envelope by a massive stellar wind. In the outflow most of the heavy elements condense into tiny solid dust particles which carry the specific isotopic anomalies due to the nucleosynthetic processes in the stellar interior. These grains become part of the interstellar medium from which later molecular clouds condense. If in these clods new stars are formed, the dust grains produced by dying stars which have survived all destructive processes in the interstellar medium enter into the freshly formed planetary systems and become part of the solid bodies formed in it. In our Solar System we find therefore in the fine grained matrix material of meteorites tiny (0.1 ... 10 µm) dust grains with exotic isotopic abundance ratios of certain elements, which can only originate from circumstellar shells. These presolar dust grains can be analyzed in the laboratory and give direct information on the nuclear burning processes in stars.
The seminar talk shall briefly discuss the properties and the origin of meteorites and the properties and origin of presolar dust grains found in them.
The atmospheres of Venus, Earth and Mars have a completely different composition from the gaseous component of the matter in the protoplanetary accretion disk from which our Solar System formed. A possible primary atmosphere with solar composition has completely dissipated, if it ever existed. The present atmospheres of the terrestrial planets must have been formed by subsequent processes. Presently these atmospheres are thought to be most likely formed by degassing of volatile components during impacts of big planetesimals and protoplanets during planetary growth and by continued degassing of the planetary bodies. The initial composition of the atmospheres of Venus, Earth, and Mars then should have been very similar (including the existence of early oceans). The present strong differences in their compositions and properties are thought to result from a different subsequent evolution due to different planetary masses (resulting in rapid or slow loss of some gases), to different solar distances (H2O present as vapor, water, or ice), and to the evolution of life.
The seminar talk shall discuss the processes responsible for the origin and early evolution of the atmospheres of terrestrial planets.