Cosmic chemical evolution observations

4 Cosmic chemical evolution observations 12.4.1 High-z absorption-line systems 

An interesting feature of damped Ly-a absorbers is that their column densities (upwards of about 1 M0 pc-2) are comparable to those of the outer parts of the 

Cosmic chemical evolution and diffuse background radiation 

Damped Ly-a absorbers are well suited to abundance analysis because the large column density ensures that O and N are essentially all in their neutral state while sulphur and metals are essentially all singly ionized, with column densities leading to readily measurable spectral features. Also, the degree of depletion from the gas phase by the formation of dust is lower than in the local ISM. Some typical metallicities (based on zinc which is not seriously depleted even in the local 

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Timmes, Woosley Weaver (1995) developed a chemical evolution model of the solar neighbourhood in an attempt to account for the observed abundances of elements from H to Zn in metal-rich and metal-poor stars. The (/-process contributions were included. With their predicted yields of nB and excluding 10B and nB from cosmic ray driven spallation, they were able to reproduce the then fragmentary data on the run of the boron abundance with metallicity (see their Fig. 9) from [Fe/H] —2.5 to [Fe/H] cz 0 and including a fit to the meteoritic abundance. Newer data on the B abundances is equally... 

These stars have been of central concern in a myriad of observational and theoretical works. No wonder They indeed play a key role in many chapters of astrophysics. In particular, they influence the physical and chemical states of their circumstellar environments or of the interstellar medium through their intense radiation and mass losses during their non-explosive phases of evolution, and even more so, as a result of their final supernova explosions. They may act as triggers of star formation, are essential agents of the evolution of the nuclidic content of the galaxies, accelerate particles to cosmic ray energies, and leave neutron stars or black holes at the end of their evolution. They are also the progenitors of certain 7-ray bursts. 

Chemical models are important for the analysis of observations of molecules in the interstellar medium and to make predictions for molecules that have not or cannot be directly observed. Modem models are able to compute the evolution of the chemical composition of a mixture of gas and dust taking into account a large number of processes including bimolecular gas phase reactions, interactions with cosmic-ray particles and UV (and X-ray) photons, interactions with interstellar... 

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