Cosmic chemical evolution models

More light on cosmic chemical evolution in general is shed by large-scale hydro-dynamical cosmological simulations (Cen Ostriker 1999) based on an updated 

Cosmic chemical evolution and diffuse background radiation 

Universe model specifically 2m = 0.37, Qt = 0.037, 2a = 0.63, h = 0.7. The key lies in the dependence of star formation rate on ambient density and temperature, roughly parameterized by the relative overdensity 8 = p/(p) — 1, the change in physical density from expansion being partly compensated by the drop in ambient temperature. Galaxies and clusters of stars are deemed to be formed in a cell in the computation when three criteria are satisfied (Cen Ostriker 2000)  

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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... 

The lack of evolution in both the neutral gas and metal content of DLAs was unexpected and calls into question the notion that these absorbers are unbiased tracers of these quantities on a global scale. On the other hand, the paucity of data at redshifts z < 1, that is over a time interval of more than half of the age of the universe (Table 1), makes it difficult to draw firm conclusions and it may yet be possible to reconcile existing measurements with models of cosmic chemical evolution (Pei, Fall, Flauser 1999 Kulkarni Fall 2002). 

Fig. 9.9. Galactic chemical evolution of 6Li, according to models with and without cosmological cosmic rays, the former providing a plateau at low metallicities. After Rollinde, Vangioni-Flam Olive (2005).

Figure 8 also shows the evolution of the Li abundance in a standard model of galactic chemical evolution. In the case of Li, new data [77, 78, 79, 80, 81, 82] lie a factor - 1000 above the BBN predictions [83], and fail to exhibit the dependence on metallicity expected in models based on nucleosynthesis by Galactic cosmic rays [84, 85, 86]. On the other hand, the Li abundance may be explained by pre-Galactic Population-Ill stars, without additional overproduction of Li [87, 88]. Some exotic solutions to both lithium problems involving particle decays in the early universe have been proposed [89, 90, 91, 92, 93,94, 95,96,97,98], but that goes beyond the scope of this review. 

Plate 6. The Greenberg model of interstellar ice mantle formation and chemical evolution. The mantle grows by condensation of gas phase species onto the cold dust grains. Simultaneously, surface reactions between these species, ultraviolet radiation and cosmic ray bombardment drive a complex chemistry. These ice-mantled grains are thought to be micron sized at most. Plate reproduced with permission from (37). (See page 6 of color inserts.)... 

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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