Modelling chemical evolution

The simplest model of chemical evolution is the Simple Model for the chemical evolution of the solar neighbourhood. The basic assumptions of the Simple Model are  

In the following we will adopt the formalism of Tinsley (1980) and define  

When substituted in (4.37) the equation can be solved analytically with the previous initial conditions and the solution is  

The meaning of the effective yield can be understood with the following example if Vieff /i(true yield) then the actual system has attained a higher abundance for the element i at a given gas fraction p. 

The Simple Model predicts too many stars with metallicity lower than [Fe/H]= -1.0 dex relative to observations. This is known as G-DWARF PROBLEM . However, the G-dwarf is no more a problem since several solutions have been suggested. 

---

The analysis of bright nebular emission lines of Hu regions has been the most frequent approach to modeling chemical evolution of more distant galaxies to... 

From the viewpoint of a model of prebiotic chemical evolution and of the primitive atmosphere of the Earth,174175 photosynthetic reactions of C02 were also examined, and formaldehyde with various nitrogen-containing products was obtained. 

Theoretical models of galactic chemical evolution offer varying predictions of how the gradient should change with time, so using open clusters to measure that time-dependence can provide important constraints on model parameters. 

Abstract. We present the results from our non-LTE investigation for neutral carbon, which was carried out to remove potential systematic errors in stellar abundance analyses. The calculations were performed for late-type stars and give substantial negative non-LTE abundance corrections. When applied to observations of extremely metal-poor stars, which within the LTE framework seem to suggest a possible [C/O] uprise at low metallicities (Akerman et al. 2004), these improvements will have important implications, enabling us to understand if the standard chemical evolution model is adequate, with no need to invoke signatures by Pop. Ill stars for the carbon nucleosynthesis. 

Abstract. The most recently discovered Galactic component - thick disk - still needs high-resolution spectral investigations since its origin and evolution is not understood enough. Elemental abundance ratios in the metallicity range —0.68 < [Fe/H] < —0.10 were determined in a sample of 10 thick-disk dwarfs and compared with results of other stars investigated as well as with models of thin disk chemical evolution. 

We can clearly see on this figure a spread of the [Sr/Ba] ratio. This spread is larger than the expected errors, confirming the results found by previous authors (see Honda et al. 2004 [2] and reference therein). Such a large scatter found in the [Sr/Ba] ratio can be explained by inhomogeneous models of chemical evolution which predict the existence of such a large variation (see for example Ishimaru et al. 2004). 

Abstract. We present here the results of the measurement of the sulphur abundance in very metal-poor stars. Our sample covers the [-4 -2] range of metallicity, and thus allows us to constraint the chemical evolution models and also to put some key constraints... 

Massive stars play an important role in numerous astrophysical contexts that range from the understanding of starburst environments to the chemical evolution in the early Universe. It is therefore crucial that their evolution be fully and consistently understood. A variety of observations of hot stars reveal discrepancies with the standard evolutionary models (see [1] for review) He and N excesses have been observed in O and B main sequence stars and large depletions of B accompanied by N enhancements are seen in B stars and A-F supergiants [2,3,4,5], All of these suggest the presence of excess-mixing, and have led to the development of a new generation of evolutionary models which incorporate rotation (full reviews in [1], [6], [7]). 

In a closed box or leaky-box chemical evolution model the most metal-poor stars would have formed before the majority of the Type la SN or the AGB stars... 

Abstract. We present metallicities for 487 red giants in the Carina dwarf spheroidal (dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT) spectroscopy. We find a mean [Fe/H] of —1.91dex with an intrinsic dispersion of 0.25 dex, whereas the full spread in metallicities is at least one dex. The analysis of the radial distribution of metallicities reveals that an excess of metal poor stars resides in a region of larger axis distances. These results can constrain evolutionary models and are discussed in the context of chemical evolution in the Carina dSph. 

The accretion history of a parent galaxy is constructed using a semi-analytical code. The full phase-space evolution during each accretion event is then followed separately with numerical simulations [2]. Star-formation and chemical evolution models are implemented within each satellite. The star formation prescription matches the number and luminosity of present-day galaxies in the Local Group, whereas the chemical evolution model takes into account the metal enrichment of successive stellar populations as well as feedback processes. Below we present results of a sample of four such simulated galaxy halos, denoted as Halos HI, H2, H3 and H4. 

Abstract. Observed large scatters in abundances of neutron-capture elements in metal-poor stars suggest that they are enriched a single or a few supernovae. Comparing predictions by an inhomogeneous chemical evolution model and new observational results with Subaru HDS, we attempt to constrain the origins of r-process elements. 

Although Ba and heavier elements seem to fit the solar r-process pattern, lighter elements show wide varieties [5]. In particular, a large dispersion has been found in [Sr/Ba] at low metallicity[l], suggesting that lighter elements such as Sr does not come from a universal process, which produces Ba and Eu, but from weak r-process. An inhomogeneous chemical evolution model suggests that the dispersions in [Sr/Ba] are well-explained, when weak r-process produces 60% of Sr but only 1% of Ba in metal-poor stars. Furthermore, intermediate mass elements such as Pd must provide clues to understand the weak r-process yield. 

We focus our attention on the role which the treatment of convection may play on the physical and chemical evolution of massive AGBs, and how their chemical yields depend on the efficiency of the convective model. 

Abstract. We recall the emergence of the 3He problem , its currently accepted solution, and we summarize the presently available constraints on models of stellar nucleosynthesis and studies of Galactic chemical evolution from observations of the He isotopic ratio in the Galaxy. 

Our chemical evolution model for the MW predicts [4] that the abundance gradients for different elements were almost flat in the very early phases of the thin disk formation (Fig. 2). Moreover we predict little evolution of the abundance gradients in the last 5 to 8 Gyrs. 

Given the scatter in the data (Fig. 2) and the difficulties involved in each one of these determinations, it is clear that the observations available at the present moment are not inconsistent with the two main predictions of our chemical evolution models outlined above. 

We present chemical evolution models for NGC 6822 computed with five fixed parameters, all constrained by observations, and only a free parameter, related with galactic winds. The fixed parameters are i) the infall history that has produced NGC 6822 is derived from its rotation curve and a cosmological model ii) the star formation history of the whole galaxy based on star formation histories for 8 zones inferred from H-R diagrams iii) the IMF, the stellar yields, and the percentage of Type la SNe progenitors are the same than those that reproduce the chemical history of the Solar Vicinity and the Galactic disk. 

Only one or two first stellar generations containing normal stars plus very massive stars included in the best model of PM04, produce negligible effects on the subsequent photo-chemical evolution, when either the yields of HW02 or those of UN are adopted. Therefore, these models are acceptable and we cannot assess Pop III existence nor disproved it. 

Multipopulation Models for Galactic Star Formation and Chemical Evolution. Effects of Stochastic Accretion and Collisional Stripping (Especially for Deuterium)... 

Multipopulation Models for Galactic Star Formation Chemical Evolution 379... 

---