Chemical evolution model

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. 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... 

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... 

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. 

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. 

Fig. 4.12. Stellar lithium abundances (log of the number per 1012 H atoms) among main-sequence stars as a function of metallicity. The full-drawn curve shows the prediction of a numerical Galactic chemical evolution model, while the broken-line curve gives the sum of a primordial component and an additional component proportional to iron and normalized to meteoritic abundance. Adapted from Matteucci, D Antona and Timmes (1995).

For a description of additional numerical Galactic chemical evolution models, as well as aspects of nucleosynthesis (e.g. from novae) not discussed in this book, see the monograph by Matteucci (2001). 

Fig. 12.14. Metallicity evolution in DLAs. Curves show predicted mean metallic-ity in the interstellar gas relative to solar predicted by chemical evolution models of Pei, Fall and Hauser (1999), Pei and Fall (1995), Malaney and Chaboyer (1996) and Somerville, Primack and Faber (2001) respectively. Data points giving column-density weighted metallicities based on zinc only (filled circles) or other elements (open circles) are plotted in the upper panel taking upper limits as detections and in the lower panel taking upper limits as zeros. Horizontal error bars show the redshift bins adopted. After Kulkarni et al. (2005).

The third parameter in chemical evolution models of the galaxy, the star formation rate, serves to define the rate at which evolution proceeds. Unfortunately, there is a great deal of uncertainty in its estimation. There is no theory of star formation worthy of the name. In decline as time goes by, the star formation rate is often assumed to be proportional to the gaseous fraction of the Galaxy, which is itself in permanent decline, or to some power of it, but less than 2. 

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 ultimate goal of any chemical evolution model is to account for the global and local metallicity within a galaxy, the gas and stellar mass distributions, and the stellar luminosity self-consistently. Thus, any discussion of abundances and chemical evolution should include a few words about observational determinations of gas and stellar masses and mass surface densities. 

Supplementary Parameters- Infall of extragalactic gas, radial flows and galactic winds are important ingredients in building galactic chemical evolution models. 

In order to include the results from nucleosynthesis into chemical evolution models we need to define the stellar yields. The stellar yield of an element i is defined as the mass fraction of a star of mass m which has been newly created as species i and ejected ... 

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