Model evolution

As expected, a response to the hypercycle criticisms appeared, in fact in the same issue of the Journal of Theoretical Biology (Eigen et al., 1980). According to this, the Freiburg investigations refer to one particular evolution model, in which the occurrence of mutants with different, selective values is ignored. In such realistic models, the error threshold loses its importance for the stability of the wild type. If the latter reaches a finite fitness value, it can always be the subject of selection, as no rivals are present. 

We have observed 3He towards several PNe that have been selected to maximize the likelihood of 3He+ detections. First epoch observations with the GBT are discussed in [3]. Figure 2 shows a 4 a detection towards the PNe J 320 with the VLA. Both of these results are consistent with 3He/H abundances between 10 4 — 10 3 by number and standard stellar evolution models. Observations with Arecibo are planned for winter 2005. Our goal is to be able to make a connection between some of the selection criteria and a high 3He abundance. In this way we can use subsidiary measures, e.g., the N abundance, to estimate what fraction of PNe have preserved their 3He. 

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. 

We have now to go one step further and to build stellar evolution models where the transport of angular momentum will be followed self-consistently under the action of meridional circulation, shear turbulence, and internal gravity waves. In this path some important aspects still need to be clarified Can we better describe the excitation mechanisms and evaluate in a more reliable way the quantitative properties of the wave spectra What is the direct contribution of 1GW to the transport of chemicals, especially in the dynamical shear layer produced just below the convective envelope by the wave-mean flow interaction What is the influence of the Coriolis force on IGW How do 1GW interact with a magnetic field Work is in progress in this direction. 

How much the ages of young PMS object depend on the starting stellar evolution model Baraffe et al. (2002) show that the first million year(s) are very uncertain for low mass stars (the statement refers to masses O.IMq or smaller), as they... 

Practically all sophisticated stellar evolution models predict the existence of processes altering photospheric abundances on long timescales (see e.g. Pinson-neault, these proceedings). For example, Richard et al. [6] predict iron abundances in turnoff stars of NGC 6397 to be lower by 0.2 dex than in red giants. 

Red giant stars, both in the field and in globular clusters, present abundance anomalies that can not be explained by standard stellar evolution models. Some of these peculiarities, such as the decline of 12C/13C, and that of Li and 12C surface abundances for stars more luminous than the bump, clearly point towards the existence of extra-mixing processes at play inside the stars, the nature of which remains unclear. Rotation has often been invoked as a possible source for mixing inside Red Giant Branch (RGB) stars ([8], [1], [2]). In this framework, we present the first fully consistent computations of rotating low mass and low metallicity stars from the Zero Age Main Sequence (ZAMS) to the upper RGB. 

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. 

Accurate isochrones from stellar evolution modelling. 

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).

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