Stellar model calculations

Explosive astrophysical environments invariably lead to the production of nuclei away from stability. An understanding of the dynamics and nucleosynthesis in such environments is inextricably coupled to an understanding of the properties of the synthesized nuclei. In this talk a review is presented of the basic explosive nucleosynthesis mechanisms (s-process, r-process, n-process, p-process, and rp-process). Specific stellar model calculations are discussed and a summary of the pertinent nuclear data is presented. Possible experiments and nuclear-model calculations are suggested that could facilitate a better understanding of the astrophysical scenarios. 

Comparison of our stellar abundance data with [Th/Eu] vs. [Fe/H] curves obtained from the GCE models, calculated for four different Galactic disk ages - 6, 9, 12, and 15 Gyr Tq = 8.2 1.9 Gyr. The uncertainty is relative only to the abundance ratio uncertainties, not considering the uncertainties intrinsic to the GCE model itself, which are difficult to evaluate. 

Our multi-level carbon model atom is adapted from D. Kiselman (private communication), with improved atomic data and better sampling of some absorption lines. The statistical equilibrium code MULTI (Carlsson 1986), together with ID MARCS stellar model atmospheres for a grid of 168 late-type stars with varying Tefj, log g, [Fe/H] and [C/Fe], were used in all Cl non-LTE spectral line formation calculations, to solve radiative-transfer and rate equations and to find the non-LTE solution for the multi-level atom. We put particular attention in the study of the permitted Cl lines around 9100 A, used by Akerman et al. (2004). 

Where Ay (IS) is the correction for the ionization structure [6] by model calculations, depending on a star effective temperature (Teft) and dust y+= N(He+)/N(H+), y = N(He)/N(H). Correction for a stellar nucleosynthesis He production was either using Y Z linear dependence with the slope value of [3] or for distant source Ay = -(0.5 0.5)% being accepted as half of [2] calculation. 

Programs to construct synthetic models combining stellar evolutionary calculations with flux or spectrum libraries include PEGASE, accessible via http //www2.iap.fr/users/hoc/PEGASE.html. 

A good account of the chemical evolution of the Galaxy, giving details of the phases of stellar evolution and model calculations, is provided in the monograph by Matteucci (2001). 

Mishustin et al. (2003). The results of calculation of hadronic (H) and quark stellar models (SS, QC and MC) in Hard-Dense-Loop approach are represented in Fig. 9 from Thoma et al. (2003), where one of the model parameters is changing. The free quarks exist in the state of deconfined quarks, and the density when deconfined quarks become energetically preferable is also rather indefinite (Berezhiani et al., 2003). 

By coupling the nuclear reaction network to stellar models, we may calculate the compositions resulting from these nuclear processes, under any imaginable... 

Three important parameters enter the model calculations the mass loss rate M, the stellar radius R and the temperature parameter T . The fit of the helium line profiles of HD 50896 requires a final wind velocity of 1700 km/s. Hence, we now calculate a small grid of models in the appropriate range of R and T and with the specially adapted v. The mass-loss rate is kept at log (M/(M /yr)) = -4.4 as an arbitrary choice. The results are presented in the form of contour lines in the log T, -log R,-plane. Those of the contours which match the observed equivalent widths or peak intensities are extracted and yield a "fit diagram". We obtain a well-defined intersection region centered about R = 2.6 R, T = 60 kK (hereafter quoted as "model B"). °... 

Three different structures are found for stellar models with identical luminosity, total mass, core mass, and core radius as the effective temperature is varied. Two of these solutions can correspond to effective temperatures in the range pertinent to SK -69 202. This implies that models for the progenitor of SN 1987A will prove very sensitive to physical assumptions and numerical treatment in structural and evolutionary calculations. 

An estimate of the iron core mass before collapse is found as follows. We need 1.6 to 1.7 M0 to explain the 12 second signal and a few tenths of a solar mass for the accretion phase luminosity. From a series of stellar evolution calculations of stars producing different iron core masses by Weaver and Woosley (private communication), we find that for models with iron core masses less than 1.5 M0 the density exterior to the core falls so rapidly with radius that appreciable accretion could occur not in a few seconds. For high mass iron cores the density doesn t fall off... 

Another major and as yet unresolved issue centers upon precisely why SK-202-69 was a blue supergiant, and not a red one. This issue has been recently reviewed by Woosley (1987) and will be briefly summarized here. The essential problem is that there exist multiple solutions to the structure equations for the stellar atmosphere (see also Wheeler, this volume). Two stars having the same helium core mass and only slightly different luminosities, for example, can have radically different envelope structures, either a convective red supergiant or one that is radiative and blue (Woosley, Pinto, and Ensman 1987). There are several physical parameters that may break this symmetry and cause the star to chose one solution and not the other. Among them axe metallicity, (extreme) mass loss, and the theory of convection used in calculating the stellar model. 

Table 5. Surface mass fractions of various isotopes in stellar evolution models in the initial mass range 15M0 < Minitiai < 50M (Langer Henkel 1995) at the pre-SN stage. The pre-SN configuration is also indicated, where RSG means red supergiant and WC stands for Wolf-Rayet star of the carbon sequence. The last column gives the initial abundances used in the stellar evolution calculations.

A unified stellar atmosphere/wind model calculation has been carried out for the AF star, showing the brightest Hel emission (Najarro et al. 93). The He and H (Bry and Pa) lines as well as the continuum brightness... 

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. 

We then use a Feautrier scheme [4] to perform spectral line formation calculations in local thermodynamic equilibrium approximation (LTE) for the species indicated in table 1. At this stage we consider only rays in the vertical direction and a single snapshot per 3D simulation. Abundance corrections are computed differentially by comparing the predictions from 3D models with the ones from ID MARCS model stellar atmospheres ([2]) generated for the same stellar parameters (a microturbulence = 2.0 km s-1 is applied to calculations with ID models). 

In models alc-a2c the predicted stellar distributions are almost indistinguishable (see Figure 1), except for the absence of stars with [Fe/H]< —3.0 in the case with Pop III stars. The reason for this resides in the fact that the Pop III phase is very short and at the same time the star formation rate is small at early stages. In the closed-box cases (models bl and b2) the difference is more noticeable since at the beginning the star formation is quite high. In both models, in fact, no stars with metallicity lower than -3.0 and -2.0, respectively, are predicted. The distribution of stars with metallicity, in turn, influences the calculation of [< Mg/H > ] and [< Fe/H > ]. For the ale and a2c models there is very little difference in these average values, whereas for bl and b2 model the [< Mg/Fe > ] varies from... 

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