 Cosmological formation of low-mass objects; Thoul, Anne  ; in Astrophysical Journal (1996), 464(2), 523-538 We investigate the early formation of bound objects with masses comparable to the cosmological Jeans mass (similar to 10(5) M.). We follow the growth of isolated spherically symmetric density peaks ... [more ▼] We investigate the early formation of bound objects with masses comparable to the cosmological Jeans mass (similar to 10(5) M.). We follow the growth of isolated spherically symmetric density peaks starting from the linear perturbative regime. The initial parameters correspond to density peaks of various widths and heights in a cold dark matter cosmology. We use a one-dimensional spherical Lagrangian hydrodynamics code to follow the dynamical, thermal, and nonequilibrium chemical evolution of the gas. The system includes a collisionless dark matter component and a baryonic component composed of the nine species H, H-, H+, He, He+, He++, H-2, H-2(+), and e(-). All relevant chemical reactions between these species and their cooling mechanisms are included in the calculations. We explore the dependence of the dynamical evolution of the gas on two parameters: the initial mass scale and the initial overdensity of the system. We follow the evolution of the density, temperature, and abundance profiles within the cloud, assuming two types of central boundary conditions for the collisionless component: in one the infalling dark matter virializes through a reflection from a hard sphere, while in the other it accretes onto a central sink. We find that in both cases, radiative cooling by H-2 affects the collapse dynamics of the gas only after it has already virialized and become part of the bound object. Therefore, radiative cooling is unlikely to have triggered the initial collapse of perturbations at redshifts z>10. Nevertheless, baryonic objects with masses well below the linear theory Jeans mass (less than or similar to 10(3) M.) form at high redshifts because of shell crossing by the dark matter. Such objects could be the progenitors of a primordial population of high-mass stars in the intergalactic medium. [less ▲] Detailed reference viewed: 5 (2 ULg) ELEMENT DIFFUSION IN THE SOLAR INTERIORThoul, Anne ; ; in Astrophysical Journal (1994), 421(2), 828-842 We study the diffusion of helium and other heavy elements in the solar interior by solving exactly the set of flow equations developed by Burgers for a multicomponent fluid, including the residual heat ... [more ▼] We study the diffusion of helium and other heavy elements in the solar interior by solving exactly the set of flow equations developed by Burgers for a multicomponent fluid, including the residual heat-flow terms. No approximation is made concerning the relative concentrations, and no restriction is placed on the number of elements considered. We give improved diffusion velocities for hydrogen, helium, oxygen, and iron, in the analytic form derived previously by Bahcall & Loeb. These expressions for the diffusion velocities are simple to program in stellar evolution codes and are expected to be accurate to similar to 15%. We find that the inclusion of the residual heat flow terms leads to an increase in the hydrogen diffusion velocity. We compare our numerical results-with those obtained analytically by Bahcall & Loeb using a simplified treatment, as well as with those derived numerically by Michaud & Proffitt. We find that for conditions characteristic of the Sun, the results of Bahcall & Loeb for the hydrogen diffusion velocity are smaller than our more accurate numerical results by similar to 30%, except very near the center where the error becomes larger. The Michaud & Proffitt results differ from the numerical results derived here by less than or similar to 15%. Our complete treatment of element diffusion can be directly incorporated in a standard stellar evolution code by means of an exportable subroutine, but, for convenience, we also give simple analytical fits to our numerical results. [less ▲] Detailed reference viewed: 6 (1 ULg) |
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