Microscopic diffusion of partly ionized metals in the Sun and metal-poor stars

Abstract

Abstract. An improved microscopic diffusion in stars is presented considering in detail the partly ionized stages of metals. Besides, the influence of degenerate electron-gas and of the contribution of radiation to the total pressure has been accounted for. The solution of the diffusion equations is then performed following the scheme of Thoul et al. (1994). By defining one mean charged ion per element very few modifications are necessary to solve the improved diffusion scheme. (A portable FORTRAN routine is provided.) The change in the sound-speed profile of a solar model obtained with the new diffusion description is at most about 25% at r = 0.6 R . The biggest effect on low-mass stars is expected near the turn off, where the convective envelope is shallowest. However, only a difference of at most 40 K in the effective temperature could be observed when assuming either fully or partly ionized metals in the diffusion equation. Nevertheless, the surface metal distribution is strongly altered.

GARSTEC—the Garching Stellar Evolution Code

“…α conv is the free parameter of the convection model employed, R bcz /R the depth of the convective zone. TBL stands for (H/He/metal-) diffusion a la Thoul et al (1994), and QSchl if the improvements by Schlattl (Schlattl and Salaris 2003;Schlattl 2002) are included. The column "Comp."…”

“…While in the standard treatment of diffusion, all elements are assumed to be fully ionized, an extension of the diffusion treatment (Schlattl 2002) is available which considers the actual ionization stage of each element (if this information is provided by the EoS). Furthermore, more accurate diffusion constants can be computed from improved collision integrals with additional quantum corrections (Schlattl and Salaris 2003).…”

Gravitational settling in pulsating subdwarf B stars and their progenitors

“…Furthermore, we have not included diffusion coefficients of neutral atoms, which are due to atomic polarizability rather than Coulomb scattering. To avoid these complications, we simply assume full ionization for the calculation of the diffusion velocities, although we are aware that this might lead to a slight underestimation of the diffusion velocities (Turcotte et al 1998;Schlattl 2002). For the equation of state the ionization of H, He, C, N, O and Ne are calculated using an approximate pressure ionization model (Pols et al 1995).…”

The surface carbon and nitrogen abundances in models of ultra metal-poor stars

“…We have also tested the effect of implementing a new EOS by Irwin (2000), which covers the entire stellar structure in all relevant evolutionary phases, and closely reproduces the OPAL EOS in the common validity range (see, e.g., Schlattl 2002). The inclusion of this new EOS in the models does not modify the value of the largest critical heavy elements abundance for which the HEFM occurs.…”