October 18, 2010, 12:00 pm - 12:30 pm
October 18, 2010, 12:00 pm - 12:30 pm
Stellar Opacities and Atomic Processes
Werner Eissner (Universität Stuttgart)
We shall focus on key atomic processes contributing to the bulk property of monochro- matic opacity кν (at photon energy hν). This quantity is a cross section per unit mass of plasma, as it appears in a damping or attenuation factor exp(-кνρ·x) of the light intensity with penetration depth x, where the matter density ρ is a function of chemical composition. In a stationary plasma temperature controls further decomposition of ρ into ionization and excitation stages. Since an atom or ion offers a scattering cross section of order α·πa02 or Mb = 10-18 cm2 to electromagnetic radiation, the hydrogen mass of 2·10-24g leads to к = 2·10-6cm2/g, which represents sort of unit value, typical results for chemical mixtures falling into the range of cm2/kg and peaks into the cm2/kg range. So on a microscopic scale we are dealing with electric dipole radiation exciting an ion from the ground state (or a thermally excited state) to an energetically higher atomic state. Below the respective ionization limit such a state is still bound, and we speak of line absorption. And if the photon energy suffices an electron is ejected into a continuum state, a process known as photoionization — expected to be more important for the opaqueness of a plasma, if only for its vast energy range. Modern approaches treat both processes as essentially the same: initially all electrons of the ion bound, finally the affected electron either still bound or free as a ‘continuum’ electron. They exploit elaborate computer programs such as the Breit-Pauli R-matrix code — and sizable data archives to keep results for processing. The concept looks simple enough when applied to the H or He iso-electronic sequence. But in a stellar atmosphere we have a background of medium ionized iron modified by elements up to oxygen, a key topic of the current solar abundance debate, and of some selected ‘metals’. Inner shell excitation – as distinct from exciting an outer or valence electron – contributes decisively to opacities. Resonance structure due to intermediate capture of the ejected electron in an excited ‘core’ state and core excitation can vastly enhance a photoionization cross section.