A Hibbert

Estimation of inaccuracies in oscillator strength calculations

In recent years, the quality and resolution of observational data, notably from the Hubble Space Telescope, has so improved that the accuracy of atomic data is frequently the limiting factor in progress. It is therefore imperative not merely that atomic data of improved accuracy are determined, but also that a realistic assessment of that accuracy is provided. Calculations of oscillator strengths are necessarily approximate. We discuss various indicators of accuracy, how semi-empirical corrections can be made to improve ab initio results, and how this data can be used to assess the level of accuracy of calculations for specific transitions. By way of illustration, these ideas are applied to set estimates of accuracy of a recent calculation of the lifetime of the 2s2p3 5S2o level of N II.

An extremely peculiar hot subdwarf with a 10 000-fold excess of zirconium, yttrium and strontium

Helium-rich subdwarf B (He-sdB) stars represent a small group of low-mass hot stars with luminosities greater than those of conventional sdB stars, and effective temperatures lower than those of subdwarf O (sdO) stars. By measuring their surface chemistry, we aim to explore the connection between He-sdB stars, He-rich sdO stars and normal sdB stars. LSIV-14°116 is a relatively intermediate He-sdB star, also known to be a photometric variable. High-resolution blue-optical spectroscopy was obtained with the Anglo-Australian Telescope. Analysis of the spectrum shows LSIV-14°116 to have effective temperature T eff= 34000 ± 500K, surface gravity logg= 5.6 ± 0.2 and surface helium abundance n He= 0.16 ± 0.03 by number. This places the star slightly above the standard extended horizontal branch, as represented by normal sdB stars. The magnesium and silicon abundances indicate the star to be metal poor relative to the Sun. A number of significant but unfamiliar absorption lines were identified as being due to germanium, strontium, yttrium and zirconium. After calculating oscillator strengths (for Ge, Y and Zr), the photospheric abundances of these elements were established to range from 3 dex (Ge) to 4 dex (Sr, Y and Zr) above solar. The most likely explanation is that these overabundances are caused by radiatively driven diffusion forming a chemical cloud layer in the photosphere. It is conjectured that this cloud formation could be mediated by a strong magnetic field.

Transition probabilities for some spectral lines of singly ionised nitrogen

Sophisticated configuration interaction wave functions have been used to obtain transition probabilities for transitions between the fine structure levels of the thirty-two lowest lying 2s22p2, 2s2p3, 2s22p3s, 2s22p3p, 2s22p3d, 2s22p4s, 2s22p4p states together with the 2s2p23s state of singly ionised nitrogen. The calculations were performed using the CIV3 code and small empirical adjustments were introduced to the diagonal Hamiltonian matrix elements in order to achieve accurate energy splittings between the levels and thereby the CI mixing coefficients. The present results are believed to be the most accurate available. Significant differences are found when the present lifetimes of the more highly excited states are compared with recent experimental data.

Oscillator strengths of transitions in the beryllium sequence

Configuration interaction wavefunctions have been used to evaluate absorption oscillator strengths, using both the length and velocity forms, for transitions between the 1s22s2p and 1s22p2 states: 3P0 to 3Pe; 1P0 to 1De; 1P0 to 1Se. It is found that the length values stabilize fairly quickly while the corresponding velocity values are more sensitive. Both L-shell and K-L inter-shell correlations are considered. The latter make small changes to length values, but alter velocity values by up to 10%. All-external configurations are important for neutral beryllium and to some extent for B II also. The 1s22p2 1S state of neutral beryllium appears to be about 0.01 a.u. above the Be+ 2s threshold. The calculations of Burke (1972) for the 1s22s2 1S to 1s22s2p1P0 transition are extended, and compared with the accurate calculations of Sims and Whitten (1973). Agreement to within 2% is achieved.

Oscillator Strengths for Transitions in O III

Energy levels and oscillator strengths have been calculated for the fine-structure transitions among the levels of the (1s2) 2s22p2, 2s2p3, 2p4, 2s22p3s, 2s22p3p, and 2s22p3d configurations of O III using the CIV3 program. The extensive configuration interaction and relativistic effects have been included while generating the wave functions. The results are compared with other recent theoretical estimates, and their accuracy is assessed.

Atomic data for opacity calculations. XXI. The neon sequence

Photoabsorption processes are studied for the ground and excited terms of the members of the neon isoelectronic sequence considered to be of significant astrophysical abundance. A two-state, LS close-coupling calculation is carried out. Oscillator strengths and total photoionization cross section data are calculated for even and odd LS terms with S=0,1;L<or=4, and an effective principal quantum number nu <or=10.

Atomic polarizabilities and polarized pseudo-states in the multiconfigurational approach. II. First row atoms and ions

For pt.I see ibid., vol.9, p.1065 (1976). Mean static dipole polarizabilities are calculated for the ground states of neutral and singly ionized atoms from Be to Ne. The polarizability is expressible in terms of the second-order energy correction in a formalism with the static dipole field being treated as a perturbation. The radial functions used in the calculation are determined by treating this second order energy as a variational functional, rather than by using the unperturbed atom variational principle. Because of this, the use of quite simple wavefunctions gives results which compare well with the much larger-scale calculations also performed.

Accurate transition probabilities for some spectral lines of singly ionized oxygen

Transition probabilities based on sophisticated configuration interaction wave functions have been obtained for transitions between the fine structure levels of the 26 lowest lying 2s22p3, 2s2p4, 2s22p23s, 2s22p23p, 2s22p23d states of singly ionized oxygen. The calculations were performed using the CIV3 code of Hibbert and small empirical adjustments were introduced to the diagonal Hamiltonian matrix elements in order to achieve accurate energy splittings between the levels. The present results are believed to be the most accurate available and, for several allowed transitions, a significant departure from LS coupling, due to relativistic effects, is found.

Wavefunctions and oscillator strengths of the beryllium iso-electronic sequence

Configuration interaction wavefunctions for the 1s22s2 1S, 1s22s2p 3P and 1s22s2p 1P states are calculated for Be I, B II, C III, N IV, O V, F VI and Ne VII and are used to calculate oscillator strengths for the 1S to 1P transition.

The 1909 Å intercombination line in C III

The configuration interaction code CIV3 is used to calculate the A-value for the 1909 A 2s2 1S0-2s2p 3Po1 intercombination line in C III. An extensive calculation of valence shell correlation is augmented by the inclusion of core polarization effects both explicitly through further configurations with one or both 1s orbitals replaced by correlation orbitals, and also by the use of model potentials. Our recommended value of 104 ± 4s−1 for the A-value lies somewhat lower than the experimentally determined value of 121 ± 7s−1. We have estimated the expected error in our work by considering the extent of the convergence of our calculation. We also calculated the resonance transition and obtain an oscillator strength in excellent agreement with experiment.