Tomas Brage

Very high resolution ultraviolet spectroscopy of a chemically peculiar star: Results of the chi Lupi Pathfinder project

We summarize here the results of a major eight-year investigation of the extraordinarily detailed UV spectrum of the sharp-lined, nonmagnetic, main-sequence, chemically peculiar star chi Lupi (B9.5p HgMn + A2 Vm). The UV observations are composed of 345 Angstrom of the spectrum acquired with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope at an average resolution of 0.023 Angstrom. The complete set of echelle spectrograms is presented as an atlas in a companion paper. These data were supplemented by optical-wavelength spectra obtained at the Angle-Australian Telescope. Quantitatively accurate analysis and theoretical interpretation of these data required major improvements in the accuracy and completeness of available atomic data-wavelengths, transition probabilities, hyperfine structure, and isotope shifts-for the lowest ionization states of many elements. A large, international group of theoretical and experimental atomic physicists has collaborated in this investigation, and their results are summarized or referenced in this paper. In turn, the GHRS observations of chi Lupi have become a useful source of data for atomic spectroscopy, displaying many transitions that are difficult to observe in a laboratory setting. Measured abundances or upper limits are presented for 72 ions of 51 chemical elements, spanning the periodic table. We have confirmed and refined previously identified isotopic abundance anomalies in mercury and platinum and have discovered similar isotopic anomalies in thallium and, tentatively, in lead. Large discrepancies among the LTE abundances derived, using a chemically homogeneous model atmosphere, from two or three ionization states of the same element are found to be common. In some cases these are due to departures from LTE in the ionization equilibria, but the largest such discrepancies probably result from chemical stratification within the photosphere. We find qualitative trends in the abundances of the elements that clearly signify radiatively driven diffusion and gravitational settling as the primary mechanism producing abundance anomalies. However, detailed non-LTE diffusion calculations for mercury and thallium show that there is insufficient unsaturated radiative force within the chemically enriched atmosphere to sustain the observed huge overabundances of these elements in equilibrium with gravity. Either other hydrodynamic processes, such as slow mass motions or unexpectedly strong stellar winds must assist radiation pressure in supporting the enriched material, or the observed abundance patterns simply provide a snapshot in time of a nonequilibrium, time-variable phenomenon. (Less)

Mercury in the HgMn stars chi Lupi and HR 7775

Observations of mercury lines in the HgMn stars chi Lupi and HR 7775 made with the Hubble Space Telescope Goddard High Resolution Spectrograph are presented and analyzed. In chi Lupi we find that all observed lines are consistent with the same isotopic mixture (essentially pure (204)Hg). Strong ionization anomalies are present, with UV Hg I lines being too weak and Hg III lines too strong for the abundance derived from lines of the majority ionization state, Hg rr. Observations of mercury in I;IR 7775 show less extreme isotope and ionization anomalies. We find that the ionization anomaly in the Hg I resonance lines can be plausibly explained as a non-LTE effect, but the same non-LTE calculations show that the Hg III ionization anomaly in chi Lupi cannot be explained in this way. Radiative force calculations show that the observed mercury abundance cannot be supported in the atmosphere by the radiative forces alone. We suggest that weak mixing brings mercury into the line-forming region from below the photosphere, while a wind of order 10(-14) M. yr(-1) supports a cloud of Hg III at very small optical depths. (Less)

Theoretical oscillator strengths for Sr II and Y III, with application to abundances in the HgMn-type star chi Lupi

Oscillator strengths for selected transitions of Sr II and Y III have been determined using ab initio multiconfiguration Hartree-Fock techniques. The importance of including an accurate treatment of the core-valence correlation is emphasized. The results are used to determine the abundances of Sr and Y in the chemically peculiar star chi Lupi from HST/GHRS Echelle spectra. Overabundances of 2.09 dex for Sr and 2.8 dex for Y relative to the solar abundance are derived, and an ionization imbalance of +1.1 dex is evident from abundance determinations using Y III and Y II.

The abundances of Pt, Au, and HG in the chemically peculiar HgMn-type stars kappa CANCRI and chi LUPI

Echelle mode spectra obtained with the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope have been used to determine the abundances of the heavy elements Pt, Au, and Hg in the chemically peculiar HgMn-type stars kappa Cancri and chi Lupi. The abundances were determined by fitting observed line profiles with synthetically generated spectra and are found to be enhanced relative to solar system values by between three and five orders of magnitude in both stars. The Hg isotope mixture in kappa Cancri is found to resemble the terrestrial mixture while that of chi Lupi is dominated by the heaviest isotope. As determined from multiple ionization states, the abundances place constraints upon theories attempting to explain the large surficial abundances of heavy elements. (Less)

Plunging configurations and J-dependent lifetimes in Mg-like ions

The results of configuration interaction calculations for Mg-like P IV, S V and Cl VI, concerning energy levels, oscillator strengths and lifetimes, are presented with emphasis on the isoelectronic trends. J-dependent lifetimes of 3s4p3P0 levels and energies of doubly excited states are discussed and compared with earlier results. In particular, discrepancies between earlier calculations are explained.

A Goddard High Resolution Spectrograph Atlas of Echelle Observations of the HgMn Star χ Lupi

Observations of the ultra-sharp-lined, chemically peculiar star chi Lupi taken by the Goddard High Resolution Spectrograph in echelle mode are presented. Thirty-six intervals of the spectral region between 1249 and 2688 Angstrom are covered with resolving powers in the range 75,000-93,000. Line identifications are provided, and the observed spectra are compared with synthetic spectra calculated using the SYNTHE program and associated line lists with changes to the line lists. The significance of these spectra for the chi Lupi Pathfinder Project and the closely related atomic physics effort is discussed in a companion paper.

Theoretical oscillator strengths and hyperfine structure in HgII

We present a theoretical model ion for Hg II, including oscillator strengths and hyperfine structure constants for a large number of transitions and levels. Different computational models have been used, depending on the observability of the lines and their importance in spectral synthesis of stellar atmospheres. For the resonance lines, we use highly systematic, accurate, and fully relativistic multiconfiguration Dirac-Fock methods. These methods are well suited to an accurate treatment of both the relativistic problem and the strong correlation effects in this ion. The predicted gf-values are probably accurate to within a few percent. A larger number of transitions are treated with a more flexible, but less accurate, version of the method. This is based on the idea of crosswise optimization to represent a number of states in the same calculation. The results are used in stellar atmosphere models, assuming local thermodynamic equilibrium (LTE), where a line-by-line investigation is important. A larger set of levels are treated with semiempirical methods, for use in large scale non-LTE calculations. The lower accuracy of these are well-suited to a more statistical treatment of the structure of the ion. We discuss the importance of a correct treatment of core-valence correlation and relativistic effects for predicting accurate oscillator strengths. These tend to reduce their values by as much as a factor of 2 for the resonance line.

Non-variational, spline-Galerkin calculations of resonance positions and widths, and photodetachment and photo-ionization cross sections for H- and He

A non-variational Galerkin method, completely based on B-spline methods is introduced and applied to calculations of resonance positions and widths in H- and He. The results agree very well with other theories and experiments. Photodetachment in H- and photoionization of He are also considered.

MCDF calculations for the lowest excited states in the Zn-like sequence

We report on large scale ab initio calculation for the 4s(2) S-1(0)-4s4p P-1,3(0,1,2) transitions in the zinc-like sequence, using the multiconfiguration Dirac Fock ( MCDF) method. Our attention is focused on the spin-forbidden transition 4s(2) S-1(0)-4s4p P-3(1) and the hyperfine- induced ( HPF) transition 4s(2) S-1(0)-4s4p P-3(0) for ions between Z = 30 ( Zn) and Z = 47 ( Ag). Our results provide a base for settling the existing conflicts in previous experimental and theoretical data for these transitions. We also report on rates of forbidden lines within the 4s4p P-3 term and between this term and the ground state. Accurate values for the hyperfine constants, showing excellent agreement with experimental values where available, are also given. We also give transition energies and lifetimes for the 4s2 S-1(0)-4s4p P-1(1) resonance line.

Systematic Studies of N IV Transitions of Astrophysical Importance

E1, M2, M1 and E2 rates of transitions between n = 2 levels of N IV have been calculated using the tw independent codes CIV3 and MCHF. Convergence of each of the approaches has been studied and comparisons made as the complexity of the calculations increases to include valence, core-valence and core-core correlation. The agreement between the two methods is sufficiently good to allow us to set quite narrow uncertainty bars. For the S-1-P-1 degrees resonance line, our recommended f-value is 0.609 with an estimated uncertainty of 0.002, while our recommended A-value for the S-1(0)-P-3(1) degrees intercombination line is 580 s(-1) with an estimated uncertainty of 10 s(-1).