Non-LTE spectral analyses of the lately discovered DB-gap white dwarfs from the SDSS

For a long time, no hydrogen-deficient white dwarfs have been known that have effective temperature between 30 kK and < 45 kK, i.e. exceeding those of DB white dwarfs and having lower ones than DO white dwarfs. Therefore, this temperature range was long known as the DB-gap. Only recently, the SDSS provided spectra of several candidate DB-gap stars. First analyses based on model spectra calculated under the assumption of local thermodynamic equilibrium (LTE) confirmed that these stars had 30 kK < Teff < 45 kK (Eisenstein et al. 2006). It has been shown for DO white dwarfs that the relaxation of LTE is necessary to account for non local effects in the atmosphere caused by the intense radiation field. Therefore, we calculated a non-LTE model grid and re-analysed the aforementioned set of SDSS spectra. Our results confirm the existence of DB-gap white dwarfs.Comment: 4 pages, 2 figures, to appear in: Proceedings of the 16th European Workshop on White Dwarf

Hellman-Feynman operator sampling in Diffusion Monte Carlo calculations

Diffusion Monte Carlo (DMC) calculations typically yield highly accurate results in solid-state and quantum-chemical calculations. However, operators that do not commute with the Hamiltonian are at best sampled correctly up to second order in the error of the underlying trial wavefunction, once simple corrections have been applied. This error is of the same order as that for the energy in variational calculations. Operators that suffer from these problems include potential energies and the density. This paper presents a new method, based on the Hellman-Feynman theorem, for the correct DMC sampling of all operators diagonal in real space. Our method is easy to implement in any standard DMC code

Quantum Monte Carlo modelling of the spherically averaged structure factor of a many-electron system

The interaction and exchange-correlation contributions to the ground-state energy of an arbitrary many-electron system can be obtained from a spherical average of the wavevector-dependent diagonal structure factor (SF). We model the continuous-k spherically averaged SF using quantum Monte Carlo calculations in finite simulation cells. We thus derive a method that allows to substantially reduce the troublesome Coulomb finite-size errors that are usually present in ground-state energy calculations. To demonstrate this, we perform variational Monte Carlo calculations of the interaction energy of the homogeneous electron gas. The method is, however, equally applicable to arbitrary inhomogeneous systems.Comment: 4 pages, 5 figure

Interaction-Induced Spin Polarization in Quantum Dots

The electronic states of lateral many electron quantum dots in high magnetic fields are analyzed in terms of energy and spin. In a regime with two Landau levels in the dot, several Coulomb blockade peaks are measured. A zig-zag pattern is found as it is known from the Fock-Darwin spectrum. However, only data from Landau level 0 show the typical spin-induced bimodality, whereas features from Landau level 1 cannot be explained with the Fock-Darwin picture. Instead, by including the interaction effects within spin-density-functional theory a good agreement between experiment and theory is obtained. The absence of bimodality on Landau level 1 is found to be due to strong spin polarization.Comment: 4 pages, 5 figure

Time-dependent Internal DFT formalism and Kohn-Sham scheme

We generalize to the time-dependent case the stationary Internal DFT / Kohn-Sham formalism presented in Ref. [14]. We prove that, in the time-dependent case, the internal properties of a self-bound system (as an atomic nuclei) are all defined by the internal one-body density and the initial state. We set-up a time-dependent Internal Kohn-Sham scheme as a practical way to compute the internal density. The main difference with the traditional DFT / Kohn-Sham formalism is the inclusion of the center-of-mass correlations in the functional.Comment: 13 pages. To be published in Phys. Rev.

Metal abundances in PG1159 stars from Chandra and FUSE spectroscopy

We investigate FUSE spectra of three PG1159 stars and do not find any evidence for iron lines. From a comparison with NLTE models we conclude a deficiency of 1-1.5 dex. We speculate that iron was transformed into heavier elements. A soft X-ray Chandra spectrum of the unique H- and He-deficient star H1504+65 is analyzed. We find high neon and magnesium abundances and confirm that H1504+65 is the bare core of either a C-O or a O-Ne-Mg white dwarf.Comment: To be published in: Proceedings 13th European Workshop on White Dwarfs, NATO Science Series, 4 pages, 1 figur

Spectral Analyses of DO White Dwarfs and PG1159 Stars from the Sloan Digital Sky Survey

SDSS (DR1 and DR2) has recently proposed 7 new DO white dwarfs as well as 6 new PG1159 stars. This is a significant increase in the known number of DOs and PG1159 stars. Our spectral analyses provide stellar parameters which can then be used to derive constraints for the evolution of H-deficient white dwarfs. A comprehensive understanding of these objects is still severely hampered by low-number statistics.Comment: 4 pages, 4 figures, 1 table, requires asp2004.sty, to appear in the proceedings of the 14th European Workshop on White Dwarfs, eds. D. Koester and S. Moehler, ASP Conf. Se