Uncertainties in (E)UV model atmosphere fluxes

During the comparison of synthetic spectra calculated with two NLTE model atmosphere codes, namely TMAP and TLUSTY, we encounter systematic differences in the EUV fluxes due to the treatment of level dissolution by pressure ionization. Systematic differences may occur due to a code-specific cutoff frequency of the H I Lyman bound-free opacity. This is the case for TMAP and TLUSTY. Both codes predict the same flux level at wavelengths lower than about 1500 A for stars with effective temperatures below about 30000K only, if the same cutoff frequency is chosen. In the case of Sirius B, we demonstrate an uncertainty in modeling the EUV flux reliably in order to challenge theoreticians to improve the theory of level dissolution. The theory of level dissolution in high-density plasmas, which is available for hydrogen only should be generalized to all species. Especially, the cutoff frequencies for the bound-free opacities should be defined in order to make predictions of UV fluxes more reliable.Comment: 3 pages, 5 figure

Strain Hardening in Polymer Glasses: Limitations of Network Models

Simulations are used to examine the microscopic origins of strain hardening in polymer glasses. While traditional entropic network models can be fit to the total stress, their underlying assumptions are inconsistent with simulation results. There is a substantial energetic contribution to the stress that rises rapidly as segments between entanglements are pulled taut. The thermal component of stress is less sensitive to entanglements, mostly irreversible, and directly related to the rate of local plastic arrangements. Entangled and unentangled chains show the same strain hardening when plotted against the microscopic chain orientation rather than the macroscopic strain.Comment: 4 pages, 3 figure

Non-LTE spectral models for the gaseous debris-disk component of Ton 345

For a fraction of single white dwarfs with debris disks, an additional gaseous disk was discovered. Both dust and gas are thought to be created by the disruption of planetary bodies. The composition of the extrasolar planetary material can directly be analyzed in the gaseous disk component, and the disk dynamics might be accessible by investigating the temporal behavior of the Ca II infrared emission triplet, hallmark of the gas disk. We obtained new optical spectra for the first helium-dominated white dwarf for which a gas disk was discovered (Ton 345) and modeled the non-LTE spectra of viscous gas disks composed of carbon, oxygen, magnesium, silicon, sulfur, and calcium with chemical abundances typical for solar system asteroids. Iron and its possible line-blanketing effects on the model structure and spectral energy distribution was still neglected. A set of models with different radii, effective temperatures, and surface densities as well as chondritic and bulk-Earth abundances was computed and compared with the observed line profiles of the Ca II infrared triplet. Our models suggest that the Ca II emission stems from a rather narrow gas ring with a radial extent of R=0.44-0.94 Rsol, a uniform surface density Sigma=0.3 g/cm2, and an effective temperature of Teff=6000 K. The often assumed chemical mixtures derived from photospheric abundances in polluted white dwarfs - similar to a chondritic or bulk-Earth composition - produce unobserved emission lines in the model and therefore have to be altered. We do not detect any line-profile variability on timescales of hours, but we confirm the long-term trend over the past decade for the red-blue asymmetry of the double-peaked lines.Comment: 7 pages, 6 figures, 2 table

The rotational velocity of the sdOB primary of the eclipsing binary system LB 3459 (AA Dor)

We present an analysis of the rotational velocity of the primary of LB 3459 based on 107 new high-resolution and high-S/N ESO VLT UVES spectra. 105 of them cover a complete orbital period (0.26 d) of this binary system. We have determined an orbital period of P = 22600.702 +/- 0.005 sec, a radial velocity amplitude of A_1 = 39.19 +/- 0.05 km/sec, and T_0 = 2451917.152690 +/- 0.000005. From simulations of the He II 4686A line profile (based on NLTE model atmosphere calculations), we derive v_rot = 47 +/- 5 km/sec. We present an animation which shows the orbital movement of the binary system, its synthetic lightcurve, and compares the phase-dependent variation of the predicted with the observed He II 4686A line profile. The radius of the cool component is almost the same size like Jupiter but its mass is about 70 times higher than Jupiter's mass. Thus, from its present mass (M_2 = 0.066 M_sun), the secondary of LB 3459 lies formally within the brown-dwarf mass range (0.013 - 0.08 M_sun). It might be a former planet which has survived the previous common-envelope phase and even has gained mass.Comment: 7 pages, 11 Postscript figures, to appear in A&