Abstract.We investigate the sample of 1175 new nonmagnetic DA white dwarfs with the effective temperatures T eff ≥ 12 000 K, which were extracted from the Data Release 1 of the Sloan Digital Sky Survey. We determined masses, radii, and bolometric luminosities of stars in the sample. The above parameters were derived from the effective temperatures T eff and surface gravities log g published in the DR1, and the new theoretical M − R relations for carbon-core and oxygen-core white dwarfs. Mass distribution of white dwarfs in this sample exhibits the peak at M = 0.562 M (carbon-core stars), and the tail towards higher masses. Both the shape of the mass distribution function and the empirical mass-radius relation are practically identical for white dwarfs with either pure carbon or pure oxygen cores.

Abstract. We present in this paper a catalogue of 112 massive isolated white dwarfs, with masses M > 0.8 M . Mass determinations and other parameters of white dwarfs were compiled from the available literature. For each star we present averaged values of mass, effective temperature, logarithm of surface gravity log g, radius, distance, and the surface magnetic field for magnetic white dwarfs. The mass distribution of our sample is a slowly decreasing continuum function for masses larger than 0.9 M , with an overlapping secondary maximum at 1.04 M . We conclude that the mass distribution of known massive magnetic white dwarfs is flat, whereas nonmagnetic WDs exhibit a steeper mass distribution towards the highest masses. The secondary maximum at 1.04 M is caused exclusively by nonmagnetic white dwarfs. We note that the 4 most massive stars with masses M ≥ 1.3 M are magnetic white dwarfs. Our results show also, that the occurrence of magnetism in massive white dwarfs does not depend on the cooling age (above T eff = 5000 K).

Abstract. We report discovery of 3 new faint variables in the globular cluster NGC 288: 2 SX Phe stars and 1 contact binary. Both SX Phe variables are blue stragglers. The contact binary is located below cluster turnoff, slightly to the red of the main sequence. New photometry of 6 previously known variables is also presented. We note that 26 out of a total 43 SX Phe stars identified recently in ω Cen and NGC 288 exhibit V -band light curves with full amplitudes smaller than 0.10 mag. The sample of known SX Phe stars is likely to be significantly incomplete in regard to the low amplitude variables 1 .

Compton scattering is the dominant opacity source in hot neutron stars, accretion disks around black holes and hot coronae. We collected here a set of numerical expressions of the Compton scattering redistribution functions for unpolarized radiation (RF) , which are more exact than the widely used Kompaneets equation. The principal aim of this paper is presentation of the RF by which is corrected for the computational errors in the original paper. This corrected RF was used in the series of papers on model atmosphere computations of hot neutron stars. We have also organized four existing algorithms for the RF computations into a unified form ready to use in radiative transfer and model atmosphere codes. The exact method by Nagirner and Poutanen (1993) was numerically compared to all other algorithms in a very wide spectral range from hard X-rays to radio waves. Sample computations of the Compton scattering redistribution functions in thermal plasma were done for temperatures corresponding to the atmospheres of bursting neutron stars and hot intergalactic medium. Our formulae are also useful to the study Compton scattering of unpolarised microwave background radiation in hot intra-cluster gas and the SunyaevZeldovich effect. We conclude, that the formulae by and the exact quantum mechanical formulae yield practically the same redistribution functions for gas temperatures relevant to the atmospheres of X-ray bursting neutron stars, T ≤ 10 8 K.

Abstract.We explore photometric properties of hypothetical iron core white dwarfs and compute their expected colors in UBVRI Johnson broadband system. Atmospheres of iron core WDs in this paper consist of pure iron covered by a pure hydrogen layer of an arbitrary column mass. LTE model atmospheres and theoretical spectra are calculated on the basis of Los Alamos TOPS opacities and the equation of state from the OPAL project, suitable for nonideal Fe and H gases. We have also computed UBVRI colors of the models and determined an area on the B − V vs. U − B and B − V vs. V − I planes, occupied by both pure Fe, and pure H model atmospheres of WD stars. Finally, we search for iron core white dwarf candidates in the available literature.