We have investigated in detail the uncertainties in deriving beryllium abundances for metal-poor stars from the BeII resonance doublet at λ 3131A. These uncertainties are related to the quality of the observations, the physical data used, and the adopted stellar parameters. We have taken into account the NLTE effects on the lines formation for the

We present a spectroscopic analysis of the O9.7 Iab star HDE 226868, which is the optical counterpart of Cygnus X-1. We use this extreme supergiant to test the differences in the stellar parameters derived using plane parallel hydrostatic model atmospheres and spherical non-hydrostatic models ("Unified Model"). We find that the difference is

In the framework of the morphological survey of northern sky planetary nebulae, in progress at the IAC we investigate in detail those nebulae that show peculiarities or that have not been studied before. By means of narrowband imaging and accurate two-dimensional spectroscopy we analyzed the peculiar nebula M1-75. Its morphology is very complex

By describing the dynamical evolution of a test charged particle in the presence of an electromagnetic field and a succession of infinitesimal Lorentz boosts and rotations it is possible to obtain the Lorentz force of electrodynamics. A consequence of this heuristic derivation at the classical level is that, given the existence of electric and

Intermediate-resolution optical spectra of the X-ray transient J0422+32, obtained during the 1993 December mini-outburst, show a strong modulation in radial velocities and equivalent widths of all the emission lines, with a period of 0.2127+/-0.0013d (5.1h). This modulation is in good agreement with the photometric period of 0.2121d reported during

Bolometric fluxes for 118 dwarfs and subdwarfs with 4000K less than Teff less than 7000K and +0.5 less than (Fe/H) less than -3.5 are derived by integrating UBVRIJHK photometry. For these stars (typical of the galactic and globular cluster main sequences) at least 80% of the energy emitted is measured by those bands. A grid of theoretical model