D. Groote

Wind circulation in selected rotating magnetic early-B stars

The rotating magnetic B stars are a class of variables consisting of He-strong and some Cep stars which have oblique dipolar magnetic elds. Such stars develop co-rotating, torus-shaped clouds by channeling wind particles from their magnetic poles to circumstellar regions centered around the plane of their magnetic equators. The rotation of the cloud-star complex permits the study of absorptions from the cloud as it occults the star. In this paper we describe a quantitative analysis of archival IUE data to map the properties of these clouds over four stars (HD 184927, Ori E, Cep, and HR 6684). By computing spectral synthesis models for these stars, we nd that only Cep has a solar-like metallicity. Our analysis also shows that the metal composition across the surfaces of all these stars is at least approximately homogeneous. Using the Hubeny code CIRCUS, we demonstrate that the periodic variations of broad-band ultraviolet continuum fluxes can be explained fully by the absorptions of the co-rotating cloud. We show next that among selected lines, those arising from low-excitation states are selectively aected by cloud absorption and turbulence. Our analysis also quanties the cloud temperatures and column densities required to match the absorptions of a number of weak to moderate strength resonance lines. These temperatures increase with the ionization potential of the parent ions of these various lines, a result which is consistent with radiative equilibrium models in which temperature increases with proximity to the stars surface. Although these attributes appear stable from one epoch to another, dynamic processes are nonetheless at work. Both the strengths and widths of resonance lines at occultation phases indicate the presence of a turbulence in the cloud which increases inwards. The spectroscopic hallmark of this stellar class is the presence of strong C IV and N V resonance line absorptions at occultation phases and of redshifted emissions of these lines at magnetic pole- on phases. The emissions have characteristics which seem most compatible with their generation by high-energy shocks at the wind-cloud interface, as predicted recently by Babel (1998).

Multicomponent radiatively driven stellar winds - IV. On the helium decoupling in the wind of

We study the possibility of the helium decoupling in the stellar wind of sigma Ori E. To obtain reliable wind parameters for this star we first calculate NLTE wind model and derive wind mass-loss rate and terminal velocity. Using corresponding force multipliers we study the possibility of helium decoupling. We find that helium decoupling is not possible for realistic values of helium charge (calculated from NLTE wind models). Helium decoupling only seems possible for a very low helium charge. The reason for this behavior is the strong coupling between helium and hydrogen. We also find that frictional heating becomes important in the outer parts of the wind of sigma Ori E due to the collisions between some heavier elements and the passive components - hydrogen and helium. For a metallicity ten times lower than the solar one, both hydrogen and helium decouple from the metals and may fall back onto the stellar surface. However, this does not explain the observed chemical peculiarity since both these components decouple together from the absorbing ions. Although we do not include the effects of the magnetic field into our models, we argue that the presence of a magnetic field will likely not significantly modify the derived results because in such case model equations describe the motion parallel to the magnetic field.

\mathsf{\sigma}

The Hamburg Quasar Survey is carrying out an objective-prism survey on Schmidt plates taken at the Spanish-German Astronomical Centre (DSAZ) on Calar Alto/Spain. We use a 1.7 deg objective-prism providing unwidened spectra with a dispersion of 1390 A/mm at Hy on hypersensitized KODAK IIIa-J plates. The field size is 5.5 x 5.5 deg. For each field, two prism plates are taken to improve the recognition of faint spectra. A direct plate is taken to determine accurate positions, and to recognize overlaps and extended objects. The coverage of the extragalactic fields up to 1993 is given in Engels et al. (1993).

Orionis E

We report about the progress of two ongoing wide-angle survey projects: the Hamburg Quasar Survey (HQS) in the northern hemisphere, and the Hamburg/ESO survey (HES), its southern counterpart. Prime scientific goals of these projects are: to compile large samples of high-redshift (1.5 ≲ z ≲ 3.2), bright (B≲18) QSOs suitable for high-resolution spectroscopy; to provide targets with unabsorbed lines-of-sight for ultraviolet spectroscopy with HST; to discover new gravitationally lensed systems; to directly determine the local luminosity function of QSOs; to study evolution of the most luminous part of the QSO population; to construct a spectroscopic database for the identification of ROSAT X-ray sources.An ESO Key programme dedicated to an Homogeneous Bright QSO Survey (HBQS) has been completed. 327 QSOs (Mb<-23, 0.3<z<2.2) have been selected over 555 deg^2 with 15<B<18.75. For B<16.4 the QSO surface density turns out to be a factor 2.2 higher than what measured by the PG survey, corresponding to a surface density of 0.013+/-.006 deg^{-2}. If the Edinburgh QSO Survey is included, an overdensity of a factor 2.7 is observed, corresponding to a density of 0.016+/-0.005 deg^{-2}. In order to derive the QSO optical luminosity function (LF) we used Monte Carlo simulations that take into account of the selection criteria, photometric errors and QSO spectral slope distribution. The LF can be represented with a Pure Luminosity Evolution (L(z)\propto(1+z)^k) of a two power law both for q_0=0.5 and q_0=0.1. For q_0=0.5 k=3.26, slower than the previous Boyles (1992) estimations of k=3.45. A flatter slope beta=-3.72 of the bright part of the LF is also required. The observed overdensity of bright QSOs is concentrated at z<0.6. It results that in the range 0.3<z<0.6 the luminosity function is flatter than observed at higher redshifts. In this redshift range, for Mb<-25, 32 QSOs are observed instead of 19 expected from our best-fit PLE model. This feature requires a luminosity dependent luminosity evolution in order to satisfactorily represent the data in the whole 0.3<z<2.2 interval.