1,721,007 research outputs found
Grundsätze über die Anlagen neuer Sternwarten - Mit Beziehung auf die Sternwarte der Universität Göttingen. Von Georg Heinrich Borheck
No full textThe Göttingen Observatory, workplace of the famous scholar Carl Friedrich Gauss, is an important architectural monument. In 2005, it has been reconstructed jointly by the Georg-August-University and the Academy of Sciences in Göttingen. Since then it serves as a representative building for the University and is the seat of the Academy. With the use of the historic rooms for exhibitions this imposing building is open to the public for the first time. 200 years ago the observatory represented the recent scientific findings of its time and moved the University of Göttingen into the leading group of the foremost research institutions in Europe at this time. Also due to its architecture it has been a big leap for Georg Heinrich Borheck, architecture of the University of Göttingen. The war of the Napoleonic era disrupted the attempt of Borheck to publish his description of the construction of the Göttingen observatory in 1805. But his writing is still relevant today and will be finally published with this band for the first time accessible to a wide public. It shows the principles on which public buildings were designed then, explains the importance of the construction in art- and science-historical perspectives and informs on the plans for the restoration of the observatory in a separate article and in the preface by the President of the Georg-August University Prof. Dr. Dr. hc Kurt von Figura
Secondary stars in CVs - the observational picture
No full textRecent theoretical and observational progress has substantially improved the definition of the lower main sequence and established a new basis fur a comparison of main sequence stars and the secondaries in CVs. The evolutionary sequences of Kolb & Baraffe (1999) imply that the secondaries in many CVs are expanded compared with main sequence stars of the same mass as a consequence of unusually high mass transfer rates and/or pre-CV nuclear evolution. We show that the location of the secondaries of all well-studied CVs in the spectral type period diagram implies that they are consistent with having near-solar metallicities. We show, furthermore, that the surface brightness of K/M stars depends on gravity and metallicity and present new Barnes-Evans relations valid fur dwarfs of near-solar metallicity and the secondaries in CVs of the galactic disk population. Distances derived by the surface brightness method agree with recent measurements of the trigonometric parallaxes of a few selected systems. (C) 2000 Elsevier Science B.V. All rights reserved
The high-field magnetic white dwarf LP790-29: Not a fast rotator
No full textWe have investigated the nature of the magnetic white dwarf LP 790-29 = LHS 2293 by polarimetric monitoring, searching for short-term variability. No periodicity was found and we can exclude rotation periods between 4 s and 1.5 hour with a high confidence. Maximum amplitudes of sinusoidal variations are DeltaR < 0.009 mag and Delta V-R < 0.7% for a mean value of the R-band circular polarization of V-R = -9.1 +/- 0.3%. Combined with earlier results by other authors, our observation suggests that LP 790-29 is, in fact, an extremely slowly rotating single white dwarf and not an unrecognized fast rotator and/or disguised cataclysmic variable
Eclipse mapping of the accretion stream in UZ Fornacis
No full textWe present a new method to map the surface brightness of the accretion streams in AM Herculis systems from observed light curves. Extensive tests of the algorithm show that it reliably reproduces the intensity distribution of the stream for data with a signal-to-noise ratio greater than or similar to 5. As a first application, we map the accretion stream emission of C IV lambda 1550 in the polar UZ Fornacis using HST FOS high state spectra. We find three main emission regions along the accretion stream: (1) On the ballistic part of the accretion stream, (2) on the magnetically funneled stream near the primary accretion spot, and (3) on the magnetically funneled stream at a position above the stagnation region
Eclipse mapping of the accretion stream in UZ Fornacis
No full textWe present a new method to map the surface brightness of the accretion streams in AM Herculis systems from observed light curves. Extensive tests of the algorithm show that it reliably reproduces the intensity distribution of the stream for data with a signal-to-noise ratio greater than or similar to 5. As a first application, we map the accretion stream emission of C IV lambda 1550 in the polar UZ Fornacis using HST FOS high state spectra. We find three main emission regions along the accretion stream: (1) On the ballistic part of the accretion stream, (2) on the magnetically funneled stream near the primary accretion spot, and (3) on the magnetically funneled stream at a position above the stagnation region
Accretion physics of AM Herculis binaries
No full textWe have solved the one-dimensional stationary two-fluid
hydrodynamic equations for post-shock flows on accreting magnetic
white dwarfs simultaneous with the fully frequency and angle-dependent
radiative transfer for cyclotron radiation and bremsstrahlung.
Magnetic field strengths to 100 MG are considered. At given
B, this theory relates the properties of the emission region to a
single physical parameter, the mass flow density (or accretion rate
per unit area) . We present the normalized temperature
profiles and fit formulae for the peak electron temperature, the
geometrical shock height, and the column density of the post-shock
flow. The results apply to pillbox-shaped emission regions. With a
first-order temperature correction they can also be used for narrower
columns provided they are not too tall
A limit-cycle model for the binary supersoft X-ray source RX J0513.9-6951
No full textWe present new results of our X-ray monitoring of the transient binary supersoft X-ray source RX J0513.9-6951 in the LMC and of our re-analysis of optical light curves obtained during the MACHO project. We have covered a complete X-ray outburst cycle with the ROSAT HRI detector. From the amplitude and timescale of the soft X-ray variability, tight limits are derived for the temporal behaviour of the white-dwarf radius and the effective temperature of its envelope. A limit-cycle model is proposed to explain the observed optical and X-ray variability, the characteristic timescales of the durations of the X-ray on and off states, and those of the transitions between both states. Our observations confirm that the radius changes of the white-dwarf envelope occur on the Kelvin-Helmholtz timescale. The duration of the X-ray on and off states is compatible with the viscous timescales of the inner and outer accretion disk, respectively
Supersoft X-ray binaries: an observational update
No full textRecent observational results on supersoft X-ray binaries are reviewed, with emphasis on the galactic source QR And and on the enigmatic LMC source RX J0439.8-6809. (C) 2000 Elsevier Science B.V. All rights reserved
9856
No full textUsing XMM-Newton we have obtained the first continuous X-ray observation covering a complete orbit of the longest period polar, V1309 Ori. The X-ray light curve is dominated by a short, bright phase interval with EPIC pn count rates reaching up to 15 cts s(-1) per 30 s resolution bin. The bright phase emission is well described by a single blackbody component with kT(bb) = ( 45 +/- 3) eV. The absence of a bremsstrahlung component at photon energies above 1 keV yields a flux ratio F-bb/F-br >= 6700. This represents the most extreme case of a soft X-ray excess yet observed in an AM Herculis star. The bright, soft X-ray emission is subdivided into a series of individual flare events supporting the hypothesis that the soft X-ray excess in V1309 Ori is caused by accretion of dense blobs carrying the energy into sub-photospheric layers. On average, the flares have rise and fall times of 10 s. In addition to the bright phase emission, a faint, hard X-ray component is visible throughout the binary orbit with an almost constant count rate of 0.01 cts s(-1). Spectral modelling indicates that this emission originates from a complex multi-temperature plasma. At least three components of an optically thin plasma with temperatures kT = 0.065, 0.7, and 2.9 keV are required to fit the observed flux distribution. The faint phase emission is occulted during the optical eclipse. Eclipse ingress lasts about 15 - 20 min and is substantially prolonged beyond nominal ingress of the white dwarf. This and the comparatively low plasma temperature provide strong evidence that the faint-phase emission is not thermal bremsstrahlung from a post-shock accretion column above the white dwarf. A large fraction of the faint-phase emission is ascribed to the spectral component with the lowest temperature and could be explained by scattering of photons from the blackbody component in the infalling material above the accretion region. The remaining hard X-ray flux could be produced in the coupling region, so far unseen in other AM Herculis systems
Multi-wavelength spectrophotometry of EX Hydrae
No full textWe present phase-resolved infrared and optical spectrophotometry of the intermediate polar EXHya supplemented by archival ultraviolet data. The spin-modulated emission from the accretion funnel and the emission from the accretion disk or ring contain substantial optically thin components. The white dwarf dominates the unmodulated flux in the ultraviolet and is identified by numerous absorption lines. Metal absorption in the accretion curtain may add to the observed spectral features. The secondary star is of spectral type M4 +/- 1 and is detected by its ellipsoidal modulation. We derive a distance of 65 +/- 11 pc which makes EX Hydrae one of the closest cataclysmic variables with a known distance. The luminosity derived from the integrated overall spectral energy distribution is 3 x 10(32) erg s(-1). The accretion rate of 3 x 10(15) g s(-1) (for an 0.6 M-. white dwarf) is in reasonable agreement with the rates expected from angular momentum loss by gravitational radiation and from the observed spin-up of the white dwarf
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