Theodor Schmidt-Kaler

The spiral structure of our Galaxy— A review of current studies

A review of present knowledge of the spiral structure of the Galaxy is given, concentrating on work published since 1970. The principal methods of determining spiral structure in the Galaxy are described and the value of different spiral tracers is assessed. From direct mapping, spiral features within about 4 kpc from the Sun can at present be located and in regions of low absorption up to 10 kpc distance. The main features are the inner (−I) arm from Sagittarius to Carina at a distance of 1.7 kpc, the Cygnus (local) arm with the Orion Spur, the Perseus (+I) arm at a distance of 2.4 kpc, and the arm −II at 4 kpc distance. Wolf-Rayet stars and stellar rings are the only far-reaching optical tracers. The structure indicated by the Wolf-Rayet stars, as based on a revised luminosity calibration, agrees very well with that given by the stellar rings, suggesting a late Sb galaxy. All-sky photographs of the Milky Way in the colours UBVR yield the classification Sb(−b+) I-II. Star polarizations as well as characteristics of dark nebulae (studied on all-sky photographs) are predominantly due to rather local structure. The kinematic approach to deriving spiral structure is plagued by the difference in the kinematics of the young stars and of the gas, and the problem of crowding in velocity space. The second part of this review deals with the internal structure of spiral features. The extension normal to the plane is very considerable. A wave-like structure in the Z-dimension (“shingles”) with about 1.4 kpc wavelength and only 70 pc thickness is observed. It can be quantitatively explained as a density wave normal to the plane, excited by a two-stream instability due to the relative flow of disc and extreme population I stars. In the third part some remarks have been made supplementing this review by considering theoretical possibilities to explain spiral structure apart from the density-wave theory, and summarizing some facts of observation left unexplained by the latter. In addition, various observations present strong evidence for expanding motions in our Galaxy, from about 130 km/sec at 0.3 kpc to about 15 km/sec at 5 kpc from the centre. A density-wave theory using the long-wave mode is sketched. It gives a better fit than the short-wave mode to the optical and to Weavers HI data, and allows transport of energy and momentum from the inner regions since the wave is outward-going from the material arms which are formed as trailing synchrones by continuous ejection from the nucleus.

Telescope costs and cost reduction

The prices of more than 60 telescopes of the most diverse types and sizes are analyzed according to telescope aperture. Actualized prices in 1995 were calculated taking inflation into account, as well as fluctuations in the DM exchange rate. We discuss the dependence of prices on factors like size, mass, optical system, number of construction parts (similarity laws), as well as on special technical requirements. The relationship between telescope aperture and costs shows interesting differences between the classical astronomical telescopes and the new, large telescope projects. The number of repeated construction parts and the number of realized instruments with the same design is of great importance. Cost reductions are best achieved in the conception and design phase of telescope development.

TIME ANALYSIS OF THE CO COMA OF COMET P/HALLEY BY IMAGE PROCESSING TECHNIQUES

Photographic and photoelectric observations of comet P/Halleys ion gas coma from CO+ at 4250 Å were part of the Bochum Halley Monitoring Program, conducted from 1986 February 17, to April 17 at the European Southern Observatory on La Silla (Chile).In this spectral range it is possible to watch the continuous formation, motion and expansion of plasma structures.To observe the morphology of these structures 32CO+ photos (glass plates) from P/Halleys comet have been analysed. They have a field of view of 28°.6× 28°.6 and were obtained from 1986 March 29, to April 17 with exposure times between 20 and120 minutes. All photos were digitized with a PDS 2020 GM (Photometric Data System) microdensitometer at the Astronomisches Institut derWestfälischen Wilhelms-Universität in Münster (one pixel= 25 µm × 25 µm ≈ 46′.88×46′.88). After digitization the data were reduced to relative intensities, and the part with proper calibrations were also converted to absolute intensities, expressed in terms of column densities using the image data systems MIDAS (Munich Image Data Analysis System; ESO – Image Processing Group, 1988) and IHAP (Image Handling And Processing; Middleburg, 1983).With the help of the Stellingwerf-Theta-Minimum-Method (Stellingwerf, 1978) a period of (2.22 ± 0.09) days results from analysis of structures in the plasma-coma by subtracting subsequent images. This method is also compared with the Fourier method. There may be a second cycle with a period of about 3.6 days. The idea behind subtracting subsequent images is that rotation effects are only 10% phenomena on gas distribution. Difference images are than used to suppress the static component of the gas cloud.

Black hole tunnel phenomenon

The potentials of spin-weighted wave equations in various Kerr-Newman black holes are analyzed. They all form singular potential barriers at the event horizon. Applying the WKB approximation it is shown that no particle can tunnel out of the interior of a static black hole. However, photons inside a nonextremely rotating Kerr black hole may tunnel out into the outer space, whereas neutrinos, electrons, and gravitons may not. If the rotation is extremal, any particle may tunnel out, under restrictive conditions. It is unknown whether photons and gravitons may tunnel out if the black hole is charged and rotating.

Seeing Conditions at Sutherland. Results of the 1992/93 seeing campaign

In order to decide whether the seeing conditions at SAAO/Sutherland justify the erection of a 3.5 m telescope and also to compare Sutherland with the Gamsberg/Namibia site, a seeing campaign covering 15 months has been carried out. For direct comparison with the results of the seeing campaign at Gamsberg twenty years before the same QUESTAR telescope was employed. The seeing is determined by the scattering of the star-trail exposed on a film in the focal plane of the telescope. The campaign commenced in February 1992. Up to May 1993, data for 204 nights, that is 47.3% of the total number of nights, were collected. Due to wind speeds above 30 km h-1, 25 out of the 204 nights were not considered in the final reduction. The useful 179 nights are evenly distributed over the campaign period. The median seeing value for the whole period is σ = 0″.52. There are differences during the year: the best season gives σ = 0″.42, the worst σ = 0″.67. Each night was divided into three intervals, although data for each of the three intervals were not always available. Generally, there is an improvement in the seeing during the course of a night. The results are compared to the seeing values of Gamsberg/Namibia and ESO/La Silla.

Relativistic Rotational Darkening of Lightlike Radiation and Von Zeipel's Theorem for Radially Emitting Spheroids

It is shown that an outgoing null radiation field in the outer space of a Kerr-Newman black hole is darkened by the rotation of the black hole. This rotational darkening is calculated for a spheroid emitting null radiation normally to its surface, yielding the von Zeipel-like effectthat the equatorial region is darkened more strongly than the polar regions.This effect is not confined to the case of black holes but is also observable for relativistically rotating fluid spheroids such as atmospheres of pulsars or neutron stars. Moreover, application to Hawking radiation suggests that the black hole cannot be viewed as a classical black body but that the Hawking radiationis a global geometric effect.

RUBIKON: a new photon-counting multichannel detector system for spectroscopy

The RUBIKON is a photon-counting multi-element detector system of the Digicon type, using a Spectracon with an S20- photocathode and a linear silicon diode array of 512 elements of 38 by 500 micrometer each, plus two 200 by 200 micrometer diodes at each end for adjusting, each diode having its own charge-coupling amplifier. The fast, low-noise electronic read-out system includes computer control of the spectrograph and data pre-reduction. Spectra obtained with the RUBIKON attached to the 61 cm-Bochum telescope demonstrate the high sensitivity and the large dynamic range.

Site selection for an astronomical observatory on Madeira

Possible sites for an astronomical observatory have been explored. Maritime stable air is expected to reach Pico de Paul (1640 m) almost perfectly unaltered, 4 km windward from the sea, providing there is extremely good seeing. About 62% of the nights in summer and 41% of the nights in winter are expected to be clear. Seeing measurements have been started.

The Optical Science and Technology Centre at Bochum — A New Partner for Observational Astronomy

The activities of the Optical Science and Technology Centre (Optikzentrum) in developing and applying astronomical technologies are described. Examples are given of research and development projects, design and construction, new technologies, new materials and consulting and support.