The Astronomical Observatory operates a 1.54-m and a 50-cm telescope at the European Southern Observatory, ESO, La Silla, in Chile, a meridian circle on La Palma and also telescopes in Brorfelde. All of the telescopes have run highly satisfactorily throughout 1994. Throughout the entire period of time, the 50-cm telescope has been used for Strömgren four-colour and H-beta photometry either in an observer-controlled or fully automatic mode. The observational data are transferred to the Observatory using the data-network, the Internet, immediately after the observation on the night in question. In this way the data can be reduced immediately. This means that the telescope can be operated interactively.
The 1.54-m telescope: The " DFOSC " instrument (Danish Faint Object Spectrograph and Camera) underwent a test on delivery, after which ESO will offer the instrument for regular use. ESO will be responsible for the general maintenance of this instrument.
In 1994 a new Filter-And-Shutter Unit was constructed and built. This unit makes it easy to change filter in front of the DFOSC instrument. In addition to this a new cooling unit for the 1.5-m telescope main mirror has been constructed and mounted, and thermal microturbulence is expected to be removed this way. Hereby it is expected that the telescope will produce even better images which means that even fainter astronomical objects will be observable. A new Sky Baffle has been build which effectively removes undesired scattered light when observing. Finally, a new type of cryostat has been developed and built to cool a new, highly sensitive thinned CCD with 2048x2048 pixels for the DFOSC instrument.
In 1994 the 1.54-m telescope was primarily used with a CCD camera for direct observations in selected spectral bands and also with the DFOSC spectrograph and the CORAVEL instrument for measurements of radial velocities.
The photoelectric 25-cm and 40-cm telescopes and the Schmidt telescope in Brorfelde were all running very well, and have been used in 1994. The Schmidt telescope, which is equipped with a 1024x1024 CCD from NASA, has during the past year been of great scientific importance in several research projects running in parallel and has in addition been used for test observations with a camera using adaptive optics. This camera is being developed for use on the Nordic 2.5-m telescope, NOT , on La Palma. Finally a large number of student projects were carried out (Introductory Physics, Master and Ph.D. projects).
The automated Carlsberg Meridian Circle has been in regular operation during the entire year of 1994. Observation were carried out in approximately 2800 hours on 295 nights. Of the remaining 70 nights 52 were lost because of bad weather, whereas only 18 were lost because of technical maintenance, problems with instrumentation etc. In December yet another spare computer was needed after a breakdown of the computer which controls the instrument.
The Observatory witnessed in 1994 yet again a strong growth in research areas related to the structure and evolution of the Universe, remote clusters of galaxies and also individual galaxies.
In the very early phase of the Universe - the inflationary phase - socalled quantum fluctuations occur. These fluctuations, which at later epochs leads to tiny variations in the matter density, are the seeds of galaxies and clusters of galaxies, which are formed because of the gravitational interaction. They are also responsible for the tiny fluctuations in the almost homogene cosmic background radiation. In general clusters of galaxies and individual galaxies contain much more mass than the mass which is related to the ordinary baryonic mass which we know from our immediate surroundings. The physical nature of the main part of the matter in the Universe is thus unknown. One of the possibilities is that the early quantum fluctuations resulted in primordial black holes. If this is the case then it is possible to determine the physical conditions at this very early epoch and determine the distribution of mass associated with the black holes. These studies were concluded in 1994.
A number of processes altered the spectrum of the initial fluctuations, because of the formation of oscillations prior to the transition between ionised and neutral matter (the recombination era). The traces of these oscillations are to the found in the present distribution of the clusters of galaxies and especially in the small variations in the cosmic background radiation. During 1994 special statistical filtering methods were developed, designed to determine the ionisation history of the Universe. A underlying question in whether the Universe was re-ionised by the very powerful quasars.
The Universe is not static. In fact it is expanding: the spectral lines of the galaxies are shifted towards the red end of the spectrum with an amount which is proportional to the distance to the galaxy. The redshift is here interpreted as a Doppler-effect caused by the receding motion of the galaxies. There is no convincing reason to doubt the validity of this interpretation of the measured redshifts. However, it is extremely difficult to prove that the Universe is actually expanding. According to the theory the brightness is expected to decrease stronger with redshift in an expanding Universe than in a static Universe. The tool needed to test this is the "Fundamental Plane" - a relation between the surface brightness of an elliptical galaxy and certain other properties of the elliptical galaxy. The relation makes it possible to relate the measured surface brightness to equal conditions. To do the test some 30 clusters of galaxies are observed out to a redshift sufficiently large to determine the difference in surface brightness in an expanding Universe and the static Universe. The first observations - three relatively close clusters of galaxies - have recently been carried out with the Danish 1.5-m telescope in ESO, La Silla.
Small variations in the intensity of the galaxies light distribution may in certain cases be caused by the presence of globular clusters. The identification of globular clusters have continued.
Detailed investigations of selected clusters of galaxies, which contain large amounts of hot gas (10 - 100 million degrees) have been continued. The hot gas emits x-ray radiation and is thus slowly cooled. In some clusters of galaxies the cooling time is less than the age of the cluster, and cold matter is "cooling out" on to the central dominant galaxy at a rate of up to more than 500 solarmasses per year.The radiogalaxy Hydra A is such an example. The problem is what happens to the cooled-out gas: The mass of the cold gas phases is inadequate, as is normal star formation; the occurrence of young massive stars is generally low. Therefore it is normally assumed that the main fraction of the mass is turned into low mass stars, but as these are extremely difficult to detect it is hard to verify the hypothesis. We find that the infrared radiation from Hydra A could be due to interstellar dust grains that are heated and destroyed in the x-ray gas. A model that accounts for how dust can form in connection with starformation in the cooling gas have been developed. The models explains the surprising occurrence of dust and indirectly shows that stars are forming in the cooling x-ray gas. The observations have been made both with the Danish 1.5-m telescope and the ESO telescopes at the La Silla observatory. Extensive observing programmes have in the course of the year been planned for the european ISO satellite, which is expected to be launched in 1995.
As mentioned, the quasars probably play a considerable role in the evolution of the Universe. Quasars are not a homogeneous class of objects, and a large amount of work has been done in classifying and correlating the properties of the different types. At the observatory a project aiming at a determination of the differences and similarities between spectral properties of radio-loud and radio-quiet quasars is carried out. To avoid a dependence on evolution and luminosity in the properties of the emission lines, a dependence that has plagued earlier studies, the radio-quiet quasars have been selected carefully to ensure that these have similar redshifts and luminosities as the radio-loud quasars, for which both optical and radio-data are available. The first 20 of a total of 70 objects have been observed using the Danish 1.5-m telescope, ESO telescopes and the American multiple mirror telescope.
The photometric monitoring of selected gravitationally lensed quasars with the Brorfelde Schmidt telescope and the Nordic Optical Telescope have been continued in an attempt to verify the previously measured time difference for the quasar DQS00957 and to obtain similar time differences for other candidates.
The enormous amounts of energy emitted by the quasars are usually thought to be connected to the existence of a massive black hole in the center of the galaxy. The energy is emitted by matter which is attracted to the black hole and initially form a hot rotating accretion disc around the black hole. To power this central engine about 0.1 solar mass needs to be deposited per year. This is quite a lot - a fact that makes it hard to explain this phenomenon. Especially the capture of the stars and the interaction of the stars and the black hole have been studied detailed. The tidal interactions makes the star oscillate. When the star in its elongated orbit passes the black hole, the amplitude of the oscillations will increase and the stars are disrupted. The matter then ends in the hot rotating disc. Furthermore hot rotating discs have been studied in general. A new general equation for the cooling has been derived and the maximum capture rate has been determined as a function of the viscosity of the disc.
In 1993 a search for supernovae in a precisely defined volume of the Universe out to the distance of the Virgo cluster was initiated on the Brorfelde Schmidt telescope. The project was described in the last annual report. The telescope as well as the Danish weather is well suited for this project, since large variations in brightness are expected and because a supernova will be visible in several months. Based on the experience from 1993 all galaxies observable from Brorfelde in the volume mentioned have been included. Almost 600 CCD exposures of 334 galaxies were made.
Detailed studies of binary stars in the two closest companion galaxies, the Magellanic clouds, are possible today, and can contribute with new information about the evolution of massive stars. In addition to this, the distances of the companion galaxies are determined - this continues to be a strongly debated issue in the process of establishing the cosmological distance scale. Accurate data (masses, radii, temperatures, distances) for approximately 10 binary stars are aimed for. Results for 2 of these have been found, 3 are still being analyzed, and in 1994 CCD observations were made on yet a handful of candidates. In addition to this, ultraviolet spectra with the IUE satellite of essentially all the candidates were obtained.
Finally, the phenomenon of cosmic gamma ray bursts should be mentioned, because these probably originates in very distant extragalactic objects. Satellite observations show, that burst of gamma rays are received from space, normally lasting for seconds or minutes, in some cases a few hours. A burst is roughly observed once a day, and the distribution on the sky is completely random. The reason for cosmic gamma ray bursts is still unknown and is today considered one of the big mysteries in modern astrophysics. Until now no gamma ray burst source has been identified optically . Data from the Compton Observatory satellite has been used to show that two sources had the same position on the sky. Supplementary optical observations were made with the Brorfelde Schmidt telescope. If the suggested repetition can be confirmed, the discovery will promote the understanding of the physical cause for this phenomenon.
The people working in the field of cosmology and extragalactic astronomy are: I. Artemova, J.V. Clausen, P. Diener , L. Hansen, B. Helt, H.E. Jørgensen, R.F. Nielsen, N. Kotok, I. Novikov, H.J. Fogh Olsen, H. Pedersen, P.K. Rasmussen , J. Teuber, M. Vestergaard and R. Østensen.
The Milky Way is the prototype of a spiral galaxy. As is the case for other galaxies, it has developed from a primordial cloud of solely hydrogen- and heliumgas into the complicated system of stars, gas and dust which we observe today. During this evolution, all known elements were created by nuclear reactions in the stellar interiors, distributed into the interstellar medium and partly recycled in the creation of new generations of stars. At the same time the distribution of the matter was altered to a thin disk with a central bulge, surrounded by a depleted halo. Only in our own galaxy are we able to identify the precise physical processes in this evolution through the imprints of the processes on the detailed chemical composition and motions of the stars that were created in the course of this evolution and the distribution of stars, gas and dust. These questions are the subject of a number of large and well coordinated investigations.
One of the main goals is to obtain a complete knowledge of the ages, chemical compositions and space velocities for a complete and well defined sample of approximately 15.000 F, G and K dwarf stars over the entire sky within approximately 150 light years. The required observation have been made, and in 1994 the construction of the database itself and entering of proper motions was initiated. Radial velocities are complete for a large fraction of the stars; the remaining stars are observed under an ESO programme of some 25.000 stars, which are also observed by the HIPPARCOS-satellite (ESA). A detailed chemical analysis of appr. 200 of the stars (publ. in 1993) showed the existence of a number of completely new and interesting ways to attack the problem; these have been followed up in 1994 with spectroscopic studies of strategically selected types of stars. Another part of this investigation is a new cooperative project for identification and investigation of stars with extremely low metallicity, from the very first and as yet unexamined childhood of the galaxy.
The ages and metallicities of F stars at greater distances (approx. 5000 light years) are studied photometrically. The different stellar populations is now distinguishable in stars from 150 to 5000 light years, with ages between 2x10^9 years and 15x10^9 years and with a metallicity ranging from 0.001 to 3 times solar. The chemical composition of these stars is apparently changing drastically as close as approx. 2500 light years from the Sun. A programme designed to search for very young stars in the halo has also been initiated. The main fraction of the mass of the MilkyWay is invisible and may possibly consist of compact objects (MACHOs) with a mass which may cause a measurable relativistic deflection of the light from a star which is farther away, and seen in almost the same direction. The conditions for photometric and simultaneously astrometrical measurements of MACHOs have been discussed theoretically. The center of the Milkyway contains a small, powerful radiosource, which may be caused by a black hole with a mass of a million times the Sun. While this source is readily detected in radio observations, it is difficult to identify it at infrared wavelengths. An identification is very important in order to understand the processes in the center. Therefore it is desirable to determine both the position of the radiosource and the positions of a number of infrared sources with a precision better than 0.1 arc second. We have combined observations made with the meridian telescope on La Palma and CCD frames in the near infrared made with the 1.5-m telescope on La Silla. In this way it was possible to establish a reference map of faint stars in the area around the center with an absolute precision of 0.06 ars seconds. To make a certain identification we now need good radio observations. Finally the nearby interstellar clouds are studied by combining ultraviolet data from the 'Extreme Ultraviolet Explorer' satellite with Strömgren photometry from Chile. The most recent measurements show that the interstellar matter in the vicinity of the Sun is not in pressure equilibrium and also can not be considered as a homogeneous structure, which is of importance in order to understand the circulation of matter into stars and back to the instellar medium in the galaxy.
The people working on the structure and evolution of the MilkyWay are: J. Andersen, C. Fabricius, E. Høg, H.E. Jørgensen, J. Knude, B. Nordström, E.H. Olsen and I. Novikov.
The theory of stellar structure and evolution is a vital part of modern astrophysics: Not only does the stars by far make up the main part of the luminous mass in the Universe, they also create almost all of the of the elements heavier than helium in their interiors, and the theory forms the basis of almost all determinations of ages and in some cases also of distances in the Universe. Therefore it is of importance to continuously test if the best available evolutionary models are in agreement with the most accurate observations of real stars. Two types of test objects are specially well suited for this: Binary stars, for which mass can be determined accurately, and clusters of stars, in which the numerous members are stars with the same age, but with different masses.
Earlier comprehensive investigations have been continued and a number of young binary stars with masses of 2 - 5 times the Sun (AR Aurigae, beta Aurigae, LZ Centauri, AG Persei, AO Velorum). An important result is that these stars, which are very much younger than the Sun, all seem to have the same or lower metallicity than the Sun. This is of significance in calculating evolutionary models and in comparing these with observations, confirming a growing realization that models for the chemical evolution of the galaxy can not be based on the earlier assumption of a monotonically growing metallicity with time.
Certain types of stars are rare in well known binary systems, e.g. stars of one solar mass or less, and very young newly formed stars. A large photometric search for binary stars like the Sun or cooler was initiated in 1994 and have resulted in the discovery of new systems. Detailed studies of a couple of very young binary stars have lead to whole new results about the distribution and dynamics of such systems. Furthermore new and more accurate methods for spectroscopy of such cool binary systems have been developed in order to obtain more accurate stellar masses and -radii, and also temperature, chemical composition etc.
Clusters of stars are important in order to make a detailed control of stellar models. A controversy in the literature about the interpretation of data for two open clusters have been resolved by systematic velocity measurements that showed "pollution" by field- and binary stars, after which some of the competing models were rejected.
Studies of pulsating variables have continued partly using asteroseismology, and partly by analyzing lightcurves. Approx. 30 individual stars with observational data taken from the literature have been examined in order to determine the periods and identifying the pulsations. The most interesting case is a bright variable in the globular cluster Omega Centauri. It has a period of 1.637 days and is part of a small family of stars for whom the physical properties are not yet solved conclusively. To obtain modern and more accurate data extensive CCD observations have been prepared for the spring of 1995. These data will give both very precise light curves and pulsation periods, to be used in asteroseismology.
The people working on the structure and evolution of the stars are: J. Andersen, M. Andersen, J.V. Clausen, B.E. Helt, K.T. Johansen, P. Maxted, B. Nordström, E.H. Olsen and J. Otzen Petersen.
Under the supervision of Å Nordlund supercomputer simulations and 3D-visualizations on graphical workstations are performed. The general aim is qualitatively to understand turbulence and chaotic magnetic fields, with an emphasis on astrophysical relevance. Some of the concrete projects are: Formation of structure on cosmological scales, supersonic mhd-turbulence in cold molecular clouds with star formation, turbulence in accretion discs, dynamo-effects in stars and accretion discs, turbulent convection and excitation of solar and stellar pulsations, dissipation of turbulent magnetic fields and spectralline formation in three dimensions.
The people working with plasmaphysics and magnetohydrodynamics are: A. Brandenburg , K. Galsgaard, Bernard Jones , Å Nordlund and P. Padoan.
Determining the fundamental coordinate system, the small angular motions of the stars (proper motions) on the sky, trigonometric distances (parallax) to the stars is the primary task of astrometry. These data are of decisive importance for a number of astrophysical projects. 25 astrophysical projects form the basis for the observations that are performed with the observatory's automatic Carlsberg Meridian Circle, CAMC, on La Palma. The members of the staff at the observatory are PI (Principal Investigator) on 10 astrophysical projects in relation to the HIPPARCOS astrometric satellite. The astrometrical results are used in the above mentioned astrophysical projects. In addition to this the following should be mentioned: In 1994 a new large project was initiated in cooperation with the Space Telescope Science Institute in Baltimore. 12.000 faint stars near equator have been selected for observations with the CAMC. With the help of these new precise reference star positions, it is planned to re-reduce a number of the Schmidt plates, that are the foundation of the Hubble Space Telescope Guide Star Catalogue and hereby obtain a much improved accuracy. Carlsberg Meridian Catalogue Vol.8 has been finished, and it contains positions for 18145 stars and 2143 observations of objects in the solar system. Again it has been possible to determine proper motions for a vast majority of these stars (93 %), and the high precision have been maintained as before.
ESA's astrometrical mission Hipparcos-Tycho have for three years until the end of August 1993 measured positions, proper motions and distances for a million stars distributed over the entire sky with a much better accuracy than possible from the surface of the Earth. The result will be of fundamental importance for our knowledge of the structure and dynamics of the MilkyWay and for our understanding of the formation and evolution of the stars. The Danish group is part of an international cooperation which performs the very demanding task of reducing the data. Analyses of the observations have shown that the satellite will fulfill all expectations. A new interferometry/astrometry mission has been approved as a key project in the planning of future ESA projects after the year 2000. The project is an astrometrical and photometrical satellite which uses CCDs and an interferometric technique. It will therefore be a million times more effective than the Hipparcos satellite. It will be possible to measure 50 million stars with astrometric errors 100 times smaller. In addition to this, the luminosity of the stars will be observed in at least 5 colors, spanning from ultraviolet to infrared.
The people working with positional astronomy are: C. Fabricius, L. Helmer, E. Høg, V.V. Makarov, H. Pedersen, and C.S. Petersen .
Observational astrophysics depends on the best possible telescopes and observational instruments . At ESO Danish astronomers have access to the largest, most modern and well equipped telescopes, whereas observational instruments have to be developed for the Nordic Optical Telescope, NOT , and the Danish telescopes. Among these instruments the priorities in 1994 were focused developing the largest and most sensitive detectors and the highest possible resolution. In 1994 a CCD camera developed in Brorfelde was mounted on NOT with a 1048x1048 pixel detector. A camera with a 2048x2048 pixel CCD and an efficiency of nearly 90 % over a wide range of wavelengths will be ready for use on the Danish 1.5-m telescope in Chile in January 1995. More cameras will also be delivered in 1995. Furthermore, a cooperation with the French EROS-group (Etude et Recherche d'Objets Sombres), which is searching for dark matter in the Universe using the gravitational lensing effect, has been started; cameras with a total of 16 of these large CCDs will be built. In addition to this, ESO have signed a contract to test and optimize new CCD-detectors in the Brorfelde laboratory for use on La Silla.
The highest image quality is obtained by cooling the main mirror of the telescope during the day to night temperature, and by performing a rapid correction of the remaining image motions. A mirror cooling system for the 1.5-m telescope was developed and tested during 1994 and will be installed in 1995. A special small CCD-camera, which can perform image analysis with up to 0.006 seconds of time resolution, has also been developed. Tests at the NOT shows that a very considerable improvement in the quality of the images (almost as the Hubble Space Telescope) can be obtained by correcting the image position approx. 50 times a second. Such a HiRAC (High Resolution Adaptive Camera) is under construction and will be installed on NOT in 1995. The camera on the 1.5-m telescope is supplemented by an all-round wide angle instrument which have caused much enthusiasm among colleagues (among others at the NOT ) and an agreement has been made to build more copies of the instrument.
In connection with the automated Carlsberg Meridian Circle on La Palma a new software system for collecting and storing the measured meteorological data was developed and implemented. The system is based an a PC interfaced to the meteorological sensors and to the telescope control computer. Measurements are made every 20 seconds and a mean is made over 15 measurements. In addition to this maximum and minimum values for the windspeed and -direction are determined
The people working with instrumentation are: J. Andersen, M.I. Andersen , L. Helmer, H. Jønch-Sørensen, J. Klougart, R. Florentin Nielsen, P. Nørregard and staff at the observatory workshops.
Calculations in connection with the issuing of almanacs for Denmark, Faroe Islands and Greenland have been carried out. In addition a number of special calculations for public authorities etc. have been performed.
The person working with the Almanac and related issues is O.H. Einicke.
Einicke, O.H.: Almanak, Skriv og Rejsekalender for det år efter Kristi fødsel 1995. - Færøsk Almanak 1995. - Grønlandsk Almanak 1995. -Sunrise/Sunset Tables for selected Stations in Greenland and on the Faroe Islands 1995.
Brandt, Søren K.: Observations of Galactic Binary x-ray Sources and Gamma Ray Bursts with WATCH
Jønch-Sørensen, Helge: CCD uvby,beta photometry of faint F-stars. Spatial and temporal variations of the metallicity in the disk of the Galaxy.
Tirado, Alberto J.C.: The Watch experiment: 1000 days observing the X-ray universe. A study of the X-ray sky from dec. 1989 to sept. 1992.
P. Diener : The affine model approximation of the tidal interaction of stars with a super massive black hole.
Troelsgaard Jensen, N.: Ages of Globulars: The case of NGC 6717.
Last update march 14 1995.