Institute for Astronomy and Astrophysics

Some Scientific Highlights of the ORFEUS I Mission

During the first ORFEUS-SPAS mission we were able to get valuable spectra of more than 50 different celestial objects (mostly stars). ORFEUS operates in the so far very rarely observed Far and Extreme Ultraviolet wavelengths region which because of complete light absorption in the terrestrial atmosphere is only accessible from space. ORFEUS superceded main former missions: The COPERNICUS- satellite flown in the early 70th, was restricted to bright objects not fainter than 5th magnitude, whereas ORFEUS at slightly smaller spectral resolution could reach more than hundred times fainter objects. The Hopkins Ultraviolet Telescope HUT flown in 1990 on the Space Shuttle was somewhat more sensitive but its spectral resolution was about a factor of ten lower than that of ORFEUS.

Spectral resolution is of very crucial importance, because hundreds of resonance absorption lines of molecular Hydrogen and of the most abundant atomic species like atomic Hydrogen, Helium, Oxygen, Carbon, Nitrogen, Sulfur, Phosphorus a.s.o. are only to be found in the FUV and EUV. They often are so densly crowded in this region that only a wide spread of the spectral colours (colours in an applied sense) can help to separate the absorption influence of the different species.

Almost all the spectra which we got from the ORFEUS observations are unique.

Interstellar Matter

For the first time we could definitely confirm identifications of intergalactic hydrogen clouds in the direction of the BL Lac object PKS 2155-304, an active galactic nucleus at a distance of about 2 billion light years: In addition to the expected interstellar lines we detected higher quantum number counterparts of the intergalactic Lyman Alpha lines discovered earlier with IUE and HST. The Lyman discontinuities indicate for three of the redshifted clouds a combined column density of 2-5x1016 cm-2, while the column density of another cloud appears to be well below 5x1015 cm-2. Another important result from this measurement is that we could not detect significant absorption from five times ionized Oxygen, OVI. The resulting upper limit for the column density of this hot gas component falls significantly below the value predicted by the galactic fountain models of the hot halo gas.

For the first time we could trace the interstellar molecular Hydrogen throughout our galaxy, the halo of the Galaxy and in the Large and Small Magellanic Clouds, small galaxies accompaniing our own galaxy. Our knowledge about molecular hydrogen is of crucial importance for the total mass of the Milky Way stellar system, because it is, besides atomic hydrogen, the far most abundant species of the interstellar gas. Abundance and velocity analysis of the Lyman and Werner bands of H2 give additional unique insight into structure, density, temperature, evolution and composition of the interstellar gas.

Cataclysmic Variables

Far Ultraviolet spectra of the Cataclysmic Variable VW Hyi could be observed two times during and shortly after an outburst. An IUE spectrum was taken simultaneously with the second ORFEUS measurement. This gives us new insight into the mechanisms which guide these sudden enhancements of radiation, because we can observe the phenomenon in the light of many newly observed diagnostic absorption lines, such as C II and III, N III, O VI, S IV and VI, P V and others. The spectra offer for the first time the possibility to separate the absorption line components of the accretion disk and the wind of the system. One of the remarkable findings is that the continuous flux at wavelengths below Lyman Beta was significantly higher during the second measurement after the maximum of the lightcurve in visual light. Presently a model is in preparation which shall describe the system more precisely than previously possible.

Planetary Nebulae

Spectra of several different Planetary Nebulae were observed. The Spectra of the central stars of NGC 6826 and NGC 6543 show prominent windprofiles of highly ionized species like P V, S VI and O VI. From these line profiles a wind model was derived which gives much improved data about the mass loss rates and temperature structure within the wind extending far out into the interstellar medium. Very surprisingly in the line of sight towards NGC 6826 we find quite a lot of very strong molecular Hydrogen lines. At a first glance this seems to be very strange, because we do not expect to find interstellar H2 at the very low interstellar extinction with EB-V = 0.03 in the direction of NGC 6826. Together with the search for the spatial distribution of the emission in the IRAS infrared bands we can conclude that this molecular component arises in the circumstellar environment of the PN. This means that there exist neutral clumps within the the ionized halo of NGC 6826 within which dust and molecules are able to survive within the timescales of the nebular development.

White Dwarfs

The subdwarf O star BD +28 4211 has been investigated for the first time in very detail with the help of ORFEUS FUV spectra. This object is an important standard star for a wide spectral range from infrared to ultraviolet. It has been widely used for flux calibrations of IUE, HST and many optical observations. A model atmosphere has been calculated which together with a much improved distance determination gives a very precise flux density profile in the FUV range between 912 and 1150 Angstrom. For the first time all elements from hydrogen up to order number 28, Nickel, have been investigated in all relevant ionisation stages. We find some of our results in good agreement with our previous knowledge, others disagree significantly and will give rise for further improved modelling. The results will also help to improve the model calculations for stellar evolution which are still far from being perfect for this class of objects. For the first time we could precisely measure the column density of the interstellar atomic Hydrogen which gives us a reliable value for the very low mean density of the interstellar medium. We could not find significant absorption of molecular hydrogen in the direction to this object.

Early Type Stars

Spectra of several early type stars with many absorption lines in their spectra give more insight into the mechanisms of their stellar winds (velocity, temperature and mass loss). Those data are very important for our understanding of the development of those most luminous and massive stars.