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38 7 l~L~ E~g O~a Ana1~s Finally, we see the need for a substantial expansion of funding for data analysis and interpretation. Adequate support for correlative optical and radio observations and for theoretical interpretation and related studies should be provided in order to obtain the full scientific benefits of the investment of resources in x-ray astronomy. V. EXTREME-ULTRAVIOLET ASTRONOMY A. Introduction The achievements and promise of extreme-ultraviolet (E W) astronomy, based on observations in the spectral range from 10 to 100 eV (1200 to 120 L), have grown dramati- cally in recent years. Techniques for carrying out E W observations have been developed and tested in rocket flights and satellites. These techniques were used in an exploratory survey of a selected list of potential E W sources, which was carried out on the Apollo-Soyuz satel- lite mission in 1975. This very limited survey revealed five discrete extrasolar sources of E W radiation and demonstrated the feasibility and potential scientific importance of E W astronomy. In addition, various observations have led to the conclusion that the average density of interstellar matter is less than had been previously assumed and is highly nonuniform. Conse- quently, the transparency of the interstellar medium at E W wavelengths is much higher than previously estimated, and observations out to 300 parsec or more are now known to be possible. Further evidence of the importance of E W observations has been provided by results from HEAD-1 and the Einstein x-ray observatory, which show that hot, x-ray emitting coronas are present in stars of all spectral types. These coronas and their associated chromospheres must also emit E W radiation. Thus, all the millions of stars within several hundred parsecs of the Sun must now be considered as possible candidates for E W observations. Just as the study of the Sun at E W wavelengths has provided unique information on the nature of the solar chromosphere and corona, the study of stellar spectra in the E W region will provide unique information on the structure and dynamics of stellar atmospheres. However, less than 1
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39 percent of the sky has been surveyed for E W sources, and only a few spectroscopic measurements of extrasolar EUV sources have been made. Thus the development of E W astronomy lies almost entirely in the future. B. Scientific Goals for the 1980's A primary goal in E W astronomy is to carry out an all-sky survey. Such a survey will discover many new E W sources and will determine what types of objects and how many of them are detectable in the E W portion of the spectrum. In addition to the survey, detailed studies of indivi- dual sources should be carried out. The most urgent requirement is high-resolution spectroscopy of specific sources. Eventually, detailed spectroscopic studies of all major classes of objects detected in all-sky surveys should be carried out. In the immediate future, however, these studies should be concentrated in areas already clearly identified as being of high interest. A number of these are discussed below. 1. Stellar Chromospheres, Transition Regions, Coronas, and Flares Observations of stellar spectra in the ultraviolet range from 1150 to 3000 R. made with the Copernicus and International Ultraviolet Explorer (IUE) satellites. have provided detailed information about the temperatures, densities, and velocity structures in the chromospheres and lower transition regions of stellar atmospheres where the temperatures range up to 105 K. In upper transition regions and coronas, where the temperatures are in the range from 105 to 107 R. important spectral lines for the diagnosis of physical conditions occur in the E W region of the spectrum. Therefore, to carry out similar studies on the outer regions of stellar atmospheres it is essential that observations be made in the E W region. Estimates of the densities and emission measures of a corona can be derived from such observations. From these one can deduce the emitting volumes and the volume filling factors and thereby achieve a better understanding of the geometry, temperature, and density of the emitting regions. Continuous observations of the intensities of E W lines will provide sensitive measurements of the variability of stellar coronas that may be caused, for
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40 example, by passage across the line of sight of large coronal loops or holes. For stars that exhibit flare activity, studies of the variations of their spectra will place significant constraints on flare theories as they have in the case of solar studies carried out in the Solar Maximum Mission. 2. Cataclysmic Variable Stars and Magnetic White Dwarfs One of the most exciting prospects for E W astronomy is the study of cataclysmic variables, which are close binary systems containing a degenerate dwarf and a late-type non- degenerate star. Earlier theoretical predictions of the E W intensities of these objects have proved to be too small by factors of 20 or more. On the other hand, the cyclotron resonance features that were expected to be present in the spectra of strongly magnetic (108 gauss) white dwarfs are absent. Soft x-ray observations of a number of cataclysmic variables have shown that their spectra rise sharply with decreasing energy near 0.1 keV, while far- W observations by IUE have shown that the ultraviolet flux of many cataclysmic variables and magnetic white dwarfs rises with increasing energy near 0.01 keV. One must conclude that a major component of the emission of these stars lies in the E W spectral region between 0.01 and 0.1 keV, and that the high- and low-energy tails of this spectrum are being observed by the soft x-ray and W detectors, respectively. It has been suggested that the E W flux is thermal radiation from the surface of the white dwarf that is heated to about 105 K by continuous nuclear burning of accreted material. Alternatively, the E W flux may be produced by cyclotron cooling of the shock-heated accretion flow in the magnetic field of the degenerate dwarf, with addi- tional E W radiation emitted from the heated surface. Spectroscopic and polarimetric measurements of E W emissions of these objects will clarify the mechanism of E W emission by providing unique information on the accretion processes and on the magnetic field strengths and other properties of the degenerate dwarf. Analysis of the pulsating component of optical emis- sion from the cataclysmic variable star, DQ Her, indicates that it is reprocessed E W emission. It appears likely, therefore, that DQ Her and other stars of this type will be found to have more pronounced pulsations in the E W than in the optical range. This raises the possibility
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41 that the phase of E W pulsations could be measured with sufficient accuracy to permit an analysis of orbital Doppler effects. This would make it possible to deter- mine parameters of the binary systems and measure their secular variation as the systems evolve. 3. Hot White Dwarfs - Considerable progress has been made in the last decade in understanding the final stages of stellar evolution; none- theless, many key questions remain unanswered. The study of highly evolved stars is likely to be one of the most important research topics of the 1980's. White dwarfs are the end products of the evolution of stars with initial masses less than about 5 solar masses, which includes the vast majority of stars, yet we know surprisingly little of their own evolution and properties. Observations of the immediate progenitors of white dwarfs, which are believed to be the central stars of planetary nebulae, are in an especially uncertain state. Indeed, their evolutionary tracks in the H-R diagram are still largely unknown. These stars radiate primarily in the E W. so that E W observations are necessary to determine their properties and thereby to gain a better understanding of such fundamental problems as the role of neutrino cooling in stellar evolution. In addition, E W spectrometry will provide a sensitive probe of white dwarf atmospheres and thereby shed new light on the processes of differential settling of elements and accretion of interstellar material. 4. The Interstellar Medium Various observations lead to the conclusion that the interstellar medium is very inhomogeneous in temperature and density, with conditions ranging from cold dense clouds near absolute zero to ratified intercloud gas with temperatures up to a million kelvins. However, the tem- peratures, densities, and spatial distribution of these various components are still very uncertain. Measurements in the E W will provide valuable data on this topic. The spectra of a few E W-emitting stars have already been analyzed to determine the total amounts of absorbing cool gas along the lines of sight to these stars. The likely existence of many such stars will -
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42 permit a major extension of this technique to probe the cooler gas in the nearby interstellar medium. Particu- larly powerful will be observations of the absorption edge of He I at 504 ~ and He II at 228 A. These observations will provide unique information on the ionization state of the interstellar medium. High-velocity shock waves have been detected in the interstellar medium through W absorption-line studies. E W spectroscopy of stars behind such shocks, or of the emission of the shocks themselves, could detect trace elements that have strong E W lines. The hot component of the interstellar gas produces a pervasive background of E W radiation. Since the spectrum of this gas is dominated by line emission, high-resolution spectral observations will be a rich source of information on the temperature and composition of the gas. Interven- ing neutral hydrogen imposes a low-energy cutoff on the spectrum of this background radiation at wavelengths determined by the amount of absorbing material along the line of sight. Large cool clouds embedded in the hot emitting gas are completely opaque in the E W and should reveal their presence as dark silhouettes against the background. C. Inventory of Present or Approved Resources A number of research groups in the United States and Europe have available or are developing rocketborne grazing-incidence telescopes and imaging detectors for E W observations. m e first spectroscopic observations have been carried out in the 100-500 ~ band from sounding rockets and from 500 to 900 ~ by a spectro- meter on the Voyager 2 spacecraft. The Extreme Ultraviolet Explorer (E WE) has been selected by NASA for development within the existing Explorer program. It is an essential step in the development of E W astronomy. It will survey the entire sky for E W sources in three broad energy bands at sensitivity levels that are greater by factors of 10 to 100 than those of the previous exploratory measurements. D. New Facilities Proposed for the 1980's The detection of very soft x-ray sources with HEAD-1 and the Einstein x-ray observatory reinforces the conclusion
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