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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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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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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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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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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
Representative terms from entire chapter:
white dwarfs
