In addition to the topics described above, HUT has been used to address a wide range of problems that can only be mentioned briefly here. For example, observations were made of several cataclysmic variables, or dwarf novae, which consist of a white dwarf accreting matter from a normal red dwarf in a close binary system. The accretion disk surrounding the white dwarf is of considerable interest in its own right and also for what we might learn that would be applicable to the much larger accretion disks in active galactic nuclei. The quasi-periodic outbursts seen in dwarf novae are probably due to an increased rate of mass transfer resulting from an instability in the disk. The American Association of Variable Star Observers (AAVSO) monitored 10 dwarf novae throughout the Astro-1 mission and alerted us to each of the outbursts that occurred. As a result, we succeeded in measuring the UV spectrum of Z Camelopardalis at the peak of an outburst (Figure 4) (Long et al. 1991). The result is presumably the spectrum of an optically thick accretion disk and shows a strong continuum whose intensity peaks in the far UV around 1050 Å with numerous strong, broad absorption lines of high-ionization species, including CIV, NV, OVI, SiIV, PV, and SiVI. These observations provide the first detailed look at the far-UV spectral region of such an outburst. Attempts to model the spectrum with standard accretion disk theory met with limited success and clearly revealed the need for more detailed theoretical calculations of the expected emission.
Another substantial part of the HUT scientific program involves the
study of the emission of supernova remnants (SNR) in the far UV. In
older remnants, such observations reveal the response of the
ISM to the passing blast wave from a stellar
explosion. Interstellar gas is compressed and heated by the shock
wave, and the subsequent radiation may be used to characterize physical
conditions of the gas. Thus, for example, the detection with HUT of
strong OVI emission in two filaments in the Cygnus Loop SNR
(Figure 5) indicates a shock velocity of about
and, when combined with optical data, provides
new information on the process by which energy is transferred between
electrons and ions behind the shock (Blair et al. 1991; Long et al. 1992). We also succeeded in
detecting OVI emission from an SNR in another galaxy, N49 in
the Large Magellanic Cloud, for the first time (Vancura et al. 1992).
We also obtained an interesting spectrum of the Crab Nebula (Blair et al. 1992), the
famous remnant of the supernova of A.D. 1054. Although the Crab is one
of the brightest X-ray sources in the sky, emitting synchrotron
radiation from high-energy electrons accelerated by the pulsar created
when the star exploded, it is a difficult target in the UV, because of
strong extinction caused by interstellar dust along this line of sight.
Nevertheless, HUT detected the UV continuum radiation produced by the
electrons, and also observed a few spectral lines arising in filaments
that are photoionized by this radiation. Two velocity-shifted
components of the CIV line reveal expansion along the line
of sight at 
. These observations are the
first to demonstrate that relative UV line intensity variations are
present in the Crab Nebula. This probably indicates varying carbon
or helium abundances in the material observed by HUT (Blair et al. 1992).
A potentially exciting measurement made with HUT was an attempt to
detect line radiation from decaying dark matter particles that might
dominate the mass of the Universe (Davidsen et al. 1991). The hypothesis of decaying dark matter
(Sciama 1990) is a clever attempt to explain a number of disparate
facts and ideas from astrophysics, cosmology, and particle physics by
attributing a mass of about 30 eV to the 
neutrino and assuming
that it decays with a lifetime 
s. The assumed mass would be
sufficient to close the Universe if the Hubble constant had a value
near 
, and the hypothesized
radiation would explain the degree of ionization attributed to various
components of diffuse gas in the Universe. We used HUT to search for
the emission line that would be expected from a massive cluster
of galaxies that would contain a large (and calculable) quantity of
dark matter if the theory were valid. No emission was found, limiting
the lifetime of any such decaying dark matter particles to at least
several times 
s (Davidsen et al. 1991).
Closer to home, HUT has also been used for observations within
the solar system. Observations of comet Levy (1990XX) revealed an extended
source of carbon monoxide emission and set upper limits on the
abundance of argon and neon in the comet (Feldman et al. 1991). A spectrum of the Io
plasma torus surrounding Jupiter displayed a large number of lines of
sulfur and oxygen in various stages of ionization (Moos et al. 1991). The data have
sufficient resolution to allow the ionic abundance ratios to be deduced
directly, without recourse to modeling. Even Earth's upper atmosphere
has provided interesting new data for HUT. The O
recombination
spectrum has been resolved for the first time, and the shape of the
continuum provides a direct measure of the electron temperature in the
ionosphere, which was about 1000-1200 K during the Astro-1 mission
(Feldman et al. 1992).
