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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).