In addition to the programs described in previous sections, there are a number of planned observations dealing with other varieties of hot stars that are strong sources of FUV emission. Wolf-Rayet stars are some of the most massive stars known, and show evidence for strong stellar winds and peculiar abundances. Program G32 includes both HUT and WUPPE observations to make a comprehensive study of these stars and their short timescale variability in the ultraviolet. Program G15 involves observations of normal O stars, close cousins to the Wolf-Rayet stars. HUT's 900 - 1200 angstrom coverage in addition to Hubble observations of the same stars at longer wavelengths will permit strong constraints to be placed on the atmospheres and winds of these stars. To broaden the results of this GI program, the PI team instituted program H14, which will include observations of additional hot stars of different temperatures and surface gravities than those in program G15. This will produce a benchmark spectral atlas of selected hot stars.
Program G31 is another joint HUT/WUPPE investigation, this one to measure the polarization and extinction properties of dust grains in the interstellar gas of the Large Magellanic Cloud. The dust extinction in the LMC is known to be different from that in our galaxy, although the reasons for these differences are not known with certainty. The combined HUT/WUPPE data will permit the grain size distribution, composition, and polarization properties to be studied with unprecedented accuracy.
Program G11 will combine HUT and WUPPE observations of symbiotic stars, which are a special kind of binary star system usually involving a red giant star and a very hot white dwarf star, each shining with a typical intensity of several thousand times that of our sun. Both components have evolved from stars similar to the sun and are near the endpoint of their evolution. The stars are separated by a distance similar to that of the earth from the sun and form the most widely separated of interacting binaries. Study of these systems therefore has much to tell us about the properties of isolated stars similar to our sun which form the bulk of the normal stellar population of our galaxy.
A number of symbiotic systems undergo flaring outbursts that resemble those of stars classed as novae -- either single or binary systems which shine at a level of 10,000 - 450,000 times that of our sun for a short period of time. Indeed, some of the targets of the G11 program are classed as "recurrent novae" -- novae which have been observed to repeat their outbursts more than once. It has also been speculated that some symbiotic systems may eventually develop into Type Ia supernovae, which are (briefly) the brightest stellar objects known.
Models for symbiotic systems have been restricted in the past by our limited knowledge of the hot white dwarf component. This star's spectrum peaks in the far ultraviolet but is faint at optical wavelengths where the red giant star dominates. Using the HUT spectrometer, it will be possible to measure the continuum emission from the white dwarf and so provide the first systematic measurements of white dwarf temperatures for a sample of symbiotics.
HUT will also be used to study a unique interaction between the the ionizing radiation and the stellar winds in these systems. The UV radiation from the white dwarf ionizes the stellar wind from the red giant star, producing a strong emission line spectrum. Low resolution observations with instruments on the Voyager spacecraft have shown the flux of O VI 1035 angstroms may be a factor of twenty stronger than predicted by models. Indeed, for some of these objects the O VI line is the strongest high ionization line that can be seen in the entire UV to optical spectrum. The O VI feature falls into the HUT wavelength range and will be observed with higher resolution and sensitivity than could be achieved with Voyager. By combining the HUT observations with simultaneous WUPPE data, diagnostic lines from the entire UV range will be accessible simultaneously, permitting astronomers to understand the relative importance of heating by the hot star's UV emissions and by shocks waves in the stellar winds.
Several eclipsing symbiotic binary systems will be studied to better understand the extended atmosphere of the red giant star. As the red giant moves in front of the white dwarf (from our perspective), the hot star's light is progressively dimmed as it passes through the giant star's atmosphere. By studying the way the hot star's light varies with eclipse phase, scientists can probe these normally inaccessible regions of a red giant star and test computer models of the structure of these regions.
By using a combination of the complementary properties of the HUT
and WUPPE instruments it will be possible for the first time to
study a representative sample of symbiotic systems in great detail.
The results of program G11 will have implications for stellar
evolution, mass loss, stellar winds, accretion events and even,
albeit indirectly, implications for our attempts to define improved
yardsticks with which to measure the universe around us.
