Program G11 will combine HUT and WUPPE observations to study symbiotic stars, which are a special kind of binary star system involving a red giant star and a very hot white dwarf star. Each star shines with a typical intensity of several thousand times that of our sun, but one is very hot and the other is very cool. 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, can tell us much about the properties of isolated stars similar to our sun.
Many symbiotic binaries have irregular outbursts which resemble those of stars classed as novae, flaring to levels 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." It has also been speculated that some symbiotic systems may produce Type Ia supernovae, which are the brightest stellar objects known. Supernovae can shine as bright or brighter than an entire galaxy -- up to several billion times as luminous as our sun -- for a short period of time.
Models for symbiotic systems have been restricted in the past by our limited knowledge of the hot white dwarf component. The star's continuum emission peaks in the far ultraviolet due to its high temperature. Its longer wavelength emission is contaminated by the contribution from its red giant companion and by a component of emission from hot gas blowing away in a wind from the giant. Using the HUT spectrometer it will be possible to isolate and measure the continuum emission from the white dwarf and so provide the first systematic measurements for a sample of white dwarf stars in symbiotic binaries.
When the ultraviolet radiation from the white dwarf ionizes the stellar wind from the red giant star, it produces a strong emission line spectrum that has so far eluded interpretation with the models available. It is known from low resolution observations with instruments on the Voyager spacecraft that the flux of the OVI 1035 Angstrom feature may be up to a factor of twenty stronger than these models predict. Indeed, for some objects, the OVI line is the strongest high ionization line that can be seen in the entire UV to optical spectrum. The OVI feature falls into the HUT waveband and will be observed at a resolution six times higher than could be achieved with the Voyager spectrometer. The higher signal-to-noise data from HUT will also reveal lower intensity lines in the same spectral region. By combining the HUT observations with simultaneous WUPPE data, emission lines from the entire UV range will be accessible simultaneously.
One possible model suggests that at least a portion of the line emission comes from shocked gas which originates when a hot, low density wind from the hot star runs into the slower moving cool wind blowing away from the red giant. Using the continuum measurements of the white dwarf together with the emission line observations it will be possible to predict how much emission could be due to photoionization alone and how much is produced by shock waves.
Some symbiotic systems are oriented such that we sometimes see the hot star eclipsed by its cooler companion. When this occurs the hot star's light is progressively dimmed as it passes through the giant star's atmosphere and an increasing amount of scattered and reprocessed light should be seen. By comparing the dependence of the resulting spectrum on the phase of the eclipse a unique probe of the red giant's atmosphere can be made. These observations will provide information on the ionization level, density, and abundances in these zones for comparison with the predictions of stellar structure models.
By using a combination of the complementary capabilities 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. This may allow us to understand how these very different "beasts" came to live together in these unusual symbiotic binary systems. The results of program G11 will have implications for our understanding of stellar evolution, stellar winds, and mass loss from stars.
Brian Espey