What follows is a brief description of the HUT Team's overall Science Program for Astro-2, including both the original Principal Investigator team's programs (indicated by program numbers beginning with "H") and Guest Investigator programs (indicated by program numbers beginning with "G"). Where feasible, we group various programs together in an attempt to show the larger picture. >From this, it should be obvious that there is a large amount of synergism between the programs as we try to use observations of a relatively few number of objects to understand larger issues in today's astrophysics.
The descriptions are intended to be at an understandable level for the general reader, but do include some unavoidable "technical jargon." Please refer to a good introductory astronomy textbook or ask a HUT team member for further clarification if necessary.
--------------------------------------------------------------------------- High Redshift Quasars and the Intergalactic Medium HUT Science Programs H02: High Redshift Quasars HUT program H02 is the premiere science program for Astro-2, and embodies the heart of the original science goal envisioned for the telescope when it was first proposed back in 1978. The goal of this program is grand indeed--to study the vast but exceedingly tenuous regions of space between the galaxies using one or more distant, high redshift quasars as a background source of light, to better understand the conditions in the medium from which all of the structure in the universe subsequently formed. This intergalactic medium (IGM) is thought to consist almost entirely of hydrogen and helium that was the first material to form in the cooling phase after the Big Bang that created our expanding universe. This medium has proven very difficult to detect or study. No evidence has been found for the expected hydrogen, leading to speculation that the conditions must be such that the hydrogen is all ionized. Helium atoms are harder to ionize; partially ionized helium would reveal itself as a telltale absorption at certain extreme UV wavelengths, not only confirming the presence of the IGM, but also providing information on its temperature and density. As we look at high enough redshifts, these EUV absorptions will get redshifted into the far-ultraviolet portion of the spectrum above 912 angstroms where they can be observed with HUT. The same measurement can (in principle) by made by the Hubble Space Telescope, but only for higher redshift objects that shift the helium absorption above about 1216 angstroms. The problem then becomes finding an appropriate high redshift quasar to use as a background source against which to measure this helium absorption. Even though a large number of quasars with appropriate redshifts for this study are known, almost all of these fade to invisibility before reaching the wavelengths where the helium absorption could be observed. It is a testament to the incredible distances of these high redshift objects that their light gets completely absorbed by an intervening medium whose density is many orders of magnitude less than the best vacuum we can create on Earth! The higher redshift (and hence much more distant) objects needed to make this observation with HST also, then, stand a considerably higher probability of being completely absorbed at the appropriate wavelengths. The shorter UV wavelengths available to HUT could make a crucial difference in detecting the IGM. There are only a couple of quasars known with redshifts that place the expected helium absorption in the HUT primary 900 - 1200 wavelength range AND that still have detectable UV background light at the shortest UV wavelengths observed to date (as observed by HST or the IUE satellite). The brighter of these, an object called HS1700+64, is the primary target of this program. Even so, it is expected to be very near the faint detection limit of HUT and will require multiple pointings during the mission to gather sufficient data to make this measurement. Although a fledgling attempt to perform this observation was made during Astro-1, the project had to be abandoned because of the pointing problems and other difficulties encountered on that mission. With HUT's improved sensitivity, hopes are high that data from Astro-2 will provide a definitive measurement of this elusive component of the early universe. Active Galaxies and Quasars HUT Science Programs H03: Active Galactic Nuclei and QSOs H04: Seyfert 1 Galaxies H05: Seyfert 2 Galaxies H06: BL Lacertae Objects G13: Starburst Galaxies (C. Leitherer et al.) HUT programs H03 - H06 all deal with understanding various aspects of the active galaxy phenomenon. It is widely believed today that quasars and active galaxies are powered by supermassive black holes which produce copious (indirect) luminosity even as they gobble their prey. Quasars (or QSOs) are thought to be the most extreme (both in distance and luminosity) form of the phenomenon. Some quasars are observed to have associated "fuzz" which is believed by some to be light from a surrounding galaxy or proto-galaxy. Lower redshift objects sometimes show clear galactic structures surrounding very bright central regions, leading to the term active galactic nuclei, or AGN. This leads to a picture whereby AGN may be a closer and less luminous manifestation of the same phenomenon occurring in quasars, and could even represent an evolutionary step to normal galaxies. The picture developed for these objects includes the idea that a thick accretion disk of material forms around the central black hole, an idea that is supported by some tantalizing glimpses of such structures provided by the Hubble Space Telescope. It is the goal of program H03 to test this picture for distant AGN and QSOs by searching for the spectral shapes predicted by theories of these accretion disks. Results from HUT on Astro-1 have already provided some evidence for accretion disks in a few key objects, and HUT scientists have actively pursued the generation of more sophisticated computer models to help interpret such data. Because each object is unique, observations of additional objects on Astro-2 will permit these ideas to be tested more thoroughly. Programs H04 and H05 study two additional kinds of active galaxies called Seyfert 1 and Seyfert 2 galaxies. The numbers are indicative of an observational classification scheme, and some scientists think that the two types are basically the same except for the line-of- sight geometry to the central black hole. These galaxies are relatively nearby, which permits the black hole/accretion disk paradigm to be studied at a more detailed level than for the more distant examples. For instance, some Seyfert galaxies show evidence for cones of radiation which are thought to be collimated by the thick accretion disk around the central black hole, as discussed above in the AGN section. Astro-1 HUT observations were obtained for the brightest members of the Seyfert 1 and 2 classes, NGC 4151 and NGC 1068, respectively. The spectrum of NGC 4151 showed a broad line of O VI never before observed for such a low redshift Seyfert, and showed evidence for absorption in the far-UV spectrum thought to arise in the accretion disk itself. This galaxy will be re-observed on Astro-2, using multiple pointings spread through the mission to study the variability of the UV emission on timescales of days to weeks. The spectrum of NGC 1068 showed many narrow emission lines, including several in the 900 - 1200 range that had not been observed previously. These new lines, in conjunction with the data above 1200 , implied a significant contribution from heating by shock waves, a process once proposed to be important but which had fallen out of favor as the leading cause of the emission. During Astro-2, pointings at several positions near the active nucleus of NGC 1068 are planned to try and isolate the region responsible for this emission. One additional member of each class is slated for observation, although even these "next best" candidates are considerably fainter and more difficult to observe. BL Lacertae objects (H06), named after the prototype of the class, are thought to be active galaxies where our line-of-sight to the nucleus is looking right down the ionizing cone of radiation. HUT astronomers hope to observe the brightest available member of this class to round out their sampling of active galaxies. Program G13 involves HUT observations of yet another class of active galaxy, although the motivation of this program is somewhat different. Starburst galaxies is a term used to describe a class of galaxies that show evidence for very intense on-going star formation activity. The HUT spectra of these objects will be useful in comparison with HUT spectra of other AGN and normal galaxies to understand the differences in the types of stars in each class of object. However, the primary motivation is more closely associated with the H02 IGM program. The IGM should consist almost entirely of hydrogen and helium that were the first elements formed after the Big Bang. The main way the hydrogen in the IGM could have escaped detection is if it is highly ionized . If this is the case, then some source (or sources) of high energy light must have been available in the early universe that caused this ionization to occur. The source of this energy has been debated. One source might be the light from the quasars themselves. However, another substantial source might have been the UV light from bursts of star formation as galaxies were forming early in the universe, IF some fraction of this light could escape from the local region of star formation and into the IGM. It is the primary goal of program G13 to test this idea by looking at (relatively) nearby starburst galaxies. Photons of light capable of ionizing hydrogen must have wavelengths shorter than 912 angstroms, which is a region of the spectrum normally unobservable due to absorption by hydrogen gas in our own galaxy. However, the galaxies in program G13 are sufficiently redshifted that the region below 912 angstroms in the rest frame of these galaxies is observable in the HUT spectrum above 912 angstroms. Hence, the HUT spectra will be searched carefully to investigate whether a substantial fraction of the light from the hot stars in these galaxies can "leak out" at short enough wavelengths to ionize hydrogen. If so, and if these relatively nearby galaxies are similar to the starburst galaxies in the early universe, they could have been substantial contributors to the ioniation of the IGM. Stellar Populations in Elliptical Galaxies HUT Science Programs H07a: Elliptical Galaxies H07b: Subdwarf OB Stars H14: OB Star Spectral Atlas Elliptical galaxies are conglomerations of old stars that show little if any associated ionized gas or dust. Because all of the young, hot, massive stars have apparently evolved, cool low mass stars dominate the optical spectra of these galaxies. It was with much surprise, then, that the earliest ultraviolet observations of these galaxies showed many of them to maintain a faint glow of light at ultraviolet wavelengths, even increasing at the shortest UV wavelengths observed. The exact cause of this UV upturn has long been a mystery: could there be a small population of young, hot stars present in elliptical galaxies, or was there some evolved population of stars that was responsible for this emission? HUT observations of one elliptical galaxy and one nuclear bulge of a spiral galaxy (which shows many similarities to an elliptical galaxy) on Astro-1 made a significant contribution to understanding this problem by both extending the spectral coverage to shorter wavelengths and obtaining higher signal-to-noise ratio spectral data to attack this problem. They ruled out the possibility that young stars were responsible for the UV emission in the observed objects, and were able to argue for a contribution from a new branch of stellar evolution that had received little study until recently. The UV emission from elliptical galaxies is difficult to study because it is so faint. Also, with observations of only one or two objects from Astro-1, it is difficult to reach convincing general conclusions. The improvements made to HUT since Astro-1 make HUT an even more powerful tool for this program now, and observations of several additional key galaxies are planned for Astro-2. These observations will indeed shed "new light" on this 25 year old problem in modern astrophysics. Another crucial piece of information for this program is to obtain FUV spectra of different types of hot stars to use for comparison against the galaxy spectra. It is dangerous to have to compare against theoretical models of evolved stars that have not been tested against direct observation. Since HUT is observing in a largely unexplored region of the spectrum, it can provide "ground truth" to test these stellar models. Hence, observations of sub-dwarf OB stars (H07b), normal (hot) OB stars (H14), and even white dwarfs and planetary nebula central stars (H01) have been added to the program. Of course, the spectra of these objects will be of direct scientific interest as well as being applicable to the elliptical galaxies study. Nebulae and Interstellar Medium HUT Science Programs H10: Supernova Remnants G14: Interstellar Shocks (J. Raymond et al.) H11: Herbig-Haro Objects H12: Molecular Hydrogen in the ISM H08: Probing the Galactic Halo This category actually contains a number of related, and yet very different, programs to understand the interstellar medium (ISM), the tenuous regions of gas and dust between the stars. Programs H10, H11, and G14 are closely related programs of observe shock waves in a range of astrophysical situations. The bulk of programs H10 and G14 deal with FUV observations of selected filaments in older galactic supernova remnants (SNRs) to understand the physics of shock waves and their interaction with the ISM. Certain kinds of shock waves have been found that emit very little optical light but can be very bright at UV wavelengths. These so- called non-radiative shocks are of great interest because they are thought to occur right at the interface between the primary blast wave and the ISM. Only one such shock was observed on Astro-1. HUT scientists hope to observe several other shocks with higher velocity on Astro-2 to understand how the characteristics of these shock waves change with the speed of the shock wave. HUT will also be used to observe optically-bright filaments in several SNRs to better understand what happens when a shock wave encounters "density enhancements" (sometimes called "clouds") in the ISM. Observations of the Cygnus Loop, Vela, and Puppis A SNRs and the objects N49 and N63a in the nearby galaxy called the Large Magellanic Cloud will study this phenomenon in detail. The observations will seek to understand the range of shock velocities present, the density of the material, and whether or not the shock waves are able to destroy the grains of interstellar dust they encounter. This affects the perceived abundances that are derived for interstellar gas. Some young SNRs are being observed for a totally different reason. (Any age less than about 1000 - 2000 years is considered "young" for a SNR.) When a star explodes as a supernova, it tears the aging star apart and sends the star's outer layers expanding off into the surrounding space at velocities of thousands of kilometers per second. Until this material (called "ejecta") encounters sufficient interstellar material to slow it down, it maintains the chemical composition of the precursor star. Hence, by observing this material spectroscopically, scientists can investigate the chemical abundances of the material processed by the star during its lifetime. Understanding these processes is an important facet of astrophysics; since the Big Bang is thought to have produced mainly hydrogen and helium, nearly all of the heavier elements in the Universe, including the material in the earth and all of its inhabitants, have been generated by nuclear fusion in the cores of stars. This process is known as nucleosynthesis. Ultraviolet observations are important in addition to optical and other observations because some important elements (such as carbon and silicon for example) only have strong spectral features in the UV. Other elements (such as oxygen and nitrogen) show spectral features arising from higher energy ionization states than are available in the optical. Hence, UV spectra provide unique information about these fascinating objects. One particular observation is made possible by a coincidental alignment of a young SNR with an appropriate background object. A young SNR, called SN 1006 (because the supernova was observed in 1006 A.D. by Chinese astronomers), is seen today as an expanding circular bubble of X-ray and radio emission, with only a few very faint optical emission filaments. Seen in projection near its center is a hot, so-called subdwarf star (which is an evolved type of star that is burning helium in its core instead of hydrogen). This star was originally thought to be associated with the SNR, but today we know that it lies beyond the SNR; hence, to reach us the star's light must shine directly through the middle of the young remnant. The star is very faint at optical wavelengths, but because its temperature is about 38,000 K its spectrum gets brighter at UV wavelengths. This is the only known example of a hot star so closely aligned with a young SNR. This star is sometimes called the Schweizer-Middleditch star (or S-M star for short) after the two astronomers who discovered it. Observations with the IUE satellite and with HST have detected a number of absorption lines in the spectrum of the S-M star that are believed to arise due to the expanding supernova ejecta. Of particular interest are some broad absorption lines of once-ionized iron (designated Fe II). Iron is predicted to be the end product of nucleosynthesis in stars, and supernovae of Type 1a (such as SN 1006 was purported to be) should produce up to half a solar mass of iron! This iron should be ejected at high velocities in the supernova explosion. Iron does not appear to be over-abundant in X-ray analyses of SN 1006, meaning it can't be hot. No strong neutral iron lines (Fe I) are detected in the optical spectrum, so the iron can't be completely cold. The amount of iron inferred from the observations of Fe II absorption in HST spectra is significant, but falls at least a factor of 20 below expectations. Where is the rest of the iron? An obvious possible answer is that there is "warm" iron, more highly ionized than Fe II, but not heated to X-ray temperatures. A key absorption line of twice-ionized iron (Fe III) is expected at 1123 in the far-UV spectrum. This line is at too short a wavelength to be observed with Hubble, but is directly in HUT's prime wavelength range. Hence, a HUT observation of the S-M star will either go a long way toward settling the mystery of the missing iron or deepen the mystery even further. The star is faint, but the absorption should be strong if it is present near the expected level, making the HUT observation feasible. An interesting aside: Astronomers would like to be able to use type 1a supernovae as cosmological "standard candles" because they are luminous and visible at great distances. However, it is difficult to trust the predictions that all type 1a supernovae have the same intrinsic luminosity (and hence can be used to judge relative distances) if one of the other basic predictions of the models is incorrect. Hence, confirming that the models of type 1a supernovae are correct in their predictions about iron is of more than passing interest. Program H11 deals with Herbig-Haro objects, which are dense clumps of material that are heated by shock waves in regions near newly forming stars. Because the conditions are very different than seen in supernova remnants, observations of HH objects test a whole different astrophysical regime. Unfortunately, these objects are often found in dusty regions, making them difficult objects to observe in the UV (since dust absorbs and scatters UV light very effectively). The brightest HH objects are not well placed for night observation in March, and only one such object is in the premission observation timeline. In program H12, HUT scientists are attempting to understand the interactions between young stars and the cocoons of dust and gas that still surround them. Much of this gas is in the form of molecular hydrogen, which has important transitions in the prime HUT wavelength range. While molecular hydrogen is also observable in the infrared spectrum, the UV lines arise from the dominant "ground- state connected" transitions of the molecule and the observations can be interpreted in a more straightforward manner. Observations of the nebulae will permit a better understanding of the effects of the stars on their surroundings, while observations of the stars in the nebulae will provide information on the characteristics of the over-lying interstellar dust, which absorbs some of the star's radiation. Finally, program H08 investigates a very special portion of the ISM, the halo of our Galaxy. The galactic halo is a tenuous region of gas that extends both above and below the plane of our Milky Way galaxy. Supernovae and stellar winds from regions of star formation are thought to drive material out of the plane of the galaxy and into the halo. This material subsequently should cool down, and fall back onto the galactic plane in a process called the "galactic fountain." One of the main methods of probing the conditions in the galactic halo is to measure it's absorption of light coming from more distant sources. HUT offers the opportunity to observe absorption by five-times ionized oxygen (O VI), if it is present in a sufficient amount, which arises in hotter gas than any other UV line available for study. During Astro-1, scientists were able to obtain data of sufficient quality along only one line-of-sight through the halo to constrain the presence of O VI. While the anticipated absorption was apparently detected, there are reasons to believe that the line-of-sight studied may be peculiar or unrepresentative of the halo as a whole. With HUT's increased sensitivity for Astro-2, astronomers hope to study several additional directions through the halo to confirm and/or expand the Astro-1 result. Interestingly, this program is done "piggy back," using observations primarily motivated by other scientific programs. White Dwarfs and Related Stars HUT Science Programs G12: White Dwarf Stars (D. Finley et al.) H01a: Calibration H01b: PG1159 and Related Stars All of the scientific results that arise from HUT observations rely to a large extent on the calibration of the instrument (H01a), which means the ability to take the observed counts from the detector as a function of wavelength and convert them into physical units of measurement, such as the flux of energy received from the source per unit wavelength and per unit of time. The various components of HUT are carefully measured in the laboratory prior to launch to verify the expected performance on orbit. However, the integration of the payload and preparations for launch take considerable time and the sensitivity of various components is expected to change with time, and possibly with conditions on orbit. Hence, a method of calibration during the mission is required. Laboratory measurements again after the flight are then used to confirm the on-orbit measurements. Hot white dwarf stars have long been used for this purpose for various UV instruments like Hubble and the IUE satellite. White dwarfs have relatively simple spectra, and theoretical calculations of their spectral shapes are thought to be the most reliable stellar models available. Hence, by observing certain white dwarfs that have the best existing information about their intrinsic fluxes, scientists can derive a conversion between the instruments performance on orbit and the "real world." This conversion is then used to "calibrate" observations of other objects. (Note: there are many other steps or aspects of calibration that are not discussed here.) Of course, as the endpoint of stellar evolution for stars of a few solar masses or less, white dwarf stars are of interest scientifically in their own right. These unusual stars contain anywhere from about 0.5 to 1.2 solar masses (a few are known to be more massive, but 1.4 solar masses is the theoretical upper limit for a white dwarf), all packed in an object smaller than the size of the Earth. White dwarf stars are thought to represent just the cores of their original stars. When red giant stars lose their outer layers and expose their cores, we see an expanding nebula (called a planetary nebula) with a blue star at the center. The nebula expands and disperses in a matter of thousands of years, leaving behind what is essentially a fledgling white dwarf. These young white dwarfs are the hottest known stars in the universe, with temperatures in excess of 150,000 K in some cases. These stars cool down quickly at first (by astronomical standards), so quickly that very few of them have been identified for study. Of the stars that are known, most show small optical brightness variations on regular time periods of 5 - 15 minutes or so that are indicative of pulsations; these are called PG1159 stars, after PG1159-053, the prototype of the class (program H01b). A few stars apparently do not seem to vary, meaning (presumably) that they do not pulsate. HUT will use its "photon counting" detector to make sensitive searches for pulsations in the far UV (where these stars are brightest) for comparison with optical data. HUT scientists will try and understand why some stars of this category pulsate and others do not, as well as comparing these stars with the older, cooler white dwarfs discussed above. Program G12 will use high quality HUT observations of the hydrogen Lyman absorption lines in white dwarf stars, which are all located between 912 and 1216 angstroms in the HUT prime wavelength range, to derive very accurate measurements of the temperatures and surface gravities of the stars. HUT observations are planned for white dwarfs over a large range of intrinsic temperatures and surface gravities. Interestingly, this will not only improve our knowledge of the white dwarf star properties, but will ultimately lead to an improved calibration by providing better inputs to the theoretical models of the stars used for this purpose. Cataclysmic Variables and Related Stars HUT Science Programs H09: Cataclysmic Variables and Related Stars "Cataclysmic variables" is a general term used to describe several classes of binary stars that undergo major brightenings or outbursts. The sub-class called dwarf novae are of particular interest because their outbursts, while not strictly regular in period, recur on timescales from a couple of weeks to months or years. Dwarf novae (DNe) consist of a white dwarf star and a normal low mass star locked in tight orbit about each other with orbital periods of (typically) 3 - 6 hours. Because of the proximity of the two stars, the normal star fills its Roche lobe (a theoretical surface where the gravity of the two stars balance one another) and transfers matter onto its companion. Because this material has angular momentum, it does not simply drop onto the companion star but rather spirals in, forming an accretion disk around the white dwarf. The accreting material is heated as it spirals in toward the white dwarf, generating copious UV and in some cases X-ray emission in its final plunge onto the white dwarf. The outbursts are thought to be due to instabilities in the rate of mass transfer, although whether this occurs due to changes in the transfer rate from the normal star or whether the mass transfer rate within the disk is responsible is a matter of debate. HUT was used to observe several such systems during Astro-1, including one object that was observed during its outburst phase. Each of these observations provided new and different information on the processes occurring in these systems, and demonstrated the power of the HUT wavelength range to provide new diagnostics for these stellar systems. However, these "snapshots" could not provide the comprehensive coverage needed to understand fully the cause of the outbursts and the heating that occurs on the white dwarf. The longer mission planned for Astro-2 should make it possible to study several DNe in more detail, and at least one throughout and entire outburst cycle. The American Association of Variable Star Observers (AAVSO), and international clearing house for compiling amateur astronomer observations of variable stars, will provide monitoring of key objects before, during, and after the Astro-2 mission and will provide quick notice of any objects entering the outburst phase. In some cataclysmics, the white dwarf star has a strong magnetic field; this field either partially or totally disrupts the accretion disk and causes the accretion to take place instead onto the magnetic poles of the white dwarf. HUT astronomers want to learn about how accretion occurring in this way differs from disk accretion in DNe. By measuring the light variations with time, the heating processes occurring on the surface of the white dwarf star can be studied. Although some magnetic systems were planned for Astro-1, none were observed successfully. Hence, this will be a new area of investigation for Astro-2. Other Stellar Programs HUT Science Programs G11: Symbiotic Stars (B. Espey et al.) G15: FUV Spectral Atlas of O stars (N. Walborn et al.) H14: OB Star Spectral Atlas G31: Extinction in the Large Magellanic Cloud (G. Clayton et al.) G32: Wolf-Rayet Stars (R. Schulte-Ladbeck et al.) 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 instuments 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. Solar System Investigations HUT Science Programs H13: Solar System Investigations Last but not least are the observations of solar system objects. As with Astro-1, the dynamic processes occurring in the Jovian system (Jupiter and its moons) will be a primary target for HUT. These include the moon Io, the torus of plasma surrounding Io's orbit, the planet itself, and the polar aurorae. The UV emissions from all of these sources are time variable, and are related to one another, at least indirectly. In addition, the separation in time between Astro-1 and Astro-2 is sufficiently large to show possible secondary differences in the state of the Jovian system due to the 11 year solar cycle. (We are close to the minimum of the solar activity cycle now, whereas we were near the maximum back in late 1990.) The HUT wavelength coverage and resolution are almost ideal for this project, which is a favorite of HUT payload specialist Sam Durrance. In addition, HUT scientists plan to observe Venus toward the end of the Astro-2 mission. Venus is somewhat closer to the sun than the Astro telescopes are normally pointed, but not dramatically so, being near 40 degrees away. (The telescopes were pointed about 40 degrees from the sun once on Astro-1, to observe a comet, with no ill effects.) Investigators will search for emission from various molecular species that are excited by incoming solar radiation and particles, as well as evidence of trace constituents such as the noble gases argon and neon. ----------------------------------------------------------------------- EOT