End of Mission PAO Report for the Hopkins Ultraviolet Telescope

Huntsville, Alabama March 16, 1995

Arthur F. Davidsen Principal Investigator Hopkins Ultraviolet Telescope

The Astro-2 Mission has exceeded the most optimistic expectations held for it by members of the Hopkins Ultraviolet Telescope team. Based on the quantity and quality of the unique new scientific data gathered on this flight, as evaluated with the quick-look data available to us in the Payload Operations Control Center in real time, Astro-2 must be judged an unqualified scientific success.

More than 200 separate successful observations were made of objects selected by HUT investigators, including studies of more than 100 different objects. All of these observations will contribute to answering the questions posed by 14 PI team science programs and 7 guest investigator programs. The observations made with HUT form a unique set of spectrophotometric data in the far ultraviolet band of the electromagnetic spectrum, providing information that has never been obtained previously, and for which no other telescopes are suitable. These data are highly complementary to related data obtained at longer wavelengths with the spectrographs aboard the Hubble Space Telescope.

Some highlights of the observations include:

Spectra of two high redshift quasars will be analyzed to search for evidence of the existence of the primordial intergalactic medium formed in the big bang. Absorption of the quasar light by helium ions that may be spread throughout the vast space between the galaxies may be revealed by the HUT spectra.

Several lower redshift quasars and Seyfert galaxies were observed to study details of their ultraviolet radiation that could help confirm the theory that such objects contain supermassive black holes swallowing matter from their surroundings. Additional observations of specific objects will shed light on the dynamics of the gas clouds in the nuclei of such active galaxies.

Observations of several ellipticals galaxies will help establish what types of stars contribute most significantly to the ultraviolet light emitted by these old stellar systems. Pioneering data obtained with HUT on Astro-1 has led to new views concerning the importance of previously unknown evolutionary paths for old low mass stars. The greatly improved data gathered on Astro-2 will allow us to test these ideas and extend them substantially.

The possible existence of very hot gas in the halo of our galaxy will be studied using several excellent observations of extragalactic objects that were obtained during the mission. Absorption lines produced by highly ionized oxygen and other elements may indicate the presence of substantial quantities of interstellar gas with temperatures of several hundred thousand degrees.

Numerous cataclysmic variable stars were observed in various stages of their outburst cycles. One system was monitored throughout an outburst cycle, while others were observed in quiescence or near the peak of an outburst. The highly magnetic system AM Herculis was observed throughout its binary orbital period and its ultraviolet light was seen to flicker significantly on short time scales. Many of these observations were made with simultaneous coverage by other satellite telescopes sensitive to different wavelength regions.

Supernova remnants in our own galaxy and in the Magellanic Clouds have been observed with HUT, and previously unobserved emission lines of highly ionized gas have been found. These data will provide important new insights into the physics of the interstellar shock waves created when stars explode. Observations of very young remnants will reveal the composition of the material ejected by the exploding stars.

Studies of aurorae on Jupiter were conducted by HUT, with simultaneous high resolution imagery being obtained with the Hubble Space Telescope. Studies of the atmosphere of Jupiter's moon Io and the torus of ionized gas it produces around Jupiter were also made, at a time when volcanic eruptions onIo were actively spewing material into Jupiter's magnetosphere.

Ultraviolet emissions from the atmospheres of Venus and Mars were also observed. Detailed analysis may reveal the presence of several interesting elements, including the noble gases.

Numerous symbiotic star systems were studied to understand the physics of these binary stars containing a hot white dwarf or subdwarf and a cool red giant star. The strong ultraviolet radiation from the hot component illuminates the red giant atmosphere and produces large effects, including strong emission lines of highly ionized oxygen and other elements.

Several isolated hot white dwarf stars were also studied with HUT, which provides the most significant test of theoretical models for the atmospheres of these compact stars. The temperatures and densities of these highly evolved stars will be accurately determined from the HUT data. Exotic hot pre-white dwarf stars were also observed. These rare stars are in a rapidly changing evolutionary state that will lead eventually to the white dwarf stage.

Several starburst galaxies were observed with HUT, providing information on the formation of new stars in such systems. The radiation produced by the hot massive young stars in these galaxies will be studied to determine whether such systems contribute significantly to the metagalactic radiation field and possibly to the ionization of the intergalactic medium.

Several very massive hot young O stars were studied in the Small and Large Magellanic Clouds. These stars emit most strongly in the HUT wavelength region, where they have never before been observed. One of the objects studied is considered to be the most massive star known in the Universe, weighing in at about 190 times the mass of the sun.

Joint observations with WUPPE were conducted to study the extinction of ultraviolet light by interstellar dust in the Magellanic Clouds. The dust in these galaxies is known to differ from that in our own galaxy, and observations with HUT will help illuminate the nature and cause of such differences.

Finally, joint HUT and WUPPE observations of several Wolf-Rayet stars were made, yielding crucial new information concerning these rapidly evolving hot stars and the strong winds that emanate from them.

Conclusion

The surprises which await discovery in the incredible set of data obtained with HUT on the Astro-2 mission remain unknown to us, of course. We look forward to uncovering those surprises in the months and years ahead. Some of those surprises will undoubtedly overshadow anything that we were expecting to see in the data that has now been obtained and stored in our computers. The secrets of the universe remain safe for now, but it is just a matter of time before they will be exposed for all to see.