HUT scientists have developed more than a dozen distinctly different scientific goals to be pursued on the Astro-2 mission. Many of these are new programs that take advantage of HUT's increased sensitivity and the longer planned mission duration. Others represent further detailed investigation of topics already addressed with Astro-1 observations. A few of the topics to be investigated are described below.

For a more in-depth look at the various programs there is the HUT Science Program Overview for Astro-2. This includes links to science program descriptions written by other astronomers that will be using HUT during ASTRO-2.

The HUT science team's primary goal is to search for the primordial intergalactic medium (IGM), a hypothesized gas that may be spread throughout the Universe between the galaxies. This gas was created in the Big Bang and subsequently condensed to form the galaxies and stars we see today. Astronomers have long been interested in observing the IGM directly, as it forms a key link in the chain of events leading from the origin of the Universe to its present-day structure. A detailed description of this goal by HUT project director Arthur Davidsen is included elsewhere in this information packet.

Active galactic nuclei (AGN) and quasars are thought to produce their extraordinary luminosities by the accretion of matter onto a black hole with a mass exceeding 1 billion suns in some cases. Theory and observation both suggest that most of the energy generated in these objects is emitted as far- and extreme-ultraviolet radiation from an accretion disk surrounding the black hole. Astronomers will use HUT's unique wavelength sensitivity to check the validity of this picture for several relatively nearby, bright AGN, where the most detailed data can be obtained.

The 16-day length of the Astro-2 mission will make it possible to study time variations in the amount of ultraviolet light emitted by the active galaxy NGC 4151. A half dozen separate pointings will enable Hopkins astronomers to learn more about the location, composition, density, and temperature of the cold gas absorbing the ultraviolet radiation in this active galaxy. Additional HUT observations will attempt to map the location of the shock-heated gas in NGC 1068 discovered during Astro-1.

While most galaxies appear to have developed their star populations billions of years ago, some are currently forming new stars at a prodigious rate in a process known as a starburst. The massive, hot young stars being formed are strong emitters of far-ultraviolet light. Scientists will use HUT's sensitivity to the shortest UV wavelengths to characterize the emission of starburst galaxies, whose radiation may play an important role in ionizing the intergalactic medium. Studies of these galaxies may also yield clues about conditions much earlier in the history of the Universe, when the majority of galaxies may have gone through a starburst phase in their initial formation.

The tenuous regions of gas and dust between the stars is difficult to study directly. On Astro-2 HUT astronomers will use several approaches to learn about this interstellar medium. They will observe old supernova remnant shock waves as they encounter interstellar "clouds"; they will observe molecular hydrogen gas in regions excited by shock waves and by bright ultraviolet starlight; and they will observe the effects of interstellar dust grains as they absorb the light from background stars. All of these should lead to improved understanding of these vast, normally "invisible" regions between the stars.

Supernova explosions represent the death throes of certain stars. As they tear themselves apart, they also expose their interiors for direct inspection. Astro-2 scientists will observe several young supernova remnants where this material is still visible, and try to understand the processes that created the elements in the stars. In particular, HUT will observe a 1000 year old supernova remnant called SN 1006, using a chance alignment with a background star to search for iron in the expanding ejecta. If found, this unique observation would confirm one of the most basic and important predictions of supernova models.

Massive young stars and old, highly evolved low-mass stars are both much hotter than the majority of stars and consequently emit strongly at the far-UV wavelengths that can be studied with HUT. Several of the hottest young stars in our Galaxy and in the nearby Magellanic Clouds will be observed in order to determine their temperatures, masses and radii. HUT will also be used to investigate a number of very old stars, including white dwarfs, which have shed their outer layers to reveal the very hot, dense cores that exist inside even relatively cool stars like the sun.

Certain cataclysmic variables undergo occasional outbursts in which their brightness increases by factors of 100 or more. The time between outbursts is not regular, but varies from two weeks to months or longer. The cause of these outbursts is poorly understood. On Astro-2, HUT scientists will study several such binary stars, hoping to follow at least one system through its entire outburst cycle to understand what happens and why. They will also observe binaries containing white dwarfs with strong magnetic fields that affect the way material is accreted by the white dwarf star. The astronomers will rely on a world-wide network of amateur astronomers to monitor the brightest cataclysmic variables and provide information on outbursts.

These objects are composed of two stars in the late stages of evolution orbiting each other at a distance similar to that of the Earth from the Sun. One star is a hot white dwarf which irradiates its cooler red giant companion. Hopkins astronomers will make far-ultraviolet observations of several symbiotic stars with both HUT and WUPPE. They will study the effect of the hot star's intense ultraviolet radiation on the outer layers and stellar wind of the red giant star. This will provide a unique perspective on the structure and evolution of red giant star atmospheres.

HUT scientists hope to study the atmosphere of Venus to understand the processes that excite the ultraviolet emissions from this planet. HUT has the sensitivity and spectral resolution to identify emissions from numerous molecular and atomic species in the atmosphere, and can search for the presence of important trace elements such as argon and neon. Venus will be closer to the sun than normally allowed for HUT observations (only about 40 degrees away). However, HUT observed a comet on Astro-1 at about this angular separation from the sun, with no ill effects. Observations of Jupiter and Io will also be made to study differences due to the changing solar input compared with Astro-1 observations.

Dr. Blair also provides a HUT Science Program Overview for Astro-2. This is a more in-depth look at the various programs and includes links to science program descriptions written by other astronomers that will be using HUT during ASTRO-2.

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