JOHNS HOPKINS U N I V E R S I T Y Center for Astrophysical Sciences

FUSE

The Far Ultraviolet Spectroscopic Explorer

FUSE will explore the Universe through high-resolution (lambda/delta lambda = 24,000-30,000) spectroscopy at far ultraviolet wavelengths (905-1195Å), to address fundamental questions related to the origin of the universe. FUSE is scheduled as a three year mission within the NASA Origins program.

Far-UV Spectroscopy: An Unexplored Frontier

The FUSE wavelength region is hitherto largely unexplored. In the 1970s, the Copernicus mission opened the far-UV universe by obtaining spectra of bright, nearby hot stars (within 1 kiloparsec or about 3000 light years of the Sun). Two telescopes, the Hopkins Ultraviolet Telescope (HUT) and the Orbiting Retrievable Far and Extreme Ultraviolet Spartan (ORFEUS) payload, flown on Shuttle missions, have also provided brief but tantalizing glimpses into the FUSE wavelength range. FUSE will be able to observe sources more than 10,000 times fainter than Copernicus, reaching faint, distant objects in our Galaxy and beyond.

The spectral window opened by FUSE will permit the study of many important atoms, ions, and molecules that cannot be investigated otherwise. (Hubble Space Telescope/STIS observations, for instance, only extend only to 1150Å). The wavelength range that FUSE will explore, 905-1195Å, is extremely rich in spectral lines arising within the interstellar gas, the material from which stars and planets form. The FUV range also provides an opportunity to answer important questions about many types of astrophysical objects, such as the nuclear regions of active galaxies and quasars, massive stars, supernovae, planetary nebulae, and the outer atmospheres of cool stars and planets.

Three fundamental parameters, the Hubble expansion rate, the microwave background spectrum, and the abundances of light elements, are keys to our present understanding of how the universe formed and how it evolves, according to the Big Bang theory. The Hubble Space Telescope is making progress in measuring the expansion of the universe, and the Cosmic Background Explorer (COBE) has yielded important information about the microwave background. FUSE will tackle the third parameter through studies of deuterium, or "heavy hydrogen," which was created solely in the Big Bang.

FUSE will measure the abundance of deuterium in a variety of astrophysical environments, from local gas clouds to distant clouds along the lines of sight toward quasars and active galactic nuclei. The measurements will determine the extent to which stellar processing has modified the primordial abundance of deuterium, thereby providing a better understanding of the amount produced in the Big Bang and the subsequent chemical evolution of the universe. This provides a direct means for estimating the "baryonic" (or normal matter) content of the universe.

FUSE and the Interstellar Medium

Much of astronomy is spent looking at different kinds of objects and trying to understand the physical processes going in in the universe. But what if you wanted to learn about the vast regions out there between the stars? These regions, collectively called the interstellar medium (or ISM) are not entirely empty, but contain small amounts of gas and dust. These regions can be studied indirectly by the absorption they cause in the light coming from distant stars and galaxies.

So far, from spectral observations at longer UV and optical wavelengths, we know that there are hot and cold regions in in the ISM, but the extent and distribution of these hot and cold regions throughout the Milky Way are only poorly known. The spectral range covered by FUSE contains a number of key absorption features from the ISM, which makes FUSE an ideal instrument for studying these exceedingly tenous regions.

The FUSE Science Team has taken on the large task of determining the structure of the ISM in the disk and halo of our Galaxy. Furthermore, FUSE is the first satellite with sufficient sensitivty to perform similar studies in the nearby galaxies known as the Magellanic Clouds. These studies will require observations of many stars and objects at many distances and along many lines of sight to piece together an overall picture of the ISM. In the process of doing this, the FUSE spectra of the objects being used as "background" sources will also provide a wealth of information about objects themselves! Each FUSE spectrum will provide information on a wide variety of of science programs.

Other FUSE Science Objectives

Besides measuring the abundance of deuterium and the distribution of hot and cold gas in the ISM, FUSE will make significant contributions to many areas of astronomy as it opens this important region of the spectrum to detailed scrutiny. A few of these areas that are being pursued by the FUSE Science Team are listed below. However, at least half of the observing time with FUSE will be available to Guest Investigators that NASA will select annually. These users will no doubt define many new and exciting observations with FUSE that are not represented here.

*Investigations of highly ionized gas associated with active galactic nuclei, to provide insight into the mechanisms for ionizing gas clouds near massive black holes.

*Searches for the observational signature of the hot intergalactic medium, to determine how the universe evolved at high redshifts.

*Studies of nova and supernova explosions and their remnants, to test theories of heavy element nucleosynthesis and study how supernova shock waves heat the interstellar gas.

*Studies of the hottest atmospheric layers of stars, to provide information about mass loss through stellar winds (hot stars) and the structure of stellar coronae (cool stars).

*Measurements of molecular hydrogen, the primary constituent of the cold interstellar medium, from which protostars and their planetary systems form.

*Investigations of jets and circumstellar disks, to understand the properties of stars in early stages of their evolution.

*Determinations of the abundances of primordial gases in comets and planetary atmospheres, to understand the origin and evolution of our solar system.

For more information:

e-mail: Ken Sembach

The FUSE Project
The Johns Hopkins University
Center for Astrophysical Sciences
Bloomberg Center, Rm 144
3400 North Charles Street
Baltimore, Maryland 21218-2686
USA

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Last modified: 1/98