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Science Team Observing Program for the
Far Ultraviolet Spectroscopic Explorer (FUSE) Satellite

S. Friedman, W. Moos, K. Sembach, G. Kriss, E. Murphy, W. Oegerle
The Johns Hopkins University

and

The FUSE Science, Instrument, and Operations Teams

Introduction

The Far Ultraviolet Spectroscopic Explorer (FUSE) is a Principal Investigator (PI) class NASA astronomy mission that will explore the far-ultraviolet Universe through high-resolution (R=24,000-30,000) spectroscopy in the 905-1195å spectral bandpass. The FUSE mission was designed to provide answers to fundamental questions in several areas of astronomy, and to this end the FUSE Science Team has been given approximately 43% of the observing time in the first three years of the mission to address these problems. (For comparison, Guest Investigators will receive approximately 45% of the observing time over the same period).

The FUSE PI and Science Team will pursue several science objectives that require substantial amounts of observing time and coordinated efforts to obtain and interpret the large data sets that will be produced. These comprehensive programs include:

Studies of deuterium and metal abundances in a wide variety of environments, to test theories of stellar astration of deuterium and to provide Milky Way reference values of D/H for comparison with measurements of D/H in high-redshift quasar absorption line systems.

Investigations of the properties of the hot interstellar medium in the Milky Way and the Magellanic Clouds, to understand how matter and energy are transferred throughout galaxies.

Measurements of the absorption properties of singly ionized helium in the intergalactic medium at ~1 resolution, in order to better understand the column density distribution of these clouds and determine the opacity in the intercloud regions at high redshift.

The Instrument

The FUSE instrument consists of four coaligned telescope mirrors, each with an area of ~1400 cm². Two mirrors are coated with SiC to reflect light from 905-1100Å, and two mirrors are coated with Al+LiF to reflect light longward of 1100Å. Light reaching these mirrors is reflected to four spherical, aberration-corrected holographic diffraction gratings. The dispersed spectra are recorded on two delay-line microchannel plate detectors; each detector is illuminated by one SiC and one LiF channel. The effective area of the instrument changes as a function of wavelength and ranges from 20 to 80 cm².

Three spectrograph entrance slits are available: a 1.25 × 20 arcsec high resolution slit; a 4 × 20 arcsec high throughput slit; and a 30 × 30 arcsec slit for extended objects. A CCD-based Fine Error Sensor (FES), which views a 20 × 20 arcmin field around the slits, maintains pointing to an accuracy of approximately 1 arcsec.

FUSE will be launched in late 1998 and is currently scheduled for a mission duration of three years.

The following table contains a brief summary of predicted exposure times to achieve S/N = 30 per resolution element at the wavelength of the O VI lines (1031.9, 1037.6Å) for typical types of objects that will be observed by FUSE.

Estimated FUSE Exposure Times for S/N = 30 at 1032 Å

Object	Description	V	E(B-V)	F(1425Å) (erg cm^-2 s^-1 Å^-1)	Texp(1032Å) (ksec)	Torb(1032Å) ((orbits)
HD 165955	B3 V	9.19	0.15	1.0x10^-11	1.6	0.8
HD 18100	B1 V	8.46	0.02	5.5x10^-11	0.3	0.1
HD 168941	O9.5 II-III	9.34	0.37	6.8x10^-12	7.0	3.5
HD 12993	O6.5 V	8.95	0.52	4.0x10^-12	12.4	6.2
SK 82	B0 Ia	12.16	0.15	1.0x10^-12	12.8	6.4
3C 273	QSO	12.90	0.03	2.2x10^-13	42.3	21.1
Mrk 79	Sy 1	13.30	0.11	3.0x10^-14	98.0	49.0

Note: On average, we estimate that there will be approximately 2 ksec of integration time available per orbit. Observations of objects that exceed a flux of 10^-10 erg cm^-2 s^-1 Å^-1 at any wavelength within the FUSE bandpass will not be allowed in the first year of operations due to detector safety requirements.

Additional information about the FUSE spacecraft, instrument, and mission can be found on the FUSE homepage at: http://fuse.pha.jhu.edu.

Deuterium Abundances

One of the fundamental questions in astronomy that remains to be answered conclusively is whether the standard Big Bang model (or variant thereof) provides an acceptable description of the origin and evolution of the Universe and, if so, whether the Universe is open or closed. A critical test for the Big Bang paradigm can be provided by measuring the abundances of light elements and their isotopes at different locations in the Universe and determining whether the measured values are consistent with Big Bang nucleosynthesis and the subsequent chemical evolution of the Universe. One such isotope is deuterium, which is a sensitive tracer of the baryonic density in the hot, early Universe. Deuterium locked into stars during their formation is destroyed by stellar nucleosynthesis. Since no significant deuterium production mechanisms exist, it is believed that the net abundance of deuterium in the Universe should decrease with time. Measurements of deuterium in high-redshift (z > 2) quasar absorption line systems hold a great promise for providing an estimate of the primordial abundance of deuterium. Though some of these high-redshift observations imply that the amount of deuterium relative to hydrogen in the early Universe is high (D/H ~ 2 × 10^-4) (e.g., Carswell et al. 1994, MNRAS, 286, L1; Songaila et al. 1997, Nature, 385, 137), recent work using higher quality data for some sight lines (e.g., Tytler et al. 1997, preprint, astro-ph/9612121) indicates that lower ratios inferred for some of the same systems cannot be so easily dismissed. These high redshift observations are plagued by the possibility that some of the absorption attributed to D I may be due to absorption by H I interlopers at a similar velocity, though a recent statistical analysis by Hogan (1997, preprint) suggests that many of these detections may indeed be attributable to D I. Recent measurements of deuterium abundances in the local ISM (e.g., Linsky et al. 1993, ApJ, 402, 694 and 1995, ApJ, 451, 335) have been obtained using the Hubble Space Telescope. Combined with previous results from the Copernicus satellite, these measurements show a roughly uniform ratio of D/H ~ 1.5 × 10^-5 in the local ISM, or about a factor of 10 lower than the higher high-redshift estimates. Chemical evolution models have some difficulty accounting for this difference (but see Scully et al. 1996, ApJ, 462, 960), and therefore it is important to check the validity of the local result for other regions of the Galaxy as well as to test more accurately the idea that there may be variations in the local ratio. Vidal-Madjar (1996, preprint, astro-ph/9612020) has provided a useful discussion of the implications of the local results. FUSE will provide absorption line measurements of deuterium abundances in a wide range of Galactic environments having varying degrees of metallicity and different evolutionary histories. Regions to be explored include the local interstellar medium, distant gas clouds in the disk of the Galaxy, the Milky Way halo, and low redshift (z < 0.3) intergalactic clouds and galactic halos. These measurements will be used to test current theories of the chemical evolution of galaxies and the resulting degree of astration of deuterium. Since heavy element (e.g., O, Mg, S, Fe) production is intimately linked to the rate and degree of chemical processing within galaxies, these studies will include estimates of the abundances of these important elements whenever possible. The D I Lyman series lines are shifted by -82 km s^-1 relative to the H I Lyman series lines. This separation is approximately 8 times the nominal resolution of FUSE and should allow a clean separation of the D I and H I absorption in many situations. The FUSE wavelength region covers all of the Lyman lines except Ly-a, which can be observed with HST. It is important to have access to a large number of lines having a large dynamic range in strength (log fl) since this makes it possible to measure both D I and H I abundances to greater accuracy than is possible with only a single line.

Hot Gas Properties

In the last two decades, considerable efforts have been made to understand the distribution, ionization, and kinematics of hot gas within the Milky Way. The two principal means by which this has been accomplished are X-ray emission observations of the hot (T ~ 10⁶ K) gas created by energetic heating events such as supernovae (e.g., Snowden et al. 1995, ApJ, 454, 643) and ultraviolet absorption line studies with Copernicus, IUE, and the HST (Sembach etal. 1997, ApJ, 480, 216; Savage et al. 1997, ApJ, in press). The X-ray studies have focused primarily on the hot gas emission located within the Local Bubble or on isolated hotspots where there is enhanced emission associated with known structures such as the North Polar Spur or supershells in the Large Magellanic Cloud. Observations of O VI absorption have also been recorded for local material (Jenkins 1978, ApJ, 219, 845), but it is only through measurements of Si IV, C IV, and N V absorption toward distant background sources that it has been possible to begin to characterize the widespread distribution of hot gas within the Milky Way. The absorption line observations have provided a picture of an extended, ionized gaseous halo with a vertical scale height of 3-4 kpc. While it is clear that such a halo must be supplied with energy from the Galactic disk, perhaps from supernova explosions, it is still not understood exactly how this energy is transferred into the halo or how the highly ionized species are formed. It is quite plausible that a combination of formation mechanisms, including conductive interfaces, radiatively cooling gas, and turbulent mixing layers play a role in the creation and destruction of the hot gas (e.g., Shull & Slavin 1994, ApJ, 427, 784), though quantifying their relative contributions and the role of large interstellar structures remain active areas of interest. Of the species suitable for absorption line studies, O VI is the best diagnostic of gas at temperatures between that of the X-ray emission (10⁶ K) and that of the warm ionized medium (10^4-5 K). O VI peaks in abundance in collisional ionization equilibrium near a temperature of ~3x10⁵ K, and its large ionization potential (114 eV) makes it very difficult to photoionize by starlight from hot stars. Other ultraviolet ionized species, in particular Si IV and C IV, have peak abundances at lower temperatures and are subject to ionizing starlight since their ionization potentials lie below the He+ threshold at 54 eV. The FUSE Science Team will explore the properties of the highly ionized interstellar medium through absorption line observations of the O VI doublet (1031.926, 1037.617 Å) along many sight lines through the Galactic disk and halo. A portion of the O VI program will focus on understanding the vertical extent and global distribution of O VI compared to lower ionization species such as C IV and Si IV observable with HST. Both halo stars and extragalactic sources (AGNs, QSOs) will be used as background sources so that entire paths through the halo can be explored. A number of pointings toward stars in the Magellanic Clouds will be used to determine the relationship of O VI absorption and the X-ray properties of known shells and supershells in these galaxies. This program will also characterize the extent, distribution, and kinematics of O VI in the Galactic disk, which will lead to a better understanding of how matter and energy are transferred within the Galaxy. As part of this study of hot gas in the disk, we will obtain observations of O VI in the directions of interesting large scale structures (e.g., X-ray hotspots, SNRs, HI shells) to determine how the in situ interstellar gas properties are modified through the creation of these structures.

Team Projects

In addition to the large investigations previously described, the FUSE Science Team has identified nine important programs to pursue which require more modest exposure times.