Erik Wilkinson

Overview of the Far Ultraviolet Spectroscopic Explorer Mission

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905-1187 Angstrom, with a high spectral resolution. The instrument consists of four co-aligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al :LiF coatings for optimum reflectivity between approximately 1000 and 1187 Angstrom, and the other two channels use SiC coatings for optimized throughput between 905 and 1105 Angstrom. The gratings are holographically ruled to correct largely for astigmatism and to minimize scattered light. The microchannel plate detectors have KBr photocathodes and use photon counting to achieve good quantum efficiency with low background signal. The sensitivity is sufficient to examine reddened lines of sight within the Milky Way and also sufficient to use as active galactic nuclei and QSOs for absorption-line studies of both Milky Way and extragalactic gas clouds. This spectral region contains a number of key scientific diagnostics, including O VI, H I, D I, and the strong electronic transitions of H-2 and HD.

On-Orbit Performance of the Far Ultraviolet Spectroscopic Explorer Satellite

The launch of the Far Ultraviolet Spectroscopic Explorer (FUSE) has been followed by an extensive period of calibration and characterization as part of the preparation for normal satellite operations. Major tasks carried out during this period include the initial coalignment, focusing, and characterization of the four instrument channels and a preliminary measurement of the resolution and throughput performance of the instrument. We describe the results from this test program and present preliminary estimates of the on-orbit performance of the FUSE satellite based on a combination of these data and prelaunch laboratory measurements.

THE COSMIC ORIGINS SPECTROGRAPH

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F ? 1.0 ? 10?14?erg?cm?2?s?1 ??1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly? absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

Abundances of Deuterium, Nitrogen, and Oxygen in the Local Interstellar Medium: Overview of First Results from the FUSE Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D i ,N i, and O i along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 to 179 pc. Five of the sight lines are within the Local Bubble, and two penetrate the surrounding H i wall. Reliable values of N(H i) were determined for five of the sight lines from Hubble Space Telescope (HST) data, International Ultraviolet Explorer (IUE) data, and published Extreme Ultraviolet Explorer (EUVE) measurements. The weighted mean of D i/H i for these five sight lines is ð1:52 � 0:08 Þ� 10 � 5 (1 � uncertainty in the mean). It is likely that the D i/H i ratio in the Local Bubble has a single value. The D i/O i ratio for the five sight lines within the Local Bubble is ð3:76 � 0:20 Þ� 10 � 2 .I t is likely that O i column densities can serve as a proxy for H i in the Local Bubble. The weighted mean for O i/H i for the seven FUSE sight lines is ð3:03 � 0:21 Þ� 10 � 4 , comparable to the weighted mean ð3:43 � 0:15 Þ� 10 � 4 reported for 13 sight lines probing larger distances and higher column densities. The FUSE weighted mean of N i/H i for five sight lines is half that reported by Meyer and colleagues for seven sight lines with larger distances and higher column densities. This result combined with the variability of O i/N i (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O i ,N i cannot be used as a proxy for H i or as a metallicity indicator in the LISM. Subject headings: cosmology: observations — Galaxy: abundances — ISM: abundances — ISM: evolution — ultraviolet: ISM

The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in and near the Galaxy

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the O VI λλ1031.926, 1037.617 absorption lines associated with gas in and near the Milky Way, as detected in the spectra of a sample of 100 extragalactic targets and two distant halo stars. We combine data from several FUSE Science Team programs with guest observer data that were public before 2002 May 1. The sight lines cover most of the sky above Galactic latitude |b| > 25°—at lower latitude the ultraviolet extinction is usually too large for extragalactic observations. We describe the details of the calibration, alignment in velocity, continuum fitting, and manner in which several contaminants were removed—Galactic H2, absorption intrinsic to the background target and intergalactic Lyβ lines. This decontamination was done very carefully, and in several sight lines very subtle problems were found. We searched for O VI absorption in the velocity range -1200 to 1200 km s-1. With a few exceptions, we only find O VI in the velocity range -400 to 400 km s-1; the exceptions may be intergalactic O VI. In this paper we analyze the O VI associated with the Milky Way (and possibly with the Local Group). We discuss the separation of the observed O VI absorption into components associated with the Milky Way halo and components at high velocity, which are probably located in the neighborhood of the Milky Way. We describe the measurements of equivalent width and column density, and we analyze the different contributions to the errors. We conclude that low-velocity Galactic O VI absorption occurs along all sight lines—the few nondetections only occur in noisy spectra. We further show that high-velocity O VI is very common, having equivalent width >65 mA in 50% of the sight lines and equivalent width >30 mA in 70% of the high-quality sight lines. The central velocities of high-velocity O VI components range from |vLSR| = 100 to 330 km s-1; there is no correlation between velocity and absorption strength. We discuss the possibilities for studying O VI absorption associated with Local Group galaxies and conclude that O VI is probably detected in M31 and M33. We limit the extent of an O VI halo around M33 to be 200 km s-1 occurs along all sight lines in the region l = 180°-300°, b > 20°.

The Discovery of Argon in Comet C/1995 O1 (Hale-Bopp)

On 1997 March 30.14, we observed the EUV spectrum of the bright comet C/1995 O1 (Hale-Bopp) at the time of its perihelion, using our Extreme Ultraviolet Spectrograph sounding-rocket telescope/spectrometer. The spectra reveal the presence H Lyβ, O+, and, most notably, argon. Modeling of the retrieved Ar production rates indicates that comet Hale-Bopp is enriched in Ar relative to cosmogonic expectations. This in turn indicates that Hale-Bopps deep interior has never been exposed to the 35-40 K temperatures necessary to deplete the comets primordial argon supply.

Optical Structure and Proper-Motion Age of the Oxygen-rich Supernova Remnant 1E 0102–7219 in the Small Magellanic Cloud*

We present new optical emission-line images of the young SNR 1E 0102-7219 in the SMC obtained with the ACS on HST. This object is a member of the oxygen-rich class of SNRs showing strong oxygen, neon, and other metal-line emissions in its optical and X-ray spectra, and an absence of hydrogen and helium. The progenitor of 1E 0102-7219 may have been a Wolf-Rayet star that underwent considerable mass loss prior to exploding as a Type Ib/c or IIL/b supernova. The ejecta in this SNR are generally fast-moving (V > 1000 km s-1) and emit as they are compressed and heated in the reverse shock. In 2003 we obtained optical [O III], Hα, and continuum images with the ACS Wide Field Camera. The [O III] image through the F475W filter captures the full velocity range of the ejecta and shows considerable high-velocity emission projected in the middle of the SNR that was Doppler-shifted out of the narrow F502N bandpass of a previous WFPC2 image from 1995. Using these two epochs separated by ~8.5 yr, we measure the transverse expansion of the ejecta around the outer rim in this SNR for the first time at visible wavelengths. From proper-motion measurements of 12 ejecta filaments, we estimate a mean expansion velocity for the bright ejecta of ~2000 km s-1 and an inferred kinematic age for the SNR of ~2050 ± 600 yr. The age we derive from HST data is about twice that inferred by Hughes et al. from X-ray data, although our 1 σ error bars overlap. Our proper-motion age is consistent with an independent optical kinematic age derived by Eriksen et al. in 2003 using spatially resolved [O III] radial-velocity data. We derive an expansion center that lies very close to conspicuous X-ray and radio hot spots, which could indicate the presence of a compact remnant (neutron star or black hole).

On-orbit performance of the Far Ultraviolet Spectroscopic Explorer (FUSE)

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched into orbit on June 24, 1999. FUSE is now making high resolution ((lambda) /(Delta) (lambda) equals 20,000 - 25,000) observations of solar system, galactic, and extragalactic targets in the far ultraviolet wavelength region (905 - 1187 angstroms). Its high effective area, low background, and planned three year life allow observations of objects which have been too faint for previous high resolution instruments in this wavelength range. In this paper, we describe the on- orbit performance of the FUSE satellite during its first nine months of operation, including measurements of sensitivity and resolution.

FAR-ULTRAVIOLET SENSITIVITY OF THE COSMIC ORIGINS SPECTROGRAPH

We demonstrate that the G140L segment B channel of the Cosmic Origins Spectrograph (COS) recently installed on the Hubble Space Telescope (HST) has an effective area consistent with ∼ 10 cm in the bandpass between the Lyman edge at 912 Å and Lyman β, rising to a peak in excess of 1000 cm longward of 1130 Å. This is a new wavelength regime for HST and will allow opportunities for unique science investigations. In particular, investigations seeking to quantify the escape fraction of Lyman continuum photons from galaxies at low redshift, determine the scale-length of the hardness variation in the metagalactic ionizing background over the redshift range 2 < z . 2.8, measure the ratio of CO to H2 in dense interstellar environments with AV > 3, or harness the high temperature diagnostic power of the O VI λλ 1032, 1038 doublet can now be carried out with unprecedented sensitivity. Subject headings: instrumentation: spectrographs, ultraviolet: generalWe demonstrate that the G140L Segment B channel of the Cosmic Origins Spectrograph recently installed on the Hubble Space Telescope (HST) has an effective area consistent with ~10 cm2 in the bandpass between the Lyman edge at 912 ? and Lyman ?, rising to a peak in excess of 1000 cm2 longward of 1130 ?. This is a new wavelength regime for HST and will allow opportunities for unique science investigations. In particular, investigations seeking to quantify the escape fraction of Lyman continuum photons from galaxies at low redshift, determine the scale length of the hardness variation in the metagalactic ionizing background over the redshift range 2 3 can now be carried out with unprecedented sensitivity.

Identification of Fe II emission lines in FUSE stellar spectra

We identify two complexes of Fe II emission lines in far-ultraviolet spectra of the stars α TrA and HD 104237. Using spectra from both the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST), we show that these emission lines, which represent the majority of previously unidentifed emission features in cool star spectra between 912 and 1180 A, are fluorescent decays in Fe II following excitation by H Lyα. Specifically, following photoexcitation from the third term (4s a 4D) of Fe II, subsequent decays are observed to the two lowest terms (4s a 6D and 3d7 a 4F) which are observed near 1100 and 1135 A, respectively. Decays to higher terms, and hence longer wavelengths, also are clearly seen in the STIS spectra. Differences in the fluorescent Fe II spectra of α TrA and HD 104237 are tentatively identified as resulting from differences in the intrinsic width of the density-weighted H Lyα radiation fields. The additional Fe II lines observed in α TrA result from a broadened H Lyα profile. Two features near 1060 A appear to be fluorescent lines of Cr II, also excited by H Lyα.