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

http://iopscience.iop.org/article/10.1086/312795/pdf

Overview of the Far Ultraviolet Spectroscopic Explorer Mission

This compilation extends the 1991 listing of atomic data by Morton to the heavier stable elements from germanium to bismuth Technetium, thorium, and uranium are added because they can live long enough to be astrophysically detectable The tabulation emphasizes resonance lines, ie, lines the lower level of which is the ground state, or an excited fine-structure state of the ground term, and is restricted to wavelengths longward of the H I Lyman limit at 91175 A This paper has attempted to review all data published by mid-1999 and includes some later material     The tables contain the best data available to the author on level designations, ionization potentials, vacuum and air wavelengths, lower and upper energy levels, statistical weights, transition probabilities, natural damping constants (reciprocal lifetimes), oscillator strengths, and the often-used combinations of log gf and log λf All ion stages with classified lines are included Individual components resulting from isotope shift and hyperfine structure are listed explicitly for Rb I, Cs I, Hg I and Hg II, Ti II, and Pb II The accompanying text provides references, explanations for the critical selection of data, and notes indicating where new measurements or calculations are needed     This compilation should be particularly useful in the analysis of interstellar and quasar absorption lines and other astrophysical sites where the density of particles and radiation is low enough to excite only the lowest atomic levels The data also are relevant to the study of stellar atmospheres, particularly those with enhanced abundances of heavy elements

https://iopscience.iop.org/article/10.1086/317349/pdf

Atomic Data for Resonance Absorption Lines. II. Wavelengths Longward of the Lyman Limit for Heavy Elements

The Solar System is embedded in a flow of low-density, warm, and partially ionized interstellar material that has been sampled directly by in situ measurements of interstellar neutral gas and dust in the heliosphere. Absorption line data reveal that this interstellar gas is part of a larger cluster of local interstellar clouds, which is spatially and kinematically divided into additional small-scale structures indicating ongoing interactions. An origin for the clouds that is related to star formation in the Scorpius-Centaurus OB association is suggested by the dynamic characteristics of the flow. Variable depletions observed within the local interstellar medium (ISM) suggest an inhomogeneous Galactic environment, with shocks that destroy grains in some regions. Although photoionization models of the circumheliospheric ISM do an excellent job of reproducing the observed properties of the surrounding ISM, the unknown characteristics of the very low-density hot plasma filling the Local Bubble introduces uncertainty about the source of ionization and nature of cloud boundaries. Recent observations of small cold clouds provide new insight into the processes affecting the local ISM. A fuller understanding of the local ISM can provide insights into the past and future Galactic environment of the Sun, and deeper knowledge of the astrospheres of nearby stars.

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The Interstellar Medium Surrounding the Sun

We use the absorption features from the resonance transitions of neutral argon at 1048 and 1066 A to determine interstellar argon abundances or their lower limits toward nine early-type stars. These features were observed with the interstellar medium absorption profile spectrograph (IMAPS) along sight lines with low reddening and low fractional abundances of molecular hydrogen. Four of the sight lines in our sample have reliably measured interstellar Ar abundances, of which three also have good determinations of N(H I). We find that the interstellar Ar is below its solar and B-star abundance with respect to hydrogen toward ζ Pup, γ2 Vel, and β Cen A with (logarithmic) reduction factors D = -0.37 ± 0.09 dex, D = -0.18 ± 0.10 dex, and D = -0.61 ± 0.12 dex, respectively. While Ar can condense onto the surfaces of dust grains in the interiors of dense clouds, it is unlikely that argon atoms are depleted by this process in the low-density lines of sight considered in this study. Instead, we propose that the relatively large photoionization cross section of Ar makes it much easier to hide in its ionized form than H. In regions that are about half ionized, this effect can lower Ar I/H I by -0.11 to -0.96 dex, depending on the energy of the photoionizing radiation and its intensity divided by the local electron density. We apply this interpretation for the condition of the gas in front of β Cen A, which shows the largest deficiency of Ar. Also, we determine the expected magnitudes of the differential ionizations for He, N, O, Ne, and Ar in the partly ionized, warm gas in the local cloud around our solar system. For the local cloud and others that can be probed by UV studies, the observed Ar I-to-H I ratio may be a good discriminant between two possible alternatives, collisional ionization or photoionization, for explaining the existence of partly ionized regions.

Interstellar Medium Absorption Profile Spectrograph Observations of Interstellar Neutral Argon and the Implications for Partially Ionized Gas

Studies of the abundances of deuterium in different astrophysical sites are of fundamental importance to answering the question about how much deuterium was produced during big bang nucleosynthesis and what fraction of it was destroyed later. With this in mind, we used the Interstellar Medium Absorption Profile Spectrograph (IMAPS) on the ORFEUS-SPAS II mission to observe at a wavelength resolution of 4 km/s (FWHM) the L-delta and L-epsilon absorption features produced by interstellar atomic deuterium in the spectrum of delta Ori A. A chi-square analysis indicated that 0.96 < N(D I)< 1.45e15 cm^{-2} at a 90% level of confidence, and the gas is at a temperature of about 6000K. To obtain an accurate value of N(H I) needed for a determination of the atomic ratio of D to H, we measured the L-alpha absorption features in 57 spectra of delta Ori in the IUE archive. From our measurement of N(H I)= 1.56e20 cm^{-2}, we found that N(D I)/N(H I)= 7.4(+1.9,-1.3)e-6 (90% confidence). Our result for D/H contrasts with the more general finding along other lines of sight that D/H is approximately 1.5e-5. The underabundance of D toward delta Ori A is not accompanied by an overabundance of N or O relative to H, as one might expect if the gas were subjected to more stellar processing than usual.

Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. I. Observations toward delta Orionis by the Interstellar Medium Absorption Profile Spectrograph

The Interstellar Medium Absorption Profile Spectrograph (IMAPS) is an objectivegrating, echelle spectrograph built to observe the spectra of bright, hot stars over the spectral region 950–1150A, below the wavelength coverage of HST. This instrument has a high wavelength resolving power, making it especially well suited for studies of interstellar absorption lines. Following a series of sounding rocket flights in the 1980's, IMAPS flew on its first Shuttle-launched orbital mission in September 1993, as a partner in the ORFEUS-SPAS program sponsored by the US and German Space Agencies, NASA and DARA.

High resolution spectroscopy in the far UV: Observations of the interstellar medium by IMAPS on ORFEUS-SPAS

We have developed a video signal processing system to detect and store photoevents recorded by a magnetically focused, electron-bombarded CCD (EBCCD) image sensor of the type that flew on the Interstellar Medium Absorption Profile Spectrograph (IMAPS). The EBCCD represents a good alternative to detectors that use microchannel plates: the opaque photocathode is on a solid substrate which leads to a very uniform response with a high quantum efficiency. Our experience with sounding rocket missions has demonstrated that this image sensor can perform well in poor vacuum conditions. Previous uses of the EBCCD on IMAPS missions have worked with video signals that were accumulated in an analog fashion. To make the EBCCD more useful for viewing very faint images in the future, we have recently developed new circuitry to detect photoevents in real time and store them discretely. This photon-counting signal processing mode overcomes the CCD readout noise penalty that accumulates when many successive analog frames are added together. The new circuits also provide for an automatic, accurate subtraction of the CCD dark current pattern.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Michael A. Reale

Papers

High resolution spectroscopy in the far UV: Observations of the interstellar medium by IMAPS on ORFEUS-SPAS

Development of a photon-counting capability for the electron-bombarded far-UV image sensor