▪ Abstract The Goddard High-Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) has yielded precision abundance results for a range of interstellar environments, including gas in the local medium, in the warm neutral medium, in cold diffuse clouds, and in distant halo clouds. Through GHRS studies, investigators have determined the abundances of elements such as C, N, O, Mg, Si, S, and Fe in individual interstellar clouds. These studies have provided new information about the composition of interstellar dust grains, the origin of the Galactic high-velocity cloud system, and the processes that transport gas between the disk and the halo. Precision measurements of the interstellar D to H ratio and of the abundances of r- and s-process elements have also provided fiducial reference values for cosmological and stellar evolutionary observations and theoretical models.

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Interstellar abundances from absorption-line observations with the hubble space telescope

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

It is argued that supermassive accretion in disks may explain the flat optical continuum observed from some QSOs and Seyfert galaxies. With the aid of energy considerations it is shown that a luminous QSO may be powered by accretion onto a black hole with a mass corresponding to that of 100 million solar masses. It is proposed that the blue continuum component of 3C 273 is thermal continuum emission from the inner part of an accretion disk that powers 3C 273. The present model suggests that many QSOs and Seyfert galaxies may show a power law continuum plus thermal emission from the disk at optical or UV frequencies. Many such objects show indeed a flattening or upturn of their continua in the blue. The QSO PHL 957 shows a flat continuum and indications of a cutoff at the highest observed frequencies.

Thermal continuum from accretion disks in quasars

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

Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 z abs ≤ 4.2) Damped Lyman- α systems (DLAs) capitalising on observations performed with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). Methods. We identify 77 DLAs/strong sub-DLAs, with log N (H I) ≥ 20 and z abs >1.8, which have data that include redshifted H 2 Lyman and/or Werner-band absorption lines. This sample of H I, H 2 and metal line measurements, performed in an homogeneous manner, is more than twice as large as our previous sample (Ledoux et al. 2003) considering every system in which searches for H 2 could be completed so far, including all non-detections. Results. H 2 is detected in thirteen of the systems, which have molecular fractions of values between f $\simeq$ 5$\times$10 -7 and f $\simeq$ 0.1, where f = 2 N (H 2 )/(2 N (H 2 )+N(H I)). Upper limits are measured for the remaining 64 systems with detection limits of typically log N (H 2 )~14.3, corresponding to log f ≥ -1.3 (i.e., 1/20th solar), with X = Zn, S or Si, have molecular fractions log f > -4.5, while H 2 is detected – regardless of the molecular fraction – in ∼ 50% of them. In contrast, only about 4% of the [X/H] f > -4.5. We show that the presence of H 2 does not strongly depend on the total neutral hydrogen column density, although the probability of finding log f > -4.5 is higher for log N (H I) ≥ 20.8 than below this limit (19% and 7% respectively). The overall H 2 detection rate in log N (H I) ≥ 20 DLAs is found to be about 16% (10% considering only log f > -4.5 detections) after correction for a slight bias towards large N (H I). There is a strong preference for H 2 -bearing DLAs to have significant depletion factors, [X/Fe] > 0.4. In addition, all H 2 -bearing DLAs have column densities of iron into dust grains larger than log $N({\rm Fe})_{\rm dust}$~ 14.7, and about 40% of the DLAs above this limit have detected H 2 lines with log f > -4.5. This demonstrates the importance of dust in governing the detectability of H 2 in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H 2 -bearing DLAs nor that of the molecular fraction in systems with H 2 detections over the redshift range 1.8 z abs ≤ 3.

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Molecular hydrogen in high-redshift damped Lyman-α systems : the VLT/UVES database

A novel sounding rocket payload consisting of a slitless objective grating spectrograph with no transmission elements in the optical train (or detector) is described. This instrument, called the interstellar medium absorption profile spectrograph (IMAPS), is designed to provide continuous coverage over the wavelength range of 950-1150 A; it has an effective collecting area of about 4 sq cm and can record spectra of pointlike sources at a wavelength resolution of 0.004 A and with a sample interval of 0.002 A. The successful use of this instrument aboard a Black Brant rocket is described.

IMAPS: A High-Resolution, Echelle Spectrograph To Record Far-Ultraviolet Spectra Of Stars From Sounding Rockets

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

Spectra of the radio-quiet quasar PHL 957 were obtained at a 200-in. telescope with a Cassegrain image tube, a Cassegrain multichannel spectrometer, and an integrating television camera on the spectrograph. The image-tube spectra revealed 64 absorption lines between 3200 and 6800 A. The absolute energy distribution indicates that this quasar is one of the most luminous known. The absorption line redshifts and line profiles observed are discussed.

The spectrum of the quasi-stellar object PHL 957.

Fabry-Perot/CCD observations of /S III/ and /S II/ emissions from the Jupiter plasma torus

The recent availability of large format CCD's with high quantum efficiency makes it possible to achieve significant advances in high dispersion astronomical spectroscopy. An echelle CCD combination excels or equals other techniques presently available, and offers the advantage of complete spectral coverage of several thousand Angstroms in a single exposure. Attention is given to experiments which were conducted with a CCD camera head and an echelle spectrograph on a 4-meter telescope. It was found possible to achieve a signal-to-noise ratio of 150/1 on a 13th magnitude star at 6000 A in a two-hour exposure at 0.16 A/pixel, limited primarily by photon statistics. For fainter objects, readout noise is the limiting factor in precision. For 20 electron rms readout noise, an S/N = 15/1 at 18th magnitude is expected, all other things being equal.

Paul Zucchino

Papers

IMAPS: A High-Resolution, Echelle Spectrograph To Record Far-Ultraviolet Spectra Of Stars From Sounding Rockets

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

The spectrum of the quasi-stellar object PHL 957.

Fabry-Perot/CCD observations of /S III/ and /S II/ emissions from the Jupiter plasma torus

Echelle Spectroscopy With A Charge-Coupled Device (CCD)