▪ 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

This compilation revises the 1991 listing of atomic data for the lighter elements from hydrogen to gallium. The tabulation emphasizes resonance lines, i.e., lines whose lower level 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 911.753 ?. All but the very weakest known and predicted electric-dipole transitions are included, but no forbidden lines. This paper has attempted to review all data published by the end of 2002.?????The tables contain the best data available to the author on ionization potentials, level designations, 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 ?f. All ion stages with relevant classified lines are included. Individual components resulting from isotope shift and hyperfine structure are listed explicitly for certain species. 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, and gaseous nebulae.?????An Appendix summarizes new data relevant to the similar compilation in Paper II for the elements germanium to uranium.

https://iopscience.iop.org/article/10.1086/377639/pdf

Atomic Data for Resonance Absorption Lines. III. Wavelengths Longward of the Lyman Limit for the Elements Hydrogen to Gallium

A study of gas-phase element abundances reported in the literature for 17 different elements sampled over 243 sight lines in the local part of our Galaxy reveals that the depletions into solid form (dust grains) are extremely well characterized by trends that employ only three kinds of parameters. One is an index that describes the overall level of depletion applicable to the gas in any particular sight line, and the other two represent linear coefficients that describe how to derive each element's depletion from this sight-line parameter. The information from this study reveals the relative proportions of different elements that are incorporated into dust at different stages of grain growth. An extremely simple scheme is proposed for deriving the dust contents and metallicities of absorption-line systems that are seen in the spectra of distant quasars or the optical afterglows of gamma-ray bursts. Contrary to presently accepted thinking, the elements sulfur and krypton appear to show measurable changes in their depletions as the general levels of depletions of other elements increase, although more data are needed to ascertain whether or not these findings are truly compelling. Nitrogen appears to show no such increase. The incorporation of oxygen into solid form in the densest gas regions far exceeds the amounts that can take the form of silicates or metallic oxides; this conclusion is based on differential measurements of depletion and thus is unaffected by uncertainties in the solar abundance reference scale.

https://iopscience.iop.org/article/10.1088/0004-637X/700/2/1299/pdf

A unified representation of gas-phase element depletions in the interstellar medium*

A study of gas-phase element abundances reported in the literature for 17 different elements sampled over 243 sight lines in the local part of our Galaxy reveals that the depletions into solid form (dust grains) are extremely well characterized by trends that employ only three kinds of parameters. One is an index that describes the overall level of depletion applicable to the gas in any particular sight line, and the other two represent linear coefficients that describe how to derive each element's depletion from this sight-line parameter. The information from this study reveals the relative proportions of different elements that are incorporated into dust at different stages of grain growth. An extremely simple scheme is proposed for deriving the dust contents and metallicities of absorption-line systems that are seen in the spectra of distant quasars or the optical afterglows of gamma-ray bursts. Contrary to presently accepted thinking, the elements sulfur and krypton appear to show measurable changes in their depletions as the general levels of depletions of other elements increase, although more data are needed to ascertain whether or not these findings truly compelling. Nitrogen appears to show no such increase. The incorporation of oxygen into solid form in the densest gas regions far exceeds the amounts that can take the form of silicates or metallic oxides; this conclusion is based on differential measurements of depletion and thus is unaffected by uncertainties in the solar abundance reference scale.

A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium

Among the various processes responsible for the formation of the heavy elements in stars, the slow neutron capture process (s-process) is distinguished by the fact that it involves mostly stable isotopes. Therefore, the relevant nuclear physics data can be determined by experiments. With this rather reliable data basis, s-process nucleosynthesis offers an important testground of models for the late stages of stellar evolution, which are supposed to be the s-process site. The empirical counterpart for such models is the so-called classical s-process, a purely phenomenological picture, that is successfully used to derive the resulting abundances as well as information on the physical conditions during the s-process. The status of this classical approach is reviewed with emphasis on the implications for various stellar models of the s-process and in the light of results obtained by stellar spectroscopy. A brief account of the potential s-process chronometers is also presented.

s-process nucleosynthesis-nuclear physics and the classical model

Anisotropy in the beam-foil light source

Analysis of absorption from interstellar Ar I in lightly reddened lines of sight provides information on the warm and hot components of the interstellar medium near the sun. The details of the analysis are limited by the quality of the atomic data. Accurate oscillator strengths for the Ar I lines at 1048 and 1067 A and the astrophysical implications are presented. From lifetimes measured with beam-foil spectroscopy, an f-value for 1048 A of 0.257 +/- 0.013 is obtained. Through the use of a semiempirical formalism for treating singlet-triplet mixing, an oscillator strength of 0.064 +/- 0.003 is derived for 1067 A. Because of the accuracy of the results, the conclusions of York and colleagues from spectra taken with the Copernicus satellite are strengthened. In particular, for interstellar gas in the solar neighborhood, argon has a solar abundance, and the warm, neutral material is not pervasive.

Accurate oscillator strengths for ultraviolet lines of Ar I - Implications for interstellar material

New cascade analysis techniques for determining spontaneous atomic transition probabilities

Lifetime measurements are reported for levels arising from the 5s25d and 5s24f configurations in Sn II. Measured decay curves were jointly analyzed using the Arbitrarily Normalized Decay Curve (ANDC) method to remove the effects of cascade repopulation from the determination of the lifetimes of the 5s25d 2D3/2 level. The branching ratio of the decay of this level to the ground term fine-structure levels 2P1/2 and 2P3/2 was carefully measured, and we have obtained an accurate value for the absorption oscillator strength of the resonance transition to this level at 1400.52 A. The results are discussed in the context of interpreting vacuum ultraviolet absorption spectra observed with the Goddard High Resolution Spectrograph on board the Hubble Space Telescope.

Lifetime Measurements in Sn II

The recently installed 330 kV electrostatic positive ion accelerator at the University of Toledo is described. Experiments have been performed using ions ranging from H + to Hg 2+ and exotic molecules such as HeH + . Most of these experiments involve the beam-foil studies of the lifetimes of excited atomic states and the apparatus used for these experiments is also described. Another beamline is available for ion-implantation. The Toledo heavy ion accelerator facility welcomes outside users.

R. M. Schectman

Papers

Anisotropy in the beam-foil light source

Accurate oscillator strengths for ultraviolet lines of Ar I - Implications for interstellar material

New cascade analysis techniques for determining spontaneous atomic transition probabilities

Lifetime Measurements in Sn II

The Toledo heavy ion accelerator