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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
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NASA NTRS1986

Elemental abundance determination in gaseous nebulae from IUE lines

A method for obtaining the C/O elemental abundance ratio for high excitation H II regions is discussed. This is applied to rederive C/O for the Orion Nebula from IUE observations using improved atomic data and extinction corrections. It is found that C/H is 20 to 40% lower than previously determined. When recombinations contribute importantly to (O II) 2470 and 7325 A emission, use of these lines for assessing the O(+) ionic abundance with the so-called empirical method is inadvisable.

Rubin, Robert H.↗
NASA NTRS1987

Oscillator strengths for S I, S II, and S III

A series of calculations for atomic data of various sulfur and oxygen ions is examined. Recent observations of the Io plasma torus obtained with the Voyager UV Spectrometer, the IUE satellite short wavelength spectrograph, and the rocket-borne faint object telescope are discussed. The calculation of oscillator strengths for S II, the P I sequence, S I, and S III in terms of configuration interaction effects is described. The derivation of orbital wave functions is considered. The use of the close coupling method to estimate collision strengths is studied. The accuracy of these calculations depends on: (1) the number of states used in the close coupling expansion; (2) resonance contributions to the thermally averaged collision strength; and (3) the quality of the target state wave functions. Tables of the derived oscillator strengths are presented.

Ho, Y. K.↗
NASA NTRS1987

Atomic parameters for carbon-like S XI

Calculations of radiative rates and collision strengths of S XI for 46 levels of the six configurations 2s2 2p2, 2s 2p3, 2p4, 2s2 2p 3s, 2s2 2p 3p, and 2s2 2p 3d are presented concurrently so that all the atomic data are self-consistent. Collision strengths are calculated at an incident electron energy of 25 Rydbergs above the ground level 2p2(3P0) in order to include collisional excitation to the highest level considered. Good agreement with previous results is found, and it is suggested that the given electric dipole rates are accurate to within at least 20 percent.

Bhatia, A. K.↗
NASA NTRS1988

Electron density-sensitive line ratios for Fe XII in the solar corona

The collision strengths for electron-impact excitation of fine-structure transitions from the ground 3s23p3 configuration to the excited 3s3p4 configuration in Fe XII are presented. These are calculated by transforming the K-matrices obtained in a nonrelativistic R-matrix method to K-matrices in intermediate coupling using term-coupling coefficients which take into account the effect of breakdown of target LS coupling. Extensive configuration-interaction wave functions are used to represent the target states. The effective collision strengths are determined assuming a Maxwellian distribution of electron energies. Rydberg series of resonances converging to the excited state thresholds are explicitly included in the calculation. The relative level populations and emission-line strengths for Fe XII are calculated using the new atomic data presented in this paper and that published by Tayal, Henry, and Pradhan (1987) and Tayal and Henry (1986) over a wide range of electron densities and temperatures (1-2 x 10 to the 6th K). Present results for collision strengths and emission-line strengths are compared with the previous calculation of Flower (1977). Some significant differences are noted.

Tayal, S. S.↗
NASA NTRS1988

Io: IUE observations of its atmosphere and the plasma torus

Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent.

Ballester, G. E.↗
NASA NTRS1988

The critical densities for some emission lines

The critical densities for some emission lines of astronomical interest, especially in the UV range, are calculated on the basis of recent atomic data. Many lines can be used to estimate the densities between 100,000-100 million/cu cm. It is suggested that the intensity ratios of forbidden ArIV 2869/2854 A and forbidden NeIV 2425/2422 A line pairs are good density indicators.

Zheng, Wei↗
NASA NTRS1989

The winds of high luminosity late-type bright stars

The occurrence and characteristics of the Fe II line asymmetries were studied to determine the radial dependence of the wind velocity for each star. The dependence of the Fe II profiles on spectral type and luminosity class and thus the variation of the velocity fields with stellar type was also investigated. This allows the generality of the results reported for alpha Ori by Carpenter (1984b) to be judged. In addition, new atomic data was used along with observations of the C II (UV 0.01) multiplet to estimate N(sub e) in the stellar winds. Measures of relative Fe II fluxes can be used in a probability-of-escape model to determine the opacity and hydrogen column density versus height in the chromosphere of each star. Finally, analysis of the fluorescent Fe II lines (pumped by Ly alpha) near 2507 A will yield estimates of the intrinsic stellar Ly alpha flux that cannot be measured directly because of interstellar and circumstellar absorption. One important goal of the effort was to acquire high resolution spectra of the whole 2300 to 3200 A region of 13 luminous K and M stars as a data base that will be enormously valuable in planning observations with the Hubble Space Telescope High Resolution Spectrograph. It is also proposed to follow up the recent discovery of significant variations in the Fe II chromospheric emission line profiles from the M-giant Gamma Cru for the purpose of determining the underlying cause of the variations.

Stencel, Robert E.↗
NASA NTRS1991

Emission processes and dynamics of hot gases in astrophysics

A detailed model was developed for Kepler's supernova remnant (SNR). Observations of the SNR revealed a strong interaction with the surrounding circumstellar medium, which was studied through both analytical and numerical calculations. Effects were studied of electron thermal conduction on the structure of radiative interstellar shock waves. An explanation is sought for the observed line emission from metal rich ejecta in SNR, incorporating atomic data. Light echoes around SN 1987A was also studied. Analysis of infrared and scattered circumstellar light echoes was accomplished with early observations to set limits on the mass of circumstellar dust. Work was completed on the emission from heavy element gas ejected in the supernova explosion of massive stars. It was assumed that a radioactive energy source was present and calculated the detailed heating and ionization of the gas. The evolution was studied of SNR in the very high pressure environment of a starburst galaxy.

Chevalier, Roger A.↗
NASA NTRS1992

Improved calculations for the C III 1907,1909 and Si III 1883,1892 electron density sensitive emission-line ratios, and a comparison with IUE observations

Atomic data are used in conjunction with the statistical equilibrium code of Dufton (1977) to calculate relative C III and Si III level populations, and hence emission-line strengths for a range of electron temperatures and densities. It is assumed that photoexcitation and deexcitaton rates are negligible in comparison with the corresponding collisional rates, that ionization to and recombination from other ionic levels are slow compared with bound-bound rates, and that all transitions are optically thin. The observed values of R1 and R2 for several planetary nebulae and a symbiotic star, measured from high-resolution spectra obtained with the IUE satellite, lead to electron densities that are compatible, and are also in good agreement with those deduced from line ratios in other species.

Keenan, F. P.↗
NASA NTRS1983

Proportional counters as low energy photon detectors

We investigated the response of an argon-methane proportional counter to monochromatic X-rays in the range 99 to 277 eV. The apparent nonlinearities in mean pulse height as a function of photon energy and the detailed shape of the pulse height distribution for each energy can be predicted quite precisely using the extensive atomic data available for argon. Based on this understanding, we propose a semi-empirical system for using a limited amount of calibration data to predict the full response of counters employing less well-characterized gases. Such accurate models of proportional counters are required to maximize the spectral information that can be derived from observed pulse height distributions.

Jahoda, Keith↗
NASA NTRS1992

Electron density diagnostics for gaseous nebulae involving the O 4 intercombination lines near 1400 A

Theoretical O IV electron density sensitive emission line ratios, determined using electron impact excitation rates calculated with the R-matrix code, are presented for R(sub 1) = I(1407.4 A)/I(1401.2 A), R(sub 2) = I(1404.8 A)/I(1401.2A), R(sub 3) = I(1399.8 A)/(1401.2 A), and R(sub 4) = I(1397.2 A)/I(1401.2 A). The observed values of R(sub 1)-R(sub 4), measured from high resolution spectra obtained with the International Ultraviolet Explorer (IUE) satellite, lead to electron densities that are compatible, and which are also in good agreement with those deduced from line ratios in other species. This provides observational support for the accuracy of the atomic data adopted in the present calculations.

Keenan, F. P.↗
NASA NTRS1992

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

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.

Federman, S. R.↗
NASA NTRS1993

The optically thin C III spectrum - Line and multiplet intensities

C III line/multiplet intensities expected under optically thin conditions are presented over the density/ temperature ranges 4.0 - 12.0 and 4.6 - 5.0 (40,000 - l00,000 K). These improved values are obtained from a hybrid level/term calculation which makes use of the most recently available atomic data and extends the treatment down to lower densities than were achieved with our previous term representation. Some illustrative applications are given, including a brief description of the importance of the present data for interpretation of the strong C III line emission from carbon Wolf-Rayet stars.

Bhatia, A. K.↗
NASA NTRS1993

Infrared coronal emission lines and the possibility of their laser emission in Seyfert nuclei

Results are presented from detailed balance calculations, and a compilation of atomic data and other model calculations designed to support upcoming ISO and current observing programs involving IR coronal emission lines, together with a table with a complete line list of infrared transitions within the ground configurations 2s2 2p(k), 3s2 3p(k), and the first excited configurations 2s 2p and 3s 3p of highly ionized astrophysically abundant elements. The temperature and density parameter space for dominant cooling via IR coronal lines is presented, and the relationship of IR and optical coronal lines is discussed. It is found that, under physical conditions found in Seyfert nuclei, 14 of 70 transitions examined have significant population inversions in levels that give rise to IR coronal lines. Several IR coronal line transitions were found to have laser gain lengths that correspond to column densities of 10 exp 24-25/sq cm which are modeled to exist in Seyfert nuclei. Observations that can reveal inverted level populations and laser gain in IR coronal lines are suggested.

Greenhouse, Matthew A.↗
NASA NTRS1993

Transition probabilities for the 3s2 3p(2P0)-3s3p2(4P) intersystem lines of Si II

Intensity ratios of lines of the spin-changing 'intersystem' multiplet of S II (4P yields 2P0) at 234 nm have been used to determine electron densities and temperatures in a variety of astrophysical environments. However, the accuracy of these diagnostic calculations have been limited by uncertainties associated with the available atomic data. We report the first laboratory measurement, using an ion-trapping technique, of the radiative lifetimes of the three metastable levels of the 3s3p2 4P term of Si II. Our results are 104 +/- 16, 406 +/- 33, and 811 +/- 77 micro-s for lifetimes of the J = 1/2, 5/2, and 3/2 levels, respectively. A-values were derived from our lifetimes by use of measured branching fractions. Our A-values, which differ from calculated values by 30 percent or more, should give better agreement between modeled and observed Si II line ratios.

Calamai, Anthony G.↗
NASA NTRS1993

The International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas

This was the fourth in a series of colloquia begun at the University of Lund, Sweden in 1983 and subsequently held in Toledo, Ohio and Amsterdam, The Netherlands. The purpose of these meetings is to provide an international forum for communication between major users of atomic spectroscopic data and the providers of these data. These data include atomic wavelengths, line shapes, energy levels, lifetimes, and oscillator strengths. Speakers were selected from a wide variety of disciplines including astrophysics, laboratory plasma research, spectrochemistry, and theoretical and experimental atomic physics.

Sugar, J.↗
NASA NTRS1994

Coronal electron density diagnostic from Fe XII

We present observations of the forbidden coronal lines Fe XII 1242 A and 1349 A from active regions and from two flares, obtained by the SO82B slit spectrograph onboard Skylab. The line intensity ratio R = I(1242 A)/I(1349 A) is sensitive to electron density. We have calculated this ratio using recent atomic data, and obtained coronal electron densities at T = 1.5 x 10(exp 6) K for our observations. We find a range in N(sub e) of (0.5 to 7.2) x 10(exp 9)/cm(exp -3) for active regions, which is in good agreement with previous results from other diagnostic ratios in this temperature range, and of approximately (0.9 to 12) x 10(exp 9)/cm(exp -3) (or higher) for flares, which is generally low compared to previous flare results. The flare values employ particularly weak 1349 A observations and may not be reliable. From an observation of an active region just inside the solar limb, giving the best coverage in our data of both line profiles, we find a line width (FWHM) for both lines of 0.20 A, which corresponds to a nonthermal velocity of 18 km/sec.

Cook, J. W.↗
NASA NTRS1995

The neutral oxygen spectrum. 1: Collisionally excited level populations and line intensities under optically thin conditions

This is the first paper in a projected program to produce quantitative information on the spectrum of the neutral oxygen atom under a variety of excitation conditions. Radiative rates and effective collision strengths are assembled from the recent literature where available, or are calculated for as yet untreated transitions using the University College superstructure/distorted-wave computer package, to produce a complete set of atomic data for a 13 hybrid level model of neutral oxygen. Level populations and relative intensities for 28 allowed, inter-combination, and forbidden oxygen lines are computed, under optically thin conditions, for the electron density range 4.0 less than log N(sub e) less than 12.0 and the electron temperature values T(sub e) = 5000, 10,000, 20,000, 50,000, and 100,000 K. Preliminary applications to observed intercombination/allowed and forbidden/allowed line ratios are discussed.

Bhatia, A. K.↗