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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

The Lyman alpha forest of the high-z quasar 0000-263

Medium-resolution (delta(v) = 45 km/s) optical spectra of the bright, high-redshift (z = 4.1) quasar 0000-263 taken at the ESO 3.5-m NTT telescope were analyzed to determine the distribution of column densities, velocities and line widths of the Lyman-alpha forest absorption components. The values of NH, b, and z were determined by fitting Voigt profiles to the lines, and convolving with a Gaussian instrumental response function. Over 350 components with log N(sub H) greater than 13.2 were identified. An analysis of the dependence of the number of components with z reveals that the number evolution of components obeys the power law dN/dz varies as (1+z)(sup gamma), where gamma = 0.5 +/- 0.4 for the sample of 182 lines with log N(sub H) greater than 14.0. The distribution of component strengths is found to obey f(N(sub H)) varies as N(sub h)(sup -beta), where beta = -1.55 for components with log(N(sub H)) is greater than 14.7, and beta = -0.68 for the components with log(N(sub H)) greater than 13.5. A distinct break in the f(N(sub H)) histogram is also observed, at log(N(sub H)) is approximately 14.7. The results are briefly considered in the context of theoretical models of quasar Lyman alpha clouds and their evolution.

Penprase, Bryan↗
NASA NTRS2005

Coherent Laser Instrument Would Measure Range and Velocity

A proposed instrument would project a narrow laser beam that would be frequency-modulated with a pseudorandom noise (PN) code for simultaneous measurement of range and velocity along the beam. The instrument performs these functions in a low mass, power, and volume package using a novel combination of established techniques. Originally intended as a low resource- footprint guidance sensor for descent and landing of small spacecraft onto Mars or small bodies (e.g., asteroids), the basic instrument concept also lends itself well to a similar application guiding aircraft (especially, small unmanned aircraft), and to such other applications as ranging of topographical features and measuring velocities of airborne light-scattering particles as wind indicators. Several key features of the instrument s design contribute to its favorable performance and resource-consumption characteristics. A laser beam is intrinsically much narrower (for the same exit aperture telescope or antenna) than a radar beam, eliminating the need to correct for the effect of sloping terrain over the beam width, as is the case with radar. Furthermore, the use of continuous-wave (CW), erbium-doped fiber lasers with excellent spectral purity (narrow line width) permits greater velocity resolution, while reducing the laser s power requirement compared to a more typical pulsed solid-state laser. The use of CW also takes proper advantage of the increased sensitivity of coherent detection, necessary in the first place for direct measurement of velocity using the Doppler effect. However, measuring range with a CW beam requires modulation to "tag" portions of it for time-of-flight determination; typically, the modulation consists of a PN code. A novel element of the instrument s design is the use of frequency modulation (FM) to accomplish both the PN-modulation and the Doppler-bias frequency shift necessary for signed velocity measurements. This permits the use of a single low-power waveguide electrooptic phase modulator, while simultaneously mitigating the effects of speckle as a noise source in the coherent detection.

Chang, Daniel↗
NASA NTRS1978

XUV spectra of the 1973 June 15 solar flare observed from Skylab. III - A list of spectral lines from 1000 to 1940 A

A wavelength list of spectral lines between 1000 and 1940 A is presented for the solar flare that occurred on June 15, 1973. The spectra were recorded by the NRL spectrograph on Skylab. The spectral resolution is 0.06 A. Intensities, identifications, and estimates of line widths are given. The intensity of the continuum is also given at 50-A intervals between 1400 and 1900 A. The wavelength list includes about 1400 lines; about 30% of these lines are not indentified. Because of the high wavelength resolution, this line list will be useful as a source of identification for some stellar as well as solar spectra. In particular, the list should be a useful aid in the identification of lines in the spectra of stars with classifications close to that of the sun. Spectra of such stars may be obtained from the recently launched IUE spacecraft. It is also interesting to compare the list with some of the spectra of early-type stars recorded by Copernicus.

Cohen, L.↗
NASA NTRS1984

High-resolution, far-ultraviolet study of Beta Draconis (G2 Ib-II) - Transition region structure and energy balance

High-resolution far ultraviolet spectra of the star Beta Draconis have been obtained with the IUE satellite. The observations and emission line data from the spectra are presented, the interpretation of the emission line widths and shifts is discussed, and the implications are given in terms of atmospheric properties. The emission measure distribution is derived, and density diagnostics involving both line ratios and line opacity arguments is investigated. The methods for calculating spherically symmetric models of the atmospheric structure are outlined, and several such models are presented. The extension of these models to log T(e) greater than 5.3 using the observed X-ray flux is addressed, the energy balance of an 'optimum' model is investigated, and possible models of energy transport and deposition are discussed.

Brown, A.↗
NASA NTRS1976

Moessbauer effect measurement in single crystal iron subjected to cyclic stress

Backscattered Moessbauer spectra were recorded at different fatigue levels in cyclically fatigued high-purity single-crystal iron specimens in the form of rectangular strips with appropriate concentrators in the midplane. The Moessbauer spectra were submitted to a least-squares analysis to determine the isomer shift, quadrupole shift, hyperfine field, and peak spectral widths in each spectrum. It is shown that the quadrupole shift and the isomer shift changed little with fatigue, whereas the effective internal field and the associate line widths exhibit significant variation as a function of fatigue level. It is concluded that the impurity concentration builds up slowly with increasing fatigue level in the region of stress concentration. Since there is a direct correlation between crack initiation and defect concentration level, it is expected that Moessbauer measurements in the test specimen can provide a means of monitoring the impurity buildup presumed to be the eventual cause of fatigue crack initiation and failure.

Singh, J. J.↗
NASA NTRS1983

Positron annihilation radiation from solar flares

Positron-annihilation radiation has been observed from the June 21, 1980 and June 3, 1982 flares by the gamma-ray spectrometer on the Solar Maximum Mission satellite. The observed 0.511-MeV line fluences from the flares were 14.6 + or - 3.3 gamma/sq cm and 103 + or - 8 gamma/sq cm, respectively. Measurement of the line width establishes an upper limit to the temperature in the annihilation region of 3 x 10 to the 6th K. The time dependence of the 0.511-MeV line during the 1980 flare is consistent with the calculations of Ramaty et al. (1983) for positrons created in the decay of radioactive nuclei. The time dependence of the 0.511-MeV line for the 1982 flare is more complex and requires more detailed study.

Share, G. H.↗
NASA NTRS1973

Precise measurement of charged defects in III-V compounds (supplement 2)

Experimental methods and related theory which will permit the measurement of low concentrations of vacancies and other defects in III-V compound semiconductors are discussed. Once the nature of these defects has been determined, this information can be incorporated into a transport theory for devices constructed from these materials, and experiments conducted to test the theory. The vacancies and other defects in the III-V compounds are detected by measurement of the nuclear magnetic resonance (NMR) line width. Most of the III-V compounds have at least one isotope with a nuclear quadrupole moment. In a crystal with a cubic crystal field (characteristic of most III-V compounds) there is no quadrupole splitting of the Zeeman resonance line. However, a defect removes the cubic symmetry locally and causes splitting which result in a change of the NMR width. This change can be used to detect the presence of vacancies.

Soest, J. F.↗
NASA NTRS1991

Active region mass motions observed in soft X-ray lines

Significant coronal mass motions are deduced from soft X-ray line broadening observed in solar active regions even during quiescent periods. A large data base of Mg XI resonance line profiles near 9.17 A has been acquired with the SMM Flat Crystal Spectrometer for a variety of active region conditions and disk locations. For the X-ray bright areas, the typical profiles have an excess line width equal to or greater than the thermal Doppler width for Mg ions at the observed coronal temperatures (3-4 MK). The equivalent excess velocities of 40-60 km/s are higher than those reported previously for cooler lines in other wavebands. The excess velocities show a correlation with magnetic field strength or gradient, but no obvious dependence on the angular distance from sun center.

Saba, J. L. R.↗
NASA NTRS1991

Physics of the infrared spectrum

The IR bandpass is attractive for solar magnetic field studies in virtue of the proportionality to wavelength of the ratio of Zeeman splitting to line width. The large Zeeman splitting and optical thinness of the 12-micron observations render them especially useful for vector magnetic field derivations. The IR continuum, and many IR spectral lines, are formed in LTE and are useful in studies of the temperature structure of the solar atmosphere from the deepest observable photospheric layers to chromospheric altitudes. The far-IR continuum is an excellent thermometer for the upper photosphere and chromosphere.

Deming, Drake↗
NASA NTRS1983

Real line strength distributions for random band models

An improved random band-model method, which makes allowance for the real line-strength distribution, is proposed. The model is shown to be useful for low-resolution, infrared observational data of the outer solar system. The method can be used as easily as conventional random band calculations. In the illustrative examples cited here, the variation of line width with J, the rotational quantum number, is small. Other effects which can, in principle, cause the model to deviate from laboratory observations are discussed. These include the assumption that line positions are random, ignoring the effects of the Lorentz wings of lines immediately outside the specific interval for which the mean transmission is calculated, and ignoring the effects of instrumental slit functions.

Kim, S. J.↗
NASA NTRS1984

Incoherent Scatter Radar Studies of the Mesosphere

All current incoherent backscatter radars can make mesospheric or D region measurements under at least some conditions. The conditions under which measurements are possible are derived and shown for each radar. Conditions examined include overall system sensitivity, electron and ion spectral line widths and power distributions, and time/height averaging effects. The radar equations is derived and calibrations of the radar system is discussed. Total power only and combined total power and ion line spectral measurements are described in terms of ease of use, applicability to various radars, and parameters of aerodynamic interest derivable from the measurements.

Mathews, J. D.↗
NASA NTRS1992

Planar H2O masers in star-forming regions

The paper examines the planar geometry of shocked material, which is the key property in enabling the high brightness temperatures of H2O masers in star-forming regions. The brightness temperature, beaming angle, and the maser spot size are determined for thin, saturated planar masers under the assumption that the velocity change across the maser due to ordered motions is small compared with the thermal or microturbulent line width. For a given set of physical parameters, the brightness temperature is essentially fully determined by the length of the velocity-coherent region in the shocked plane along the line of sight. Effective aspect ratios (about 5-50) are found that are in agreement with values previously inferred from observed brightness temperatures.

Elitzur, Moshe↗
NASA NTRS2021

Calibration and Testing of Small High-Resolution Transition Edge Sensor Microcalorimeters With Optical Photons

Pulses of narrow line-width optical photons can be used to calibrate and test sub-2 eV full-width at half-maximum (FWHM) energy resolution transition-edge sensor (TES) microcalorimeters at low energies (<1 keV),where it is very challenging to obtain X-ray calibration lines comparable to (or narrower than) the detector resolution. This scheme depends on the ability to resolve the number of 3 eV photons in each pulse, which we have recently demonstrated up to photon numbers of about 300. AtLTD-18 we showed preliminary results obtained with this technique on a 0.25 eV baseline resolution TES microcalorimeter designed for the ultra-high-resolution sub-array of the Lynx mission. The line-shape was well described by a simple Gaussian. However, the difficulty of delivering photons to the small 46 μm square absorbers resulted in a large thermal crosstalk signal, whose random nature is expected to rapidly degrade the observed energy resolution towards higher photon numbers/energies. We have since improved the coupling between the optical fiber and the TES absorber and report here our current results.

Felix T Jaeckel↗
NASA NTRS1995

Near-infrared spectra of compact stellar wind sources at the Galactic center

We present high- and low-resolution, H- and K-band spectra on nine compact 2.2 micrometers Galactic center sources in which we clearly detect He I 2.058 micrometers emission, including the AF source, IRS 13, IRS 1W, IRS 16NE, IRS 16NW, IRS 16C, IRS 16SW, IRS 34, and IRS 6E. We have also obtained comparison spectra of both a luminous blue variable (LBV) and a WR star (P Cygni and HD 192163, respectively). Our H- and K-band spectrum of the LBV P Cygni strongly resembles the near-infrared spectra of known WN9/Ofpe stars. Our spectra of the Galactic center sources share many characteristics in common with our spectrum of P Cygni. The spectra all show emission lines of H I and He I with large He I/H line flux ratios. Some have permitted and forbidden lines of Fe II. Brackett line widths and ratios indicate the presence of strong stellar winds. In contrast to the spectrum of the WR star, none of the Galactic center sources show evidence of He II emission lines in their spectra, suggesting that none of the Galactic center sources are WR stars. Our high-resolution H-band spectrum of the AF source differs from previously published low-resolution H-band spectra in that it is rich in emission lines. Furthermore, we find two distinct spectral components to the AF source separated in space by a few arcseconds. We identify both the emission-line component of the AF source and an exciting source of IRS 13 as an LBV or WN9/Ofpe star. Our results, when combined with the results of others, also suggest that IRS 16NE, IRS 16C, IRS 16NW, IRS 34, and a component of IRS 6E are early-type, emission-line stars. The argument for IRS 16SW, however, is less compelling. We find no evidence for a compact He I emission-line source at IRS 1W. This result contradicts previous findings, suggesting that the He I source at IRS 1W may be variable. If the He I lines in IRS 1W are truly variable, a stellar component of IRS 1W may be an LBV, because LBVs are known to have variable line emission on short timescales. The nine Galactic center wind sources appear to contribute a significant fraction of the total luminosity of the central few parsecs of the Galaxy.

Libonate, S.↗
NASA NTRS1982

Skylab observations of H I Lyman-alpha

The H I Lyman-alpha profiles obtained with the Skylab SO82-B spectrograph at spectral and spatial resolutions of 60 mA and 2 arcsec x 60 arcsec are presented. Profile variations are illustrated from center to limb, at different heights above the limb, and over a cell and network boundary. The Lyman-alpha core exhibits weak or no limb darkening, whereas the line wings show weak limb darkening. Above the limb, the Lyman-alpha core integrated intensity and line width remain in essence constant up to plus 12 arcsec and may result from resonance scattering of chromospheric radiation by coronal neutral hydrogen or from spicules. On the disk, intensities in the line core are seen as correlating well with the H-alpha network; however, the Lyman-alpha wing intensities are found to correlate better with the transition region lines. The average of profiles acquired over the cell and network region is found to be similar to profiles obtained in the cell.

Roussel-Dupre, D.↗
NASA NTRS1981

On the enigma of FK Comae

Stellar chromospheric and coronal activity appears ubiquitous among late type stars to the left of the TR-wind boundary line (Linksy and Haisch 1979). The level of activity as measured by the X-ray surface flux is linearly proportional to the stellar angular velocity, with the exception of slowly rotating dwarfs (Walter 1981, 1982; Walter and Bowyer 1981). The peculiar rapidly rotating G giant FK Comae (Merrill 1948) appears to fit into this pattern. Line widths indicate V sin i = 120 + or - 20 km s(-1) (Bopp and Stencel 1981). FK Comae has strong Ca II H and K and H alpha emission, strong transition region UV lines (Bopp and Stencel 1981), and an X-ray surface flux in good agreement with its rapid rotation (Walter 1981). Yet, FK Comae is an enigmatic star. It is a rapid rotator, but it is not clear why it is a rapid rotator. There is no direct evidence for duplicity; indeed, the upper limit of 20 km s(-1) on the K velocity puts tight constraints on any binary configuration, especially if sin i approx 1, as indicated by the large V sin i. Bopp and Stencel (1981) have suggested that FK Comae is an example of a coalesced W UMa system (Webbink 1976), wherein the orbital angular momentum has become rotational angular momentum of the coalesced star.

Walter, F.↗
NASA NTRS1991

First results from the Goddard High-Resolution Spectrograph - Evidence for photospheric microturbulence in early O stars - Are surface gravities systematically underestimated?

GHRS spectra of two very hot stars provide evidence for the presence of microturbulence in their photospheres. In attempting to reproduce the observed spectra, theoretical models have been built in which the microturbulence is allowed to modify not only the Doppler line widths (classical 'spectroscopic' microturbulence), but also the turbulent pressure (thus mimicking a 'physical' turbulence). It is found that a corresponding modification of the temperature-pressure stratification influences the hydrogen and helium line profiles to the extent that the surface gravities of early O stars determined without considering microturbulence are too low by 0.1-0.15 dex. Thus, including microturbulence would reduce, or resolve completely, a long-standing discrepancy between evolutionary and spectroscopic stellar masses.

Hubeny, I.↗