Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “LINE WIDTHS”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Investigation of the 2-0 pressure-induced vibrational absorption spectrum of hydrogen at temperatures below ambient

A theoretical fit has been made to our laboratory measurements of the 2-0 collisionally induced H2 absorption band for temperatures of 122 and 273.3 K and at a density of 20 amagats. A Lennard-Jones 6-12 intermolecular potential and a Birnbaum-Cohen line profile have been used. The fit resulted in a chi-square of 0.2%. Line widths have also been derived as a function of temperature. The lifetimes of the states have been calculated.

Silvaggio, P. M.↗
NASA NTRS1981

Thermal instabilities in radiatively driven winds - Application to emission line clouds of quasars and active galactic nuclei

It is shown that radiatively driven, optically thin winds from active galactic nuclei are thermally unstable, provided that the mass loss rates are not smaller than about 50 solar masses/year. Clouds form at distances of less than about 1 pc, with electron densities greater than 100 million/cu cm, temperatures of more than 10,000 K, and radii between 3 x 10 to the 14th and 10 to the 15th cm. These values agree with the values deduced from observations. Since the clouds are formed in a high velocity wind, this model, as does the model by Eilek and Caroff (1979), avoids problems of disruption inherent in using radiation pressure to accelerate clouds from rest to velocities up to 0.1 c needed to explain the emission line widths. The thermal balance of the gas is discussed, the criteria for thermal instability are given, and a perturbation analysis is made. This analysis is restricted to objects with bolometric luminosities lower than approximately 2 x 10 to the 46th ergs/sec. For more luminous objects, only winds slightly optically thick to electron scattering can be unstable, provided one can extend this analysis to the optically thick case.

Beltrametti, M.↗
NASA NTRS1982

The H-alpha/H-beta ratio in solar flares

The present investigation involves the study of an extensive body of data accumulated of simultaneous H-alpha and H-beta cinematography of flares. The data were obtained with two telescopes simultaneously photographing flares in H-alpha and H-beta. The results of measurements in a number of flares are presented in a table. The flares were selected purely by optical quality of the data. That the measured ratios are not too different from those in stellar flares is suggested by the last two columns of the table. These columns show that a variety of possible line width ratios could give an integrated intensity ratio of less than unity.

Zirin, H.↗
NASA NTRS1982

Measurement of the profile of solar He I resonance lines

The analysis of data obtained by a rocket-borne helium-filled spectrometer employing a curve-of-growth technique in 1977 and 1980 is used in an investigation of the 584-A helium resonance line from the full solar disk. Between 6.5 and 13% of the Gaussian core area was found to be missing through self-reversal. Line widths from 1980 and 1977 were 101 + or - 10 mA and 128 + or - 20 mA full width at half maximum, respectively. No consistent relationship is found between the measured widths and solar activity.

Phillips, E.↗
NASA NTRS1982

Analysis of coronal H I Lyman alpha measurements from a rocket flight on 1979 April 13

It is noted that measurements of the profiles of resonantly scattered hydrogen Lyman-alpha coronal radiation have been used in determining hydrogen kinetic temperatures from 1.5 to 4 solar radii from sun center in a quiet region of the corona. Proton temperatures derived using the line widths decrease with height from 2.6 x 10 to the 6th K at 1.5 solar radii to 1.2 x 10 to the 6th K at 4 solar radii. These measurements, together with temperatures for lower heights determined from earlier Skylab and eclipse data, suggest that there is a maximum in the quiet coronal proton temperature at about 1.5 solar radii. Comparison of measured Lyman-alpha intensities with those calculated using a representative model for the radial variation of the coronal electron density yields information on the magnitude of the electron temperature gradient and suggests that the solar wind flow was subsonic for distances less than 4 solar radii.

Withbroe, G. L.↗
NASA NTRS1983

Low-temperature Positron Annihilation

For a satisfactory understanding of astrophysical annihilation radiation, especially that observed from the galactic center direction, the interaction of positrons with the ambient medium must be carefully investigated. Although hot, ionized regions may be important sources of annihilation radiation, the simpler processes occurring in low-temperature neutral hydrogen gas are mainly addressed. The goal is to set limits on conditions in the annihilation region by using the predictions of atomic theory compared with the observed gamma-ray line width, continuum strength and time dependence.

Drachman, R. J.↗
NASA NTRS1983

Two hot, low-field magnetic DA white dwarfs

Two hot stars discovered in the Palomar Green survey which were found to exhibit peculiarly broad and strong Balmer lines possibly indicative of low magnetic fields are discussed. The stars, PG 1658+441 and PG 0136+251, were found to have extended trough-shaped Balmer and Lyman-alpha line profiles when compared to nonmagnetic dwarfs of similar temperatures. Further observations of PG 1658+441 show it to correspond to a 30,000 K pure hydrogen atmosphere and confirm its nature as a magnetic object with a longitudinal field strength of about 0.7 megagauss and a mean surface field of about 2.3 megagauss. PG 0136+251 is found to be a hotter star (40,000-50,000 K) with weaker lines. Although no strong evidence for magnetic line splitting was obtained, it is argued that neither a high surface gravity nor very rapid rotation can account for the Balmer line shapes. Results thus extend the range of magnetic degenerates to include very hot white dwarfs, and demonstrate the usefulness of line-widths as indicators of possible low-field magnetic sources.

Liebert, J.↗
NASA NTRS1982

Cool luminous stars

The consequences of magnetic fields for the atmospheres of cool stars are addressed based on IUE and Einstein Observatory observations. Gross atmospheric structure data for cool luminous stars are discussed, and the relevance of the observations for the energy balance and nonradiative heating rates is considered. Data from X-ray fluxes, low dispersion UV spectra, and high dispersion spectra in the region of the Mg II resonance lines are presented. New results on cool star atmospheres provided by high dispersion UV spectra are considered, including those pertaining to spectral line identification, identification of emission components in close binary systems, densities and atmospheric extension, emission line widths, properties of stellar winds as derived from IUE data, and systematic flows of transition region plasma. An explanation for the observed spectra of cool giants and supergiants is proposed in terms of active, quiet, and hybrid stars.

Linsky, J. L.↗
NASA NTRS1982

Fluorescent excitation of photospheric Fe K-alpha emission during solar flares

The Bent Crystal Spectrometer on the NASA Solar Maximum Mission satellite provides high spectral and temporal resolution observations of the Fe K-alpha lines. Analyses have been conducted of spectra from almost 50 solar flares that occurred during 1980. These data strongly support fluorescent excitation of photospheric iron by photons of E greater than 7.11 keV emitted by the hot coronal plasma produced during the flare. After comparison of the data with a model, the observed K-alpha line widths are discussed along with estimates of the size of the emitting region, the height of the coronal source and the photospheric iron abundance.

Parmar, A. N.↗
NASA NTRS1983

Redistribution - Why half a collision is better than a whole one

The study of spectral line shapes has traditionally been mainly concerned with the measurement and interpretation of absorption or emission profiles. Often only the line widths are studied. The present investigation has the objective to evaluate the additional information which can be obtained by scattering light (usually from a laser) from an atomic system which is being perturbed by collisions. A scattering experiment is discussed. The scattered light consists of two components, a (coherent) Rayleigh component and a redistributed (fluorescent) component. In order to obtain the absorption spectrum, questions regarding the probability of photon absorption are considered. By observing the fluorescence subsequent to absorption during a collision it is found possible to obtain information on the evolution of the system from the point of absorption to the completion of the collision. The information on the intracollisional evolution is the justification for the title of the study, namely 'Why half a collision is better than a whole one'.

Cooper, J.↗
NASA NTRS1984

Ground-based observations of equatorial thermosphere dynamics with a Fabry-Perot interferometer

Fabry-Perot determinations of thermospheric temperatures from 630.0 nm nightglow line width measurements were carried out for the period April to August, 1983. The nightly variation of the thermospheric temperature measured on 53 nights is compared with MSIS model predictions and found to agree occasionally with the model but, on the average, to exceed model predictions by approximately 180 K. The largest differences, 400 to 500 K occur during strongly increasing geomagnetic activity. Significant differences occur both during high geomagnetic/low solar activity and during low geomagnetic/high solar activity.

Meriwether, J. W., Jr.↗
NASA NTRS1983

Evidence for a warm wind from the red star in symbiotic binaries

A systematic redshift of the high ionization resonance emission lines with respect to the intercombination lines is found from an examination of the ultraviolet spectra of symbiotic stars obtained with IUE. After consideration of other possibilities, this is most probably explained by photon scattering in an expanding envelope optically thick to the resonance lines. Line formation in a wind, or at the base of a wind is therefore suggested. Reasons are also given indicating line formation of the most ionized species in a region with an electron temperature of the order of 100,000 K, probably around the cool star. The behavior of the emission line width with ionization energy seems to support this model. The cool components of symbiotic stars appear to differ from normal red giants, which do not have winds of this temperature. An explanation in terms of a higher rotation velocity due to the binary nature of these stars is suggested.

Friedjung, M.↗
NASA NTRS1983

An H-alpha velocity study of S252

The H II region S252 (NGC 2175) was studied by means of H-alpha radial velocities and line widths. The velocity structure appears to be dominated by the western ionization front, abutting a dense neutral cloud which is a CO source, and by symmetry abut the ionizing star. This data, together with other studies of CO, H167-alpha, and the H II radio continuum, allow construction of a model of S252. The H II model is basically a double blister formed by a luminous hot star being located between two orthogonal, elongated neutral clouds.

Fountain, W. F.↗
NASA NTRS1983

A laboratory study of the 8.65 micron fundamental of (C-12)H3D at temperatures relevant to Titan's atmosphere

Spectral transmittance measurements have been performed in the nu6-fundamental of (C-12)H3D at 153 K. Relevant to the studies of the emission spectra of Titan recorded by the IRIS instruments on board Voyagers 1 and 2, N2 has been used as the broadening gas. Excellent agreement between experimental and theoretical spectral transmittance has been the result of using N2-broadened line widths and their dependence upon temperature measured in the nu2-fundamental and S = 63.6 per(sq cm atm) for the absolute intensity of the nu6-fundamental.

Varanasi, P.↗
NASA NTRS1984

A high-resolution study of Herbig-Haro objects 1 and 2

Observations have been conducted of Herbig-Haro objects 1 and 2 with high spectral and spatial resolution in order to test shock models for the line emission. A detailed bow shock model has been constructed which yields line widths and optical and UV spectra in reasonable agreement with observation for maximum shock velocities approximately 200 km/s. Such velocities are comparable to the transverse velocities of the knots in HH 1 and 2 inferred from proper motion studies. The observations suggest that HH objects are rapidly moving, dense clouds shocking with less dense interstellar medium. The large energy requirements of these objects, combined with their small apparent size as seen from the exciting star, seem to require unreasonably large stellar mass loss rates. However, it is possible that the HH objects subtended a much larger solid angle at the star when originally accelerated.

Hartmann, L.↗
NASA NTRS1983

The value of Ho

A brief discussion of the systematic effects of selection bias on the extragalactic distance scale is given. Distance indicators with intrinsic scatter yield only upper limits to Ho, unless the true intrinsic scatter is either small or well determined. Several distance indicators (luminosity index, diameters, globular clusters, 21 cm line widths) are discussed. It is concluded that type I supernovae, calibrated through brightest M supergiants, yield presently the most reliable determination of the large-scale value of the Hubble constant, i.e., 50 + or - 7 km/s Mpc.

Tammann, G. A.↗
NASA NTRS1983

Positron-electron annihilation radiation from the Galactic Center

Evidence and arguments are presented supporting the theory that the 0.511 MeV line observed from the Galactic Center stems from photon-photon pair production near a black hole with a mass not greater than approximately 500 solar masses. Observations are reviewed and the subsequent implications on the annihilation site and the positron source are discussed. Because of the variations and line width of the e(+) - e(-) annihilation radiation from the Galactic Center, positrons must essentially be produced by a single source and annihilate in an ambient gas of density greater than 10 to the 5th/cu cm, an ionization fraction greater than 10 percent, a temperature less than 5 x 10 to the 4th K, and be confined to a region of size less than 10 to the 18th cm. Such conditions may exist in warm clouds and other IR sources within the central parsec of the Galaxy.

Ramaty, R.↗
NASA NTRS1984

Analysis of magnesium XI line profiles from solar active regions

High-resolution solar spectra of the Mg XI 1s2 1S0-1s2p 1P1 resonance line at 9.169 A and the associated nearby satellite lines obtained from two rocket-borne crystal spectrometer measurements are presented. Comparisons with two independent sets of theoretical calculations for the 1s2nl-1s2pnl dielectronic satellite lines with n = 3-7 indicate electron temperatures of 4-4.5 million K. Measured line widths indicate either that the ion temperature exceeds the electron temperature by about a million K or that about 28 km/s of turbulence is present.

Blake, R. L.↗