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At least 685 records · Page 38
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NASA NTRS1981

Theoretical quasar emission line ratios. IV - General asymptotic escape probabilities and the effects of linear Stark broadening

Established techniques permitting the evaluation of exact asymptotic forms of photon escape probabilities for a wide range of absorption coefficient profile types are used for QSO radiative transfer effect studies, and it is shown that the exact formulae are easily evaluated. It is found that some of the commonly used expressions overestimate the true value by a factor of about two, and a computationally expedient approximate expression is derived for the asymptotic form of the photon escape probability when linear Stark broadening contributes to the line absorption coefficient profile in hydrogenic lines. Attention is given to Lyman-alpha, for the case in which partial redistribution, over the linear Stark component of the line absorption coefficient profile, dominates single-flight photon escape.

Puetter, R. C.↗
NASA NTRS1982

Infrared emission line studies of the structure and excitation of H II regions

Maps of five H II regions in one or more of the infrared fine-structure lines of Ne II (12.8 microns), Ar III (9.0 microns), and S IV (10.5 microns) have been obtained with angular resolutions ranging from 4 to 7 arcsec. The observations are used to discuss the morphology and excitation of these nebulae. Considerable diversity is found in the structures of the nebulae, probably resulting from differences in their ages and the circumstances of their formation. In all cases, more ionizing luminosity than would be provided by a single dominant ionizing star appears to be required, although uncertainties in the model nebulae make this conclusion uncertain.

Lacy, J. H.↗
NASA NTRS1982

Far-infrared photometry of optical emission-line stars. II

Far-infared photometry of dust clouds around a sample of eight optically bright stars, five of which are believed to be pre-main-sequence and three of which are probably post-main-sequence, is reported. Energy distributions consistent with the results of an earlier study of eight other stars where a good correlation was found between the existence of cool, far-infrared-emitting dust clouds and the presumed youth of the central star are found. The post-main-sequence objects, as in the previous study, were found to have energy distributions falling steeply from 10 to 100 microns implying very small amounts of cool dust relative to the amount of dust at a few hundred K.

Harvey, P. M.↗
NASA NTRS1982

S IV emission-line ratios in the sun

New atomic data are presented for transitions between the five lowest levels in S IV, taking into account the fine-structure rates between the individual J levels together with the electron impact mixing rates within the levels. The values are found to differ significantly from previously published values. Using the atomic data, ionic level populations are deduced for a range of electron temperatures and densities. The results are used to calculate theoretical line intensity ratios for S IV. Excellent agreement is found with intensity ratios for a variety of solar features observed with the NRL normal incidence spectrograph on Skylab.

Dufton, P. L.↗
NASA NTRS1982

The Mg II resonance line emission at 2800 A in stars with different rotational velocities and different metal abundances

In many investigations, a study of the Mg II 2800 A emission was conducted in order to derive more information about the correlation of chromospheric emission with the properties of the convection zone. It has been suggested by Ayres and Linsky (1980) that there is a positive correlation between rotation and chromospheric emission at least for binary stars. The present investigation is concerned with such a correlation between rotation and chromospheric emission. Since large differences in rotational velocities are observed for F stars for which also strong chromospheric emission is observable, this particular study deals mainly with F stars. On the basis of observations of the Mg II emission in F and G stars, it is found that the emission is abnormally strong in close binary F stars. This may be explained by the generation of a large differential rotation due to tidal effects which may give very efficient dynamo generation of a surface magnetic field.

Boehm-Vitense, E.↗
NASA NTRS1983

Profiles and intensity ratios of the C IV lambda 1548, 1550 emission lines in planetary nebulae

The C IV resonance doublet at lambda 1548, 1550 is an important diagnostic tool in the study of planetary nebulae. The predicted theoretical intensity ratio of 2:1 is, however, rarely observed in the high dispersion IUE spectrograms. Values of 0.8-2.0 are observed for a sampling of 11 planetary nebulae of various excitation classes and for three proto-planetary nebulae. A diversity of line profiles is observed. In addition to the C IV doublet, weak subsidiary lines are seen. The spacing between these pseudo-lines happens to correspond to the separation of the C IV lines but are believed to be due to hot pixels and do not conform to the radial velocity displacement of the C IV lines.

Feibelman, W. A.↗
NASA NTRS1982

The Orion Nebula - Large-scale distribution of far-infrared and submillimeter line emission

Attention is given to the results of a study of radiative cooling in the interstellar clouds of the Orion Nebula through the emission of far-IR and submillimeter radiation. That the gas is 'clumpy' in the ionized region of the nebula is indicated by the far-IR observations of doubly ionized oxygen. With respect to the edges of the ionized region, the excitation temperature for the 3P1 state is 228 K and the observed emission can be plausibly associated with either ionization or shock fronts at the interface between the ionized and neutral region. Consideration is also given to the possibility of an ionized carbon halo, the turbulent state of the molecular cloud, and the need for further laboratory and theoretical analyses.

Harwit, M.↗
NASA NTRS1984

Measuring EUV Line Emission from the Hot Interstellar Medium

In the local interstellar medium, the presence of hot gas was inferred from measurements of oxygen VI absorption in the ultraviolet, and from diffuse emission in the extreme ultraviolet (EUV) and soft X-ray regions. A spectrometer that is very sensitive to gas in this temperature range is described. The spectrometer uses an array of plane ruled gratings at grazing incidence in the extreme off plane mount. A set of Kirkpatrick-Baez mirrors focuses the conically diffracted light on to one of two microchannel plate detectors. The field of view of the instrument is 0.2 deg by 12 deg. The predicted sensitivity ranges from 50 to 200 ph/(sq cm sec str) with a resolving power of 15 to 50 over the 50 to 700 angstrom wavelength band.

Labov, S.↗
NASA NTRS1985

Emission-line QSOs in the region of the Hercules cluster of galaxies

Slit spectrophotometry in the 305-855-nm range of 25 QSO candidates identified by slitless spectroscopy in two 1-deg-sq fields (one centered on Abell 2151 and one in a blank region) near the Hercules cluster of galaxies, obtained using image-dissector scanners on the 3-m Lick Observatory and 4-m KPNO telescopes at resolution 5-20 A during 1976-1979, are reported. The data are presented in tables and spectrograms and characterized in detail. In the cluster field, 14 candidates are confirmed as QSOs with z = 0.89-3.24; in the blank field, six are confirmed with z = 1.52-3.00. While the strongest feature of most spectra is found to be the Lyman alpha line, five QSOs have the 154.9-nm line of C IV and one the semiforbidden 190.9-nm line of C III as strong features. The wide variety of spectral profiles and absorption features in the higher-redshift QSOs and the lack of any QSOs with broad absorption troughs are discussed, and the use of these QSOs as probes for warm or cool gas in the Hercules cluster in Space Telescope Faint Object Spectrograph observations is suggested.

Burbidge, E. M.↗
NASA NTRS1985

Optical emission-line properties of evolved galactic supernova remnants

New optical spectrophotometric data are presented for the supernova remnants CTB 1, OA 184, VRO 42.05.01, S147, the Monoceros Loop, G206.9 + 2.3, and G65.3 + 5.7. These data are combined with published spectral data to study some of the general properties of evolved galactic supernova remnants. It is found that (1) O I and O II forbidden line strengths, when used in conjunction with the usual H-alpha S II forbidden line ratio test, provide an excellent additional diagnostic for discriminating remnants from H II regions; (2) the line ratios H-alpha forbidden line N II, H-alpha forbidden line S II, and forbidden line S II 6717/6731 A generally do not vary substantially among the filaments of an individual remnant; and (3) the observed correlation of forbidden line N II/H-alpha with S II forbidden line 6717/6731 A in remnants is the result of observational selection rather than of evolutionary effects. A galactic nitrogen abundance gradient of d log (N/H)/dR = -0.088 dex/kpc, which is in agreement with that derived from H II regions. However, no abundance gradients for oxygen or sulfur are indicated from the remnant data.

Fesen, R. A.↗
NASA NTRS1986

EX-111 Thermal Emission from Hot White Dwarfs: The Suggested He Abundance-Temperature Correlation. EX-112: The Unique Emission Line White Dwarf Star GD 356

Progress in the EXOSAT data analysis program is reported. EXOSAT observations for four white dwarfs (WD1031-115, WD0004+330, WD1615-154, and WD0109-264) were obtained. Counting rates were unexpectedly low, indicating that these objects have a substantial amount of x-ray absorbing matter in their photosheres. In addition, soft x-ray pulsations characterized by a 9.25 minute cycle were discovered in the DA white dwarf V471 Tauri. A residual x-ray flux from the K dwarf companion can be seen during the white dwarf eclipse at orbital phase 0.0. Pronounced dips in the soft x-ray light curve occur at orbital phases 0.15, 0.18, and 0.85. The dips may be correlated with the triangular Lagrangian points of the binary orbit. Smaller dips at phases near the eclipse may be associated with cool loops in the K star corona. Data for the white dwarf H1504+65 was also analyzed. This object is particularly unusual in that its photoshere is devoid of hydrogen and helium. Finally, existing data on the white dwarf Sirius B were analyzed to see what constraints from other data can be placed on the properties of this star. Interrelationships between radius, rotational velocity, and effective temperature were derived.

Shipman, H. L.↗