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At least 109 records · Page 6
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NASA NTRS1978

An atlas of Copernicus ultraviolet spectra of Wolf-Rayet stars

An atlas of Copernicus UV scans is presented, and line identifications are tabulated, for the Wolf-Rayet stars Gamma-2 Vel (WC 8 + O7), HD 50896 (= EZ CMa; WN 5), and HD 92740 (WN 7). The atlas covers the wavelength ranges from 946.8 to 3182 A for Gamma-2 Vel, from 1012 to 1294 A for HD 50896, and from 1051 to 1243 A for HD 92740. The wavelengths include corrections for components of satellite velocity, earth velocity, and stellar heliocentric velocity; each spectral feature is classified as interstellar, photospheric, emission, UV-displaced P Cygni line absorption, or P Cygni line emission. UV-edge velocities of the P Cygni profiles are estimated, P Cygni profile types are discussed, and the results are compared with Copernicus scans of OB stars exhibiting UV P Cygni profiles. It is noted that: (1) the line-strength ratio of molecular hydrogen to atomic species appears to be substantially greater in the scans of the WN stars than in the Gamma-2 Vel scans; (2) some of the P Cygni profiles in Gamma-2 Vel differ significantly from the corresponding profiles in OB stars; and (3) there may be a slight inverse correlation between ejection velocities and excitation potentials in Gamma-2 Vel.

Johnson, H. M.↗
NASA NTRS1980

Emission measures derived from far ultraviolet spectra of T Tauri stars

Spectroscopic diagnostics based on UV emission line observations have been developed to study the solar chromosphere, transition region, and corona. The atmospheric properties that can be inferred from observations of total line intensities include the temperature, by identifying the ionic species present; the temperature distribution of the emission measure, from the absolute intensities; and the electron density of the source, from line intensity ratios sensitive to the electron density. In the present paper, the temperature distribution of the emission measure is estimated from observations of far UV emission line fluxes of the T Tauri stars, RW Aurigae and RU Lupi, made on the IUE. A crude estimate of the electron density of one star is obtained, using density-sensitive line ratios.

Cram, L. E.↗
NASA NTRS1981

Fluorescence equilibrium in the ultraviolet spectra of Comets Seargent (1978 m) and Bradfield (1979 l)

Detailed fluorescence calculations for OH including the Swings effect were performed. These calculations were used to reproduce the high resolution spectra of Comets Seargent and Bradfield taken with IUE. The calculations also provide the OH fluorescence efficiencies (g factors), as a function of heliocentric radial velocity, which are needed to derive OH abundances from measured fluxes. A close examination of the spectra shows no sign of the corresponding emission bands of OD allowing upper limits to be placed on the ratio N(OD)/N(OH). Preliminary attempts to reproduce the CO+ band structure by fluorescence are discussed.

Donn, B.↗
NASA NTRS1980

IUE ultraviolet spectra and chromospheric models of HR 1099 and UX Arietis

IUE spectra in the region 1150-3200 A of the RS CVn-type variables HR 1099 and UX Arietis are presented and analyzed in terms of chromospheric models. Measurements of Mg h and k lines and Ca II H-K and H alpha spectra are indicated which are found not to be correlated with orbital phase or radio flares and which suggest that the strong emission arises in the K star rather than the G star in these systems. Under the assumption that the UV emission lines are associated with the K star, surface gravities of log g = 3.6 and 3.4 and effective temperatures of 4850 and 5000 K are adopted for HR 1099 and UX Ari, respectively, along with solar metal abundances for each. Model calculations of the chromospheric structure necessary to account for observed C(+), Mg(+), Si(+) and Si(+2) line fluxes are presented which indicate that the transition region pressure lies in the range 0.18-1.0 dynes/sq cm, implying transition regions that are more extended than that of the sun and are not conductively heated. It is noted that pressure scaling laws and the use of Mg II and C II lines as pressure diagnostics may be invalid, possibly due to atmospheric inhomogeneities or gas flows.

Simon, T.↗
NASA NTRS1982

The ultraviolet spectra of R and N type stars

Low dispersion long wave radiation spectra of three early N-type carbon stars (BL Orionis, TX Piscium and T Indi) obtained with the IUE are discussed. N-star spectra were compared with IUE observations of late M-type stars and several differences were noted. The Mg II and C II chromospheric emission lines reported are the first convincing detection of a 10,000-20,000 K chromosphere in the N stars.

Obrien, G. T.↗
NASA NTRS1981

The optical and ultraviolet spectra of the high excitation planetary nebula, CD -23 deg 12238 = Me 2-1

Planetary nebula are now recognized as a penultimate stage in the evolution of many stars. Me 2-1 offers advantages meriting a detailed analysis. A description is presented of measurements regarding this object which have been conducted with a photoelectric scanner and a 3 m telescope Cassegrain image tube scanner. The chemical composition of Me 2-1 appears to fit well that of other planetaries. The values for N, O, and Ne fall slightly above those for an 'average' planetary, but Me 2-1 would not qualify as a nitrogen-rich object. When a comparison is made with the sun, there appears to be an enhancement of carbon, possibly slight enhancements of nitrogen and neon, but a slight reduction in oxygen.

Aller, L. H.↗
NASA NTRS1982

The far-ultraviolet spectra and geometric albedos of Jupiter and Saturn

Spectra and geometric albedoes in the range 1200 to 1940 A are compiled for Jupiter and Saturn on the basis of IUE observations. The spectra of both planets are dominated by H Lyman-alpha emission line at 1216 A, although absorption bands of C2H2 are apparent at longer wavelengths, particularly in the spectrum of Saturn, and the C I line at 1657 A is also observed. Geometric albedoes show emission features corresponding to the weak H2 Lyman and Werner bands around 1230-1280 A, auroral Lyman band emission, C I emission, and C2H2 absorption from 1600 to 1900 A. A model of atmospheric absorption in homogeneously mixed atmospheres of H2 and trace molecular absorbers is then presented and fit to the Jupiter albedo, resulting in a predicted atmosphere containing C2H2 and an unidentified molecular or particulate absorber. Finally, north-south maps of Jupiter continuum emission show limb darkening, and a comparison of equatorial and polar spectra indicates a polar increase in C2H2 absorption and weaker polar H2 emissions than previously reported.

Clarke, J. T.↗
NASA NTRS1983

Redshifts of high-temperature emission lines in the far-ultraviolet spectra of late-type stars

High-dispersion IUE spectra of six late-type stars exhibit small but statistically significant differential redshifts of high-temperature emission lines, like Si IV and C IV, with respect to low-temperature lines like S I and O I. A well-exposed, small-aperture spectrum of the active chromosphere binary Capella (Alpha Aurigae A: G6 II+F9 III) establishes that the high-temperature lines are redshifted in an absolute sense with respect to the accurately determined photospheric velocity of the system at single-line phase 0.50. Several possible explanations for the stellar redshifts are discused, including a warm wind (100,000 K) in which aparent redshifts are produced in optically thick lines by an accelerating outfow, and the downflowing component of a vertical circulation system for which the up-leg portion of the flow is too cool, too hot, or too tenuous to be visible in Si IV and C IV. If the second scenario is true, the stellar redshifts may provide an important phenomenological link to the downflows observed in 100,000 K species over magnetic active regions in the sun.

Ayres, T. R.↗
NASA NTRS1984

Comparison of variations in the visible and ultraviolet spectra of a Centauri

Short and long wavelength, high resolution IUE spectrographs of the He I spectrum variable star a Cen are the basis of the present equivalent width and central depth measurements of UV absorption lines. The period for cyclic intensity variations is determined by means of a four-parameter, least squares fit to a sinusoid. Other spectrographic and photometric intensity determinations of the He I 4026 A line are combined with the present data to yield a period of 8.81620 + or - 0.00006 days.

Fahey, R. P.↗
NASA NTRS1984

The ultraviolet spectra of the O and B stars in the young galactic cluster NGC 6530

The UV spectra between 1200 and 3000 A of stars in the young galactic cluster NGC 6530 and the surrounding association are studied. From the UBV colors and empirical as well as theoretical calibrations, the T(eff) and L for those stars which follow a sequence in the H-R diagram corresponding to the main sequence are determined. From a comparison with theoretical evolutionary tracks, the age of the cluster is estimated to be 5 + or - 2 x 10 to the 6th yr, with a very small scatter for the different stars. The UV extinction is determined for the stars from a comparison of theoretical model energy distributions for the stellar T(eff)s and the observed energy distributions. The stellar wind lines are studied, and strong stellar winds are found for bolometric magnetidues less than -8.

Boehm-Vitense, E.↗
NASA NTRS1985

Identification of the 1400 and 1600 A features observed in the ultraviolet spectra of DA white dwarfs

The 600 A absorption feature in the spectra of cool DA white dwarf stars has been identified as a resonance broadening of Ly-alpha due to the hydrogen quasi molecule. Likewise, the 1400 A absorption feature in the spectra of cool and moderately warm DA white dwarf stars appears to be due to a Ly-alpha satellite line arising from the hydrogen ion quasi molecule. The strength of both features is gravity sensitive and therefore promises to be an excellent indicator of surface gravity.

Nelan, E. P.↗
NASA NTRS1985

The ultraviolet spectra of intermediate-redshift quasars

In the present sample of 21 intermediate redshift quasars, for which IUE Lyman-alpha line flux and UV continuum observations have been conducted, an extrapolation of the continuum to the X-ray region is noted to fall at least 2-sigma above the measured X-ray flux, at 2 keV, for two-thirds of the sample. The strength of Lyman-alpha is well correlated with the luminosity at 1450 A, and an estimate of the covering factor on the basis of the ratio of the number of Lyman-alpha to ionizing protons yields a value for the sample which, at about 0.17, is consistent with the number of observed Lyman discontinuities.

Kinney, A. L.↗
NASA NTRS1985

The ultraviolet spectra of active galaxies with weak optical Fe II lines

Very weak optical Fe II lines are noted in the present simultaneous optical and IUE spectra of the two Seyfert galaxies Mrk 290 and Mrk 915. Strong and distinct UV features are also observed in the spectrum of Mrk 290. A synthetic spectrum fitting is conducted for all spectral features between 1900 and 5500 A, using the calculations of Netzer and Wills (1983). IUE archival spectra of the broad line radio galaxy 3C 390.3 is compared with optical data obtained during the same year, and evidence is found for strong UV Fe II lines. The present results demonstrate several new aspects of the spectra of active galactic nuclei, including the great amount of energy emitted by Fe II lines and the possible importance of dust and/or very high density clouds in such objects.

Netzer, H.↗
NASA NTRS1987

Infrared-ultraviolet spectra of active galactic nuclei

Data from IRAS and IUE were combined with ground based optical and infrared spectrophotometry to derive emission line free spectral energy distributions (SEDs) for 29 active galactic nuclei (AGNs) between 0.1 and 100 microns. The IRAS data were scaled down to account for extended emission. These correction factors, determined by comparing small aperture ground based 10.6 micron data with large aperture IRAS 12 micron fluxes, were usually less than 25%. These corrected SEDs are shown.

Malkan, M. A.↗
NASA NTRS1987

The ultraviolet spectra of intermediate-redshift quasars. II

This paper completes the analysis of an IUE survey of intermediate-redshift QSOs for which the results on Ly-alpha and continuum properties were given previously. The weaker lines were measured and are discussed in the context of an extended sample including Seyfert galaxies and high-redshift QSOs. The EW(C IV) anticorrelates with the luminosity of the continuum at 1450 A, showing the well-known Baldwin effect. The slopes of the two relations differ slightly, which can be explained by a decrease in the ionization parameter with increasing luminosity. Weak lines of Ly-beta + O VI, N V, and semiforbidden Si IV + O IV were measured in improved spectra for two of the sample objects, 1100 + 772 and 2201 + 315. The Ly-alpha line in both objects is highly symmetrical, with N V contribution of 5 percent or less. The line shapes were fitted with a two-component profile; 1100 + 772 has a distinct narrow component which contributes 20-25 percent of the Ly-alpha flux.

Kinney, A. L.↗