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At least 415 records · Page 23

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NASA NTRS1982

IUE observations of four BL Lacertae-type objects

Spectral fits to IUE observations of four BL Lacertae-type objects are presented. Combined with near-simultaneous measurements at longer wavelengths, positive spectra curvature within the infrared to ultraviolet energy range were found. It is shown that the data provide strong evidence for relativistic beaming in the sources. Lyman alpha emission and possible C IV and N V absorption features in the spectrum of 3C 371 are discussed.

Worrall, D. M.↗

NASA NTRS1982

Evidence of redshifts in the average solar line profiles of C IV and Si IV from OSO-8 observations

Line profiles of C IV and Si V obtained by the Colorado spectrometer on OSO-8 are presented. It is shown that the mean profiles are redshifted with a magnitude varying from 6-20 km/s, and with a mean of 12 km/s. An apparent average downflow of material in the 50,000-100,000 K temperature range is measured. The redshifts are observed in the line center positions of spatially and temporally averaged profiles and are measured either relative to chromospheric Si I lines or from a comparison of sun center and limb profiles. The observations of 6-20 km/s redshifts place constraints on the mechanisms that dominate EUV line emission since it requires a strong weighting of the emission in regions of downward moving material, and since there is little evidence for corresponding upward moving materials in these lines.

Roussel-Dupre, D.↗

NASA NTRS1983

Photoionization models for the winds from cataclysmic variables

Photoionization models are presented for the relative abundances of Si IV, C IV, and N V as a function of the ionization parameter and shape of the ionizing continuum spectrum in the EUV and X-ray energy range. These models are applied to the interpretation of the UV resonance lines observed from the cataclysmic variable TW Virginis by Cordova and Mason. The observed lines are assumed to be formed in gas flowing supersonically out from the continuum source in the form of a stellar wind. It is shown that the shape of the ionizing spectrum can be determined without assuming that the outflow is spherical by fitting to ratios of the line strengths. Models which fit the observed ratios include an X-ray continuum with a low-energy cutoff and may also include a lower luminosity component in the EUV spectral region. Under the added assumption of spherical outflow the derived ionization parameter for the X-ray component can be used to constrain the total mass flux in the wind.

Kallman, T. R.↗

NASA NTRS1983

Observations of two peculiar emission objects in the Large Magellanic Cloud

Ultraviolet and visual wavelength spectra were obtained of two peculiar emission objects, Henize S63 and Sanduleak's star in the Large Magellanic Cloud. Previously not observed in the near- or far-ultraviolet, both objects exhibit strong permitted and semiforbidden line emissions. Estimates based on the absolute continuum flux of the hot companion star in Hen S63 indicate that it rivals the luminosity of the carbon star primary. The emission-line profile structure in both objects does not suggest Wolf-Rayet type emission. Carbon in Sanduleak's star (LMC anonymous) is conspicuously absent, while N V, semiforbidden N IV, and semiforbidden N III dominate the UV emission-line spectrum. Nitrogen is overabundant with respect to carbon and oxygen in both objects. The large overabundance of nitrogen in Sanduleak's star suggests evidence for CNO processes material similar to that seen in Nu Car.

Kafatos, M.↗

NASA NTRS1984

Two multifrequency observations of 3C 371

Observations of 3C371 made at frequencies from the radio to the ultraviolet and coordinated during two short time intervals separated by 3 months are presented. Also presented are 1 keV X-ray flux densities measured at a different time. The multifrequency measurements indicate spectral steepening at visual wavelengths, and that an extrapolation of the ultraviolet continuum falls below the X-ray data. The infrared through X-ray data is explained as relativistically beamed synchrotron self-Compton emission, and source parameters are derived for two possible models. The present ultraviolet spectra both show strong Lyman-alpha emission at the same redshift as weak optical emission lines reported previously. Production of these lines by recombination of gas, after its ionization by the ultraviolet to X-ray continuum radiation, is favored as an explanation. C IV and N V absorption lines are tentatively identified in one ultraviolet spectrum, which, if real, suggest the presence of a hot (about 300,000 K) gaseous halo in 3C 371.

Puschell, J. J.↗

NASA NTRS1984

High-resolution IUE observations of interstellar absorption lines in the Vela supernova remnant

Ultraviolet spectra of 45 stars in the vicinity of the Vela supernova remnant were recorded by the short-wavelength echelle spectrograph aboard the International Ultraviolet Explorer (IUE). Over one-third of the stars show interstellar absorption lines at large radial velocities (greater than 60 km/s). The mapping of these high-velocity components in the sky suggests the motions are chaotic, rather than from a coherent expansion of the remnant material. In accord with earlier conclusions from Copernicus data, the gas at high velocity exhibits higher than normal ionization and shows substantially less depletion of nonvolatile elements than normal interstellar material at low velocities. Relatively strong lines from neutral carbon in the two excited fine-structure states indicate that the neutral clouds within the remnant have had their pressures enhanced by the passage of the blast wave from the supernova. Also, the remnant seems to show a significant enhancement in the abundances of low-velocity Si IV, C IV, and N V over those found in the general interstellar medium.

Jenkins, E. B.↗

NASA NTRS1984

An unprecedented UV/optical flare in TV Columbae

A surprising, 2 mag, short time scale (hr) outburst of TV Col (2A 0526-328) was observed simultaneously at IUE and optical wavelengths in 1982 November. During this 'flare', the IUE emission lines of N v 1240, C IV 1550, and He II 1640, intensified by more than an order of magnitude and developed P Cygni profiles, indicating mass loss. Continuum fits with a power-law plus a blackbody from the UV through the optical showed a steepening of the UV power-law component and an increase in the temperature and size of the blackbody component during the flare activity. This unusual behavior is discussed in terms of an accretion disk instability.

Szkody, P.↗

NASA NTRS1986

A Penning discharge source for extreme ultraviolet calibration

A Penning discharge lamp for use in the calibration of instruments and components for the extreme ultraviolet has been developed. This source is sufficiently light and compact to make it suitable for mounting on the movable slit assembly of a grazing incidence Rowland circle monochromator. Because this is a continuous discharge source, it is suitable for use with photon counting detectors. Line radiation is provided both by the gas and by atoms sputtered off the interchangeable metal cathodes. Usable lines are produced by species as highly ionized as Ne IV and Al V. The wavelength coverage provided is such that a good density of emission lines is available down to wavelengths as short as 100A. This source fills the gap between 100 and 300A, which is inadequately covered by the other available compact continuous radiation sources.

Finley, David S.↗

NASA NTRS1987

PG 1411 + 442 - The nearest broad absorption line quasar

IUE observations reveal strong, moderately broad absorption troughs in the blue wings of the C IV and N V emission lines of the quasar PG 1411 + 442. No absorption from weakly ionized gas is detected. The emission-line strengths and overall shape of the ultraviolet/optical/near-infrared/far-infrared continuum of the new broad absorption line quasar are within the range normally measured in quasars. Its redshift is low enough to allow the morphology of the host galaxy to be studied in deep broad-band and intermediate-band CCD images. The galaxy appears to be a large spiral with a very long arm or tail. The inclination angle is 57 deg, which rules out the possibility that the line of sight to the nucleus intersects a large path length in a galactic disk.

Malkan, Matthew A.↗

NASA NTRS1988

Fast variations in the ultraviolet resonance lines of Alpha Camelopardalis (O9.5 Ia) - Evidence for blobs in the wind

The 72 IUE spectra of Alpha Cam and 19 IUE spectra of Kappa Cas, obtained during 72 hours of continuous IUE time in September 1978 were searched for variations in the profiles of the resonance lines of Si IV, C IV, and N V, and the results are discussed. The UV resonance lines in the spectra of Alpha Cam showed variations at the 2 percent level near -1800, -700, and +700 km/s. The first two variations can be explained by absorption components of outward-accelerated blobs or shells with an average acceleration of 1.5 cm/sq s. The characteristics of the blobs and shells are discussed, including the column densities and masses. No variations were found in the spectra of Kappa Cas.

Lamers, Henry J. G. L. M.↗

NASA NTRS1988

Stellar winds in the Small Magellanic Cloud

According to the predictions of line-driven stellar wind theory, the mass loss from luminous stars should be proportional to some power of the metallicity, yet previous studies have not shown this effect clearly. The authors have observed 22 main-sequence and giant O-type stars n the SMC with the low-resolution mode of the IUE in order to study the stellar winds in a particularly metal-poor galaxy. These stars are compared with carefully selected Galactic counterparts for wind terminal velocities and line strengths of C IV and N v. While the wind velocities in the SMC stars are confirmed to be systematically lower than in Galactic stars, as theory predicts, the mass-loss rates which are inferred from these data are comparable to those in the Galactic stars. The possible reasons for this result are discussed, including systematic effects in the data, assumed metal abundance, different ionization structure, and different physical parameters for the SMC stars.

Garmany, Catharine D.↗

NASA NTRS1988

Companions to peculiar red giants: HR 363 and HR 1105

Recent IUE observations of two Tc-deficient S-type peculiar red giants that are also spectroscopic binaries, HR 363 and HR 1105 are reported. A 675 min SWP exposure of HR 363 shows emission lines of O I 1304 and Si II 1812 and a trace of continuum. Compared to the M giants, the far UV flux may be relatively larger, indicating a possible contribution from a white dwarf companion, but no high temperature emission lines are seen to indicate that this is an interacting system where mass-transfer recently occurred. However, HR 1105 appears to have a highly variable UV companion. In 1982, no UV flux was discerned for this system, but by 1986 C IV was strong, increasing by a factor of 3 in 1987 with prominent lines of Si III, C III, O III, Si IV, and N V. Using orbital parameters, these observations are consistent with high activity occuring when the side of the S-star primary illuminated by the companion faces the Earth, but since the IUE data were taken over 3 orbits, a secular change in the UV component cannot be excluded.

Ake, Thomas B., III↗

NASA NTRS1988

Arches showing UV flaring activity

The UVSP data obtained in the previous maximum activity cycle show the frequent appearance of flaring events in the UV. In many cases these flaring events are characterized by at least two footpoints which show compact impulsive non-simultaneous brightenings and a fainter but clearly observed arch developes between the footpoints. These arches and footpoints are observed in line corresponding to different temperatures, as Lyman alpha, N V, and C IV, and when observed above the limb display large Doppler shifts at some stages. The size of the arches can be larger than 20 arcsec.

Fontenla, J. M.↗

NASA NTRS1989

Sanduleak's Star (LMC Anonymous) - Its similarity in the far-ultraviolet with the luminous object Eta Carinae and SN 1987A

Low-resolution IUE SWP spectra of the peculiar object LMC Anonymous (Sanduleak's Star) indicates strong evidence for CNO-processed nebula in the vicinity of the star. The far-UV spectrum of LMC Anonymous closely resembles that of the S Condensation of Eta Carinae. The similarity between LMC Anonymous and the S Condensation is apparent from the absolute intensity of the N V, semiforbidden N IV, and semiforbidden N III emission lines compared with the reduced strength of C IV or semiforbidden C III emission. IUE spectra of the S Condensation and SN 1987A may provide important clues concerning the nature of LMC Anonymous, which indicates departures from normal cosmic abundances of nitrogen relative to carbon that are extreme. This may suggest that carbon envelope burning and dredge-up occurred simultaneously during the helium-burning stage.

Michalitsianos, A. G.↗

NASA NTRS1989

Some acyclic analogues of nucleotides and their template-directed reactions

Bismonophosphoimidazolides of acyclic analogues of guanosine IV and adenosine V were synthesized. They undergo oligomerization in the presence of complementary polynucleotide templates. Details of their synthesis and their subsequent template- and nontemplate-directed reactions are described, and their possible relevance to the origin of life is discussed.

Tohidi, Mahrokh↗

NASA NTRS1990

A search for evidence of past and ongoing mass loss in the hot DO degenerates PG 1159 - 035, PG 1034 + 001, and KPD 0005 + 5106

The results are reported of a search for evidence of mass loss from three very hot, helium-rich degenerate stars, PG 1034 + 001, PG 1159 - 035, and KPD 0005 + 5106, based upon a search for weak, velocity-displaced line features in high-resolution IUE spectra. The search was entirely negative for PG 1159 - 035 and resulted in the detection of Si IV in PG 1034 + 001 at the same redshifted velocity as the other detected far-ultraviolet resonance doublets. The presence of photospheric silicon in a DO star at 80,000 K is not predicted by current radiative acceleration theory. For KPD 0005 + 5106 there is no convincing far-ultraviolet spectroscopic detection of either past or ongoing mass loss. The high-ionization resonance doublets of C IV, Si IV, and N v share the same velocity broadening as the interstellar lines but are velocity displaced by + 10 km/s relative to the local interstellar medium. Their origin is ascribed to an expanding, photoionized H II region around KPD 0005 + 5106.

Fritz, Matthew L.↗

NASA NTRS1990

Ultraviolet interstellar absorption toward HD 163522, a halo star at 9 kiloparsecs in the direction l = 350 deg and b = -9 deg

UV interstellar absorption line measurements obtained by the IUE satellite toward HD 163522 in the direction l = 349.6 deg and b = -9 deg are analyzed. The UV stellar spectrum of HD 163522 places the star beyond the Galactic center at a distance away from the Galactic plane of z = -1.5 kpc. An absorption-line spectrum with S/N approximately equal to 15-25 is produced. The UV interstellar absorption lines for neutral, and weakly and highly ionized gas are examined, as well as the correlation between profiles and the Galactic rotation. Simple model calculations are performed to quantitatively determine the characteristics of the profiles. The observed absorption-line profiles for Al III, Si IV, C IV, and N V are converted into plots of optical depths vs velocity. A comparison of these plots for the weak and strong member of each doublet reveals that the profiles are not strongly influenced by the presence of unresolved saturated absorption, and the optical depth profiles can therefore be directly converted into measures of N(v), the column density per km/s. The converted column densities are examined.

Savage, Blair D.↗

NASA NTRS1990

The oxygen emission lines in Wolf-Rayet spectra

The statistical equilibrium of oxygen in the line-emitting regions of WR stars is studied using the one-representative point theory of Castor and van Blerkom (1970) for a wider range of the radiation temperature, electron temperature, and electron density. Radiative processes, collisional processes, and electronic recombination are studied for a 59-level model atom. The level populations depart by large factors from those expected for LTE at the electron temperature and electron density. When the parameters have the values found previously by Bhatia and Underhill to be representative for WR stars, the oxygen atoms are strongly concentrated in the lower levels of O(2+), O(3+), and O(4+). The predicted relative energies in typical lines of O III, O IV, and O V for a solar abundance ratio of O to H are compatible with what is observed in WC and WN spectra.

Bhatia, A. K.↗