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At least 811 records · Page 45
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NASA NTRS2005

Monitoring the Intrinsic Absorption Complex Toward RXJ123.8+0115

The main focus of this study was changed after it became clear that the object to be observed by the Far Ultraviolet Spectroscopic Explorer (FUSE) could not be observed because of spacecraft attitude control system limitations. Therefore, we replaced the prime object for the program (RXJ1230.8+0115) with a secondary object (H1821+643) and began an analysis of a similar quasar spectrum for comparison with the extant RXJ1230.8+0115 spectrum obtained previously by FUSE. Using high-resolution spectra of the radio-quiet, X-ray bright quasar HE0226-4110 obtained with the Hubble Space Telescope (HST) and FUSE, we have examined the remarkable associated absorption line system. The redshift of the quasar is roughly 0.5 so the FUSE+HST spectra cover the rest-frame wavelength in the extreme ultraviolet range 610-1 160A. In this range, we detect transitions from a wide range of ionization species, including the H I Lyman series, Ne VIII, and, for the first time at this resolution, four adjacent stages of oxygen O III-VI. The high quality of these spectra allow us to disentangle the structure of the gas using the complex kinematics to guide the assessments of the multi-component and multi-phase nature of the system. The kinematics of the absorber indicate that all the H I, C 111, and 0 III reside in a single component, while higher ionization species largely arise in a separate, more kinematically complex structure. An analysis of a ground-based spectrum of the quasar covering the H-beta and [0 III] 5007 emission lines reveal excellent agreement in redshift between the 0 III absorption and emission. Thus, for the first time, we are able to use absorption line diagnostics to place interesting constraints on the location and structure of the narrow emission line region around a quasar. In addition, we detect a narrow absorption line that is only seen O VI and lies roughly 50 km/s redward of the narrow emission line component. Absorption from 0 V, and Ne VIII is also detected at this velocity, but as part of broader, more redshifted, component. The line width of this narrow component imply a maximum temperature that is too small to produce O VI through collisional process, suggesting instead that photoionization from the quasar is the dominant ionization mechanism. The photoionization constraints imply that O VII should be the dominant ionization stage in the component.

Sembach, Kenneth↗
NASA NTRS1984

Ultraviolet interstellar absorption toward stars in the Small Magellanic Cloud. II - Sk 159

On the basis of a study of high-dispersion spectra of stars in the Large and Small Magellanic Clouds (LMC and SMC) obtained with the International Ultraviolet Explorer (IUE) satellite, Savage and de Boer (1979, 1981) have detected the Milky Way halo gas. The existence of gaseous coronae around both the LMC and SMC was also suggested. The evidence for this included the presence of strong interstellar absorption lines from high-ionization stages (e.g., C IV, Si IV, and Al III). Fitzpatrick and Savage (1983) concluded that the strongest C IV and Si IV features in the spectrum of the SMC star HD 5980 are probably nebular in origin, produced by stellar photoionization in NGC 346. However, other evidence indicated that at least one additional region of high-ionization stage gas is present in the HD 5980 line of sight. The present investigation is concerned with a reobservation of the star SK 159. It is found that the Milky Way disk and halo absorption features toward Sk 159, except for Al III are very similar to those seen toward HD 5980, also in the SMC.

Fitzpatrick, E. L.↗
NASA NTRS1982

Ultraviolet properties of the symbiotic stars

A general discussion of the UV spectra of symbiotic stars, including both the emission lines and the continua, is presented, with AG Pegasi considered as an illustrative example. It is noted that the IUE observations of the symbiotics have revealed UV properties which rival the diversity of the optical features. Nevertheless, the UV data have for the first time permitted the hot component to be studied relatively uncontaminated by the giant companion, which dominates the optical regime. The UV observations provide convincing evidence that many of the symbiotics have hot stellar companions embedded in the enshrouding nebula or accretion shell formed from the wind from one or possibly both of the components.

Slovak, M. H.↗
NASA NTRS1991

Characterization of low albedo asteroids

Three asteroid classes were defined from the early spectral surveys of the asteroids: C for carbonaceous, M for metallic, and S for stony. Subsequent spectral studies have defined new asteroid classes and have shown that the original classes can be divided into a number of additional asteroid types. The low albedo asteroids are now classified as types B, C, D, F, G, K and P. These types are concentrated in the middle asteroid belt and beyond. Their visible and near infrared spectra are generally featureless and dark, with a red slope in the ultraviolet. Some of them show water absorption features in the mid-infrared, indicating the presence of bound water. The lack of absorption features in the telescopic reflectance spectra has made it difficult to determine the mineralogy of the low albedo asteroids. Some of the asteroid spectra closely resemble laboratory spectra of carbonaceous chondrite meteorites, implying similar mineralogy. These meteorites are chemically primitive, composed of fine-grained matrix material which includes hydrous silicates, complex carbon compounds, olivine and pyroxene, combined with olivine and/or pyroxene chondrules. The main belt low albedo asteroids are probably very similar to these asteroids, but no meteoritic analogues exist for the most distant asteroid types.

Lebofsky, Larry A.↗
NASA NTRS1992

Ultraviolet observations of stellar coronae - Early results from HST

The first GHRS spectra of two very different late-type giant stars - Capella and Gamma Dra are reported. Capella is a 104 day period binary system consisting of two stars (G9 III and GO III) each of which shows bright emission lines formed in solar-like transition region and coronae. By contrast, Gamma Dra is a hybrid-chromosphere star with very weak emission lines from high-temperature plasma. Low-dispersion spectra of these stars covering the 1160 to 1717 A spectral range show unresolved emission lines from neutral species through N V. The very different surface fluxes detected in the spectra of these stars suggest different types of heating mechanisms.

Linsky, Jeffrey L.↗
NASA NTRS1993

Analysis of the IUE spectra of the strongly interacting binary beta Lyrae

The six-band ultraviolet light curves of beta Lyrae obtained with the Orbiting Astronomical Observatory A-2 in 1970 exhibited a very unusual behavior. The secondary minimum deepened at shorter wavelength, indicating that one was not observing light variations caused primarily by the eclipses of two stars having a roughly Planckian energy distribution. It was then suggested that the light variations were caused by a viewing angle effect of an optically-thick, ellipsoidal circumbinary gas cloud. Since 1978 beta Lyrae has been observed with the International Ultraviolet Explorer (IUE) satellite. We have constructed ultraviolet light curves from the IUE archival data for comparison with the OAO-A2 results. We find that they are in substantial agreement with each other. The Voyager ultraviolet spectrometer was also used to observe this binary during a period covered by IUE observations. The Voyager results agree with those of the two other satellite observatories at wavelengths longer than about 1350 A. However, in the wavelength region shorter than the Lyman-alpha line at 1216 A, the light curves at 1085 A and 965 A show virtually no light variation except an apparent flaring near phase 0.7, which is also in evidence at longer wavelengths. We suggest that the optically-thick circumbinary gas cloud, which envelops the two stars completely, assumes a roughly spherical shape when observed at these shorter wavelengths.

Mccluskey, George E., Jr.↗
NASA NTRS1999

Ultraviolet Evidence for a Binary Nucleus of SH 2-71

Archival IUE spectra of Sh 2-71 show a variable Mg II lambda 2800 emission line superposed on a variable stellar continuum. The Mg II feature is thought to be excited in the chromosphere of the cooler component of a binary nucleus, although other causes are also plausible.

Feibelman, W. A.↗
NASA NTRS1989

IUE observations of the Jovian dayglow emission

IUE spectra of Jupiter are examined in light of recent models put forward to explain the anomalously bright ultraviolet emissions seen from the upper atmospheres of the outer planets. Chi-squared fits of the IUE spectra with model spectra produced by two proposed excitation mechanisms, electron impact and fluorescence of solar radiation, result in consistently higher chi-squared values for the solar fluorescence model. No conclusive evidence is found in the IUE data for the dominance of solar fluorescence over electron excitation in producing the Jovian dayglow emission.

Mcgrath, M. A.↗
NASA NTRS1998

Black Hole X Ray Nova Outburst with XTE and HST

We obtained multiwavelength coverage of the soft X-ray transient. GRO J1655-40 during its 1996 outburst, using HST (Hubble Space Telescope), RXTE (Rossi X Ray Timing Explorer), CGRO (Compton Gamma Ray Observatory), and ground-based facilities. This outburst was qualitatively different from other SXT (Soft X Ray Telescope) outbursts and from previous outbursts of this source. The onset of hard X-ray activity occurred very slowly, over several months, and was delayed relative to the soft X-ray rise. During this period, the optical fluxes declined steadily. This apparent anticorrelation is not consistent with with the standard disk instability model of SXT outbursts, nor is it expected if the optical output is dominated by reprocessed X-rays, as in persistent low-mass X-ray binaries. Based on the strength of the 2175-A interstellar absorption feature, we constrain the reddening to be E(B - V) = 1.2 plus or minus 0.1, a result which is consistent with the known properties of the source and with the strength of other interstellar absorption lines. After this dereddening we find that the spectra are dominated by a component peaking in the optical, with the expected v (sup l/3) disk spectrum seen only in the ultraviolet. Bowen fluorescence lines of NIII and OIII are also seen, as well as possible P Cyg profiles in the ultraviolet resonance lines. These features suggest an accretion-disk wind. The X-ray spectra broadly resemble the high/soft state commonly seen in black hole candidates, but evolve through two substates.

Patterson, Joseph↗
NASA NTRS1974

Cross sections and band strengths for the N2O/+/ /A 2Sigma+ to X 2Pi/ system produced by vacuum ultraviolet radiation

Analysis of cross sections that have been obtained for the production of the N2O(+) (A 2Sigma+ to X 2Pi) fluorescence, using vacuum ultraviolet radiation between 462 and 755 A. The fluorescence spectra produced using incident photons of 715.6- and 754.9-A wavelengths are presented, as well as the relative fluorescence cross sections for the individual observed bands of the above-mentioned N2O(+) system. Finally, absolute cross sections for the production of the N2O(+) (A 2Sigma+ to X 2Pi) system are presented, as well as band strengths for the A 2Sigma+(0,0,0) to X 2Pi(n1,n2,0) fluorescence.

Lee, L. C.↗
NASA NTRS1980

Ultraviolet spectroscopic observations of VV Cephei

Observations with the International Ultraviolet Explorer satellite (IUE) of the recent egress of the eclipsing binary VV Cep(M2Iabep + B) are presented. Egress in the ultraviolet lagged behind the visible egress by 2 to 3 months, as a result of higher opacity of the M star atmosphere in the ultraviolet. Shortly after visible egress, the low-dispersion spectrum from 1200-1900 wavelengths was dominated by fluorescent emission lines arising in the atmosphere of the M star and excited by the ultraviolet continuum of the hot companion. High-dispersion spectra show a hot continuum blanketed by absorption lines, some of which weaken or disappear as the egress continues. A year after eclipse, the absorption spectrum is still too complex to permit determination of a spectral type for the hot companion. The Mg II h and k lines both exhibit asymmetric double-peaked emission features indicative of an expanding chromosphere in the M supergiant.

Hagan, W.↗
NASA NTRS1980

Velocity fields and spectrum peculiarities in Beta Cephei stars

The acquisition of short wavelength spectra of Beta Cephei variable stars from the International Ultraviolet Explorer is reported. A total of 122 images of 10 variable stars and 3 comparison stars were obtained. All of the images were observed in the high dispersion mode through a small aperture. The development of image processing methods is also briefly discussed.

Lesh, J. R.↗
NASA NTRS1982

Ultraviolet observations of four symbiotic stars

Observations were obtained with the International Ultraviolet Explorer (IUE) of four symbiotic stars. The UV spectra of YY Her, SY Mus, CL Sco, and BX Mon are characterized by varying degrees of thermal excitation. These low resolution spectra have been analyzed in terms of line-blanketed model atmospheres of early A, B, and F type stars in order to identify the nature of the hot companion in these systems. The expected emission from early main sequence stars does not fully explain the observed distribution of UV continuum energy over the entire IUE spectral range (1200-3200 A). More likely the observed continuum may be originating from an accretion disk and/or hot subdwarf that photoionizes circumstellar material, and gives rise to the high excitation lines that have been detected. The Bowen fluorescent excited lines of O III in SY Mus exhibit slightly broadened profiles that suggest possible turbulent motions in an extended circumstellar cloud with characteristic velocities of approximately 300 km/s.

Michalitsianos, A. G.↗
NASA NTRS1982

Optogalvanic wavelength calibration for laser monitoring of reactive atmospheric species

Laser-based techniques have been successfully employed for monitoring atmospheric species of importance to stratospheric ozone chemistry or tropospheric air quality control. When spectroscopic methods using tunable lasers are used, a simultaneously recorded reference spectrum is required for wavelength calibration. For stable species this is readily achieved by incorporating into the sensing instrument a reference cell containing the species to be monitored. However, when the species of interest is short-lived, this approach is unsuitable. It is proposed that wavelength calibration for short-lived species may be achieved by generating the species of interest in an electrical or RF discharge and using optogalvanic detection as a simple, sensitive, and reliable means of recording calibration spectra. The wide applicability of this method is emphasized. Ultraviolet, visible, or infrared lasers, either CW or pulsed, may be used in aircraft, balloon, or shuttle experiments for sensing atoms, molecules, radicals, or ions.

Webster, C. R.↗
NASA NTRS1989

Interstellar and cometary dust

Aspects of interstellar dust which are known from direct observation will be discussed. Some specific difficulties that various theories have in explaining the observations will be presented. Several theoretical interpretations which have been advanced will be discussed, highlighting first their similarities and then their differences. Also discussed will be the author's ideas about the conditions of interstellar dust throughout its life cycle, from origin to incorporation in pre-cometary ices. Dust is primarily observed by its effects on the spectra of background stars, so observations at optical and ultraviolet (UV) wavelengths are confined to the diffuse interstellar medium (ISM) or to the outer regions of dense clouds. Within this somewhat limited range of environments there are very few lines of sight which show any evidence for icy mantles, but there are major variations in the wavelength dependence of the extinction. In the infrared region of the spectrum, it is possible to observe a few stellar sources deeply embedded within molecular clouds.

Mathis, John S.↗
NASA NTRS1991

Radiative transfer in the dynamic atmospheres of Mira-type variables

This NASA grant covers our ADP research program, which involved detailed radiative transfer calculations of hydrodynamic models of pulsating asymptotic giant branch stars. Synthetic spectra resulting from these calculations are compared with International Ultraviolet Explorer (IUE) observations of these stars to test the validity of the models.

Linsky, Jeffrey L.↗
NASA NTRS1994

On planetary nebula nuclei of the spectral type 'continuous'

A detailed reexamination of existing and new International Ultraviolet Explorer (IUE) low-and high-resolution spectra, plus a search of recent literature, shows that for most of the central stars of planetary nebulae the spectral classification previously called 'continuous-type' does not really exist. The stellar continua show primarily photospheric absorption features of N V, O V, C IV, H, He II, Fe VII, Fe VI, and Fe V, and/or P Cygni profiles.

Feibelman, Walter A.↗