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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

The ultraviolet spectrum of the high redshift BL lac object 0215+015

The highly variable object 0215+015 is studied. It has a range in the optical band of more than 4 magnitudes, and exhibits radidly variable polarization. At optical wavelengths it shows no emission lines but has a rich absorption spectrum with at least six, separate redshift systems. Combined are 3 LWP low-dispersion images of the object to produce a high signal-to-noise spectrum between 2400 - 3200 A which is use to study the hydrogen Lyman lines of the various redshift systems.

Blades, J. C.↗
NASA NTRS1986

The outer atmosphere of the carbon star TX Piscium

A high-resolution LWP IUE spectrum of the bright N-type carbon star TX Psc demonstrates that the Mg II h and k emission profiles are strongly affected by absorption from Mg II, Mn I, probably Fe I, and possibly from molecules. The indication that the absorbing matter has a column density of not less than 10 to the 20th H atoms or molecules per sq cm is consistent with absorption in a slowly expanding envelope. The integrated Mg II line flux is found to be much greater than in 1981, and the radio CO (J = 1 - 0) line from the circumstellar shell is detected. Results for a column density of not larger than 10 to the 22nd H2 molecules/sq cm, and a radial velocity close to that of the star, are in agreement with those obtained from UV data. Some dust emission from carbon grains is suggested by the far infrared flux distribution, and a mass-loss rate estimation for the star of 10 to the -6th to 10 to the -8th solar masses is obtained.

Eriksson, K.↗
NASA NTRS1987

IUE observations of Mg2 emission form Miras

A total of 61 well exposed low dispersion LWP observations were obtained from the brightest normal M-type Mira variables around their pulsation cycles. By observing Mg II throughout the cycle, the intent is to determine the shock luminosity and duration, and the density in the upper atmospheres of Miras. The data also provide a mechanism for probing the radiative cooling in postshock regions of the atmosphere as a function of time. A summary of data acquired for IUE programs SRHLW and MGIEB is presented.

Brugel, Edward W.↗
NASA NTRS1987

Ultraviolet Spectra of Two Magnetic White Dwarfs and Ultraviolet Spectra of Subluminous Objects Found in the Kiso Schmidt Survey

Low resolution International Ultraviolet Explorer (IUE) spectroscopic observations of two magnetic white dwarfs BPM25114 and K813-14 were obtained using both the SWP and LWP cameras. The first object has an observed magnetic field of 4 x 10(7) Gauss and the second has one of 3 x 10(7) Gauss. Both objects have overall spectral energy distributions appropriate for cool DA white dwarfs with T(eff) near 10,000 K and accordingly show strong lambda lambda 1400 and 1600 absorption in their spectra. Compared to non-magnetic DA white dwarfs of comparable effective temperature, there are some differences in the profiles, presumably produced by the magnetic fields in these objects. In addition, the ultraviolet spectra of a number of hot subluminous stars in the Kiso Schmidt survey were observed.

Wegner, Gary A.↗
NASA NTRS1986

The ultraviolet IUE spectrum of the population II horizontal-branch A star HD 109995

Eight SWP and six LWP high dispersion IUE images of the field horizontal-branch A star HD 109995 have been averaged to increase the signal-to-noise ratio. The line density in the UV is greater than that of the optical region, but less than that of comparable population I A stars. Many of the identified lines are those of Fe II. Lines of C I, C II, N I, O I, Mg I, Mg II, Al II, Al III, Si I, Si II, P II, S I, Cl I, Ti II, V II, Cr II, Mn II, Fe I, Co II, Ni II, Zn II, and Ge II are also present. Definite interstellar lines of C II, Mg I, Mg II, Al II, Si II, and Fe II were found.

Adelman, S. J.↗
NASA NTRS1988

International ultraviolet explorer spectral atlas of planetary nebulae, central stars, and related objects

The International Ultraviolet Explorer (IUE) archives contain a wealth of information on high quality ultraviolet spectra of approximately 180 planetary nebulae, their central stars, and related objects. Selected are representative low-dispersion IUE spectra in the range 1200 to 3200 A for 177 objects arranged by Right Ascension (RA) for this atlas. For most entries, the combined short wavelength (SWP) (1200to 1900) and long wavelength (LWR) (or LWP, 1900 to 3200 A) regions are shown on 30 cm by 10 cm Calcomp plots on a uniform scale to facilitate intercomparison of the spectra. Each calibrated spectrum is also shown on an expanded vertical scale to bring out some of the weaker features.

Feibelman, Walter A.↗
NASA NTRS1987

Intrinsically variable stars

The characteristics of intrinsically variable stars are examined, reviewing the results of observations obtained with the IUE satellite since its launch in 1978. Selected data on both medium-spectral-class pulsating stars (Delta Cep stars, W Vir stars, and related groups) and late-type variables (M, S, and C giants and supergiants) are presented in spectra, graphs, and tables and described in detail. Topics addressed include the calibration of the the period-luminosity relation, Cepheid distance determination, checking stellar evolution theory by the giant companions of Cepheids, Cepheid masses, the importance of the hydrogen convection zone in Cepheids, temperature and abundance estimates for Population II pulsating stars, mass loss in Population II Cepheids, SWP and LWP images of cold giants and supergiants, temporal variations in the UV lines of cold stars, C-rich cold stars, and cold stars with highly ionized emission lines.

Bohm-Vitense, Erika↗
NASA NTRS1988

Direct UV observations of the circumstellar envelope of alpha Orionis

Observations were made in the IUE LWP camera, low dispersion mode, with alpha Ori being offset various distances from the center of the Long Wavelength Large Aperture along its major axis. Signal was acquired at all offset positions and is comprised of unequal components of background/dark counts, telescope-scattered light, and scattered light emanating from the extended circumstellar shell. The star is known from optical and infrared observations to possess an extended, arc-minute sized, shell of cool material. Attempts to observe this shell with the IUE are described, although the deconvolution of the stellar signal from the telescope scattered light requires further calibration effort.

Stencel, R. E.↗
NASA NTRS1988

Mapping of surface activity on the W UMa-type system VW Cephei

After multifilter photometry of the W UMa-type contact binary VW Cep (P = 6.67 hr; G5V + K0V) in 1986/87 revealed large asymmetries in the light curves believed to be caused by large, cool starspot regions on the surface of the larger stars, in Apr. 1987 it was observed with IUE to study the chromospheres and transition regions of the components. During one complete orbital cycle, three SWP and four LWP low dispersion spectra were obtained, including and then excluding the suspected active region. Phase dependent TR line emission strengths were found, most notably C IV, which is 50 percent stronger when the spot region is most visible. The results could be important because VW Cep represents an extreme case for studying stellar dynamo theory and observations can play a crucial role in the unterstanding of magnetic fields and activity cycles in rapidly rotating solar-like stars.

Bradstreet, David H.↗
NASA NTRS1989

Rotational modulation and flares on RS Canum Venaticorum and BY Draconis stars. XI - Ultraviolet spectral images of AR Lacertae in September 1985

High-resolution IUE SWP and LWP spectra of the bright eclipsing RS CVn system AR Lac, obtained during 40 h of continuous observations on September 18-19, 1985, are compiled and used to derive chromosphere images. The data and results are presented graphically and discussed in detail. The K 0 IV star is found to have a globally brighter trailing hemisphere and at least three distinct plage regions; the G 2 IV star has a flaring region with significant redshift and line-emission broadening and exhibits factor-of-3 variability in total emission, attributed to the presence of a large chromospherically inactive region.

Neff, J. E.↗
NASA NTRS1990

Ultraviolet observations of the enigmatic bipolar nebula M1-92

IUE observations of the bipolar nebula M1-92 indicate that this object is an evolved object and possibly a preplanetary nebula. Data acquired in the SWP camera clearly show a flat UV continuum indicative of a relatively unobscured O subdwarf. Data acquired in the long wavelength region of the SWP and in the LWP camera reveal an object which mimics the characteristics of an F2 supergiant. These results, combined with other data at visual and infrared wavelengths, strongly favor that the central object of M1-92 is an evolved binary system. The He I 5876 and 6678 A absorption seen in optical spectra is extremely problematic. This, plus the presence of Fe II, Mg II, and Mg I absorption arising from resonance transitions and transitions from low-lying metastable levels, leads to the suggestion that the object resembling an F2 star is actually an early-type subdwarf cloaked in a thick circumstellar nebula. This interpretation and the similarities of the IR flux distribution to Vy 2-2, a preplanetary prototype, is consistent with M1-92 being in the process of forming a planetary nebula. The bipolar flow, the optical Fe II and ultraviolet Mg II emission of M1-92 are also characteristics of T Tauri stars. Although constraints definitely rule out that M1-92 is a pre-main-sequence object, these similarities imply that in many cases it may be difficult to distinguish between T Tauri stars and protoplanetary nebulae.

Feibelman, Walter A.↗
NASA NTRS1990

Camera artifacts in IUE low-dispersion spectra

Sky-background images obtained by the International Ultraviolet Explorer (IUE) were analyzed to study artificial spectral features (camera artifacts) in low-dispersion spectra. The artifacts mimic emission features and have been present in long-exposure spectra since the launch of the IUE satellite. The camera artifacts are strong in spectra characterized by long exposure times because they scale in time-integrated flux with the background level, which increases during the exposure due to camera phosphorescence. The artifacts cannot be detected in spectra obtained from short, direct exposures of flat-field lamps or standard stars. Plots of average sky-background spectra for the three operational IUE cameras (SWP, LWP, and LWR) are given to aid scientists in the identification of artifacts in their spectra.

Crenshaw, D. Michael↗
NASA NTRS1990

IUE observations of WX Ceti in outburst

IUE observations of the peculiar dwarf nova WX Ceti roughly two weeks into a rare outburst, the first since 1963, are obtained. The SWP and LWP observations are very similar to other erupting dwarf novae, both in the line and continuum spectrum. The C IV 1549 A doublet displays blueshifted absorption, indicating the presence of a wind which has a terminal velocity of either 3150 km/s or 5450 km/s; the two possible values are due to an uncertain interpretation of features in the blue wing of the line. The line may also show a weak, redshifted emission, completing a P Cygni profile. The derived mass-loss of 0.7-2.2 x 10 to the -11th solar masses/yr is quite typical of dwarf novae in outburst. Thus, despite the unusual photometric properties of this system, long recurrence time, large amplitude outbursts, and long duration eruptions, the IUE observations reveal that the physical characteristics of the outburst are fairly typical of dwarf novae.

Downes, Ronald A.↗
NASA NTRS1991

IUE spectra of the eclipsing binary NN Serpentis

Low-resolution SWP and LWP IUE spectra are used to fit the temperature and angular radius of the white dwarf in the detached eclipsing binary NN Ser. It is found that the redenning to the system has E(B-V) of 0.05 +/-0.05, the white dwarf temperature is 60,000 +/-10,000 K, and the age of the white dwarf is less than 10 exp 7. The shape of eclipse and the K-magnitude of the secondary star are used to constrain the inclination of the binary and the masses and radii of the two stars. The size of the secondary star relative to its Roche lobe and the age of the white dwarf indicate that mass transfer has not yet occurred and that the system is a precataclysmic variable rather than a cataclysmic variable which has entered the period gap. Fitting the observed magnitude of the sinusoidal modulation with a reprocessing model shows that only when i is approximately equal to 90 deg is the required temperature of the secondary star consistent with these results. For this solution the white dwarf temperature is also consistent with those obtained from the IUE spectra.

Wood, Janet H.↗
NASA NTRS1993

Ultraviolet spectra of QSOs, BL Lacertae objects, and Seyfert galaxies

Motivated by the possibility of detecting strong absorption features toward extragalactic background objects, we present optimally extracted and co-added IUE spectra of QSOs, BL Lac objects, and Seyfert galaxies. These spectra together with those presented (in a similar work) by Kinney et al. form an essentially complete sample of the SWP, LWP, and LWR observations of QSOs, BL Lac objects, and Seyfert galaxies with redshifts z greater than 0.05 available in the IUE merged observation log as of 1991 December.

Lanzetta, Kenneth M.↗
NASA NTRS1993

The analysis of global cloud and radiation data for the study of cloud-climate interactions

During the past year, ISCCP C1 data has been analyzed for two regions, the stratocumulus area off California and the tropical Atlantic. The first part of the study dealing with stratocumulus clouds is complete and the results form the technical portion of this report. A manuscript that has recently been prepared covering this subject is attached as an Appendix. The abstract describes the key finding that cloudy mean liquid water path (LWP) is invariant with cloud fraction for cloud cover between 20% - 80%. Similar conclusions are reached with ISCCP and LANDSAT data. This has important implications for General Circulation Models. A possible application is described in the Discussion section of the Appendix. An idea that has been proposed for dealing with GCM hydrology can be used.

HARSHVARDHAN↗
NASA NTRS1993

A regional analysis of cloudy mean spherical albedo over the marine stratocumulus region and the tropical Atlantic Ocean

Since clouds are the largest variable in Earth's radiation budget, it is critical to determine both the spatial and temporal characteristics of their radiative properties. The relationships between cloud properties and cloud fraction are studied in order to supplement grid scale parameterizations. The satellite data used is from three hourly ISCCP (International Satellite Cloud Climatology Project) and monthly ERBE (Earth Radiation Budget Experiment) data on a 2.5 deg x 2.5 deg latitude-longitude grid. Mean cloud spherical albedo, the mean optical depth distribution, and cloud fraction are examined and compared off the coast of California and the mid-tropical Atlantic for July 1987 and 1988. Individual grid boxes and spatial averages over several grid boxes are correlated to Coakley's theory of reflection for uniform and broken layered cloud and to Kedem, et al.'s findings that rainfall volume and fractional area of rain in convective systems is linear. Kedem's hypothesis can be expressed in terms of cloud properties. That is, the total volume of liquid in a box is a linear function of cloud fraction. Results for the marine stratocumulus regime indicate that albedo is often invariant for cloud fractions of 20% to 80%. Coakley's satellite model of small and large clouds with cores (1 km) and edges (100 m) is consistent with this observation. The cores maintain high liquid water concentrations and large droplets while the edges contain low liquid water concentrations and small droplets. Large clouds are just a collection of cores. The mean optical depth (TAU) distributions support the above observation with TAU values of 3.55 to 9.38 favored across all cloud fractions. From these results, a method based upon Kedem, et al's theory is proposed to separate the cloud fraction and liquid water path (LWP) calculations in a general circulation model (GCM). In terms of spatial averaging, a linear relationship between albedo and cloud fraction is observed. For tropical locations outside the Intertropical Convergence Zone (ITCZ), results of cloud fraction and albedo spatial averaging followed that of the stratus boxes containing few overcast scenes. Both the ideas of Coakley and Kedem, et al. apply. Within the ITCZ, the grid boxes tended to have the same statistical properties as stratus boxes containing many overcast scenes. Because different dynamical forcing mechanisms are present, it is difficult to devise a method for determining subgrid scale variations. Neither of the theories proposed by Kedem, et al. or Coakley works well for the boxes with numerous overcast scenes.

Ginger, Kathryn M.↗
NASA NTRS1994

Relationship between ice water path and downward longwave radiation for clouds optically thin in the infrared: Observations and model calculations

A vertically pointing 3.2-cm radar is used to observe altostratus and cirrus clouds as they pass overhead. Radar reflectivities are used in combination with an empirical Z(sub i)-IWC (ice water content) relationship developed by Sassen (1987) to parameterize IWC, which is then integrated to obtain estimates of ice water path (IWP). The observed dataset is segregated into all-ice and mixed-phase periods using measurements of integrated liquid water paths (LWP) detected by a collocated, dual-channel microwave radiometer. The IWP values for the all ice periods are compared to measurements of infrared (IR) downward fluxes measured by a collocated narrowband (9.95-11.43 microns) IR radiometer, which results in scattergrams representing the observed dependence of IR fluxes on IWP. A two-stream model is used to calculate the infrared fluxes expected from ice clouds with boundary conditions specified by the actual clouds, and similar curves relating IWP and infrared fluxes are obtained. The model and observational results suggest that IWP is one of the primary controls on infrared thermal fluxes for ice clouds.

Uttal, Taneil↗