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At least 955 records · Page 53
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NASA NTRS1993

Ultraviolet properties of individual hot stars in globular cluster cores. I - NGC 1904 (M79)

As part of an observing program using the IUE satellite to investigate the properties of stars within the cores of Galactic globular clusters, we have obtained three spectra of the cluster NGC 1904 (M79). All three were long-integration-time, short-wavelength (SWP) spectra obtained at the so-called 'center-of-light', and all three showed evidence of multiple sources within the IUE large aperture. We describe the analysis of these spectra and present evidence that the UV sources represent individual hot stars in the post-horizontal-branch stage of evolution. We see more UV-bright objects in the core of this cluster than expected from surveys of similar objects discovered in the outer regions of other globulars, leading us to conclude that dynamical effects in the core may significantly alter the path of evolution off the horizontal branch. The spectra also appear to be fitted more closely by models using Population I metal abundances than by Population II abundance models.

Altner, B.↗
NASA NTRS1974

Optical grating evaluator - A device for detailed measurement of diffraction grating efficiencies in the vacuum ultraviolet

A device for detailed measurement of diffraction grating efficiencies and over-all performance in the VUV has been designed and constructed at the Naval Research Laboratory. The system employs semiautomated mechanisms to scan the face of the grating with a narrow monochromatic beam, and an efficiency map of the grating surface is produced on a strip chart recorder. Grating efficiency in the various diffracted orders and intensity of light scattered between orders may also be measured. A unique feature is the ability to determine the angle and effectiveness of grating blaze and variations in blaze under different conditions of illumination.

Michels, D. J.↗
NASA NTRS1993

The first ultraviolet spectropolarimetric study of NGC 1068

We present the first ultraviolet spectropolarimetric observations of the Type 2 Seyfert galaxy NGC 1068 obtained by the Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE). In this communication we discuss the continuum polarization in the ultraviolet. Shortward of 2500 A the polarization becomes constant at 16 percent at a position angle of 112 deg. This is in general agreement with the 1983 prediction of Miller and Antonucci (1983) and their thick torus model, in which a hidden Type 1 Seyfert nucleus is seen in light scattered by electrons outside the obscured nuclei.

Code, Arthur D.↗
NASA NTRS1993

Spectroscopic observations of the detached binary PG 1413 + 015

We present improved estimates of the stellar parameters of the eclipsing, precataclysmic binary system PG 1413 + 015 (GH Vir), which has an orbital period of only 8h16m. Model atmosphere fits a Balmer line profiles yield T(eff) = 48,800 +/- 1200 K and log g = 7.70 +/- 0.11 for the DAO white dwarf primary star, from which a mass of 0.51 +/- 0.04 solar mass is inferred using evolutionary models. An ultraviolet spectrum obtained with the IUE Observatory has a slope consistent with this temperature and the assumption of no interstellar extinction. A red CCD spectrum of the secondary star during the 12-minute total eclipse indicates a spectral type of M3 V-M5 V. Reanalysis of the eclipse light curve leads to an inferred radius of 0.15 solar radius and a mass of 0.10 solar mass for the secondary, the latter being marginally consistent with the spectral type. Reprocessing on the facing side of the secondary produces phase-dependent Balmer line emission and detectable variations in the continuum from 6500-9000 A. The observed levels of reprocessing are consistent with expectations based on the above stellar parameters.

Fulbright, Michael S.↗
NASA NTRS1994

Globular cluster photometry with the Hubble Space Telescope. 3: Blue stragglers and variable stars in the core of M3

This paper describes Hubble Space Telescope (HST)/Planetary Camera-I images of the core of the dense globular cluster M3 (NGC 5272). Stellar photometry in the F555W (V) and F785LP (I) bands, with a 1-sigma photometric accuracy of about 0.1 mag, has been used to construct color-magnitude diagrams of about 4700 stars above the main-sequence turnoff within r less than or approximately equal to 1 min of the cluster center. We have also analyzed archival HST F336W (U) images of M3 obtained by the Wide Field/Planetary Camera-I Instrument Definition Team. The UVI data are used to identify 28 blue straggler (BS) stars within the central 0.29 sq. arcmin. The specific frequency of BSs in this region of M3, N(sub BS)/N(sub V less than (V(HB)+2)) = 0.094 +/- 0.019, is about a factor of 2 - 3 higher than that found by Bolte et al. in a recent ground-based study of the same region, but comparable to that seen in the sparse outer parts of the same cluster and in HST observations of the core of the higher density cluster 47 Tuc. The BSs in M3 are slightly more centrally concentrated than red giant branch stars while horizontal branch stars are somewhat less concentrated red giants. The radial distribution of V-selected subgiant and turnoff stars is well fit by a King model with a core radius r(sub core) = 28 arcmin +/- 2 arcmin (90% confidence limits), which corresponds to 1.4 pc. Red giant and horizontal branch stars selected in the ultraviolet data (U less than 18) have a somewhat more compact distribution (r(sub core) = 22.5 arcmin). The HST U data consist of 17 exposures acquired over a span of three days. We have used these data to isolate 40 variable stars for which relative astrometry, brightnesses, colors, and light curves are presented. A Kolmogorov-Smirnov test indicates that, typically, the variability for each star is significant at the 95% level. We identify two variable BS candidates (probably of the SX Phe type), out of a sample of approximately 25 BSs in which variability could have been detected. Most of the variables are RR Lyrae stars on the horizontal branch. All of them have periods P greater than or approximately equal 8 h.

Guhathakurta, Puragra↗
NASA NTRS2002

A New Diagnostic Technique for the Solar Corona

Over the last 30-40 years spectroscopic observation of the EUV (extreme ultraviolet) line emission has proved invaluable as a diagnostic of the solar coronal plasma state. Line ratios have been used to determine electron density, electron temperature and ion flow velocity. In this paper, we present results obtained with a new measurement technique that uses spectroscopic observations of the white light corona to obtain the electron density, temperature, and flow velocity. A prototype instrument has been designed and built to obtain visible light spectra (3800-4300 A) with modest resolution. This instrument was used to obtain coronal observations during the June 2001 eclipse in Zambia. The data were corrected for sky and instrument transmission to derive the electron temperature and flow speed. Results from these measurements will be discussed.

Nelson, R.↗
NASA NTRSDate not supplied

The NASA TEMPO Mission: Hourly Daytime Air Pollution Observations for Enhanced Health and Air Quality Applications

The Tropospheric Emissions: Monitoring of Pollution (TEMPO) mission was launched into Geostationary Earth Orbit (GEO) to 91°W longitude on April 7, 2023, and had a successful 3-day First Light period starting July 31 with TEMPO making its first hourly daytime scans across the TEMPO Field of Regard (FoR) covering greater North America on August 2. TEMPO will provide hourly daytime observations of aerosols and trace gases, including criteria pollutants of nitrogen dioxide (NO2), sulfur dioxide (SO2), and ozone (O3), at high spatial resolution (e.g., 2.0 x 4.75 km2) across the FoR. The hyperspectral ultraviolet-to-visible measurements from the TEMPO imaging grating spectrometer will enable an O3 profile product capable of monitoring the daytime diurnal evolution of O3 pollution in the planetary boundary layer and assessing related health impacts. The non-standard or special scan operations of TEMPO at sub-hourly frequency (e.g., 2-10 minutes) over selected slices of the FoR will further enhance air quality monitoring capabilities during disasters (e.g., wildfires, volcanic eruptions, dust storms, industrial accidents) and other episodic events for better understanding emission sources and the evolution of harmful air pollutants. A large diversity of stakeholders and end-users have been engaged in the TEMPO Early Adopters Program, supported by the NASA Applied Sciences Program, which aims to enhance health and air quality applications and maximize the societal benefit of TEMPO data. Key outcomes from the Early Adopters Program include enhanced knowledge and preparation for using TEMPO data, tailoring the data dissemination and visualization tools for TEMPO based on user needs, and broadening the health and air quality applications facet of the mission. This presentation will provide the latest TEMPO mission and Early Adopters Program updates, insight into the health and air quality applications enabled through TEMPO data, and showcase the First Light NO2 imagery and provisional data from the mission.

Remote Sensing↗
NASA NTRS1972

Mg II 2800 A emission in late type stars

The largest body of data on ultraviolet spectra of late-type stars now available is the series of scans made with the long wavelength spectrometer onboard OAO-2. Some features of selected scans from this series and estimates of Mg II emission fluxes were reported earlier. Since that time, the effects of sky background, scattered light and variable instrumental sensitivity have become better understood. Additional stars are used to define more clearly the transition from Mg II 2800 A absorption to emission with advancing spectral type, and additional scans of alpha Sco provide a better estimate of Mg II emission strength for this supergiant in OAO observations.

Doherty, L. R.↗
NASA NTRS1992

Voyager 2 ultraviolet spectrometer solar occultations at Neptune - Constraints on the abundance of methane in the stratosphere

The study compares Voyager 2 ultraviolet spectrometer (UVS) solar occultation lightcurves at wavelengths 125-138 nm acquired during the Neptune encounter with 1D methane photochemical transport models. For the p-T models under consideration, acceptable fits to the UVS lightcurves are obtained with eddy mixing coefficient values (K sub 1/2) near the half-light altitudes of 2-15 x 10 exp 6 sq cm/s (ingress) and 4-35 x 10 exp 6 sq cm/s (egress) and lower stratospheric methane mixing ratio values of 5-100 x 10 exp -5. For the nominal p-T models and a criterion based on replicating the spacing in altitude of the 125-138 nm UVS lightcurves, K sub 1/2 values of 10 exp 7 sq cm/s (ingress) and 1-2 x 10 exp 7 sq cm/s (egress) and methane mixing ratios of about 0.0004 (ingress and egress) are indicated.

Bishop, James↗
NASA NTRS1978

Unusual satellite data: A black hole?

Data obtained by the NASA-launched European Space Agency's International Ultraviolet Explorer satellite suggests the possibility of a massive black hole at the center of some globular clusters (star groups) in our galaxy. Six of these clusters, three of them X-ray sources, were closely examined. Onboard short wavelength UV instrumentation penetrated the background denseness of the clusters 15,000 light years away where radiation, probably from a group of 10 to 20 bright blue stars orbiting the core, was observed. The stars may well be orbiting a massive black hole the size of 1,000 solar systems. The existence of the black hole is uncertain. The dynamics of the stars must be studied first to determine how they rotate in relation to the center of the million-star cluster. This may better indicate what provides the necessary gravitational pull that holds them in orbit.

Source record↗
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 NTRS1988

Visible and near-ultraviolet spectroscopy at Thule AFB (76.5 N) from January 28 - February 15, 1988

Near-ultraviolet and visible spectrographs identical to those employed at McMurdo Station, Antarctica (77.8 S) during the austral spring seasons of 1986 and 1987 were used to study the stratosphere above Thule, Greenland (76.5 N) during early spring, 1988. Observations were carried out both at night using the direct moon as a light source, and during the day by collecting the scattered light from the zenith sky when solar zenith angles were less than about 94.5 degrees. Excellent meteorological conditions prevailed in the troposphere and stratosphere at Thule. Surface weather was extremely clear over most of the period, facilitating measurements of the direct light from the moon. The lower stratospheric arctic polar vortex was located very near Thule throughout the observing period, and temperature at the 30 mbar level were typically below -80 C above Thule, according to the National Meteorological Center daily analyses. Thus conditions were favorable for polar stratospheric cloud formation above Thule. Total column ozone abundances were about 350 to 400 Dobson units, and did not suggest a clear temporal trend over the observing period. Stratospheric nitrogen dioxide measurements were complicated by the presence of a large component of tropospheric pollution on many occasions. Stratospheric nitrogen dioxide could be identified on most days using the absorption in the scattered light from the zenith sky, which greatly enhances the stratospheric airmass while suppressing the tropospheric contribution. These measurements suggest that the total vertical column abundance of nitrogen dioxide present over Thule in February was extremely low, sometimes as low as 3 x 10 to the 14th per sq cm. The abundance of nitrogen dioxide increased systemically from about 3 x 10 to the 14th in late January to 1.0 x 10 to the 15th per sq cm in mid-February, perhaps because of photolysis of N2O5 in the upper part of the stratosphere, near 25 to 35 km.

Mount, G. H.↗
NASA NTRS1995

Space qualification of IR-reflecting coverslides for GaAs solar cells

Improvements to GaAs solar array performance, from the use on solar cell coverslides of several reflecting coatings that reject unusable portions of the solar spectrum, are quantified. Blue-red-rejection (BRR) coverslides provide both infrared reflection (IRR) and ultraviolet rejection (UVR). BRR coverslides were compared to conventional antireflection (AR) and ultraviolet (UV) coated coverslides. A 2% improvement in peak-power output, relative to that from Ar-coated coverslides, is seen for cells utilizing BRR coverslides with the widest bandpass. Coverslide BRR-filter bandpass width and covered-solar-cell short-circuit current is a function of incident light angle and the observed narrower-bandpass filters are more sensitive to change in angle from the normal than are wide-bandpass filters. The first long-term (3000 hours) UV testing of unirradiated and 1 MeV electron-irradiated GaAs solar cells, with multilayer-coated coverslides to reduce solar array operating temperature, has indicated that all multilayer coatings on coverslides and solar cells will experience degradation from the space environment (UV and/or electrons). Five types of coverslide coatings, designed for GaAs solar cells, were tested as part of a NASA-sponsored space-flight qualification for BRR, multi-layer-coated, coverslides. The reponse to the different radiations varied with the coatings. The extent of degradation and its consequences on the solar cell electrical characteristics depend upon the coatings and the radiation. In some cases, an improved optical coupling was observed during long-term UV exposure to the optical stack. The benefits of multi-layered solar cell optics may depend upon both the duration and the radiation environment of a mission.

Meulenberg, Andrew↗
NASA NTRS1977

Photoelectrolysis of water at high current density - Use of ultraviolet laser excitation

The behavior of TiO2 and SrTiO3 photoanodes in cells for the photoelectrolysis of H2O has been investigated for high-intensity 351-,364-nm excitation from an Ar ion laser. Intensities up to 380 W/sq cm have been used. For TiO2 a small amount of surface decomposition is found after irradiation at high intensity, whereas SrTiO3 undergoes no detectable changes. Current-voltage properties for both electrodes are essentially independent of light intensity up to the level of 380 W/sq cm, and there is little if any change in quantum efficiency for electron flow. Photocurrent densities have been shown to exceed 5 A/sq cm for O2 evolution. Data show that the energy storage rate associated with the SrTiO3 photoelectrolysis can exceed 30 W/sq cm; this represents the highest demonstrated rate of sustained optical-to-chemical energy conversion.

Bocarsly, A. B.↗
NASA NTRS1993

Ultraviolet stability and contamination analysis of Spectralon diffuse reflectance material

A detailed chemical analysis was carried out on Spectralon, a highly Lambertian, diffuse reflectance material. Results of this investigation unambiguously identified the presence of an organic (hydrocarbon) impurity intrinsic to the commercial material. This impurity could be removed by a vacuum bake-out procedure and was identified as the cause of optical changes (degradation) that occur in the material when exposed to UV light. It was found that when this impurity was removed, the Spectralon material was photochemically stable and maintained its reflectance properties even after extensive solar UV exposure.

Stiegman, Albert E.↗
NASA NTRS2002

Calibration of the Shuttle Ozone Limb Sounding Experiment (SOLSE) and the Limb Ozone Retrieval Experiment (LORE)

The calibration and characterization of two new instruments designed to retrieve ozone profiles into the lower stratosphere will be presented. These instruments will fly as a single payload on the Space Shuttle Columbia currently scheduled to lift off July 11, 2002. The purpose of SOLSE (Shuttle Ozone Limb Sounding Experiment) and LORE (Limb Ozone Retrieval Experiment) is to provide a thorough test of the limb ozone retrieval technique, which is being employed on several satellite instruments currently deployed or planned for deployment in the near future. OSIRIS (Optical Spectrograph and Infrared Imager System) and SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) are already in orbit, while OMPS (the Ozone Mapping and Profiler Suite) is planned as the primary US ozone monitoring instrument in the next decade.. SOLSE is a Czerny-Turner spectrograph utilizing a 1k x 1k cooled CCD at the focal plane and covering the spectral range of 310-380 nm in the ultraviolet and 535-865 nm in the visible to near infrared. LORE is a 5 channel filter radiometer with center band wavelengths of 322, 350, 603, 675, and 1000 nm. The focus of this paper will be on measurements of the SOLSE spectrograph performance in the limb-viewing configuration including stray light rejection, spatial and spectral resolution and absolute radiometric response.

Janz, S. J.↗
NASA NTRS2012

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope, This dataset provides unique spectral time series down to 2000 A. Significant diversity is seen in the near-maximum-light spectra (approx.2000-3500 A) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in the uvw lIF250W filter are found to correlate with the B-band light-curve shape parameter .(Delta)m15(B), but with much larger scatter relative to the correlation in the broad-band B band (e.g., approx. 0.4 mag versus approx. 0.2 mag for those with 0.8 <.(Delta)m15(B) < 1.7 mag). SN 2004dt is found as an outlier of this correlation (at> 3(sigma), being brighter than normal SNe Ia such as SN 2005cf by approx. 0,9 mag and approx. 2.0 mag in the uvwl1F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects

Wang, Xiaofeng↗
NASA NTRS2017

Accounting for the Effects of Surface BRDF on Satellite Cloud and Trace-Gas Retrievals: A New Approach Based on Geometry-Dependent Lambertian-Equivalent Reflectivity Applied to OMI Algorithms

Most satellite nadir ultraviolet and visible cloud, aerosol, and trace-gas algorithms make use of climatological surface reflectivity databases. For example, cloud and NO2 retrievals for the Ozone Monitoring Instrument (OMI) use monthly gridded surface reflectivity climatologies that do not depend upon the observation geometry. In reality, reflection of incoming direct and diffuse solar light from land or ocean surfaces is sensitive to the sun-sensor geometry. This dependence is described by the bidirectional reflectance distribution function (BRDF). To account for the BRDF, we propose to use a new concept of geometry-dependent Lambertian equivalent reflectivity (LER). Implementation within the existing OMI cloud and NO2 retrieval infrastructure requires changes only to the input surface reflectivity database. The geometry-dependent LER is calculated using a vector radiative transfer model with high spatial resolution BRDF information from the Moderate Resolution Imaging Spectroradiometer (MODIS) over land and the Cox-Munk slope distribution over ocean with a contribution from water-leaving radiance. We compare the geometry-dependent and climatological LERs for two wavelengths, 354 and 466 nm, that are used in OMI cloud algorithms to derive cloud fractions. A detailed comparison of the cloud fractions and pressures derived with climatological and geometry-dependent LERs is carried out. Geometry-dependent LER and corresponding retrieved cloud products are then used as inputs to our OMI NO2 algorithm. We find that replacing the climatological OMI-based LERs with geometry-dependent LERs can increase NO2 vertical columns by up to 50% in highly polluted areas; the differences include both BRDF effects and biases between the MODIS and OMI-based surface reflectance data sets. Only minor changes to NO2 columns (within 5 %) are found over unpolluted and overcast areas.

Lambert-equivalent reflectivity↗