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At least 541 records · Page 30
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NASA NTRS1987

Sulfur-oxygen processes on Io

Laboratory studies of irradiated sulfur dioxide frost have found that sulfur trioxide should be formed as a consequence of the irradiation process. The spectral reflectance of solid sulfur trioxide was measured in the laboratory and it was found that the compound has strong absorption features at 3.37 and 3.70 microns. These features are not present in the spectral geometric albedo of Io. This is interpreted as an indication that sulfur trioxide may exist in such limited abundance that it is undetectable in disk averaged spectrophotometry. It is suggested that the Near-Infrared Mapping Spectrometer on the Galileo spacecraft should be able to identify condensed sulfur trioxide on Io particularly in regions bordering the sulfur dioxide deposits. The presence of elemental sulfur on Io's surface has been questioned on several grounds, most notably the suggested production process (quenched molten sulfur extrusions) and the effect of radiation (particularly X-rays) on some of the allotropes. Mixtures of sulfur allotropes were produced in the laboratory by quenching molten sulfur and it was found that the spectra indicate the presence of certain red-colored allotropes which are preserved upon quenching. The color of the sulfur glass produced is redder when the temperature of the original melt is higher. This is consistent with the suggestion that Io's spectral geometric albedo can be partly explained by the presence of quenched sulfur glasses.

Nelson, Robert M.↗
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 NTRS1986

Oxygen production rates for P/Halley over much of the 1985-1986 apparition

Long slit CCD spectrophotometry of comet P/Halley in the visible region was used to measure the production rate of atomic oxygen during the 1985/86 apparition. The observations cover a large range of heliocentric distances, since the technique is applicable to apparently bright and faint comets. The cometary gas production rate for P/Halley increases rapidly with decreasing heliocentric distance toward perihelion and is systematicaly larger at a given heliocentric distance for the postperihelion observations. The average production rate for O1D on the day of the Giotto flyby is 4 times 10 to the 28th power atoms/sec giving an extrapolated total water production rate of 6 times 10 to the 29th power mols/sec. A method for comparing the absolute cometary gas production rates for different comets is discussed.

Spinrad, Hyron↗
NASA NTRS1986

The infrared astronomical satellite asteroid database

The creation, maintenance, and distribution of the set of ground-based observational data being used to help calculate the diameters and albedos of asteroids observed by IRAS during the sky survey of February-November 1983 are discussed in a status report. The database comprises orbital elements, absolute magnitudes, UBV colors, thermal radiometry, light-curve parameters, and taxonomic classifications; supplementary files include eight-color photometry, spectrophotometry, polarimetry, JHK observations, proper elements, discovery data, occultation diameters, radar characteristics, and references. Distribution in machine-readable and hard-copy formats and maintenance of the database by incorporating new data as they are published are planned.

Tedesco, E. F.↗
NASA NTRS1986

Spectral energy distributions of active galactic nuclei between 0.1 and 100 microns

The 0.1-100-micron line-free spectral energy distributions of 29 AGN are characterized by fitting simple models to data from IRAS and IUE observations and ground-based NIR and optical spectrophotometry. The data are presented in tables and graphs and discussed in detail. It is found that nuclear dust does not play a major role in most of the Seyfert 1 galaxies and QSOs, which can be described by a model with four spectral components.

Edelson, R. A.↗
NASA NTRS1986

Temperatures for hot and pulsating DB white dwarfs obtained with the IUE Observatory

Ultraviolet energy distributions are presented for 12 hot, helium-atmosphere DB white dwarfs, including the four known pulsating stars which define an empirical DB instability strip. The UV spectrophotometry is reported, and the energy distributions are matched with two separate grids of DB model atmosphere calculations, so that effective temperature estimates may be derived. The sources of uncertainty in the UV temperature fits are assessed, and a comparison with estimates from optical data is made.

Liebert, James↗
NASA NTRS1986

Variability of Markarian 1018 - Seyfert 1.9 to Seyfert 1

Simultaneous IUE and optical spectrophotometry of the Seyfert 1.9 galaxy Markarian 1018 is presented. The strength of H-beta(broad) has increased by a factor of 6.6 in 4 yr, and the galaxy now shows strong broad UV and optical line emission similar to that seen in normal Seyfert 1 galaxies, but with markedly asymmetric line profiles. As in other variable Seyfert galaxies, H-alpha emission has also increased but by a lesser amount than H-beta. Furthermore, Ly-alpha (not observed during the first epoch) is now quite strong relative to H-beta and H-alpha. Inspection of the line profiles reveals nearly identical blue asymmetric line profiles in all permitted lines; these show evidence of significant profile evolution over a 1 yr time scale. The present line ratios and the nature of their variability suggest that the enhancement in the broad-line emission is not caused by the removal of obscuring dust but by the 'lighting up' of the broad-line region.

Cohen, Ross D.↗
NASA NTRS1986

Infrared reflection nebulae in Orion Molecular Cloud 2

New observations of Orion Molecular Cloud 2 have been made from 1 to 100 microns using the NASA Infrared Telescope Facility and the Kuiper Airborne Observatory. An extensive program of polarimetry, photometry, and spectrophotometry has shown that the extended emission regions associated with two of the previously known near-infrared sources, IRS 1 and IRS 4, are infrared reflection nebulae, and that the compact sources IRS 1 and IRS 4 are the main luminosity sources in the cloud. The constraints from the far-infrared observations and an analysis of the scattered light from the IRS 1 nebula show that OMC-2/IRS 1 can be characterized by L of 500 solar luminosities or less and T of roughly 1000 K. The near-infrared albedo of the grains in the IRS 1 nebula is greater than 0.08.

Pendleton, Yvonne↗
NASA NTRS1987

The stellar population in the Wolf-Rayet knot in NGC 5430

Ultraviolet and optical spectra and optical imaging were obtained in order to probe the stellar population in the NGC 5340 WR knot and clarify its relation to star formation elsewhere in the galaxy. The barred spiral NGC 5340 contains an extremely luminous H II region showing very strong He II and N III Wolf-Rayet emission features. From spectrophotometry over the range from 1200-7000 A limits on the reddening and effective temperature have been derived: T eff is between 25,000 and 35,000 K. No UV emission lines are clearly detected. The H-alpha linewidth implies a very short timescale for star formation here.

Keel, W. C.↗
NASA NTRS1987

BE Ursae Majoris - A detached binary with a unique reprocessing spectrum

New infrared photometry, optical and UV spectrophotometry, and a photographic ephemeris are presented for the detached binary BE UMa. Results show the primary to be a DO white dwarf with an effective temperature of 80,000 + or - 15,000 K and a mass of 0.6 + or - 0.1 solar masses. No evidence is found for variability of the primary. The main-sequence secondary star is shown to be of early-M spectral type, with a formal range of M1-M5 being possible. A reflection effect in reprocessed line and continum radiation is produced by EUV radiation from the primary incident on the secondary atmosphere. It is suggested that the temperature of the reprocessed component of the secondary's atmosphere is in the 5000-8500 K range, and that emission lines of decreasing ionization form deeper in the irradiated envelope. Relatively narrow He II and high-excitation metal lines are formed from recombination and continuum fluorescence processes.

Ferguson, Donald H.↗
NASA NTRS1987

Spectroscopic observations of the symbiotic binary RW Hydrae

Ultraviolet/optical spectrophotometry and infrared photometry show that the symbiotic binary RW Hya is comprised of an M giant (with L of about 1000 solar luminosities) and a compact object (with L of about 200 solar luminosities) which resembles the central star of a planetary nebula. The luminosity of the hot component is produced by a nuclear shell source which is replenished by the wind of the red giant at a rate of about 10 to the -8th solar mass/yr. Results indicate that the binary is surrounded by an H II region (of radius of about 10 AU) which gives rise to the observed emission lines and radio emission. The He(2+) and O(2+) regions are found to be confined to the immediate vicinity of the hot component.

Kenyon, Scott J.↗
NASA NTRS1987

Methane absorption variations in the spectrum of Pluto

The lightcurve phases of 0.18, 0.35, 0.49, and 0.98 covered by 5600-10,500 A absolute spectrophotometry of Pluto during four nights include minimum (0.98) light and one near-maximum (0.49) light. The spectra are noted to exhibit significant methane band absorption depth variations at 6200, 7200, 7900, 8400, 8600, 8900, and 10,000 A, with the minimum absorption occurring at minimum light and thereby indicating a 30-percent change in the methane column abundance in the course of three days. An attempt is made to model this absorption strength variation with rotational phase terms of an isotropic surface distribution of methane frost and a clear layer of CH4 gas.

Buie, Marc W.↗
NASA NTRS1987

BE Ursae Majoris: A detached binary with a unique reprocessing spectrum

New infrared photometry, optical and UV spectrophotometry, and a photographic ephemeris are presented for the detached binary BE UMa. Results show the primary to be a DO white dwarf with an effective temperature of 80,000 + or - 15,000 K and a mass of 0.6 + or - 0.1 solar masses. No evidence is found for variability of the primary. The main sequence secondary star is shown to be of early M spectral type, with a formal range of M1 to M5 being possible. A reflection effect in reprocessed line and continuum radiation is produced by EUV radiation from the primary incident on the secondary atmosphere. It is suggested that the temperature of the reprocessed component of the secondary's atmosphere is in the 5000 to 8500 K range, and that emission lines of decreasing ionization form deeper in the irradiated envelope. Relatively narrow He II and high excitation metal lines are formed from recombination and continuum fluorescence processes.

Ferguson, Donald H.↗
NASA NTRS1987

Spectral synthesis in the ultraviolet. I - Far-ultraviolet stellar library

A library of mean stellar energy distributions for use in population synthesis of the ultraviolet spectra of active star-forming galaxies is derived from the ultraviolet spectrophotometry in the IUE Spectral Atlas. The spectra extend from 1230 to 1930 A with a resolution of 6 A. The library contains eight main-sequence groups from O3 to A7 V, four giant groups from O5 to B9 III, and three supergiant groups from O9 to A0 I. Several continuum and spectral line indices are computed, and their usefulness as temperature and luminosity discriminants is discussed. The advantages of analyzing composite stellar populations in the far-ultraviolet, where cool starlight is strongly suppressed, are examined.

Fanelli, Michael N.↗
NASA NTRS1988

The electron temperature and optical emission structure of the central component of CTB 80

Optical spectra on the central component of the galactic supernova remnant CTB 80 are presented. Long slit CCD spectra reveal relative line-intensity variations among individual filaments, the presence of Balmer-dominated emission in the core's east and west optical lobes, and no radial velocities in excess of 300 km/s. Aperture spectrophotometry of the temperature-sensitive forbiddden O III lines suggests Te = 25,000 K, thus favoring a shock-heated interpretation for the remnant's optical core emission. These results indicate that fundamental differences exist betweem the optical emission of CTB 80's central core component and the Crab Nebula, despite similarities in these remnants' radio and X-ray properties. The optical emission from CTB 80's core is likely to be a direct result of the interaction of an active pulsar with its adjacent partially neutral interstellar medium. The size of CTB 80's outer structure and the absence of either high velocity or enhanced abundances in the region local to CTB 80's pulsar argue for a considerably larger age for CTB 80 than the Crab Nebula's 1000 yr. A qualitative picture for CTB 80 that can plausibly explain many of the core's properties is proposed.

Blair, William P.↗
NASA NTRS1988

Nuclear emission-line intensities and profiles in NGC 3783

High-resolution optical spectrophotometry is presented for the nucleus of the Seyfert 1 galaxy NGC 3783. The data suggest that the broadest components of the forbidden lines have blueshifts relative to systemic velocity which are correlated primarily with ionization potential, and FWHMs which are correlated primarily with criticl density. Similar trends also exist in the line components responsible for the larger part of the line emission, but are not as strong as for the broadest line components. Comparison of the profiles of the Balmer lines indicates that the Balmer decrement measured from the H-beta/H-alpha ratio flattens in the line wings. However, the H-gamma and H-delta line profiles are more similar to the H-alpha line profile than to the H-beta line profile, suggesting that the H-beta profile may be anomalous.

Evans, I. N.↗
NASA NTRS1988

The abundances of carbon and nitrogen in I Zw 18

The first measurements of the semiforbidden C III 1909 A and forbidden N II 6583 A emission lines in I Zw 18 are reported. These data are combined with previously published spectrophotometry in order to derive the carbon and nitrogen abundances in I Zw 18 with the aid of a nebular model. The CNO abundances in I Zw 18 are found to exceed those found in the Magellanic Clouds and other well-observed oxygen-poor irregular galaxies and are more similar to those in the sun and nearby Galactic H II regions. It is argued that I Zw 18 may be older and more chemically evolved than previously believed, and that its extremely low O/H results from escape of O-rich supernova ejecta from the Galaxy or from dilution by accretion of primordial gas. Alternatively, I Zw 18 may have formed from a preenriched intergalactic medium. Spectroscopic diagnostics for composite photoionization models to fit the spectrum of I Zw 18 are discussed.

Dufour, Reginald J.↗
NASA NTRS1988

Simultaneous IUE, EXOSAT and optical observations of the unusual AM Her type variable H058+608

Simultaneous observations of the AM Her type variable H0538+608 made with IUE, EXOSAT, and a 1.3 m ground based telescope, and subsequent optical spectrophotometry at high and low resolution are discussed. The X-ray and optical data show clear evidence of a 3.30 + or - 0.03 hr period. Three SWP spectra were taken outside of eclipse and during overlapping phase intervals. The UV spectra contain strong emission lines characteristic of this class of objects and a flat continuum which appears to be deficient, given the brightness of source at optical and X-ray wavelengths. There is evidence for intensity variations in emission lines, particularly C IV. The X-ray light curves for H0538+608 reveal behavior which may be related to irregularities in its accretion flow.

Shrader, C. R.↗