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Results for “ULTRAVIOLET SPECTRA”

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At least 487 records · Page 27
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NASA NTRS1977

The extreme ultraviolet /750-1230 A/ spectrum of an aurora

Extreme ultraviolet auroral spectra have been obtained in the wavelength range of 750-1230 A resolution with a rocket-borne grating spectrometer. Atomic emissions of N I, N II, O I and O II are the dominant features along with geocoronal H I Lyman alpha. N2 emission bands, which appear strongly in laboratory electron impact spectra, appear only as a modest background continuum between 900 to 1100 A. The observed intensities are consistent with electron impact excitation of O and N2. For optically thick emissions, such as N I at 1200 A, radiation transport along the arc results in an intensity enhancement when viewed through the arc towards the zenith.

Park, H.↗
NASA NTRS1979

Catalog of far-ultraviolet objective-prism spectrophotometry: Skylab experiment S-019, ultraviolet steller astronomy

Ultraviolet stellar spectra in the wavelength region from 1300 to 5000 A (130 to 500) were photographed during the three manned Skylab missions using a 15 cm aperture objective-prism telescope. The prismatic dispersion varied from 58 A mm/1 at 1400 A to 1600 A mm/1 at 3000 A. Approximately 1000 spectra representing 500 stars were measured and reduced to observed fluxes. About 100 stars show absorption lines of Si IV, C IV, or C II. Numerous line features are also recorded in supergiant stars, shell stars, A and F stars, and Wolf-Rayet stars. Most of the stars in the catalog are of spectral class B, with a number of O and A type stars and a sampling of WC, WN, F and C type stars. Spectrophotometric results are tabulated for these 500 stars.

Henize, K. G.↗
NASA NTRS1979

Vacuum ultraviolet spectrophotometry and effective temperatures of hot stars

Absolutely calibrated ultraviolet stellar spectra from 3100 A to the hydrogen absorption edge at 912 A were obtained on 1977 February 17 by rocket observations above Woomera, Australia. Spectra taken at 15 A resolution have been compared with the observed fluxes from OAO 2 and with recent model-atmosphere fluxes of Kurucz for five hot stars: Gamma (2) Vel, Zeta Pup, Alpha Eri, Beta Cen, and Alpha Vir. The present data give fluxes which are generally lower than those obtained from OAO 2, with the largest deviations of about 20% between 1400 and 1700 A. Agreement with the models is good, although the model fluxes are substantially larger than the observed values below 1200 A. This discrepancy is greater for the higher-temperature stars. Effective temperatures are also determined and are in good agreement with previous results.

Brune, W. H.↗
NASA NTRS1980

The ultraviolet dayglow. I - Far UV emissions of N and N2

Far ultraviolet rocket spectra of N I and N2 dayglow emissions have been analyzed by using AE-E photoelectron spectra, laboratory-measured excitation cross sections, and photochemical models of atomic nitrogen. A self-consistent picture of both optically thick and thin emission features is found by using a model in which the principal production mechanism for N I 1200-A and 1493-A photons is photodissociative excitation of N2. The aeronomic data require that 50-70% of the excited 4 P atoms produced dissociatively have velocities within the Doppler core of the ambient nitrogen atoms, contrary to the expectation that those atoms are produced with large excess kinetic energy. The derived atomic nitrogen density has a maximum density of 2.7 x 10 to the 7th per cu cm at 170 km, a value that is within 40% of that from recent models of odd nitrogen photochemistry.

Meier, R. R.↗
NASA NTRS1981

IUE observations of supernova remnants

The ultraviolet emission spectra of several filaments in the Cygnus Loop and the Vela Supernova Remnant, including several which are anomalously bright in the optical lines, are discussed. The effects of internal and interstellar resonance line scattering are estimated. Shock velocities, elemental abundances, and in some cases, filament ages are given.

Raymond, J.↗
NASA NTRS1981

Ultraviolet absorption by interstellar gas at large distances from the galactic plane

Eighteen high dispersion International Ultraviolet Exploration spectra of 6 stars in the large magellanic cloud (LMC) 3 stars in the small magellanic cloud (SMC) and 2 foreground stars were studied. Fourteen spectra cover the wavelengths lambda 1150-2000 A and 4 cover lambda 1900-3200 A. All the Magellanic Cloud star spectra exhibit exceedingly strong interstellar absorption lines due to a wide range of ionization stages at galactic velocities and at velocities associated with the LMC or SMC. The analysis is restricted to the Milky Way absorption features. Toward the LMC stars, the strong interstellar lines have a positive velocity extension, which exceeds the extension recorded toward the SMC stars. The most straightforward interpretation of these velocity extensions is obtained by assuming that gas at large distances away from the plane of the galaxy participates in the rotation of the galaxy as found in the galactic disk.

Savage, B. D.↗
NASA NTRS1981

Ultraviolet emission of the Vela supernova remnant and the Cygnus Loop

Ultraviolet emission spectra of several filaments in the Vela supernova remnant and the Cygnus Loop are compared with theoretical models of radiative shocks. Shock velocities range from 85 to 130 km/s. Little or no grain depletion is apparent. Several of the observed filaments deviate from the steady-flow models in the sense that much of the photoionization-recombination zone is absent. It is shown that resonance line scattering within the emitting region can account for the weakness of the C II 1335 and C IV 1550 lines. Evaporation fronts, which are basic to the McKee and Ostriker picture of supernova remnant evolution, do not contribute significantly to the observed emission. Some discrepancies in UV emission predicted by different sets of shock models are traced to differing atomic rates used in the calculations. A fluorescent emission line of H2 may be observable in some supernova remnants.

Raymond, J. C.↗
NASA NTRS1982

Intensity changes in the UV spectrum of Beta Cephei

Fifty eight short wavelength International Ultraviolet Explorer spectra were taken over a 152 day period. Variations in the absorption lines of C IV (1550 A), Si IV (1393 and 1404 A) and N V (1239 and 1243 A) and a Cr III feature (1208 to 1211 A) were observed. A data reduction technique was developed in order to compare the changes in the equivalent widths of these absorption lines. Periods were determined by least squares fit to a sinusoid using several combinations of the data. In one case eighteen spectra for each ion taken at approximately one day intervals were used; in another, all 58 measurements of each were used; and in a third all measurements from all the ions wee combined. Using both variable metric and interactive algorithms it was determined that for all combinations mentioned a period of 6.10 + or - .06 days is the best fit to the vector of equivalent widths versus time. No other period, with or without the 6.10 day period removed from the data, was found. Analysis of the Cr III feature near 1210 A gives the same period as the others, but in antiphase.

Fahey, R. P.↗
NASA NTRS1982

OH fluorescence in comets - Fluorescence efficiency of the ultraviolet bands

The present investigation is concerned with the calculation of the fluorescence efficiency of OH over a wide range of heliocentric radial velocities. Three vibrational states in each of the two lowest electronic states are included in these calculations, producing a total of nine vibrational bands in the ultraviolet. The attempt is made to reproduce observed, ultraviolet, cometary spectra of OH by utilizing the Swings fluorescence mechanism. The inclusion of four rotational states within each of the three vibrational states and a detailed solar atlas make it possible to match observations very well, although there remain some discrepancies. This ability to reproduce observations provides confidence that the more general results obtained for the fluorescence efficiency as a function of heliocentric radial velocity can be used in interpreting both previous and future observations of OH in comets.

Schleicher, D. G.↗
NASA NTRS1982

The detection of ultraviolet photospheric absorption in the spectra of two Wolf-Rayet stars

Narrow, unshifted UV absorption lines, principally of Fe v, have been detected in the spectra of two Wolf-Rayet stars, HD 93162 (WN7) and HD 193793 (WC7). Both stars are known to show upper Balmer-line absorption but exhibit no periodic Doppler shifts that might indicate the presence of a companion. If intrinsic to the Wolf-Rayet stars, these lines presumably form near the base of the envelope in the photosphere. The potential significance of absorption-line studies of Wolf-Rayet stars is discussed.

Fitzpatrick, E. L.↗
NASA NTRS1983

Chromospheric and coronal emissions from the giants in the Hyades

The visible Ca II K and International Ultraviolet Explorer spectra from the four K0 III stars (theta-1, gamma, delta, and epsilon Tau) in the Hyades, along with a field giant of similar spectral type, beta Gem, have been measured. Among the Hyades giants, the range of the high-temperature emissions can be a factor of 6 or more for C IV and an order of magnitude in the X-ray luminosity measured by Stern et al. (1981). For these presumably coeval cluster giants with similar macroscopic parameters, such as age, mass, effective temperature, gravity, and projected rotational velocity, the observed range in chromospheric and coronal emissions is not easily explained by the dominant factors thought to control these emissions in cool stars. It is possible that the emissions are time variable, on a time scale longer than six months. This would be the first evidence that giants undergo magnetic activity cycles similar to those of dwarf stars.

Baliunas, S. L.↗
NASA NTRS1983

Outer atmospheres of cool stars. XV - High-dispersion ultraviolet studies of active chromosphere G-K dwarfs

IUE ultraviolet echelle spectra of three late-type active chromosphere dwarf stars, Chi-1 Orionis, Xi Bootis, and Epsilon Eridani, are compared with previously published observations of the quiet chromosphere dwarfs Alpha Centauri A and Alpha Centauri B. The spectra indicate a gross strengthening of chromospheric and transition-region line fluxes from quiet to active dwarf stars without a fundamental change in their line shapes. Qualitatively the same spectroscopic behavior is seen in detail on the solar surface between regions of strong and weak magnetic fields. It is argued that the correspondence between the stellar and solar behavior is not accidental, but instead affirms that enhanced chromospheric and coronal emission on late-type main sequence stars is a result of increased surface coverage of magnetic active regions.

Ayres, T. R.↗
NASA NTRS1983

Images in the rocket ultraviolet - The starburst in the nucleus of M83

Ultraviolet images of the SAB(s)c I-II galaxy M83 (NGC 5236) obtained with a rocketborne telescope in broad bandpasses centered at 1540 A and 2360 A show a bright resolved nuclear source which accounts for approximately 20 percent of the flux of the galaxy in both bandpasses. Low-resolution International Ultraviolet Explorer spectra of this source reveal an energy distribution similar to that of the starburst nucleus of NGC 7714. Strong blueshifted absorption lines can be interpreted as evidence for a nuclear wind powered by supernovae. Observations from UV, X-ray, optical, and far-infrared bandpasses are consistent with a starburst approximately one-sixth as strong as that in M82. A scaling of the M82 models of Rieke et al. (1980) predicts that the nucleus of M83 contains 10 to the 6th - 3 x 10 to the 7th solar masses in young stars and has a supernova rate of approximately 0.01 per yr.

Bohlin, R. C.↗
NASA NTRS1990

Herbig-Haro objects

The IUE (International Ultraviolet Explorer) spectra of Herbig-Haro (HH) objects and their relation to the very detailed optical spectra available for these objects are studied. Useful information about the physics of HH objects in general and especially about their hydrodynamic models which are contained in these IUE observations is outlined. The merged spectra of high excitation, low excitation HH objects (like HH43 and HH47), and peculiar HH objects are discussed. Results of the spatial variation of lines and continua in HH objects are discussed. Their compatibility with predictions for aerodynamic and especially bow shock models is discussed. The problems arising in the interpretation of the continuous energy distribution in the short wavelength range are discussed. They require the presence of other emission mechanisms in addition to the collisionally enhanced two photon continuum. In HH43 the fluorescent H2 line emission comes from a surprisingly small spatial region. Implications of these results are discussed.

Boehm, K. H.↗
NASA NTRS1989

Nitrogen abundances from UV lines of HgMn and standard late-B stars

A recent survey by Roby (1987) discovered that the relatively weak high excitation lines of N 1 near 7468 and 8680 A were undetectable in the majority of HgMn stars, leading to upper limits on the N abundance of roughly ten times below that of the solar N abundance. Standard stars with similar temperatures (10,000 to 13,000 K) did exhibit these same N lines and were found to have roughly solar N abundances. The N abundances were redetermined in two HgMn stars and four standard stars using the strong, low excitation lines of N 1 found in the ultraviolet. The observational data consisted of high quality, high resolution, co-added International Ultraviolet Explorer spectra which was previously collected and reduced. Examination of the spectra plus considerations of signal/noise and severe line blending led to the choice of three promising N 1 lines located at 1742.7, 1745.3 and 1411.9 A. The atomic data for these lines were previously calculated using the best laboratory measurements found in a search of the relevant literature. The chosen N lines turned out to be blended significantly with Fe 2 lines. To obtain the abundances for N synthetic spectra were computed to match the observed spectra. The synthetic spectra were calculated using lie-blanketed model atmospheres, stellar parameters, and abundances for the other elements based upon previous work by S. Adelman. The N abundance was then adjusted to give the best fit of the observed line profiles. In the 2 HgMn stars, the N lines were again found to be undetectable, but the stronger intrinsic strength of the new lines yield more stringent upper limits than those obtained previously. Model atmosphere and abundances were updated in two stars where new results were reported. An additional N 1 line and an additional standard star were added to the program. A line opacity model was developed for a missing feature adjacent to the N 1 line at 1745.2 A, leading to a 25 percent improvement in the determined from this line.

Roby, Scott W.↗
NASA NTRS1994

Sulfur dioxide in the atmosphere of Venus 1 sounding rocket observations

In this paper we present ultraviolet reflectance spectra obtained during two sounding rocket observations of Venus made during September 1988 and March 1991. We describe the sensitivity of the derived reflectance to instrument calibration and show that significant artifacts can appear in that spectrum as a result of using separate instruments to observe both the planetary radiance and the solar irradiance. We show that sulfur dioxide is the primary spectral absorber in the 190 - 230 nm region and that the range of altitudes probed by these wavelengths is very sensitive to incidence and emission angles. In a following paper Na et. al. (1994) show that sulfur monoxide features are also present in these data. Accurate identification and measurement of additional species require observations in which both the planetary radiance and the solar irradiance are measured with the same instrument. The instrument used for these observations is uniquely suited for obtaining large phase angle coverage and for studying transient atmospheric events on Venus because it can observe targets within 18 deg of the sun while earth orbiting instruments are restricted to solar elongation angles greater than or equal to 45 deg.

Mcclintock, William E.↗
NASA NTRS1994

Hubble Space Telescope faint object spectrograph Quasar Absorption System Snapshot Survey (AbSnap). 1: Astrometric optical positions and finding charts of 269 bright QSO

We present finding charts and optical positions accurate to less than 1 arcsec for 269 bright (V less than or = 18.5) Quasi-Stellar Objects (QSOs). These objects were selected as candidates for the Hubble Space Telescope (HST) Quasar Absorption System Snapshot Survey (AbSnap), a program designed to use the Faint Object Spectrograph (FOS) to obtain short exposure ultraviolet (UV) spectra of bright QSOs. Many quasars were included because of their proximity to bright, low redshift galaxies and positions of these QSOs are measured accurately for the first time. Data were obtained using the digitized sky survey produced by the Space Telescope Science Institute's Guide Stars Selection System Astrometric Support Program.

Bowen, David V.↗