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NASA NTRS1984

Nonequilibrium Crystal Growth During Laser Annealing of Silicon

The Systematic studies of dopant (Group III, IV, or V) incorporation into silicon as a result of pulsed-laser annealing are reviewed. At low concentrations, dopant profiles measured after laser annealing can be compared to model calculations for redistribution by liquid-phase diffusion in order to determine the interfacial distribution coefficient K('). At high concentrations there is a limit (C sub s) to dopant incorporation into substitutional lattice sites. The mechanisms which limit dopant incorporation are discussed as well as the dependence of K(') and (max) on crystal orientation. Finally, measurements of interface temperatures, temperature profiles, and thermal gradients during rapid solidification are summarized.

White, C. W.↗

NASA NTRS1983

Sharp shortward-shifted features in the spectra of O subdwarfs

Preliminary inspection of archival high-dispersion ultraviolet spectra obtained by the International Ultraviolet Explorer reveal sharp shortward-shifted features in the spectra of O subdwarfs. In three examples presented here, BD + 75.325 deg, HD 128220 B, and BD + 28.4211 deg, stable, multiple, discrete shortward-shifted components of the N V, C IV, and Si IV resonance doublets are seen. The available data suggest that some of these features are persistent, being relatively stable in velocity and intensity over at least 9 months. It is tentatively suggested that the mechanism, or mechanisms, producing these discrete features operates universally in hot stars, from luminous O and B stars down through the O subdwarfs to include the hot white dwarfs. If true, this has important ramifications for current knowledge of mass loss in hot stars.

Bruhweiler, F. C.↗

NASA NTRS1984

Variable ultraviolet emission in SY Muscae

The results of continued IUE monitoring of the symbiotic variable SY Muscae, following an earlier report of a radical enhancement in UV emission from the star, are reported. Over the course of one year, the prominent emission lines of N V, O V, C IV, and He II appear to be gradually decreasing in absolute intensity. This appears to coincide with a steady decline in electron density in the emission line forming region. The data are consistent with a sudden ejection event in which material expelled from the surface of a hot subdwarf has exposed the underlying UV continuum of the star. A number of strong emission lines that are photoexcited by the intense radiation field of the secondary also exhibit broad pedestal emission that suggests turbulent velocities of about 150-300 km/s in an expanding shell or possibly in an accretion disk.

Michalitsianos, A. G.↗

NASA NTRS1985

Line identifications, line strengths, and continuum flux measurements in the ultraviolet spectrum of Arcturus

The ultraviolet spectrum of Arcturus has been observed at high resolution with the IUE satellite. Line identifications, mean absolute 'continuum' flux measurements, integrated absolute emission-line fluxes, and measurements of selected absorption line strengths are presented for the 2250-2930 A region. In the 1150-2000 A region, identifications are given primarily on the basis of low-resolution spectra. Chromospheric emission lines have been identified with low-excitation species including H I, C I, C II, O I, Mg I, Mg II, Al II, Si I, Si II, S I, and Fe II; there is no evidence for lines of C IV, N V, or other species requiring high temperatures. A search for molecular absorption features in the 2500-2930 A interval has led to several tentative identifications, but only OH could be established as definitely present. Iron lines strongly dominate the identifications in the 2250-2930 A region, Fe II accounting for about 86 percent of the emission features and Fe I for 43 percent of the identified absorption features.

Carpenter, K. G.↗

NASA NTRS1985

On the nature of the UX Ursa Majoris-type nova-like variables - CPD-48 deg 1577

A series of low-dispersion spectra and one high-dispersion spectrum have been obtained of the catacylsmic variable CPD-48 deg 1577. Like other UX UMa stars, this variable exhibits the flat continuum of a luminous, thick, steady state accretion disk dominating its light from the far UV to the infrared. A wind mass loss rate of less than about 10 to the -9th solar mass/yr and an accretion rate of roughly 5 x 10 to the -9th solar mass/yr is roughly consistent with the range which characterizes other UX UMa stars. An SWP echelle spectrum reveals detailed line profile information on the N V, Si IV, and C IV resonance lines. He II 1640 A is remarkably weak and even absent in two of the spectra. The known properties and evolutionary status of CPD-48 deg 1577 and the other UX UMa stars are summarized.

Sion, E. M.↗

NASA NTRS1986

The R Aquarii-jet at UV and radio wavelengths

The jet and central star of the peculiar symbiotic variable R Aquarii was monitored with IUE low resolution SWP 1200 to 2000 A spectra, in order to detect possible temporal variations. The absolute line intensities of N V, C IV and He II, as well as numerous other ionic species in the jet are found to vary on a timescale of 1.5 yr. High spatial resolution radio VLA maps and SiO maser millimeter interferometer observations of the star are quoted.

Michalitsianos, A. G.↗

NASA NTRS1986

Photoionization in the halo of the Galaxy

The ionizing radiation field in the halo is calculated and found to be dominated in the 13.6-45 eV range by light from O-B stars that escapes the disk, by planetary nebulae at 45-54 eV, by quasars and the Galactic soft X-ray background at 54-2000 eV, and by the extragalactic X-ray background at higher energies. Photoionization models are calculated with this radiation field incident on halo clouds of constant density for a variety of densities, for normal and depleted abundances, and with variations of the incident spectrum. For species at least triply ionized, such as Si IV, C IV, N V, and O VI, the line ratios are determined by intervening gas with the greatest volume, which is not necessarily the greatest mass component. Column densities from doubly ionized species like Si III should be greater than from triply ionized species. The role of photoionized gas in cosmic ray-supported halos and Galactic fountains is discussed. Observational tests of photoionization models are suggested.

Bregman, Joel N.↗

NASA NTRS1987

Spectrophotometry of six broad absorption line QSOs

Spectrophotometric observations of six broad absorption-line QSOs (BALQSOs) are presented. The continua and emission lines are compared with those in the spectra of QSOs without BALs. A statistically significant difference is found in the emission-line intensity ratio for (N V 1240-A)/(C IV 1549-A). The median value of (N V)/(C IV) for the BALQSOs is two to three times the median for QSOs without BALs. The absorption features of the BALQSOs are described, and the column densities and limits on the ionization structure of the BAL region are discussed. If the dominant ionization mechanism is photoionization, then it is likely that either the ionizing spectrum is steep or the abundances are considerably different from solar. Collisional ionization may be a significant factor, but it cannot totally dominate the ionization rate.

Junkkarinen, Vesa T.↗

NASA NTRS1987

Studies of calcium emission stars. I - HD 158393 a giant star in a binary system

HD 158393 is a giant star of spectral type late G or early K which has moderately strong Ca II H and K emission cores. UBVRIJHK photometry and IUE spectra reveal a companion of type F0-F2, luminosity class IV or V. Radial velocity data yield an orbit with a period of 31 day. The K and F stars have nearly equal mass, with a minimum value of 1.7 solar masses if i of less than 72 deg is adopted, as required by the absence of eclipses. The strong Ca II emission indicates the enhanced chromospheric activity which is typical of giant stars in a binary system of this period, and the light is variable in a 31-day period, indicative of synchronous rotation as well as on a long time-scale. Analysis of the 31-day light period by fitting a spot model indicates the presence of two spots in 1975 and 1976; the major spot appears to have been present throughout the observing period (1973-1981). The potential confusion caused by the presence of the second spot for part of the observing period emphasizes the importance of fitting a model if the photometric period is to be understood in terms of the synchronous rotation. The two spots showed different periods and were at different latitudes, suggesting differential rotation of the giant star.

Evans, T. Lloyd↗

NASA NTRS1987

Einstein Observatory survey of X-ray emission from solar-type stars - The late F and G dwarf stars

Results of a volume-limited X-ray survey of stars of luminosity classes IV and V in the spectral range F7-G9 observed with the Einstein Observatory are presented. Using survival analysis techniques, the stellar X-ray luminosity function in the 0.15-4.0 keV energy band for both single and multiple sources. It is shown that the difference in X-ray luminosity between these two classes of sources is consistent with the superposition of individual components in multiple-component systems, whose X-ray properties are similar to those of the single-component sources. The X-ray emission of the stars in our sample is well correlated with their chromospheric CA II H-K line emission and with their projected equatorial rotational velocity. Comparison of the X-ray luminosity function constructed for the sample of the dG stars of the local population with the corresponding functions derived elsewhere for the Hyades, the Pleiades, and the Orion Ic open cluster confirms that the level of X-ray emission decreases with stellar age.

Maggio, A.↗

NASA NTRS1988

The complex wind of the central star of the planetary nebula Abell 78

High-dispersion IUE results of the broad P Cygni profiles of C IV, N V, and O V arising from the Abell 78 nucleus are presented. The data indicate a terminal velocity to the wind of about 3670 + or - 30 km, a positive velocity gradient, and a mass loss rate of approximately 4 x 10 to the -9th solar masses/yr. It is found that low-velocity absorption lines are superimposed upon these high-velocity features. Results imply a gradient in the decelerating wind. It is suggested that a density enhancement noted at -250 km/sec may be caused by a rebound shock.

Kaler, James B.↗

NASA NTRS1988

Determination of electron density and temperature in non-LTE plasmas from spectral lines of impurity ions

A novel approach for calculating the populations of the excited Li-like ions C IV, N V, O VI, and Ne VIII is presented. The populations of the 2(2P), 3(2S), 3(2P), and 3(2D) electronic levels in these ions in optically thin plasmas with a broad range of electron density, N(e), and temperature, T(e), are determined from the collisional-radiative model by solving the system of rate equations for the production of excited ions; the equations are linear with respect to the excited ion populations, and the N(e) and T(e) are taken as independent variables. These populations are used to determine the ratios of line intensities for dipole allowed transitions between various energy levels. This approach can be applied to impurities other than the lithiumlike ions and is especially useful for diagnostics of systems where nonintrusive spectroscopic techniques must be used.

Kunc, Joseph A.↗

NASA NTRS1988

Phase-resolved IUE observations of AM Herculis system E1405-451

The first phase-resolved IUE observations of the AM Herculis system E1405-451 is reported. The spectra show bright emission lines from N V, C IV, and He II, whose centroids vary drastically during the 101.5 minute period. The radial velocities measured from the line centroids show a sinusoidal variation with a semiamplitude of 300 km/s. An explanation for the observed emission line velocities is considered using a model for the system having inclination i = 60 deg and magnetic colatitude Delta = 20 deg.

Takalo, L. O.↗

NASA NTRS1988

Galactic interstellar abundance surveys with IUE. III - Silicon, manganese, iron, sulfur, and zinc

This paper continues a survey of intestellar densities, abundances, and cloud structure in the Galaxy using the IUE satellite. A statistical data set of 223 O3-B2.5 stars is constructed, including 53 stars in the Galactic halo. It is found that S II lines in B stars, of luminosity classes IV and V, have possible contamination from stellar S II, particular for stars with v sin i less than 200 km/s. The mean logarithmic depletions are -1.00, -1.19. -0.63, and -0.23 (Si, Mn,Fe,S, Zn). Depletions of Si, Mn, and Fe correlate with the mean hydrogen density n-bar along the line of sight, with a turnover for n-bar greater than 1/cm. Sulfur depletions correlate with n-bar along the line of sight. The slight Zn depletion correlation also appears to be statistically insignificant. No correlation of depletion is found with the physical density derived from H2 rotational states in 21 lines of sight. Depletion variations in the disk are consistent with a Galactic abundance gradient or with enhanced mean depletions in the anticenter region.

Van Steenberg, Michael E.↗

NASA NTRS1988

Observations of interstellar absorption lines towards hot white dwarfs

A method of detecting and determining parameters for narrow absorption lines in IUE high-resolution spectra of hot white dwarfs was developed. It provides a statistical estimate of the variance of the parameters measured. The statistical accuracy of narrow line measurements with IUE spectra is discussed. The method was employed to determine the column densities of low-ionization species (including N I, Si II, and C II) in the line of sight to seven hot white dwarfs. High-ionization species (N V, Si IV, and C IV) are detected towards four of the stars. The temperature of the low-ionization gas towards 4 of the stars is constrained to be less than 50,000 K. The temperature of the high-ionization gas to two of the stars is constrained to be under 100,000 K, which is well below the equilibrium ionization temperature of a collisionally ionized gas. This suggests that this gas is either in a photoionized region (most likely near the star) or is produced in the photosphere of the star.

Jelinsky, Patrick↗

NASA NTRS1988

Elemental abundance analyses with coadded DAO spectrograms. IV - Revision of previous analyses. V - The mercury-manganese stars Phi Herculis, 28 Herculis and HR 7664

Changes in chromium, manganese, and nickel abundances derived from singly ionized lines are incorporated into the elemental abundance of Adelman and Hill (1987) in order to provide more accurate gf values and damping constants for several atomic species. An improved agreement with the values from neutral lines of the same element is found. In the second part, the method is applied to an elemental abundance analysis of three mercury-manganese stars, and correlations are found between the derived abundances and the effective temperature.

Adelman, Saul J.↗

NASA NTRS1989

Narrow ultraviolet emission lines from SN 1987A - Evidence for CNO processing in the progenitor

Results from UV observations of SN 1987A with the IUE satellite are presented, showing that emission lines of He II, C III, N III, N IV, N V, and O III increase in strength with time after May 24, 1987. It is found that increased emission originated in a photoionized low-density circumstellar gas, lost by the progenitor in its red supergiant phase. A nebular analysis reveals a large nitrogen overabundance, indicating that the gas has undergone substantial CNO processing. This suggests that the progenitor of SN 1987A lost much of its hydrogen envelope before the explosion. The findings are consistent with models in which a red supergiant evolves to the blue supergiant stage before exploding.

Fransson, C.↗

NASA NTRS1989

Emission-line properties of optically and radio-selected complete quasar samples

Spectrophotometry of two complete samples of quasars, one obtained from the flat radio spectra of the objects and the other from their optical colors, is presented. It is confirmed that the equivalent widths of the major UV emission lines decrease with increasing continuum luminosity (the Baldwin effect) and that the (Ly-alpha + N V)/C IV and 1909 A/C IV intensity ratios are also luminosity-dependent by virtue of the C IV luminosity dependence. It is shown that the Mg II/C IV intensity ratio and possibly the strengths of the blue Fe II emission features and the FWHM of the 1909 A blend are luminosity-dependent. These results generally support the Mushotzky and Ferland (1984) interpretation of the Baldwin effect in terms of a weak inverse correlation between the continuum luminosity and ionization parameter.

Baldwin, J. A.↗