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At least 361 records · Page 20
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NASA NTRS2000

The Middle Ultraviolet-Visible Spectrum of H2 Excited by Electron Impact

The electron-impact-induced emission spectrum of H2 has been measured in the extended wavelength region 175-530 mm at a spectral resolution of 1.7 mm (FWHM). The laboratory spectra are characterized by underlying H2 (a (sup 3) Sigma(sup +, sub g) to b (sup 3) Sigma(sup +, sub u) continuum emission. together with many strong lines assigned to the radiative decay of the gerade singlet states of H2, and to members of the H Balmer series resulting from dissociative excitation of H2.

James, Geoffrey K.↗
NASA NTRS1998

HST Spatially Resolved Spectra of the Accretion Disc and Gas Stream of the Nova-Like Variable UX Ursae Majoris

Time-resolved eclipse spectroscopy of the nova-like variable UX UMa obtained with the Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) on 1994 August and November is analysed with eclipse mapping techniques to produce spatially resolved spectra of its accretion disk and gas stream as a function of distance from the disk centre. The inner accretion disk is characterized by a blue continuum filled with absorption bands and lines, which cross over to emission with increasing disk radius, similar to that reported at optical wavelengths. The comparison of spatially resolved spectra at different azimuths reveals a significant asymmetry in the disk emission at ultraviolet (UV) wavelengths, with the disk side closest to the secondary star showing pronounced absorption by an 'iron curtain' and a Balmer jump in absorption. These results suggest the existence of an absorbing ring of cold gas whose density and/or vertical scale increase with disk radius. The spectrum of the infalling gas stream is noticeably different from the disc spectrum at the same radius suggesting that gas overflows through the impact point at the disk rim and continues along the stream trajectory, producing distinct emission down to 0.1 R(sub LI). The spectrum of the uneclipsed light shows prominent emission lines of Lyalpha, N v lambda1241, SiIV Lambda 1400, C IV Lambda 1550, HeII Lambda 1640, and MgII Lambda 2800, and a UV continuum rising towards longer wavelengths. The Balmer jump appears clearly in emission indicating that the uneclipsed light has an important contribution from optically thin gas. The lines and optically thin continuum emission are most probably emitted in a vertically extended disk chromosphere + wind. The radial temperature profiles of the continuum maps are well described by a steady-state disc model in the inner and intermediate disk regions (R greater than or equal to 0.3R(sub LI) ). There is evidence of an increase in the mass accretion rate from August to November (from V = 10 (exp -8.3 +/-0.1) to 10(exp -8.1 +/- 0.1 solar mass yr(exp -1)), in accordance with the observed increase in brightness. Since the UX UMA disc seems to be in a high mass accretion, high-viscosity regime in both epochs, this result suggests that the mass transfer rate of UX UMA varies substantially (approximately equal to 50 per cent) on time-scales of a few months. It is suggested that the reason for the discrepancies between the prediction of the standard disk model and observations is not an inadequate treatment of radiative transfer in the disc atmosphere, but rather the presence of addition important sources of light in the system besides the accretion disk (e.g., optically thin contiuum emission from the disk wind and possible absorption by circumstellar cool gas).

Baptista, Raymundo↗
NASA NTRS1980

IUE observations of circumstellar emission from the late type variable R Aquarii /M7 + pec/

IUE observations of R Aquarii (M7 + pec) have been obtained in low dispersion in order to study its circumstellar emission. Strong permitted, semiforbidden, and forbidden emission lines are identified that are superposed on a bright ultraviolet continuum. From the analysis it is deduced that the strong emission-line spectrum that involves semiforbidden C III, C IV, semiforbidden Si III, forbidden O II, and forbidden O III probably arises from a dense compact nebula the size of which is comparable to the binary system of which R Aqr is the primary star. Low-excitation emission lines of Fe II, Mg II, O I, and Si II suggest the presence of a warm chromosphere (T less than about 10,000 K) in the primary M7 late type giant. The secondary is identified as a white dwarf, comparable to or somewhat brighter than the sun, since such a star can produce enough ionizing photons to excite the continuum and emission-line spectrum and yet be sufficiently faint to escape detection by direct observation. The UV continuum observed is attributed to Balmer recombination and not to blackbody emission from the hot companion. The general spectral properties of R Aqr between 1200 A and 3200 A are discussed in the context of the model for the circumstellar nebula, the companion, and the mass-loss rate of the primary star.

Michalitsianos, A. G.↗
NASA NTRS1981

IUE observations of circumstellar emission from the late-type variable R AQR (M6 + pec)

The IUE observations of R Aqr (M7 + pec) obtained in low dispersion are discussed with particular reference to circumstellar emission. Strong permitted, semiforbidden, and forbidden emission lines are seen, superimposed on a bright ultraviolet continuum. It is deduced that the strong emission line spectrum that involves C III, C IV, Si III, (0 II) and (0 III) probably arises from a dense compact nebula the size of which is comparable to the orbital radius of the binary system of which R Aqr is the primary star. The low excitation emission lines of Fe II, Mg II, 0 I, and Si II probably a white dwarf, comparable to or somewhat brighter than the Sun, since such a star can produce enough ionizing photons to excite the continuum and emission line spectrum and yet be sufficiently faint as to escape detection by direct observation. The UV continuum is attributed to Balmer recombination from the dense nebula and not to blackbody emission from the hot companion.

Hobbs, R. W.↗
NASA NTRS1991

First ultraviolet spectropolarimetry of Be stars from the Wisconsin Ultraviolet Photo-Polarimeter Experiment

The first UV spectropolarimetric observations of Be stars are presented. They were obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) aboard the Astro-1 mission. WUPPE data on the Be stars Zeta Tau and Pi Aqr, along with near-simultaneous optical data obtained at the Pine Bluff Observatory (PBO). Combined WUPPE and PBO data give polarization as a function of wavelength across a very broad spectral region, from 1400 to 7600 A. Existing Be star models predicted increasing polarization toward shorter wavelengths in the UV, but this is not supported by the WUPPE observations. Instead, the observations show a constant or slightly declining continuum polarization shortward of the Balmer jump, and broad UV polarization dips around 1700 and 1900 A, which may be a result of Fe-line-attenuation effects on the polarized flux. Supporting evidence for this conclusion comes from the optical data, in which decreases in polarization across Fe II lines in Zeta Tau were discovered.

Bjorkman, K. S.↗
NASA NTRS1984

The 3000 A bump in quasars

New observations of the 3000 A bump in quasars are presented and some simple considerations regarding its physical origin are addressed. Absolute spectrophotometry of 30 quasars with redshifts between 0.53 and 0.70 was obtained with the multichannel spectrometer on the 5 m Hale telescope with a rest wavelength coverage of 2000-5000 A. The continuum observations are well represented by a single power law plus a smooth, broad bump and superposed Fe II multiplets. The strength of the bump correlates fairly well with that of H-beta but poorly with those of the Mg II and Fe II lines. Models consisting of a power law and optically thick hydrogen continua with temperatures of about 12,000 K, and Balmer-edge optical depths equal to three fit the observed energy distributions well. The physical conditions and continuum emission of a dense slab of hydrogen heated by gamma rays are analyzed.

Oke, J. B.↗
NASA NTRS1987

The ionization structure of planetary nebulae. VII. New observations of the Ring Nebula

New optical spectrophotometric observations of emission-line intensities have been made in eight positions in the Ring Nebula corresponding to those observed previously with the Ultraviolet Explorer satellite; the total coverage is therefore 1400 to 7200 A. The intensities are in generally good agreement with those found previously in corresponding positions. The Oand Balmer continuum electron temperatures agree well on the average and, like the Nelectron temperatures, decrease with increasing distance from the central star. As found previously for the Ring Nebula and for other planetaries in this series, the lambda 4267 C 2 line intensity near the central star implies a Cabundance that is higher than that determined from the lambda 1906, 1909 C 3 lines. The discrepancy again decreases with increasing distance from the central star and vanishes from the outermost positions, again suggesting that the excitation mechanism from the lambda 4267 line is not understood. Standard equations used to correct for the existence of elements in other than the optically observable ionization stages give results that are consistent and in appropriate agreement with abundances calculated using UV lines. The rather high abundances of O, N, and C, and, to some extent N, indicate that some mixing of CNO processed material into the nebular shell may have occurred in the Ring Nebula.

Barker, Timothy↗
NASA NTRS1994

Optical observations of NGC 2915: A nearby blue compact dwarf galaxy

This paper presents B and R band Charge Coupled Device (CCD) images and medium resolution spectroscopy of NGC 2915, a relatively isolated BCD (blue compact dwarf) galaxy at a distance of approximately 5 Mpc. NGC 2915 contains two stellar populations: a high surface brightness blue core population and a red diffuse population. The core population contains all of the H II, and numerous embedded objects. It is the locus of current high mass star formation. The brightest embedded objects are likely to be young ionizing clusters, while many of the fainter objects are likely to be individual supergiant stars with masses up to approximately 25 solar mass, or blends of a few such stars. Curious aligned structures on the SE side of the galaxy are seen and their nature discussed. The spectrum of the core is dominated by bright narrow emission lines like that of a high excitation and low metallicity (less than half solar) H II region. The continuum is flat, with Balmer and Ca II features seen in absorption. The velocity of the Ca II features suggest contamination by galactic interstellar absorption. There is a significant velocity gradient in the spectra, probably indicative of rotation. Outside of its core, NGC 2915 resembles a dE (dwarf elliptical) galaxy, in that it has an exponential surface brightness profile, is red ((B-R)(sub 0) = 1.65), and has a low extrapolated central surface brightness (B(0)(sub c) = 22.44). NGC 2915's properties are compared with other BCDs, concentrating on two morphologically similar BCDs that are near enough to resolve into stars: NGC 1705 and NGC 5253. It is noted that the presence of winds in BCDs invalidates closed box chemical evolution models and the remaining constraints on star formation duration are relatively weak. Some BCDs, including NGC 2915, may be able to maintain their present star formation rate for Gyr time scales. This suggests that the overall evolution of these BCDs may be much slower than the approximately 10 Myr burst time scales commonly quoted. However, shortly after the formation of a massive (10(exp 6) solar mass) cluster a BCD will have all the properties of strong starburst galaxy).

Meurer, G. R.↗
NASA NTRS1986

The ionization structure of planetary nebulae. Part 7: New observations in the ring nebula

New optical spectrophotometric observations of emission-line intensities have been made in eight positions in the Ring Nebula corresponding to those observed previously with the Ultraviolet Explorer satellite; the total coverage is therefore 1400 to 7200 A. The intensities are in generally good agreement with those found previously in corresponding positions. The 0(++) and Balmer continuum electron temperatures agree well on the average and, like the N(+) electron temperatures, decrease with increasing distance from the central star. As found previously for the Ring Nebula and for other planetaries in this series, the lambda 4267 C 2 line intensity near the central star implies a C(++) abundance that is higher than that determined from the lambda 1906, 1909 C 3 lines. The discrepancy again decreases with increasing distance from the central star and vanishes from the outermost positions, again suggesting that the excitation mechanism from the lambda 4267 line is not understood. Standard equations used to correct for the existence of elements in other than the optically observable ionization stages give results that are consistent and in appropriate agreement with abundances calculated using UV lines. The rather high abundances of O, N, and C, and, to some extent N, indicate that some mixing of CNO processed material into the nebular shell may have occurred in the Ring Nebula.

Barker, Timothy↗
NASA NTRS1972

Line-blanketed model stellar atmospheres applied to Sirius

The primary goal of this analysis is to determine whether the effects of atomic bound-bound transitions on stellar atmospheric structure can be represented well in models. The investigation is based on an approach which is called the method of artificial absorption edges. The method is described, developed, tested, and applied to the problem of fitting a model stellar atmosphere to Sirius. It is shown that the main features of the entire observed spectrum of Sirius can be reproduced to within the observational uncertainty by a blanketed flux-constant model with T sub eff = 9700 K and Log g = 4.26. The profile of H sub gamma is reproduced completely within the standard deviations of the measurements except near line center, where non-LTE effects are expected to be significant. The equivalent width of H sub gamma, the Paschen slope, the Balmer jump, and the absolute flux at 5550 A all agree with the observed values.

Fowler, J. W.↗
NASA NTRS1990

Doppler imaging of the dwarf nova U Geminorum

Doppler tomography is applied to time-resolved spectra of the dwarf nova U Geminorum, and it is found that emission from the bright-spot region has neither the velocity of the gas stream nor the velocity of the disk along the stream but lies roughly midway between. It is shown that earlier observations of U Gem were consistent with the present results, and it is concluded that the line emission arises immediately after the shock as the gas from the stream mixes with the disk. The data also show evidence for Balmer emission from the secondary star which, together with the bright spot's emission, produces a complicated S-wave in trailed spectra.

Marsh, Thomas R.↗
NASA NTRS1991

First ultraviolet spectropolarimetry of hot supergiants

UV spectropolarimetric observations of the early-type supergiant stars P Cygni and Kappa Cassiopeiae obtained with the Wisconsin UV Photo-Polarimeter Experiment (WUPPE) are presented. Contrary to model predictions, the intrinsic polarization of P Cyg remains constant in the Balmer continuum except for a broad dip between 2600 and 3000 A. This decrease in polarization is discussed in terms of possible Fe line blanketing effects. The intrinsic position angle is essentially constant in the UV except for a rotation across the feature between 2600 and 3000 A. The intrinsic polarization of Kappa Cas is small; most of the observed polarization is interstellar in nature. The Serkowski curve extrapolated into the UV is found to fall consistently below the observed polarization. A slight rotation in position angle from optical to UV wavelengths is noted. Very good agreement is found between the mean continuum polarization obtained by WUPPE and the broadband UV polarization measurement obtained by Gehrels (1968).

Taylor, M.↗
NASA NTRS1992

Laser-induced fluorescence of atomic hydrogen in an arcjet thruster

A diagnostic has been developed to measure gas kinetic temperature and velocity in a 1 kW hydrogen fueled arcjet thruster. The technique is based on laser-induced fluorescence of the Balmer alpha transition (656 nm) in atomic hydrogen. A narrowband CW ring dye laser scans the excitation spectrum, permitting accurate determination of the atomic line shape and position. The velocity is inferred from the Doppler shift of the transition while the temperature is obtained from the shape. Near the exit plane, measured axial velocity varies between 4 and 14 km/s; measured radial velocity varies from 0 to 4 km/s. Temperatures were measured in the range from 1000 K near the edge of the plume to 5000 K at the centerline. Relevant broadening mechanisms are discussed.

Liebeskind, John G.↗
NASA NTRS1994

A spectrum of the veiled T Tauri star CY Tau

We present a flux calibrated spectrum of the star listed as CY Tau in the `General Catalog of Variable Stars 4th ed.' in the spectral range 3700 A less than or equal to lambda less than or equal to 6400 A with a resolution of approximately equals 15 A showing the Balmer-Series from H(sub beta) to H(sub 10) as well as the CaII H (in blend with H(sub epsilon) and K lines in emission. Apart from the emission lines the spectrum is composed of a continuum equivalent to that of an ordinary pre-main sequence star (i.e. a `naked' T Tau) of spectral type M2 V with emission lines plus a `blue' veiling continuum that can be described as black body radiation of temperature T(sub BL) approximately equals 7000K due to accretion onto a boundary layer at a rate of M-dot(sub acc) greater than or approximately = 2.18 10(exp -8) solar mass/a.

Stuewe, J. A.↗
DOE OSTI2022

The Zwicky Transient Facility phase I sample of hydrogen-rich superluminous supernovae without strong narrow emission lines

ABSTRACT We present a sample of 14 hydrogen-rich superluminous supernovae (SLSNe II) from the Zwicky Transient Facility (ZTF) between 2018 and 2020. We include all classified SLSNe with peaks Mg < −20 mag with observed broad but not narrow Balmer emission, corresponding to roughly 20 per cent of all hydrogen-rich SLSNe in ZTF phase I. We examine the light curves and spectra of SLSNe II and attempt to constrain their power source using light-curve models. The brightest events are photometrically and spectroscopically similar to the prototypical SN 2008es, while others are found spectroscopically more reminiscent of non-superluminous SNe II, especially SNe II-L. 56Ni decay as the primary power source is ruled out. Light-curve models generally cannot distinguish between circumstellar interaction (CSI) and a magnetar central engine, but an excess of ultraviolet (UV) emission signifying CSI is seen in most of the SNe with UV data, at a wide range of photometric properties. Simultaneously, the broad H α profiles of the brightest SLSNe II can be explained through electron scattering in a symmetric circumstellar medium (CSM). In other SLSNe II without narrow lines, the CSM may be confined and wholly overrun by the ejecta. CSI, possibly involving mass lost in recent eruptions, is implied to be the dominant power source in most SLSNe II, and the diversity in properties is likely the result of different mass loss histories. Based on their radiated energy, an additional power source may be required for the brightest SLSNe II, however – possibly a central engine combined with CSI.

Kangas, T. (ORCID:0000000254770217)↗
NASA NTRS1995

The spectral energy distribution of the z=3 quasar: HS 1946+7658

For the bright z = 3.02 radio-quiet quasar, HS 1946+7658, we have obtained radio to X-ray data within the past year: 5 GHz and 1415 MHz data from the Very Large Array (VLA); IR photometry at J, H, K, L prime (3.4 micrometers and N; IR spectroscopy; UBVRI photometry; optical spectrophotometry and high-resolution spectra; and an X-ray spectrum from the ROSAT Position Sensitive Proportional Counter (PSPC). The spectral energy distribution (SED) constructed from these data is compared to the mean SED for a set of low-redshift quasars, and while they appear generally similar, there are several differences. In relation to the low-redshift mean, the SED of HS 1946+7658 shows (1) only an upper limit at 10 micrometers (a rest wavelength of 2.5 micrometers, indicating that HS 1946+7658 does not have a strong near-IR excess such as hot dust would produce; (2) relatively weak Fe II and Balmer continuum emission at approximately 3000 A; and (3) a steeper turndown shortward of Ly alpha, even after correction for the Ly alpha forest absorption lines.

Kuhn, O.↗
NASA NTRS1988

A spectrophotometric study of IR Gem at outburst and quiescence

Optical spectra with 2.5-A resolution obtained throughout the orbit of IR Gem during a five-day sequence beginning at an outburst state and ending at quiescence are compared with B magnitude and IUE spectra obtained throughout other quiescent-state orbits. No obvious second component is found in the emission lines. In contrast to previous results, little high-excitation He II 4686-A emission is noted, and no variation of the equivalent widths of the Balmer emission is found over the orbital cycle. The results suggest that large changes in the excitation level and in the modulation from a heated zone can take place during different quiescent states, possibly due to the interoutburst phase.

Feinswog, Laurie↗
NASA NTRS1991

Velocity measurements in a hydrogen arcjet using LIF

A new diagnostic developed to measure the axial velocity of atomic hydrogen in a 1-kW hydrogen-fueled arcjet thruster is reported. The technique is based on laser-induced fluorescence of the Balmer alpha transition (656 nm) in atomic hydrogen. A narrow-band CW ring dye laser scans the excitation spectrum permitting accurate determination of the absorption line shape and position. The velocity is derived from the Doppler-shifted line position. A spatially resolved velocity profile is presented with a peak velocity of 12.8 km/s at the arcjet centerline near the exit plane.

Liebeskind, John G.↗