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At least 73 records · Page 4

The optical and ultraviolet spectrum of the planetary nebula NGC 2440

New measurements of the optical and ultraviolet emission-line intensities of the high-excitation planetary nebula NGC 2440 in the wavelength range 1240-8578 A are analyzed with the aid of photoionization models. The observed O III and N II forbidden-line temperatures (13,800 K and 10,000 K, respectively) differ by more than the models predict. Inclusion of charge-transfer reactions at published rates improves agreement between calculated and observed abundances of many ions, but worsens the agreement for several highly ionized ions. Nitrogen shows a larger overabundance N(N)/N(O) approximately equal to 1.0, whereas ultraviolet lines give a carbon abundance close to the solar value. The refractory elements magnesium, calcium, and iron have similar gas-phase depletions of approximately 1.5 dex. The abundances of oxygen, neon, sodium, sulfur, chlorine, potassium, and argon are rougly solar.

Shields, G. A.↗

The ultraviolet spectrum of an oxygen-rich supernova remnant in the Small Magellanic Cloud

Ultraviolet and optical spectra data are presented for the oxygen-rich supernova remnant 1E 0102-7219 in the SMC. These UV data are the first to seriously constrain the UV emission from an oxygen-rich remnant. Emission lines of O I, forbidden O II, semiforbidden O III, semiforbidden O IV, C IV, forbidden Ne IV, and Mg II have been unambiguously detected in the UV. Shock models for material with abundances appropriate to massive stellar ejecta are calculated, and a number of differences between these models and the observations are found. The UV lines are observed to be weaker relative to optical lines than expected from models. Models of photoionization by the remnant's X-ray emission are calculated, and these provide a better qualitative match to the observed spectra. Approximate abundances in the ejecta are derived and the implications these have for the precursor star are discussed.

Blair, William P.↗

An absorption line in the ultraviolet spectrum of 40 Eridani B

Two excellent low-resolution spectra show an absorption line of equivalent width 3 A, near 1391 A, in the typical DA (hydrogen atmosphere) white dwarf 40 Eri B. The line is confirmed by a high-resolution spectrum and is the first seen in any DA star. Ultraviolet fluxes and the profile of Lyman-alpha confirm an effective temperature near 17,000 K. If the line is Si IV, it requires a temperature near 40,000 K. Unattractive possibilities are a hot circumstellar absorbing envelope dependent on accretion from companions, or formation at large optical depth in a transparent atmosphere with high Si/H. A suggestion that H2 should be considered leads to the possible interpretation as the (0, 5) transition of the Lyman band, formed at small optical depth. The band should be stronger in cooler DAs.

Greenstein, J. L.↗

An unusual absorption feature in the far ultraviolet spectrum of early type supergiants

The OAO-2 satellite has been used to obtain far ultraviolet scans of six early-type supergiants. The data reveal the presence of a distinct, broad absorption feature centered near 1720 A. This feature is unique in that it remains essentially constant in strength, breadth and central position over the spectral type range B0 I to A2 I. The feature also appears in the spectrum of the B-type shell star beta Tauri, with a strength comparable to that observed for the supergiants. It appears weakly, or not at all, in the B and A main sequence spectra we have examined. The hypothesis that the feature is due to a fortuitous blend of intrinsically strong lines arising primarily from the ground configurations of abundant metallic ions is discussed in detail.

Underhill, A. B.↗

The ultraviolet spectrum of the peculiar A star HD 51418

High-dispersion observations of the peculiar A star HD 51418 have been carried out in the ultraviolet by the IUE satellite. Atomic resonance lines observed in the regions 1800-2000 and 2500-3000 A include those of Mg II, Ti II, Cr I, Cr II, Mn II, Fe II and Zn II, and possibly Si II, S I, V II, Fe I, Co II, Y II, Ce II, Ce III, Eu II, Eu III and Tb III. The Mg II 2796 and 2803 A resonance lines exhibit definite variations in equivalent width with orbital phase, which can be attributed to changes in the local atmospheric structure due to the patchy distribution of other elements. In addition, the observations confirm the well known trend for a light-heavy asymmetry in iron peak spectra similar to that observed in the rare earths.

Adelman, S. J.↗

Extreme ultraviolet spectrum of a solar active region from SERTS

We present wavelengths and absolute intensities for 243 emission lines from a single active region observed by the Solar Extreme Ultraviolet (EUV) Rocket Telescope and Spectrograph (SERTS) on 1989 May 5. For this catalog, the imaged spectra have been spatially averaged over a field of view 7 sec x 276 sec cutting through the center of AR5464 at S18 W45. Wavelength coverage is 170-450 A with a spectral resolution approaching 10,000. Most of the line positions are determined to 5 mA or better, representing the highest accuracy yet obtained for solar wavelengths throughout this spectral interval. The relative photometric calibration of the instrument is good to +/- 20% over its first-order range, and has been placed onto an absolute scale that should be correct to within a factor less than 2. Where known, identifications, atomic transitions and formation temperatures are also given. The identified lines arise from temperatures that cover the range log T greater than or equal to 4.7 and less than or equal to 6.8, providing information about the Sun's corona and upper transition region. Upper limits to the intensity of any emission line not included here can be estimated from the measured instrumental sensitivity. This averaged EUV spectrum should prove useful as a source of accurate wavelengths and intensities for emission characteristic of the high-temperature plasma associated with a solar active region and small subflare.

Thomas, Roger J.↗

A High Signal-To-Noise Ultraviolet Spectrum of NGC 7469: New Support for Reprocessing of Continuum Radiation

From 1996 June 10 to 1996 July 29, the International AGN Watch monitored the Seyfert 1 galaxy NGC 7469 using the International Ultraviolet Explorer, the Rossi X-Ray Timing Explorer, and a network of ground-based observatories. On 1996 June 18, in the midst of this intensive monitoring period, we obtained a high signal-to-noise snapshot of the UV spectrum from 1150 to 3300 A, using the Faint Object Spectrograph (FOS) on the Hubble Space Telescope. This spectrum allows us to disentangle the UV continuum more accurately from the broad wings of the emission lines, to identify clean continuum windows free of contaminating emission and absorption, and to deblend line complexes such as Ly(alpha) + N V, C IV + He II + O III], Si III] + C III], and Mg II + Fe II. Using the FOS spectrum as a template, we have fitted and extracted line and continuum fluxes from the IUE monitoring data. The cleaner continuum extractions c o n h the discovery of time delays between the different UV continuum bands by Wanders et al. Our new measurements show delays increasing with wavelength for continuum bands centered at 1485, 1740, and 1825 A, relative to 1315 A with delays of 0.09, 0.28, and 0.36 days, respectively. Like many other Seyfert I galaxies, the UV spectrum of NGC 7469 shows intrinsic, blue-shifted absorption in Ly(alpha), N V, and C IV. Soft X-ray absorption is also visible in archival ASCA X-ray spectra. The strength of the UV absorption, however, is not compatible with a single-zone model in which the same material absorbs both the UV and X-ray light. Similar to other Seyfert galaxies, such as NGC 3516, the UV-absorbing gas in NGC 7469 has a lower ionization parameter and column density than the X-ray-absorbing material. While the UV and X-ray absorption does not arise in the same material, the frequent occurrence of both associated UV absorption and X-ray warm absorbers in the same galaxies suggests that the gas supply for each has a common origin.

Kriss, Gerard A.↗

A recent time of minimum for and atmospheric-eclipse in the ultraviolet spectrum of the Wolf-Rayet eclipsing binary V444 Cygni

The 1200-1900 angstrom region and fine error sensor observations in the optical for V444 Cyg were continuously observed. More than half of a primary minimum and almost a complete secondary minimum were observed. It is found that the time of minimum for the secondary eclipse is consistent with that for primary eclipse, and the ultraviolet times of minimum are consistent with the optical ones. The spectrum shows a considerable amount of phase dependence. The general shaps and depths of the light curves for the FES signal and the 1565-1900 angstrom continuum are similar to those for the blue continuum. The FES, however, detected an atmospheric eclipse in line absorption at about the phase the NIV absorption was strongest. It is suggested that there is a source of continuum absorption shortward of 1460 angstrom which exists throughout a large part of the extended atmosphere and which, by implication, must redden considerably the ultraviolet continuua of WN stars. A fairly high degree of ionization for the inner part of the WN star a atmosphere is implied.

Eaton, J. E.↗

A study of the extreme ultraviolet spectrum of O2 by electron impact

The electron impact emission cross sections for O2 at 200 eV have been measured in the laboratory. Included in the study are all emission features in the extreme ultraviolet from 40 to 131 nm at a resolution of 0.5 nm. The features are entirely from the dissociation products (O I, O II, O III). Additionally, the excitation functions from 0 to 400 eV have been measured for characteristic O I multiplets at 98.9 and 102.6 nm and for O II multiplets at 53.9 and 83.3 nm. It is found that O I multiplets are formed near the dissociation limit whereas the O II multiplets have a threshold about 10 eV above the dissociation limit. The total VUV emission cross section of O2 is determined from 40 to 200 nm and the effects of autoionization to the measured emission spectrum are indicated.

Ajello, J. M.↗

The ultraviolet spectrum of the gravitational lens candidate UM 425 = QSO 1120+019: Evidence for broad absorption line (BAL) structure

The UV line profile structure of high-ionization resonance lines found with the International Ultraviolet Explorer (IUE) in the brightest of four multiply imaged sources (images-A) in the candidate gravitational lens UM 425 = QSO 1120+019 indicates broad absorption line (BAL) structure. The deep-broad trough associated with the O IV line extends to velocities approiximately -12,000 km/s, and contains disrete features that suggest multicomponent velocity structure. This structure may include contributions from C IV absorption from the early-type galaxy that is believed to lens UM 425. A strong absorption feature in the blue wing of the Lyman-alpha lambda 1216 emission line may be a Lyman alpha absorption system at a Z(sub Ly alpha) = 1.437 +/- 0.003, or it may be formed by the superposition of the broad N V lambda lambda 1238, 1242 absorption trough on the extended blue emission wing of the QSO Lyman-alpha line. We obtained a redshift of Z(sub QSO) = 1.471 +/- 0.003 from Lyman-alpha lambda 1215, consistent with the redshift found by Meylan and Djorgovski in the optical. The Lyman-alpha line appears unusally weak due to the presence of N V lambda 1240 BAL absorption. A Lyman-limit absorption system at lambda 912 was not observed in the QSO rest frame. The detection of BAL structure in the other weaker ground-state resonance lines of N II (l) and S IV (l) was not found, suggesting these lines are formed in a region that is distinct from the BAL component. Detection of BAL structure in the other fainter images in this system with Hubble Space Telescope (HST) instrumentation, similar to structure observed here in image A, could provide evidence that UM 425 is a gravitational lens.

Michelitsianos, A. G.↗

The ultraviolet spectrum of the O-type subdwarf HD 49798

The O-type subdwarf (O6p) single-lined spectroscopic binary HD 49798, with an orbital period of 1.5477 d, has been observed by the International Ultraviolet Explorer satellite, to aid in analyses of O type photospheres. Four high resolution (0.1A) spectra are found in the far-ultraviolet region, and two high resolution (0.2A) spectra are found in the mid-ultraviolet region. Numerous absorption lines are identified in the spectra of the star, most of which are attributable to Fe V lines at wavelengths of 1270 to 1500 A. Fe IV lines are also identified at wavelengths of 1500 to 1860 A. The strong Fe V features are found to have no corresponding analogs in the Kurucz and Peytremann (1975) list, and the total line blanketing for stars of spectral type O6 to B0 is discovered to be underestimated by at least a factor of four. In addition, mass loss from the subdwarf observed in the N V resonance doublet at wavelengths of 1238 A and 1242 A is discussed, indicating the existence of a hot stellar wind.

Bruhweiler, F. C.↗

Goddard high-resolution spectrograph observations of narrow discrete stellar wind absorption features in the ultraviolet spectrum of the O7.5 III star Xi Persei

We report the observation of transient narrow absorption components (NACs) in the stellar wind of the O giant Xi Per. Two sets of GHRS observations of the Si IV ultraviolet resonance doublet have been obtained. These features are extremely weak, with column densities of approximately 10 exp 12/sq cm and optical depths of order 0.1. The features are narrow, less than 30 km/s, and seem to occur in groups. If the NACs are due to the 1393 A component, they represent previously undetected low-velocity discrete absorption components at V(rad) below -600 km/s. If they are high-velocity features on the 1402 A doublet component, they may represent the decay phase of the discrete absorption components at the terminal velocity. In either case, they are a new aspect of the NAC phenomenon that could not have been detected with previous ultraviolet spectrographs.

Shore, Steven N.↗

The ultraviolet spectrum of the Crab Nebula

New observations of the Crab Nebula by OAO 2 stellar photometers are reported. Processed image data of the Crab give the logarithm of the integrated relative intensity per wavelength interval, corrected for sky background, and the rms error from 11 passbands in the ultraviolet range. The data are converted to logarithms of flux density per frequency interval and plotted on logarithmic scales with corrections for interstellar extinction.

Johnson, H. M.↗

A study of mass loss from the mid-ultraviolet spectrum of Alpha Cygni /A2 Ia/, Beta Orionis /B8 Ia/, and Eta Leonis /A0 Ib/

Results are presented for a study of mass loss from A and late-B supergiants based on high-resolution mid-UV spectra obtained with the echelle spectrograph of the Balloon-borne Ultraviolet Stellar Spectrometer. Spectra of Alpha Cyg, Beta Ori, Eta Leo, and Alpha Lyr are compared in selected wavelength regions; particular attention is given to previous observations of each star, the Mg II and Fe II resonance lines, lines due to other ions, and evidence for mass ejection. The results indicate that mass loss from late-B and A supergiants is variable, that a considerable fraction of envelope material is ejected in 'puffs', and that the puffs may be due to photospheric instabilities. A mass-loss rate of about 1 hundred-millionth of a solar mass per year is derived for Alpha Cyg and shown to be two orders of magnitude smaller than the value determined from the observed IR excess. This discrepancy is attributed to excess ionization in the envelope.

Lamers, H. J. G. L. M.↗

The ultraviolet spectrum of a strongly reddened, high-excitation Herbig-Haro object

Short- and long-wavelength IUE spectra of the rather strongly reddened, high radial velocity Herbig-Haro object HH 32A have been obtained. The object is well known for its large positive radial velocity and for its unusually large velocity dispersion. In consequence of the large ultraviolet extinction, the spectra are faint. The semiforbidden C III 1909 and Mg II 2800 emission lines are, however, clearly detected, and their fluxes can be measured with reasonable accuracy. C IV 1550 and Si III 1892 are very probably present. The continuum can be detected convincingly in the wavelength range 1500-1900 A. In other regions it is just barely detectable. The continuum has an energy distribution which is probably very similar to that of HH 1 and HH 2H. If an average galactic extinction curve is used, the line fluxes for semiforbidden C III 1909 and C IV 1550 indicate a shock velocity of at least 140 km/s which is higher than for HH 1 or HH 2. If a Theta Ori-type extinction curve is used instead, ultraviolet line ratios are obtained which are rather similar to those in HH 1 and HH 2.

Bohm, K. H.↗

The remarkable ultraviolet spectrum of the planetary nebula Abell 30

Abell 30 has been identified as a planetary nebula in which the nucleus has experienced a final helium shell flash after ejection of the original, outer envelope. Around the central star of the nebula, four knots of material are observed. In view of the importance of the chemical composition of the ejected material for studies of advanced stages of stellar evolution, clarification of the physical conditions in the knots is vital. Data obtained of Abell 30 with the aid of the International Ultraviolet Explorer (IUE) satellite are discussed along with aspects of extinction, the He II flux, ionic abundances, the C II lambda 4267 line, questions regarding the existence of a carbon-rich core in the knot, the heating of the nebular gas, the central star, and heating by suprathermal particles from the stellar wind. The UV spectrum of the inner nebulosity of Abell 30 indicates that an extraordinary amount of energy is being deposited in the gas.

Harrington, J. P.↗

The ultraviolet spectrum of the planetary nebula M3-27

The planetary nebula M3-27 was observed in the low-dispersion mode of the IUE satellite in the region 1150 A-3200 A. A fairly strong continuum was seen to extend down to 1150 A, superposed with a relatively small number of emission lines. A high electron density is inferred from forbidden O III but regions of lower density also exist. The ultraviolet characteristics of M3-27 are compared to those of M1-2. A carbon abundance of log C = 7.72 was estimated from the semiforbidden C III nebular line for an adopted Te = 15,000 K; no emission lines of C IV 1549 A, C II 2326 A, or He II 1640 A were detected. Based on the UV observations, M3-27 would be classified as of slightly lower excitation class than M1-2 or IC 4997.

Feibelman, W. A.↗