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A numerical procedure for empirically determining a 'continuum' level in low-dispersion spectra from the IUE is described. The technique was developed for the automated fitting of SWP-LO (1150 A - 2000 A) spectra of stars of late spectral type, but it is more widely applicable.
Low-resolution IUE SWP spectra of the peculiar object LMC Anonymous (Sanduleak's Star) indicates strong evidence for CNO-processed nebula in the vicinity of the star. The far-UV spectrum of LMC Anonymous closely resembles that of the S Condensation of Eta Carinae. The similarity between LMC Anonymous and the S Condensation is apparent from the absolute intensity of the N V, semiforbidden N IV, and semiforbidden N III emission lines compared with the reduced strength of C IV or semiforbidden C III emission. IUE spectra of the S Condensation and SN 1987A may provide important clues concerning the nature of LMC Anonymous, which indicates departures from normal cosmic abundances of nitrogen relative to carbon that are extreme. This may suggest that carbon envelope burning and dredge-up occurred simultaneously during the helium-burning stage.
High-resolution IUE SWP and LWP spectra of the bright eclipsing RS CVn system AR Lac, obtained during 40 h of continuous observations on September 18-19, 1985, are compiled and used to derive chromosphere images. The data and results are presented graphically and discussed in detail. The K 0 IV star is found to have a globally brighter trailing hemisphere and at least three distinct plage regions; the G 2 IV star has a flaring region with significant redshift and line-emission broadening and exhibits factor-of-3 variability in total emission, attributed to the presence of a large chromospherically inactive region.
An observational and data reduction technique for fixed pattern noise (FPN) and random noise (RN) in fully extracted IUE high-dispersion spectra is described in detail, along with actual empirical values of signal-to-noise ratio (S/N) achieved. A co-addition procedure, involving SWP and LWR cameras observations of the same spectrum at different positions in the image format, provides a basis to disentangle FPN from RN, allowing each average amplitude, within a given wavelength interval, to be estimated as a function of average flux number. Empirical curves, derived with the noise algorithm, make it possible to estimate the S/N in individual spectra at the wavelengths investigated. The average S/N at the continuum level in well-exposed stellar spectra varies from 10 to 20, for the orders analyzed, depending on position in the spectral format. The co-addition procedure yields an improvement in S/N by factors ranging from 2.3 to 2.9. Direct measurements of S/N in narrow, line-free wavelength intervals of individual and co-added spectra for weak-lined stars yield comparable, or in some cases somewhat higher, S/N values and improvement factors.
Equivalent widths of UV silicon lines obtained from high dispersion IUE SWP spectra for a sample of 37 mid- to early B stars are presented. These are compared to other atmospheric diagnostics and to model atmosphere calculations. It is found that some of the UV silicon lines are extremely sensitive to stellar temperature and luminosity over the range B5-B0.5, or roughly a factor of two in effective temperature. The potential usefulness of these lines is stressed, and it is demonstrated that they could determine temperatures and surface gravities to the same accuracy as other commonly used diagnostics once adequate models become available. Although the available models fail to predict the quantitative behavior of the line strengths, they do, in most instances, describe their qualitative behavior. Since the existing models are somewhat out of date, the results suggest that models based upon more complete opacity tables may be quite reliable.
The nature of the long-term evolution of magnetic activity in the RS CVn binary V711 Tauri is investigated using the complete set of available archival IUE SWP low-dispersion spectra of V711 Tau for the period covering August 1978 - December 1984. An analysis of the spectra confirmed the pattern of a long-term smooth variation of chromospheric and transition region emission found previosly by Dorren et al. (1986). An explanation of the relationship between the different facets of the magnetic activity on V711 Tau is presented.
A high-resolution IUE spectrum of the hot DO1 degenerate HZ 21 was obtained by combining US1 + European 2 low-background observing shifts. The SWP image reveals a rich spectrum of interstellar absorption lines with an average velocity in the line of sight to HZ 21 of -30 km/s. However, there is no clear evidence of any highly or lowly ionized metal features which could be attributed to circumstellar, wind, or photospheric absorption. There is, however, a broad absorption trough at He II (1640) which was not unexpected, given the clear presence of He II (4686) absorption in this star's optical spectrum. The velocity width of He II (1640) appears consistent with photospheric absorption wings which appear to flank the geocoronal Ly-alpha emission feature. The He II (1640) feature reveals what appears to be a broad (310 km/s) emission reversal. Evidence is provided that the emission reversal is probably real.
Sky-background images obtained by the International Ultraviolet Explorer (IUE) were analyzed to study artificial spectral features (camera artifacts) in low-dispersion spectra. The artifacts mimic emission features and have been present in long-exposure spectra since the launch of the IUE satellite. The camera artifacts are strong in spectra characterized by long exposure times because they scale in time-integrated flux with the background level, which increases during the exposure due to camera phosphorescence. The artifacts cannot be detected in spectra obtained from short, direct exposures of flat-field lamps or standard stars. Plots of average sky-background spectra for the three operational IUE cameras (SWP, LWP, and LWR) are given to aid scientists in the identification of artifacts in their spectra.
IUE archive obserbations were used to obtain a new and independent estimate of the effective temperature of Sirius B. The observed Lyman alpha profile of Sirius B was modeled as a function of effective temperature and T(eff) = 24,500 + or - 500 K was found. This temperature is in good agreement with a recent result obtained from analysis of Exosat soft X-ray spectra of Sirius B. In addition to providing confirmation of the soft X-ray result, this temperature implies that Sirius B is significantly cooler and larger in radius than most previous estimates. The radius, R = 0.090 + or - 0.005 solar mass, implied by the lower temperature is well in excess of the radius of fully degenerate star having the mass of Sirius B. This investigation employed a unique set of SWP archive spectra, acquired with Sirius B in the small aperture, which have proved to be free of scattered light contamination from Sirius A.
IUE observations of the peculiar dwarf nova WX Ceti roughly two weeks into a rare outburst, the first since 1963, are obtained. The SWP and LWP observations are very similar to other erupting dwarf novae, both in the line and continuum spectrum. The C IV 1549 A doublet displays blueshifted absorption, indicating the presence of a wind which has a terminal velocity of either 3150 km/s or 5450 km/s; the two possible values are due to an uncertain interpretation of features in the blue wing of the line. The line may also show a weak, redshifted emission, completing a P Cygni profile. The derived mass-loss of 0.7-2.2 x 10 to the -11th solar masses/yr is quite typical of dwarf novae in outburst. Thus, despite the unusual photometric properties of this system, long recurrence time, large amplitude outbursts, and long duration eruptions, the IUE observations reveal that the physical characteristics of the outburst are fairly typical of dwarf novae.
Results are reported from a search for Fe VII absorption lines in data from high-dispersion IUE-SWP observations of PN nuclei (PNN), hot subdwarfs (HSDs), and hot white dwarfs (HWDs). The data-reduction techniques employed are outlined, and the results are presented in extensive tables and sample spectra and characterized in detail. Absorption in at least one of the four Fe VII lines above 120 nm wavelength is found in 22 of 51 PNN, and possibly in 10 HSDs, in the pulsating HWD PG 1159 - 035, and in the PNN K1 - 16. It is concluded that Fe VII is more common in WD progenitors such as PNN than previously predicted and is especially typical of the more luminous low-gravity stars.
We have begun a study of the long term evolution of novae in outburst in order to determine the means by which they return in quiescence when nuclear burning has ended. This project involves both IUE and optical observations and theoretical predictions. Recently, in the initial observational part of this project, we have obtained IUE Short Wavelength Prime (SWP) spectra of GQ Mus 1983 and QU Vul 1984. Each spectrum was a 16 hour exposure using a combined US1 plus Vilspa shift. No novae have been studied in the UV for as long as QU Vul and GQ Mus and observations of their spectral evolution are providing unique data on the turn-off time scale. We have also obtained the spectra of old novae from the IUE archives in order to compare and contrast the existing spectra with those of GQ Mus and Qu Vul. The theoretical prediction is that a nova should be very hot just before turnoff but x ray observations from EXOSAT do not confirm this prediction.
A set of criteria developed for the classification of B stars from ultraviolet spectra alone, using MK standards drawn from the optical region is described. The observational material consists of 100 archival high dispersion spectra obtained by the SWP (Short Wavelength Prime) camera on the IUE (International Ultraviolet Explorer) spacecraft. Only photospheric absorption lines are used as classification criteria. The C IV, SI IV and N V lines, which in early B stars originate in the stellar wind, are not used in the two dimensional spectra type luminosity classification, but the appearance of these lines is compared with the standard stars, and some anomalies noted. On the whole, the ultraviolet spectral types are very consistent with the optical MK types, implying that it is possible to do a two dimensional spectral classification in the ultraviolet without any knowledge of the optical spectrum.
The long term evolution of novae in outburst is studied in order to determine the means by which they return to quiescence when nuclear burning has ended is described. The project involves both International Ultraviolet Explorer (IUE) and optical observations and theoretical predictions. In the initial observational part of this project, IUE SWP (Short Wavelength Prime) spectra of GQ Mus 1983 and QU Vul 1984 are obtained. Each spectrum was a 16 hour exposure using a combined US1 plus Vilspa shift. No novae were sudied in the UV for as long as QU Vul and GQ Mus and observations of their spectral evolution provides unique data on the turnoff time scale. The spectra of old novae from the IUE archives are used to compare the existing spectra with those of GQ Mus and QU Vul. The theoretical prediction is that a nova should be very hot just before turnoff but x ray observations from EXOSAT do not confirm this prediction.
Low and high dispersion short wavelength prime (SWP) spectra of SN 1987A show that the supernova's progenitor was in a post red giant branch stage of evolution at the time of its demise in February 1987. The development of narrow high temperature emission lines of N 5, N 4, N 3, C 3, He 2 and O 3 were followed since May 1987. Observations through April 1990 are presented. The lines are interpreted as arising in a low density gas surrounding the supernova which was photoionized by the extreme UV pulse at shock breakout. In this picture, their temporal evolution is determined primarily by light travel time effects and recombination. Comparison of models of circumstellar environment with the International Ultraviolet Explorer (IUE) observations are discussed.
Five observations of PG 1115 + 080 taken with the IUE SWP camera have been combined in order to carry out the He I Gunn-Peterson test and to search for a Lyman limit which could determine the redshift of the lens candidate reported by Christian et al. (1987). No Lyman-limit discontinuities are found, implying that the lensing galaxy does not intercept the line of sight or does not contain enough neutral hydrogen to be detected as a Lyman-limit edge. It is estimated that the lens column density for neutral hydrogen is 3 x 10 to the 16th/sq cm or less if it intercepts the line of sight.
Results are presented from observations, made with the SWP spectrograph at low resolution, of Tc-deficient S and C members of the group of chemically peculiar red giant (PRG) stars, which were carried out to search for possible companions to these stars. The results support the hypothesis of Little et al. (1987) and Smith and Lambert (1987, 1988) that the chemical peculiarities of T-deficient S and MS PRG stars have arisen due to mass transfer from white dwarf companions.
The SWP camera of the IUE satellite was used in the high-dispersion mode to search for Fe VII absorption and Fe II high-excitation emission lines in five additional very hot central stars of planetary nebulae. Some of the Fe VII lines were detected at 1208, 1239, and 1332 A in all the objects of this program, LT 5, NGC 6058, NGC 7094, A43, and Lo 1 (= K1-26), as well as some of the Fe II emission lines at A 1360, 1776, 1869, 1881, 1884, and 1975 A. Two additional objects, NGC 2867 and He 2-131, were obtained from the IUE archive and were evaluated. The present study probably exhausts the list of candidates that are sufficiently bright and hot to be reached with the high-dispersion mode of the IUE.