IUE Final Archive Research Articles

The INES system

This paper deals with the definition of the input fluxes used for the calibration of the IUE Final Archive. The method adopted consists in the determination of the shape of the detector's sensitivity curves using IUE low resolution observations with model fluxes of the DA white dwarf G191-B2B. A scale factor was then determined so that the IUE observations of some bright OAO-2 standards match the original measurements from Meade ([CITE]) in the spectral region 2100-2300 Å. The ultraviolet fluxes of six standard stars used as input for the Final Archive photometric calibration together with the model fluxes of G191-B2B normalized to the OAO-2 scale are given. A comparison with the independent FOS calibration shows that the IUE flux scale for the Ultraviolet is 7.2% lower. We consider this mainly to be caused by the different normalization procedures. It is shown that the present flux calibration applies to spectra processed with the INES low resolution extraction software.

An IUE Atlas of Pre–Main‐Sequence Stars. I. Co‐added Final Archive Spectra from the SWP Camera

We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE) stars observed in the IUE short-wavelength bandpass (1150-1980 A). Each low-resolution (R ~ 6 A) spectrum was visually inspected for source contamination and data quality, and then all good spectra were combined to form a single time-averaged spectrum for each star. Use of IUE Final Archive spectra processed with NEWSIPS reduces fixed pattern noise in individual spectra, allowing significant signal-to-noise ratio gains in our co-added spectra. For the TTS observed by IUE, we measured fluxes and uncertainties for 17 spectral features, including two continuum windows and four fluoresced H2 complexes. Thirteen of the 32 accreting TTS observed by IUE have detectable H2 emission, which until now had been reported only for T Tau. Using an empirical correlation between H2 and C IV line flux, we show that lack of sensitivity can account for practically all nondetections, suggesting that H2 fluorescence may be intrinsically strong in all accreting TTS systems. Comparison of IUE and GHRS spectra of T Tau show extended emission primarily, but not exclusively, in lines of H2. We also fit reddened main-sequence templates to 72 HAEBE stars, determining extinction and checking spectral types. Several of the HAEBE stars could not be fitted well or yielded implausibly low extinctions, suggesting the presence of a minority emission component hotter than the stellar photosphere, perhaps caused by white dwarf companions or heating in accretion shocks. We identified broad wavelength intervals in the far-UV that contain circumstellar absorption features ubiquitous in B5-A4 HAEBE stars, declining in prominence for earlier spectral types, perhaps caused by increasing ionization of metal resonance lines. For 61 HAEBE stars, we measured or set upper limits on a depth index that characterizes the strength of circumstellar absorption and compared this depth index with published IR properties.

A New Look at Carbon Abundances in Planetary Nebulae. IV. Implications for Stellar Nucleosynthesis

This paper is the fourth and final report on a project designed to study carbon abundances in a sample of planetary nebulae representing a broad range in progenitor mass and metallicity. We present newly acquired optical spectrophotometric data for three Galactic planetary nebulae, IC 418, NGC 2392, and NGC 3242, and combine them with UV data from the IUE Final Archive for identical positions in each nebula to determine accurate abundances of He, C, N, O, and Ne at one or more locations in each object. We then collect abundances of these elements for the entire sample and compare them with theoretical predictions of planetary nebula abundances from a grid of intermediate-mass star models. We find some consistency between observations and theory, lending modest support to our current understanding of nucleosynthesis in stars below 8 M☉ in birth mass. Overall, we believe that observed abundances agree with theoretical predictions to well within an order of magnitude but probably not better than to within a factor of 2 or 3. However, even this level of consistency between observation and theory enhances the validity of published intermediate-mass stellar yields of carbon and nitrogen in the study of the abundance evolution of these elements.

A UV Spectrophotometric Technique for Measuring Temperatures of CS Structures over B Stars

Do optical/UV variations of spectra of Be stars arise from pulsations or structures hanging over them? There is a need for a temperature-sensitive discriminator for circumstellar structures above Be stars to determine whether their UV flux variations are due to occulting structures or photospheric pulsations.To address this question, we acquired far-UV (SWP camera) echellograms covering λλ1150–2100 from the IUE final archive. For selected B stars we have selected suitably exposed spectra observed within a short timespan, typically 1–3 days. We summed all the valid net fluxes within each of 60 echelle orders and placed them into an order vs. time array. We identify those times corresponding to high and low fluxes over several reference orders and form the quantity 1 - Fmin(λ)/Fmax(λ), for all echelle orders. The result is a low-resolution spectrum sensitive to absorptions at certain wavelengths, either strong lines such as Lyman α and several UV resonance lines or aggregates of metallic lines. All these are potentially sensitive to temperature variations.

A Reanalysis of the Ultraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud

We have reanalyzed the Large Magellanic Cloud's (LMC) ultraviolet (UV) extinction using data from the IUE final archive. Our new analysis takes advantage of the improved signal-to-noise ratio of the IUE NEWSIPS reduction, the exclusion of stars with very low reddening, the careful selection of well-matched comparison stars, and an analysis of the effects of Galactic foreground dust. Differences between the average extinction curves of the 30 Dor region and the rest of the LMC are reduced compared with previous studies. We find that there is a group of stars with very weak 2175 A bumps that lie in or near the region occupied by the supergiant shell, LMC 2, on the southeast side of 30 Dor. The average extinction curves inside and outside LMC 2 show a very significant difference in 2175 A bump strength, but their far-UV extinctions are similar. While it is unclear whether or not the extinction outside the LMC 2 region can be fitted with the relation of Cardelli, Clayton, & Mathis (CCM), sight lines near LMC 2 cannot be fitted with CCM because of their weak 2175 A bumps. While the extinction properties seen in the LMC lie within the range of properties seen in the Galaxy, the correlations of UV extinction properties with environment seen in the Galaxy do not appear to hold in the LMC.

A High‐Dispersion Spectroscopic Survey of the Hot White Dwarfs: The IUE NEWSIPS SWP Echelle Data Set

This paper summarizes the results obtained from a comprehensive analysis of all of the SWP echelle spectra of the white dwarf stars contained in the IUE Final Archive. A total of 209 NEWSIPS spectra of 55 degenerate stars of various types have been systematically reduced and analyzed. These include, in addition to conventional white dwarfs, several examples of the hot planetary nebula central stars such as NGC 246, which represent the initial stages of He-rich degenerate evolution. A representative summary of the stellar, circumstellar, and interstellar features found to be present in these spectra is presented. For 33 of the 55 stars, multiple spectra of sufficient quality exist so that co-added spectra with improved signal-to-noise ratio can be constructed. Much previously unrecognized detail and many new features are evident in these data. In addition, it was found necessary to apply several corrections to the NEWSIPS extracted spectra. These corrections, involving the wavelength scale and flux uncertainty vector, are described.

A New Look at Carbon Abundances in Planetary Nebulae. III. DDDM1, IC 3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, and NGC 7009

This paper is the third in a series reporting on a study of carbon abundances in a carefully chosen sample of planetary nebulae representing a large range in progenitor mass and metallicity. We make use of the IUE Final Archive database containing consistently reduced spectra to measure line strengths of C III] λ1909 along with numerous other UV lines for the planetary nebulae DDDM1, IC 3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, and NGC 7009. We combine the IUE data with line strengths from optical spectra obtained specifically to match the IUE slit positions as closely as possible, in order to determine values for the abundance ratios He/H, O/H, C/O, N/O, and Ne/O. The ratio of C III] λ1909/C II λ4267 is found to be effective for merging UV and optical spectra when He II λ1640/λ4686 is unavailable. Our abundance determination method includes a five-level program whose results are fine-tuned by corrections derived from detailed photoionization models constrained by the same set of emission lines. All objects appear to have subsolar levels of O/H, and all but one show N/O levels above solar. In addition, the seven planetary nebulae span a broad range in C/O values. We infer that many of our objects are matter-bounded, and thus the standard ionization correction factor for N/O may be inappropriate for these PNs. Finally, we estimate C/O using both collisionally excited and recombination lines associated with C+2 and find the well-established result that abundances from recombination lines usually exceed those from collisionally excited lines by several times.

Identification of λ Bootis Stars Using IUE Spectra

An analysis of the stars included in the catalogue of λ Bootis by Paunzen et al. (1997) and with IUE Low Resolution observations is presented here. Using line-ratios of carbon to heavier elements (Al and Ni) allows us to establish unambiguous membership criteria for the λ Bootis group. These criteria have been used to look for new λ Bootis candidates in the IUE Final Archive.