Mid-ultraviolet Spectra Research Articles

The Line Variability of 78 Vir (A2p SrCrEu) in the Mid-ultraviolet

Abstract Line variations are studied in the mid-ultraviolet spectra of 78 Vir collected with IUE between phases of far-UV (FUV) maximum and FUV minimum. The mid-UV spectrum of 78 Vir is populated with low-lying transitions of iron-peak elements. The lines of Cr ii and Ca ii are found to be stronger at phase of FUV minimum, while the lines of Ni ii probably vary in anti phase with those of chromium and calcium. Those of Fe ii and Mn ii do not show convincing variations between the two phases. The analysis of these mid-ultraviolet spectra suggests that iron and manganese are uniformly distributed over the surface of 78 Vir, while chromium, calcium and nickel are not but more ultraviolet lines should be studied to confirm this.

The Abundances of Zinc and Copper in the Atmospheres of 78 Vir (A2p SrCrEu) and θ Leo (A2 IV)

Abstract Zinc and Copper abundances are rarely available for normal and Chemically Peculiar A stars because the strongest transitions of Zn ii and Cu ii fall in the mid-UV. Estimates of the abundances of zinc and copper are derived for 78 Vir (A2p SrCrEu) and θ Leo (A2 IV) using mean mid-ultraviolet spectra constructed by coadding individual spectra collected with the Long Wave Prime and Long wavelength Redundant cameras over the 18 yr of the IUE mission. The strong transition of Cu ii at 2135.98 Å is present in 78 Vir and θ Leo but definitely stronger in 78 Vir, whereas all Zn ii lines are blended. Spectral synthesis of the least blended lines yields estimates of the abundances of zinc and copper of about 4.92 and 4.95 respectively in θ Leo and 5.82 and 5.19 in 78 Vir (on a scale where log(H) = 12). There is no convincing evidence that these lines varied in the spectra analyzed for both stars.

The Abundances of Cobalt and Nickel in the Atmosphere of 78 Vir (A2p SrCrEu)

Abstract Cobalt and nickel abundances are rarely available for normal and Chemically Peculiar A stars because the strongest transitions of Co ii and Ni ii fall in the mid-UV. The abundances of cobalt and nickel are derived for 78 Vir using a mean mid-ultraviolet spectrum constructed by coadding 10 spectra collected with the Long Wave Prime and Long wavelength Redundant cameras over the 18 yr of the IUE mission. The strong transitions of Co ii at 2286.16 Å, 2307.86 Å, 2324.32 Å and 2580.33 Å and that of Ni ii et 2287.09 Å are present and more or less affected by blends. The least blended, λ 2286.16 Å, yields a mean overabundance of cobalt of 5 times the solar abundance, the Ni ii line at 2287.09 Å yields a 3 times solar overabundance. There is no convincing evidence that these lines varied in the spectra analyzed. The rotational period of 78 Vir estimated from its recent TESS lightcurve is 3.723 ± 0.055 days.

The Mean Abundance of Cobalt in the Atmosphere of 21 Com

Abstract Cobalt abundances are rarely available for normal and Chemically Peculiar A stars because the strongest transitions of Co ii and Co i lie in the mid-UV. The abundance of cobalt is derived for 21 Com using a mean mid-ultraviolet spectrum constructed by coadding 28 spectra collected with the Long Wave Prime camera over the 18 yr of the IUE mission. The strong transitions of Co ii at 2286.16, 2307.86, 2324.32 and 2580.33 Å are present and usually blended. The least blended, λ 2286.16 Å, yields a mean overabundance of cobalt close to 10 times the solar abundance. Over the 24 hr 1991 monitoring of 21 Com, this line may have varied suggesting that cobalt may not be uniformly distributed over the surface of 21 Com.

The Mean Overabundance of Manganese over the Surface of 21 Com from its Midultraviolet Spectra Recorded by IUE

Abstract The coaddition of 21 mid ultraviolet spectra of 21 Com taken over the International Ultraviolet Explorer (IUE) mission reveals three strong lines of Mn ii at 2576.10, 2593.72 and 2605.68 Å. The synthesis of these lines consistently yields a sizable overabundance of manganese about 10 times the solar manganese abundance. Inspection of the 10 individual spectra taken over the 24 hr monitoring of 21 Com with IUE on 1991 April 17 does not reveal convincing variations of these lines between rotational phases 0.18 and 0.65 of the 2 days period.

Modeling Mid‐Ultraviolet Spectra. I. Temperatures of Metal‐poor Stars

Determining the properties of remote globular clusters and elliptical galaxies using evolutionary population synthesis requires a library of reliable model stellar fluxes. Empirical libraries are limited to spectra of stars in the solar neighborhood, with nearly solar abundances and abundance ratios. We report here a rst step towards providing a flux library that includes nonsolar abundances, based on calculations from rst principles that are calibrated empirically. Because the mid-ultraviolet spectrum of an old stellar system is dominated by the contribution from its main-sequence turno stars, we have started by modeling these. We have calculated mid-ultraviolet spectra for the Sun and nine nearby, near-main-sequence stars spanning metallicities from less than 1/100 solar to greater than solar, encompassing a range of light-element abundance enhancements. We rst determined temperatures of eight of the stars by analyzing optical echelle spectra together with the mid-ultraviolet. Both could be matched at the same time only when models with no convective overshoot were adopted, and only when an approximate chromosphere was incorporated near the surface of relatively metal-rich models. Extensive modications to mid-UV line parameters were also required, notably the manual assignment of approximate identications for mid-UV lines missing from laboratory linelists. Without recourse to additional missing opacity, these measures suce to reproduce in detail almost the entire mid-UV spectrum of solar-temperature stars up to one-tenth solar metallicity, and the region from 2900 A to 3100 A throughout the entire metallicity range. Ramications for abundance determinations in individual metal-poor stars and for age-metallicity determinations of old stellar systems are briefly discussed, with emphasis on the predictive power of the calculations. 1 Based on observations obtained with the Hubble Space Telescope of Space Telescope Science Institute, under contract with the National Aeronautics and Space Administration (NASA). 2 Based on observations obtained with the Shane Telescope at Mt. Hamilton, UCO/Lick Observatory.

O+, O, and O2 densities derived from measurements made by the High Resolution Airglow/Aurora Spectrograph (HIRAAS) sounding rocket experiment

We present the results of an analysis of the O II 834 Å and O I 1356 Å altitude profiles measured during a sounding rocket flight on March 19, 1992. The profiles were analyzed using a new set of models that used discrete inverse theory to seek a maximum likelihood fit to the data. Both profiles were fit simultaneously to ensure consistency of the retrieved ionosphere and thermospheric neutral density. During the analysis the thermospheric neutral density and temperature were modeled using the Mass Spectrometer Incoherent Scatter (MSIS‐86) model [Hedin, 1987]. Two parameters were used to scale the absolute MSIS O and O2 densities; the exospheric temperature was altered by varying the 10.7 cm solar flux (an MSIS‐86 input). The ionospheric O+ density was modeled by a three‐parameter Chapman layer. The retrieved MSIS scalars for the O and O2 densities were 0.47 ± 0.09 and 0.58 ± 0.14, respectively. These scalars indicate that the MSIS‐86 model predicted significantly higher O and O2 densities. The inferred exospheric temperature was 1125 K in good agreement with the MSIS‐86 prediction. The derived O density is in good agreement with the O density inferred from midultraviolet spectra observed during the same rocket flight [Bucsela et al., 1998]. The retrieved F region peak density, 1.98 ± 0.63 × 106 cm−3, peak height, 291 ± 22 km, and plasma scale height, 138 ± 24 km, all agreed with coincident digisonde measurements. Thus we have demonstrated that the ionospheric state can be accurately determined by inversion of observed O II 834 Å limb radiance profiles.

Detailed Mid‐ and Far‐Ultraviolet Model Spectra for Accretion Disks in Cataclysmic Binaries

We present a large grid of computed far- and mid-ultraviolet spectra (850-2000 A) of the integrated light from steady-state accretion disks in luminous cataclysmic variables. The spectra are tabulated at 0.25 A intervals with an adopted FWHM resolution of 1.0 A, so they are suitable for use with observed spectra from a variety of modern space-borne observatories. Twenty-six different combinations of white dwarf mass M(sub wd) and mass accretion rate dot-m are considered, and spectra are presented for six different disk inclinations i. The disk models are computed self-consistently in the plane-parallel approximation, assuming LTE and vertical hydrostatic equilibrium, by solving simultaneously the radiative transfer, hydrostatic equilibrium, and energy balance equations. Irradiation from external sources is neglected. Local spectra of disk annuli are computed taking into account line transitions from elements 1-28 (H through Ni). Limb darkening as well as Doppler broadening and blending of lines are taken into account in computing the integrated disk spectra. The radiative properties of the models are discussed, including the dependence of ultraviolet fluxes and colors on M(sub wd), dot-m, and i. The appearance of the disk spectra is illustrated, with regard to changes in the same three parameters. Finally, possible future improvements to the present models and spectra are discussed.

Ultraviolet Spectral Dating of Stars and Galaxies

An echelle spectrogram (R = 30,000) of the 2300-3100 Å region in the ultraviolet spectrum of the F8 V star 9 Comae is presented. The observation is used to calibrate features in the mid-ultraviolet spectra of similar stars according to age and metal content. In particular, the spectral break at 2640 Å is interpreted using the spectral synthesis code SYNSPEC. We use this feature to estimate the time since the last major star formation episode in the early-type galaxy LBDS 53W091 at redshift z=1.55, whose rest-frame mid-ultraviolet spectrum, observed with the Keck Telescope, is dominated by the flux from similar stars that are at or near the main-sequence turnoff in that system (Spinrad et al.). Our result, 1 Gyr if the flux-dominating stellar population has a metallicity twice solar, or 2 Gyr for a more plausible solar metallicity, is significantly lower than the previous estimate and thereby relaxes constraints on cosmological parameters that were implied by the earlier work.

Rotationally Resolved Studies of the Mid-Ultraviolet Spectrum of Triton. II. HST Surface and Atmospheric Results

Rotationally Resolved Studies of the Mid-Ultraviolet Spectrum of Triton. II. HST Surface and Atmospheric Results

Detection of the hot white dwarf in the magnetic nova V1500 Cygni with the Hubble Space Telescope

A comprehensive investigation of the AM Herculis-like nova system V1500 Cyg is presented, featuring time-resolved spectrophotometry with the Faint Object Spectrograph (FOS) of the Hubble Space Telescope (HST), coeval optical circular spectropolarimetry and spectrophotometry, and new broadband polarimetry. A revised ephemeris for the rotating white dwarf refines our knowledge of the timescale for synchronization with the orbital period -- and for becoming a bona fide AM Herculis system -- to 170 +/- 8 yrs. Three features of the orbital-phase-dependent spectral energy distribution are keys for understanding the relative contributions of several radiation sources in this system: (1) the brightness and shape of the ultraviolet continuum, which rises abruptly at short wavelengths and shows no significant phase dependence for lambda less than 2200 A, (2) the greater than or equal to 1 mag variations in the optical continuum and emission-line fluxes, and (3) a spectral slope in the optical which is reddest when brightest. These data lead us to conclude that the hot, postnova white dwarf is the dominant source of ultraviolet flux and of the unusually large bolometric luminosity of the system, compared with typical active AM Herculis binaries. The flat mid-ultraviolet spectrum requires a radiation component in addition to the white dwarf photosphere. We consider three possible origins: (1) cyclotron emission, (2) relatively broad, optically thick accretion columns with lumps of dense gas, and/or (3) orbiting debris in a disklike geometry. Observational tests are suggested to distinguish among these alternatives.

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.

Evidence for mass loss in the mid-ultraviolet spectra of Be stars

The spectral region about the Mg II doublet lines near 2800 A of Be and shell stars has been observed. The star Gamma Cas shows narrow components to each resonance doublet line shifted 215 km/s toward shorter wavelengths. The resonance and subordinate lines of Zeta Tau are shifted toward shorter wavelengths by 75 km/s, and the subordinate lines are greatly strengthened. The star Kappa Dra, which is much later in spectral type, shows a comparatively normal doublet spectrum.