International Ultraviolet Explorer Observations Research Articles

Time Variability of FUV Emission from Cool Stars on Multi-year Timescales

The physical and chemical properties of planetary atmospheres are affected by temporal evolution of ultraviolet (UV) radiation inputs from their host stars at all time scales. While studies of X-ray/UV flare properties and long-term stellar evolution of exoplanet host stars have provided new constraints regarding stellar inputs to exoplanetary systems, the UV temporal variability of cool stars on the timescale of stellar cycles remains largely unexplored. To address this gap in our understanding of the UV temporal variability of cool stars, we analyze far-ultraviolet (FUV) emission lines of ions that trace the chromosphere and transition region of nearby stars (C ii, Si iii, Si iv, and N v; formation temperatures ∼ 20–150 kK) using data from the Hubble Space Telescope (HST) and International Ultraviolet Explorer (IUE) archives spanning temporal baselines of months to years. We select 33 unique stars of spectral types F-M with observing campaigns spanning over a year, and create ionic light curves to evaluate the characteristic variability of cool stars on such timescales. Screening for large flare events, we observe that the relative variability of FUV light curves decreases with increasing stellar effective temperature, from 30% to 70% variability for M-type stars to <30% variability for F and G-type stars. We also observe a weak trend in the temporal variability with the Ca ii RHK′ stellar activity indicator, suggesting that stars with lower Ca ii activity exhibit a smaller range of FUV flux variability. Screening for data sets with optimal temporal spread, and a sufficient number of individual observations, we select 5 data sets for further periodicity analysis (HST α Centauri A, HST α Centauri B, IUE α Centauri B, IUE ϵ Eri, IUE ξ Boo). Various periodic structures within the FUV flux were detected, with most significant being a 79 days frequency present within the IUE observations of ξ Boo, with a significance of 6σ, and a periodic signal in the FUV observations of α Centauri B, for both HST and IUE measurements, at ≈210 days frequency with significance of 3σ and 3.7σ, respectively. Our results suggest that extreme ultraviolet flux from cool stars varies by less than a factor of two on decade timescales, significantly smaller than variations on flare or stellar evolutionary timescales.

The Behavior of HgMn Stars in the Far UV -5: υ Her

The analysis of twelve high resolution SWP spectra of the HgMn star HD 144206 (υ Her) obtained through the large aperture recorded by the International Ultraviolet Explorer (IUE) reveals that the far ultraviolet flux of this star remained constant over the various observations with IUE, except for one spectrum. The increase of FUV flux in SWP 13513 and the simultaneous brightening of υ Her in the V band could reflect a flare similar to those reported in other late B-type stars. The analysis of two high resolution ELODIE spectra obtained four years apart does not show any variations of the lines, either in reduced intensity nor in radial velocity. The absence of far ultraviolet variability and the reproducibility of the brightness in the V band during most IUE observations do not support the presence of large chemical gradients over the surface of υ Her.

SPECTRAL BEHAVIOR OF THE SYMBIOTIC NOVA HM SGE IN THE ULTRAVIOLET

Ultraviolet observations of the symbiotic nova HM Sge were obtained from the International Ultraviolet Explorer (IUE) through the interval from 1980 - 1992. Three line profiles demonstrating the variations of some emission lines at different dates are presented. We determined the reddening of HM Sge from the 2200 Å absorption feature; the estimated value is E(B − V ) = 0.34 ± 0.02. We studied CIV at 1550 Å, He II 1640 Å, and CIII] at 1909 Å produced in the wind from the hot star. The line flux variations at different dates could be explained in terms of the variations of temperature in the emitting region as a result of mass loss variations. The IUE observations can be explained by the models of Girard &amp; Willson (1987); Formiggini et al. (1995).

Spectral evolution of the two classical novae PW Vul and V1668 Cyg using International Ultraviolet Explorer low‐resolution spectra

We studied the evolution of the normalized flux of selected ultraviolet (UV) emission lines of two classical novae (PW Vul and V1668 Cyg) using International Ultraviolet Explorer (IUE) short wavelength observations. Different phases of the outburst are studied. Emission lines covering a wide range of ionization states are investigated. We use the calculated fluxes to estimate the mass accretion rates during quiescence of both novae. We found average values of the UV luminosity for PW Vul to be of 1.0 ± 0.1 × 1035 erg s−1, and the corresponding value for V1668 Cyg is 4.0 ± 0.2 × 1035 erg s−1. In quiescence, we obtained average values for the mass accretion rates of 8.7 ± 0.4 × 10−10 M⊙ year−1 and 1.8 ± 0.1 × 10−10 M⊙ year−1, respectively. We attribute the spectral behavior of the two systems to the variation of the optical thickness and temperature of the envelope during the different phases of the outburst. Our results demonstrate the effect of the white dwarf mass on the evolution of the nova. The results of IUE observations are consistent with the theoretical CO models of classical nova of.

Ultraviolet Flux Decrease Under a Grand Minimum from IUE Short-wavelength Observation of Solar Analogs

Abstract We have identified a sample of 33 Sun-like stars observed by the International Ultraviolet Explorer (IUE) with the short-wavelength spectrographs that have ground-based detections of chromospheric Ca ii H+K activity. Our objective is to determine if these observations can provide an estimate of the decrease in ultraviolet (UV) surface flux associated with a transition from a normal stellar cycle to a grand-minimum state. The activity detections, corrected to solar metallicity, span the range , and eight stars have log . The IUE-observed flux spectra are integrated over the wavelength range 1250–1910 Å, transformed to surface fluxes, and then normalized to solar B − V. These normalized surface fluxes show a strong linear relationship with activity (R 2 = 0.857 after three outliers are omitted). From this linear regression we estimate a range in UV flux of 9.3% over solar cycle 22 and a reduction of 6.9% below solar cycle minimum under a grand minimum. The 95% confidence interval in this grand-minimum estimate is 5.5%–8.4%. An alternative estimate is provided by the IUE observations of τ Cet (HD 10700), a star having strong evidence of being in a grand-minimum state, and this star’s normalized surface flux is 23.0 ± 5.7% lower than solar cycle minimum.

ULTRAVIOLET SPECTROSCOPY OF BL Hyi AND EF Eri IN HIGH AND INTERMEDIATE STATES

ABSTRACT We present the first phase-resolved ultraviolet spectroscopy of two polar systems, BL Hyi and EF Eri, in high and intermediate states from the International Ultraviolet Explorer (IUE) during the periods between 1982–1995 and 1979–1991, respectively. The flux curves for the C iv and He ii emission lines for both systems showing variations in their fluxes at different orbital phases are presented. The emission lines are produced in the accretion stream. The reddening for the two polars is found to be 0.00. Our results show that there are variations of the line fluxes with time, similar to the light curves found for both BL Hyi and EF Eri in the optical, infrared, ultraviolet, and X-ray bands. IUE observations support a radiative shock model of BL Hyi with the heating of matter by radiation from the accretion shock and cooling by the electrons scattering off ultraviolet photons from the surface of the white dwarf. EF Eri observations support a two-temperature white dwarf model producing sufficient ultraviolet flux for orbital modulations.

Spectral behavior of Nova GQ Mus in the ultraviolet

We present ultraviolet spectroscopic study of nova GQ Mus observed with the International Ultraviolet Explorer (IUE) during the period 1983–1985. Different spectra showing the changes in line fluxes at different times are presented. The outflow velocity of the ionized ejecta was calculated to be 1010–2900 $\mbox{km}\,\mbox{s}^{-1}$ (FWHM). We estimated a new value of the reddening from the 2200 A absorption feature, $E(B-V) = 0.16 \pm 0.02$ . The models generated by Jose and Hernanz (1998) indicate that GQ Mus is a CO5/CO6 nova with WD mass of ${\sim} 1.15~M_{\odot}$ . We calculated the line fluxes of NIV], C IV and N III] emission lines produced in the expanding shell of the ejecta. The average wind mass loss rate is estimated to be ${\sim} 6 \times 10^{-5}~M_{\odot}~\mbox{yr}^{-1}$ . The average temperature of the central source is estimated to be ${\sim} 2 \times 10^{5}~\mbox{K}$ , and average ultraviolet luminosity of $2 \times 10^{36}~\mbox{erg}\,\mbox{s}^{-1}$ about 1000 days after outburst. Our results reveal that the line fluxes increase after outburst and then decrease with time. We attribute these spectral variations to the large expansion of the ejected matter from the white dwarf after outburst. The estimation of some physical parameters from the IUE observations is consistent with a photoionization model of the novae ejecta, (Diaz et al. 1995) and with an optically thick wind model of novae outburst, (Hachisu et al. 2008).

Ultraviolet spectroscopy of V Sagittae in high, intermediate and low states from HST and IUE satellites

We present the first phase resolved ultraviolet spectroscopic study of V Sge in high, intermediate and low states observed with the Hubble Space Telescope High Resolution Spectrograph (HST HRS) and International Ultraviolet Explorer (IUE) during the period 1978–1996 to diagnose the ultraviolet fluxes of C IV 1550 A and He II 1640 A emission lines originating in the accretion disk during different orbital phases. Different spectra showing the variations in line fluxes at different orbital phases are presented. The reddening of V Sge is determined from the 2200 A feature. We concentrated on calculating the line fluxes of C IV & He II emission lines. From HST and IUE data, we derived an accretion luminosity and an accretion rate for V Sge. The average temperature of the outer rim of the accretion disk ${\sim}10000~\mbox{K}$ . Our results show that there are variations in line fluxes, accretion luminosities and accretion rates with time for V Sge. These variations are attributed to the variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the white dwarf. These results from the HST and IUE observations are consistent with the binary model consisting of a white dwarf, a disk around the white dwarf, and a lobe-filling main-sequence companion (Hachisu & Kato, Astrophys. J. 598:527H, 2003).

Ultraviolet Spectroscopic Investigation of HU Aqr and AN UMa with the Data from HST and IUE

Abstract We present ultraviolet spectroscopic study of two polar systems, HU Aqr and AN UMa, observed with the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (STIS) and Faint Object Spectrograph (FOS) and with the International Ultraviolet Explorer (IUE) during the period 1979–2003, to diagnose the ultraviolet fluxes of CIV 1550 Å and He II 1640 Å emission lines originating in an accretion stream during different orbital phases. The reddening of two systems is determined from the 2200 Å absorption feature. Different spectra for both systems, showing the variations in line fluxes at different orbital phases, are presented. We concentrated on calculating the line fluxes of CIV and He II emission lines. From HST and IUE data, we derived accretion luminosities and accretion rates for the two systems. The average temperature of the accretion stream for HU Aqr and AN UMa is about 5000 K and 6000 K, respectively. Our results show that there are variations in line fluxes, accretion luminosities and accretion rates with time for the two systems. These modulations are attributed to the variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the primary star. These results from the HST and IUE observations support the irradiation model producing sufficient ultraviolet flux for orbital modulations.

Ultraviolet spectral behavior of AO Psc and V1223 Sgr from the HST and IUE satellites

We present a spectroscopic study of two intermediate polar systems, AO Psc and V1223 Sgr observed with the Hubble Space Telescope Space Telescope Imaging Spectrograph (HST STIS) and International Ultraviolet Explorer (IUE) during the period 1980–2000. The reddening of two systems is determined from the 2200 A feature. The ultraviolet emission lines are originating in the accretion disk as a result of existing an extra component of emission in the EUV/soft X-ray range with luminosity comparable to the accretion luminosity which reprocessed to produce the observed ultraviolet line strengths. Different spectra for both systems showing the variations in line fluxes at different orbital phases are presented. We concentrated on calculating the line fluxes of C IV 1550 A & He II 1640 A emission lines. From HST and IUE data, we derived an accretion luminosities and an accretion rates for two systems. Our results show that there are variations in line fluxes, accretion luminosities and accretion rates with time for two systems. These variations are attributed to the variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the white dwarf. These results from the IUE and HST observations are consistent with the models of Ko et al. (Astrophys. J., 457:363K, 1996).

Ultraviolet spectroscopic study of EU UMa and ST LMi from HST and IUE observations

We present the first orbit-resolved ultraviolet spectroscopic observations of the two polar systems EU UMa and ST LMi obtained with the Hubble Space Telescope Faint Object Spectrograph (HST FOS) and International Ultraviolet Explorer (IUE) during their intermediate- and low-luminosity states in the period between 1982 and 2003. Different line profiles of the two systems showing variations of line fluxes at different orbital phases are presented. This paper focuses on the C IV emission line at 1550Å produced in the accretion stream, presenting calculations of spectral line fluxes, ultraviolet luminosities, and accretion rates for the two systems. Our analysis of the spectroscopic data reveals changes with orbital phase of the emission line profiles that correspond to the light curve variations of both EU UMa and ST LMi in the optical and infrared bands. The variations of line fluxes are attributed to variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the white dwarf. The ultraviolet luminosity and accretion rate of EU UMa are smaller than the ultraviolet luminosity and accretion rate of ST LMi.

Ultraviolet spectral behavior of IP Peg and DW UMa in high, intermediate and low states from the HST and IUE satellites

Ultraviolet spectra of IP Peg and DW UMa observed with Hubble Space Telescope Space Telescope Imaging Spectrograph & Faint Object Spectrograph (HST STIS FOS) and International Ultraviolet Explorer (IUE) satellites were analyzed during the period between 1989–1994 and 1985–2004. Different profiles of two systems showing variations of line fluxes at different orbital phases are presented. Both IP Peg and DW UMa are characterized by emission lines in high, intermediate and low states. DW UMa is characterized by absorption lines in an unusual low state. This paper focuses on the C IV emission line at 1550Å produced in an accretion disk for two systems (Saito et al., 2005; Knigge et al., 2000), by calculating spectral line fluxes. Our results show that there are significant variations of line fluxes with time for two systems. The variations of line fluxes are attributed to the variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the white dwarf (Saito et al., 2005; Knigge et al., 2000). These results from the IUE and HST observations support a white dwarf model atmospheres of IP Peg and DW UMa producing sufficient UV flux for orbital modulations.

Similarities and differences in the spectral behavior of W Ser and UX Mon in the ultraviolet

We present ultraviolet spectra of two eclipsing interacting binary systems, W Ser and UX Mon, with good coverage over the 14.16-day and 5.9-day orbital periods, respectively, using observations taken by the International Ultraviolet Explorer (IUE) during the period between 1978–1993 and 1981–1991. Two profiles of W Ser and UX Mon showing variations of line fluxes at two orbital phases are presented. This paper focuses on the N V emission line at 1240 A, C II emission line at 1336 A, C IV emission line at 1550 A, O III emission line at 1666 A and the Si III emission line at 1892 A, produced in an extended gaseous envelope around the mass-gaining component by calculating spectral line fluxes. Our results show that there are variations of line fluxes with time, similar to the light curves found for both W Ser and UX Mon. We attribute these spectral variations to eclipse effects and to variations in the mass transfer rate. These results from the IUE observations support the thick disk model around the primary star in which variations of mass transfer affect the observed radiation from the gaseous envelope around the hot star. Future, high-resolution imaging is recommended to confirm the inferred asymmetrical circumstellar envelopes.

Ultraviolet spectroscopy of Asteroid (4) Vesta

We report a comprehensive review of the UV–visible spectrum and rotational lightcurve of Vesta combining new observations by Hubble Space Telescope and Swift Gamma-ray Burst Observatory with archival International Ultraviolet Explorer observations. The geometric albedos of Vesta from 220nm to 953nm are derived by carefully comparing these observations from various instruments at different times and observing geometries. Vesta has a rotationally averaged geometric albedo of 0.09 at 250nm, 0.14 at 300nm, 0.26 at 373nm, 0.38 at 673nm, and 0.30 at 950nm. The linear spectral slope as measured between 240 and 320nm in the ultraviolet displays a sharp minimum near a sub-Earth longitude of 20°, and maximum in the eastern hemisphere. This is consistent with the longitudinal distribution of the spectral slope in the visible wavelength. The photometric uncertainty in the ultraviolet is ∼20%, and in the visible wavelengths it is better than 10%. The amplitude of Vesta’s rotational lightcurves is ∼10% throughout the range of wavelengths we observed, but is smaller at 950nm (∼6%) near the 1-μm band center. Contrary to earlier reports, we found no evidence for any difference between the phasing of the ultraviolet and visible/near-infrared lightcurves with respect to sub-Earth longitude. Vesta’s average spectrum between 220 and 950nm can well be described by measured reflectance spectra of fine particle howardite-like materials of basaltic achondrite meteorites. Combining this with the in-phase behavior of the ultraviolet, visible, and near-infrared lightcurves, and the spectral slopes with respect to the rotational phase, we conclude that there is no global ultraviolet/visible reversal on Vesta. Consequently, this implies a lack of global space weathering on Vesta, as previously inferred from visible–near-infrared data.

New variations of Mg II emission lines of Mira AB with HST

This paper presents results from high resolution observations of the Mira AB binary system taken by the HST (Hubble Space Telescope) in the ultraviolet region through the period (1999–2004). By comparing the results using HST data with those obtained from the IUE (International Ultraviolet Explorer) observations through the period (1979–1995). we found new variations in the profiles, widths and fluxes of Mg II (k&h) emission lines. In addition to this it appears that there are another emission components at some phases in the red and blue sides of both Mg II (k&h) emission lines. These emission components may be of Mira A origin. There is a considerable shift of both Mg II (k&h) emission lines in observation taken on 2nd February 2004. The Mg II profiles extends up to more than 600 km/s, and redshifted as with IUE observations.

Dynamics of the Lyα and CivEmitting Gas in 3C 273

In this paper we study the variability properties of the Lyα and C IV emission lines in 3C 273 using archival International Ultraviolet Explorer observations. Our data show for the first time the existence of variability on timescales of several years. We study the spatial distribution and the velocity field of the emitting gas by performing detailed analyses on the line variability using correlations, one- and two-dimensional response functions, and principal component analysis. In both lines we find evidence for two components, one that has the dynamic properties of gas in Keplerian motion around a black hole with a mass on the order of 109 M☉, and one that is characterized by high, blueshifted velocities at large lag. There is no indication of the presence of optically thick emission medium in either the Lyα or C IV response functions. The component characterized by blueshifted velocities, which is comparatively much stronger in C IV than in Lyα, is more or less compatible with being the result of gas falling toward the central black hole with free-fall acceleration. We propose, however, that the line emission at high, blueshifted velocities is better explained in terms of entrainment of gas clouds by the jet. Therefore, this gas is probably collisionally excited as a result of heating due to the intense infrared radiation from the jet, which would explain the strength of this component in C IV relative to Lyα. This phenomenon might be a signature of disk-jet interaction.

The Ultraviolet Spectrum of VV Cephei Out of Eclipse

ABSTRACTThe International Ultraviolet Explorer (IUE) satellite observed VV Cephei (M2 Iab+B?) over nearly a full 20.3 year orbital period, but all the observations were obtained outside total eclipse. Egress of the chromospheric eclipse was the only portion of the eclipse observed by IUE. We have examined all 124 high‐resolution IUE spectra of VV Cephei and describe the spectrum and its variations.Contrary to initial expectations, the spectrum remained complex throughout the entire orbit, and five line components have been identified: (1) emission lines; (2) neutral, low‐excitation (e.g., Fe i) lines, which disappear as the hot component emerges from eclipse; (3) broad, high‐ionization (e.g., Si iv and Fe iii) lines arising from the hot component or nearby circumstellar material; (4) narrow absorption cores from the circumstellar envelope of the M supergiant; and (5) “shell” absorption lines with variable profiles which do not absorb below a minimum depth.Mg ii h and k emission broadens and strengthens when the system is near periastron and secondary eclipse. Most of the variation in the other emission features appears to be stochastic rather than phase dependent.The continuum shows rapid variations on timescales as short as 2 weeks. At any one time, the continuum flux between 1400 and 3000 Å is fairly constant but varies, apparently stochastically, in the range (2–7) × 10-12 ergs s−1 cm−2 Å−1 for observations obtained outside of chromospheric eclipse. The dereddened continuum fluxes vary as ∼λ−0.8, implying that the source is not purely stellar.Radial velocity measurements of the circumstellar and shell lines show that neither follows the radial velocity curve of the two stars. Circumstellar line velocities range from the systemic velocity to ∼15 km s−1 more negative, consistent with their formation in an extended circumstellar envelope of the M supergiant which is large compared to the scale of the orbit. The shell lines are redshifted from the systemic velocity by ∼30 km s−1. The last IUE observations (near phase 0.9) showed shell lines with greater redshifts and deeper absorption than around the rest of the cycle.The ultraviolet spectrum of VV Cep is compared to spectra of stars showing similar features: the Be shell star 28 Tauri, the B[e] star HD 45677, and the M supergiant+B binary systems α Scorpii and KQ Puppis. A table detailing the appearance and behavior of numerous line multiplets is presented.

IUE's detection of tenuous SO2 frost on Ganymede and its rapid time variability

International Ultraviolet Explorer (IUE) observations of Ganymded demonstrate decade length or less temporal variations at a few discreet trailing side longitude regions. The temporal changes are seen in comparisons of IUE spectra from 1984–1986 with new IUE observations from 1996. Variations are observed on the trailing side associated with the new detection of SO2 frost. A distinctive SO2 frost spectral signature at 0.32 µm is detected in the 1984–1986 spectra of trailing side regions from 225° to 285° which are absent in corresponding 1996 measurements.

A Detailed Comparison ofHubble Space TelescopeFaint Object Spectrograph andIUEUltraviolet Spectra of Selected Seyfert Nuclei

Despite the contributions of the Hubble Space Telescope Faint Object Spectrograph (FOS) to the archive of UV observations of active galactic nuclei, the vast majority of UV reference data were obtained using the International Ultraviolet Explorer (IUE) satellite. These data remain important since they provide historical information about the intensities of the UV continua and emission lines that is needed to constrain models of the active nucleus. A detailed comparison of the FOS and IUE data is critical to understanding how the measurable quantities depend on the individual instrumental calibrations, and how any conclusions derived from modeling the observations may vary depending on the source of the UV data. Rigorous comparison of FOS and IUE spectra have so far been performed only for spectrophotometric standard star observations that are acquired accurately and have high signal-to-noise ratios. We compare typical FOS spectra that were not acquired and observed with the strict regimen that is used for standard-star observations, especially in the pre-COSTAR era. All nonproprietary UV FOS spectrophotometric archival data for the Seyfert 1 galaxies Mrk 509, NGC 3783, and NGC 5548 that have near-simultaneous (within 24 hr) IUE observations are used in the analysis. These data demonstrate that the absolute photometric calibrations of the FOS and IUE agree within ~5% in absolute flux for two of the objects. For NGC 5548, the FOS and IUE flux data disagree by ~50% in the 1200-2000 A region. In this object there may be evidence for flux nonlinearity of the IUE detector and a contribution from the host galaxy redward of 2800 A. Cross-correlation of the FOS and IUE spectra reveals no zero-point wavelength shift larger than the IUE wavelength calibration errors. Comparison of line flux measurements from both the FOS and IUE spectra show that for strong emission lines (e.g., Lyα, C IV, and Mg II) the measured intensities always agree within 15%, while for moderately strong lines (e.g., N V, Si IV/O IV, He II, and C III]) the agreement is ~30% (1 σ). Weak lines (e.g., O I, C II, N IV], O III], and N III]) may not even be detected in the IUE spectra, and when they are detected the disagreement between the measured fluxes can be very large.

The Unusual UV Spectra of EUV-Discovered AM Herculis Stars

International Ultraviolet Explorer observations of the ROSATWFC-discovered AM Hers RE2107–05, RE1938–46, RE1833–74, RE1149+28, and RE0751+14 reveal UV spectra (1200 – 2000 Å) which are unique when compared with previously studied systems. Specifically, the intensities of individual emission lines due to CIV, CII, N V, HeII, and Si IV, are 2 – 4 times stronger then typically seen. However, the equivalent width ratios of these same emission lines, (CII/CIV, N V/CIV, HeII/CIV, and Si IV/CIV), are similar to those in typical AM Hers. These results indicate that the EUV-discovered AM Hers must have larger UV emitting volumes. Using additional observations of these stars made by EUVE, we speculate on the cause of their larger UV line fluxes.