International Ultraviolet Explorer Research Articles

Investigating the Temperature Sensitivity of UV Line Ratios in the 280 nm Region of Solar-like Stars

Abstract Stellar UV spectra are fundamental diagnostics of physical and magnetic properties of stars. For instance, lines like Mg ii at 280 nm serve as valuable indicators of stellar activity, providing insights into the activity levels of Sun-like stars and their potential influence on the atmospheres of orbiting planets. On the other hand, the effective temperature (T eff) is a fundamental stellar parameter, critical for determining stellar properties such as mass, age, composition, and evolutionary status. In this study, we investigate the temperature sensitivity of three lines in the mid-UV range (i.e., Mg ii 280.00 nm, Mg i 285.20 nm, and Si i 286.15 nm). Using spectra from the International Ultraviolet Explorer, we analyze the behavior of the ratios of their corresponding indices (core/continuum) for a sample of calibrating solar-like stars and find that the ratio R = Mg ii/Mg i best traces T eff through a log–log relation. The T eff estimated using this relation on a test sample of solar-like stars agree with the T eff from the literature at the 95% confidence level. The observed results are interpreted making use of response functions as diagnostics. This study extends the well-established use of line depth ratio–temperature relationships, traditionally applied in the visible and near-infrared ranges, to the mid-UV spectrum. With the growing interest in stellar UV spectroscopy, results presented in this paper are potentially relevant for future missions such as HWO, MANTIS, and UVEX.

A Comparison of Publication Counts for Four Ultraviolet Explorer Observatories

Abstract Publication counts for four space-based ultraviolet observatories are compared: International Ultraviolet Explorer (IUE), Extreme Ultraviolet Explorer, Far Ultraviolet Spectroscopic Explorer (FUSE), and Galaxy Evolution Explorer (GALEX). The first three show rapid rises to either a sharp or broad maximum. Over the peak period for IUE the papers tend to be concentrated upon data from that mission specifically. Maxima are followed by declines of various duration, after which papers based upon multiple space-based observatories have lead to extended relevancy for each mission, particularly in the case of FUSE. By contrast, papers that incorporate data from GALEX have yet to evince a turn-over. As with FUSE, there is a pronounced trend for results from the GALEX mission to have been incorporated into multi-mission papers.

TESS Asteroseismology of β Hydri: A Subgiant with a Born-again Dynamo

The solar-type subgiant β Hyi has long been studied as an old analog of the Sun. Although the rotation period has never been measured directly, it was estimated to be near 27 days. As a Southern Hemisphere target, it was not monitored by long-term stellar activity surveys, but archival International Ultraviolet Explorer data revealed a 12 yr activity cycle. Previous ground-based asteroseismology suggested that the star is slightly more massive and substantially larger and older than the Sun, so the similarity of both the rotation rate and the activity cycle period to solar values is perplexing. We use two months of precise time-series photometry from the Transiting Exoplanet Survey Satellite to detect solar-like oscillations in β Hyi and determine the fundamental stellar properties from asteroseismic modeling. We also obtain a direct measurement of the rotation period, which was previously estimated from an ultraviolet activity–rotation relation. We then use rotational evolution modeling to predict the rotation period expected from either standard spin-down or weakened magnetic braking (WMB). We conclude that the rotation period of β Hyi is consistent with WMB and that changes in stellar structure on the subgiant branch can reinvigorate the large-scale dynamo and briefly sustain magnetic activity cycles. Our results support the existence of a “born-again” dynamo in evolved subgiants—previously suggested to explain the cycle in 94 Aqr Aa—which can best be understood within the WMB scenario.

The Accelerating Decline of the Mass Transfer Rate in the Recurrent Nova T Pyxidis* * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of University for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

The recurrent nova T Pyxidis (T Pyx) has erupted six times since 1890, with its last outburst in 2011, and the relatively short recurrence time between classical nova explosions indicates that T Pyx must have a massive white dwarf (WD) accreting at a high rate. It is believed that, since its outburst in 1890, the mass transfer rate in T Pyx was very large due to a feedback loop where the secondary is heated by the hot WD. The feedback loop has been slowly shutting off, reducing the mass transfer rate, and thereby explaining the magnitude decline of T Pyx from ∼13.8 (before 1890) to 15.7 just before the 2011 eruption. We present an analysis of the latest Hubble Space Telescope far-ultraviolet and optical spectra, obtained 12 yr after the 2011 outburst, showing that the mass transfer rate has been steadily declining and is now below its preoutburst level by about 40%: Ṁ∼1−3×10−7M⊙ yr−1 for a WD mass of ∼1.0–1.4 M ⊙, an inclination of 50°–60°, reddening of E(B − V) = 0.30 ± 0.05, and a Gaia Data Release 3 distance of 2860−471+816 pc. This steady decrease in the mass transfer rate in the ∼decade after the 2011 outburst is in sharp contrast with the more constant preoutburst ultraviolet continuum flux level from archival International Ultraviolet Explorer spectra. The flux (i.e., Ṁ ) decline rate is 29 times faster now in the last ∼decade than observed since 1890 to ∼2010. The feedback loop shut off seems to be accelerating, at least in the decade following its 2011 outburst. In all eventualities, our analysis confirms that T Pyx is going through an unusually peculiar short-lived phase.

Spectroscopic Observations of Ten Galactic Wolf–Rayet Stars at Bosscha Observatory: Determination of Stellar Parameters and Mass-loss Rates

We present optical spectra of 10 Galactic Wolf–Rayet (WR) stars that consist of five WN and five WC stars. The optical observation was conducted using a low-resolution spectrograph NEO-R1000 (λ/Δλ ∼ 1000) at GAO-ITB RTS (27.94 cm, F/10.0), Bosscha Observatory, Lembang. We implemented stellar atmosphere Postdam Wolf–Rayet (PoWR) grid modeling to derive stellar parameters. The normalized optical spectrum can be used to find the best model from the available PoWR grid, then we could derive stellar temperature and transformation radius. To derive luminosity, stellar radius and color excess, we conducted a Spectral Energy Distribution (SED) analysis with additional data on the near-ultraviolet spectrum from the International Ultraviolet Explorer (IUE) database, and UBV and 2MASS JHK broadband filter data. Additional analysis to derive asymptotic terminal wind velocity was conducted from the P-Cygni profile analysis of the high-resolution IUE ultraviolet spectrum. With previously derived parameters, we could determine the mass loss rate of the WR stars. Furthermore, we compared our results with previous work that used PoWR code and the differences are not more than 20%. We conclude that the PoWR spectral grid is sufficient to derive WR stellar parameters quickly and could provide more accurate initial parameter input to the PoWR program code.

Wind-line Variability and Intrinsic Errors in Observational Mass-loss Rates

UV wind-line variability in OB stars appears to be universal. In order to quantify this variation and to estimate its effect on a mass-loss rate determined from a single observation, we use the International Ultraviolet Explorer archive to identify nonpeculiar OB stars with well-developed but unsaturated Si iv λ1400 doublets and at least 10 independent observations. This resulted in 1699 spectra of 25 stars. A simple model was used to translate the observed profile variations into optical depth variations and, hence, variations in measured mass-loss rates. These variations quantify the intrinsic error inherent in any single mass-loss rate derived from a single observation. The derived rates have an overall 1σ variation of about 22%, but this appears to differ with T eff, being as small at 8% for the hottest stars and up to 45% for the cooler ones. Furthermore, any single determination can differ from the mean by a factor of 2 or more. Our results also imply that mass-loss rates determined from nonsimultaneous observations (such as UV and ground-based data) need not agree. In addition, we use our results to examine the nature of the structures responsible for the variability. Our findings suggest that the optical depth variations result from optically very thick structures occulting more or less of the line of sight to the stellar disk. Further, the smaller optical depth variations in the hottest stars suggest that the structures responsible for the variations are disrupted in their more powerful winds.

Revisiting the accretion disc spectra of dwarf novae and nova-like variables: implications for the standard disc model

ABSTRACT Accretion discs are fundamental to much of astronomy. They can occur around stars, both young and old, around compact objects they provide a window into the extremes of physics, and around supermassive black holes in galaxy centres, they generate spectacular luminosities that can outshine the entire galaxy. However, our understanding of the inner workings of accretion discs remains far from complete. Here, we revisit a conundrum in the observations of some of the simplest accreting systems: the cataclysmic variables (CVs). The high-accretion-rate states of (non-magnetic) CVs can be divided into the short-lived outbursts (∼a week), typical of dwarf novae (DNe), and the long-lived (and sometimes perpetual) high states of nova-like (NL) CVs. Since both sorts of high-state occur in approximately steady-state accretion discs with similar properties and accretors, we would expect them to display similar spectral energy distributions. However, previous analyses based on UV spectra from the International Ultraviolet Explorer have shown that their spectral energy distributions are different. We perform a re-analysis of the data using up to date calibrations and distance (and thus dereddening) estimates to test whether this difference persists and whether it is statistically significant over the sample. We find that it does persist, and it is statistically significant. We propose routes to investigating this discrepancy further and discuss the implications this has for other accreting systems, such as X-ray binaries, active galactic nuclei, and protoplanetary discs.

The FUV Fluxes of A-type Star Members of the Hyades

The International Ultraviolet Explorer database has been searched for the FUV spectra of the A stars which are members of the Hyades. Eight out of 16 confirmed members of the Hyades have FUV absolute fluxes obtained with the SWP camera through the large aperture. The observed sharp decrease of the FUV fluxes from spectral type A2 to A9 is essentially a temperature effect. The FUV flux of the A7 IV star HD 28310, a δ Scuti star, is clearly variable.

Catalog of Ultraviolet Bright Stars: Strategies for UV Occultation Measurements, Planetary Illumination Modeling, and Sky Map Analyses Using Hybrid IUE-Kurucz Spectra

Ultraviolet spectroscopy is a powerful method to study planetary surface composition through reflectance measurements, atmospheric composition through stellar/solar occultations, transits of other planetary bodies, and direct imaging of airglow and auroral emissions. The next generation of ultraviolet spectrographs (UVS) on board ESA’s Jupiter Icy Moons Explorer and NASA’s Europa Clipper missions will perform such measurements of Jupiter and its moons in the early 2030s. This work presents a compilation of a detailed UV stellar catalog, named Catalog of Ultraviolet Bright Stars (CUBS), of targets with high intensity in the 50–210 nm wavelength range with applications relevant to planetary spectroscopy. These applications include (1) planning and simulating occultations, including calibration measurements; (2) modeling starlight illumination of dark, nightside planetary surfaces primarily lit by the sky; and (3) studying the origin of diffuse Galactic UV light as mapped by existing data sets from Juno-UVS and others. CUBS includes observations from the International Ultraviolet Explorer (IUE) and additional information from the SIMBAD database. We have constructed model spectra at 0.1 nm resolution for almost 90,000 targets using interpolated Kurucz models (which have a resolution of 1 nm) and, when available, IUE spectra. CUBS also includes robust checks for agreement between the Kurucz models and the IUE data. We also present a tool for which our catalog can be used to identify the best candidates for stellar occultation observations, with applications for any UV instrument. We report on our methods for producing CUBS and discuss plans for its implementation during ongoing and upcoming planetary missions.

Aperture and Resolution Effects on Ultraviolet Star-forming Properties: Insights from Local Galaxies and Implications for High-redshift Observations* *Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

We present an analysis of the effects of spectral resolution and aperture scale on derived galaxy properties using far-ultraviolet (FUV) spectra of local star-forming galaxies from the International Ultraviolet Explorer (R ∼ 250, field of view (FOV) ∼ 10″ × 20″) and Cosmic Origins Spectrograph on the Hubble Space Telescope (R ∼ 15,000, FOV ∼ 2.″5). Using these spectra, we measured FUV luminosities, spectral slopes, dust attenuation, and equivalent widths. We find that galaxies with one dominant stellar cluster have FUV properties that are independent of aperture size, while galaxies with multiple bright clusters are sensitive to the total light fraction captured by the aperture. Additionally, we find significant correlations between the strength of stellar and interstellar absorption lines and metallicity, indicating metallicity-dependent line-driven stellar winds and interstellar macroscopic gas flows shape stellar and interstellar spectral lines, respectively. The observed line strength versus metallicity relation of stellar-wind lines agrees with the prediction of population synthesis models for young starbursts. In particular, measurements of the strong stellar C iv λλ1548, 1550 line provide an opportunity to determine stellar abundances as a complement to gas-phase abundances. We provide a relation between the equivalent width of the C iv line and the oxygen abundance of the galaxy. We discuss this relation in terms of the stellar-wind properties of massive stars. As the driving lines in stellar winds are mostly ionized iron species, the C iv line may eventually offer a method to probe α-element-to-iron ratios in star-forming galaxies once consistent models with nonsolar abundance ratios are available. These results have important implications for the galaxy-scale, low-resolution observations of high-redshift galaxies from JWST (R ∼ 100–3500).

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.

An ultraviolet spectral study of fullerene-rich planetary nebulae

ABSTRACT Several planetary nebulae (PNe) have been found to contain both polycyclic aromatic hydrocarbon (PAH-like) species and fullerenes (C60) distinguished by their mid-infrared emission. Previous laboratory and astronomical studies suggest that the formation of both species could be related to the decomposition, by photochemical processing, of hydrogenated amorphous carbon (HAC) grains. Then, HACs and, seemingly, big-fullerene related species (e.g. carbon onions) have been suggested as potential carriers of the ultraviolet (UV) bump at 2175 Å and the far-UV rise common to interstellar extinction curves. Our goal is to investigate the UV bump with the possible presence of an HAC extinction component in the International Ultraviolet Explorer (IUE) spectra of C-rich PNe; both with detected and non-detected fullerenes. The considered sample includes three C60-PNe (Tc 1, IC 418, and IC 2501) and the non-C60-PN Hen 2-5. Independently of the presence of C60 in their circumstellar envelopes, we found that the UV bump in all sample PNe is well explained by interstellar extinction, suggesting that species different from those of the foreground interstellar medium, e.g. large fullerene-related species like carbon onions, are not the carrier. Interestingly, we found that PNe Tc 1 and Hen 2-5 show an absorption in the FUV rise. Their IUE continuum spectra may be very well reproduced by including the extinction curve of HAC-like very small grains. The possible presence of both species, HAC-like grains and fullerenes (C60), in Tc 1 could support the HAC photochemical processing scenario for the formation of fullerenes in the complex circumstellar envelopes of PNe.

Nature of UV Variability in NGC 4151 from IUE’s Intensive Monitoring Spectroscopic Data

The International Ultraviolet Explorer (IUE) satellite, launched in 1978 by NASA and ESA, has conducted highly successful long-term monitoring spectroscopic observations of many nearby (low-redshift) bright active galaxies, including NGC 1068, NGC 5548, NGC 7469, Fairall 9,MRK 335 and NGC 4151. Notably, IUE has monitored NGC 4151 more intensively than any other active galaxy, obtaining 414 spectra during its campaign from December 1 to 15, 1993. Here, we present results of our recent studies on the emission line and continuum variabilities characterised by the Rmax and Fvar parameters. From the analysis of the short timescale, intense monitoring data, we obtained Rmax values ranging between 2.14±0.02 and 1.62±0.01 corresponding to the UV continua 1270 Å, and 2705 Å, respectively. The corresponding Fvar values varied from 9.62±0.04% to 5.19±0.04 %. Furthermore, the Rmax values for the UV emission lines Lyα, C IV, He II, O III], C III] and Mg II have been found to be 4.09±0.06, 4.55±0.05, 13.65±0.68, 4.78±0.17, 2.76±0.02, respectively. The corresponding Fvar values obtained are 2.85±0.01 %, 26.51±0.06 %, 15.59±0.02 %, 48.93±1.79 %, 20.30±0.06% and 18.68±0.02 %, respectively. These results provide constraints on the amplitude of UV variability and the size of the broad-line region (BLR) in low luminosity Seyfert galaxies. The decreasing Fvar amplitudes of the UV continuum with respect to increasing wavelengths are significant results of the present study.

New Aspects of Spectral Behavior of Two Dwarf Novae SS Cyg and WX Hyi Between Quiescence and Outburst States with International Ultraviolet Explorer (IUE)

New Aspects of Spectral Behavior of Two Dwarf Novae SS Cyg and WX Hyi Between Quiescence and Outburst States with International Ultraviolet Explorer (IUE)

The Behavior of Bp He-weak Stars in the Far UV—Paper XIV: HD 143699

The analysis of seven spectra of the B6 He-weak star HD 143699 recorded by the SWP camera on board the International Ultraviolet Explorer reveals variations of the far ultraviolet spectral energy distribution shortwards of 1600 Å in spectra collected from 1994 April 12 to July 9. These variations do not seem to reflect rotational modulation. They could be caused by pulsation with the recently derived pulsation period close to 4 hr. The resonance line of Si ii at 1526.71 Å clearly shows an interstellar component. There is no evidence of C iv absorption at 1550 Å in any of the spectra.

The Behavior of the Bp Stars in the Far UV—Paper VII: HD 22316

The analysis of seven spectra of the B9 Cr Hg Si star HD 22316 recorded by the SWP camera on board the International Ultraviolet Explorer does not reveal any variations of the far ultraviolet spectral energy distribution shortwards of 1800 Å in spectra collected over 2 yr with identical exposure times. The low excitation line at 1363.45 Å and the resonance line of Cl i at 1379.53 Å are most likely present. Several lines of Al ii, Si ii, P ii but none of S i could be found at their expected wavelengths. The synthesis of several Al ii lines consistently yields a clear underabundance of aluminum of about 0.40 times the solar aluminum abundance.

The Behavior of Bp HgMn Stars in the Far UV—Paper 28: HD 110073

The analysis of three spectra of the B8 Mn star HD 110073 recorded by the SWP camera on board the International Ultraviolet Explorer does not reveal any variations of the far ultraviolet spectral energy distribution shortwards of 1800 Å in spectra collected over 8 yr with identical exposure times. The Ga ii line at 1414.40 Å is a prominent feature in the mean FUV spectrum of this star. The modeling of the optical line of Ga ii at 6334.07 Å yields an overabundance of 1600 times the solar abundance, much lower than previous determinations. The projected rotational velocity is close to 15 km s−1. The silicon abundance derived from the Si ii line at 6347.10 Å is solar.

New Abundance Determinations for the B9 Mn Star HD 53929

Analysis of coadded high resolution spectra of the chemically peculiar star HD 53929 recorded by the SWP and LWR cameras on board the International Ultraviolet Explorer yields estimates of the abundances of silicon and manganese, respectively 1.5 and 6 times the solar abundances, in the atmosphere of this star. The synthesis of the 4400–4500 Å range allows the determination of the abundances of helium, magnesium, phosphorus and iron. Compared to solar abundances, helium is very underabundant (0.05 solar), magnesium is also underabundant by a factor of 0.20 times the solar abundances. Phosphorus is overabundant by a factor of 20.

The Behavior of Bp He-weak Stars in the Far UV—Paper XIII: HD 161480

The analysis of four spectra of the B7 He-weak star HD 161480 recorded by the SWP camera on board the International Ultraviolet Explorer reveals instances of variations of the far ultraviolet spectral energy distribution shortwards of 1800 Å in spectra collected over 4 consecutive days with identical exposure times. The Si ii doublet near 1530 Å is stronger at phase of FUV minimum. The C iv doublet at 1550 Å is absent. These variations suggest that silicon and other chemical elements are probably distributed in an inhomogeneous manner over the surface of HD 161480. The radial velocity of the Si ii doublet varies over the 4 days observation, reflecting the orbital motion of this SB1 star.

The Behavior of Ap/Fp Stars in the Ultraviolet—Paper I: HD 137949 (33 Lib)

Abstract The analysis of six spectra of the F0Vp EuGdSr star HD 137949 recorded by the SWP camera on board the International Ultraviolet Explorer does not reveal any variations of the far-ultraviolet spectral energy distribution shortwards of 1800 Å over slightly more than 2 yr and a half. The five spectra taken consecutively on 1984 February 23 do not show evidence either for rapid variations of the flux over 2 hr and 50 minutes (whereas the main pulsational period is 8.27 minutes for this rapidly oscillating Fp star). This behavior remarkably differs from the 50% flux variability of δ Scuti (F2 III) in the FUV over its 4 hr and 40 minutes period.