Allan J. Willis

Overview of the Far Ultraviolet Spectroscopic Explorer Mission

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905-1187 Angstrom, with a high spectral resolution. The instrument consists of four co-aligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al :LiF coatings for optimum reflectivity between approximately 1000 and 1187 Angstrom, and the other two channels use SiC coatings for optimized throughput between 905 and 1105 Angstrom. The gratings are holographically ruled to correct largely for astigmatism and to minimize scattered light. The microchannel plate detectors have KBr photocathodes and use photon counting to achieve good quantum efficiency with low background signal. The sensitivity is sufficient to examine reddened lines of sight within the Milky Way and also sufficient to use as active galactic nuclei and QSOs for absorption-line studies of both Milky Way and extragalactic gas clouds. This spectral region contains a number of key scientific diagnostics, including O VI, H I, D I, and the strong electronic transitions of H-2 and HD.

Quantitative analysis of WC stars: constraints on neon abundances from ISO-SWS spectroscopy

Neon abundances are derived in four Galactic WC stars - gamma(2) Vel (WR 11, WC8+O7.5III), HD 156385 (WR 90, WC7), HD 192103 (WR 135, WC8) and WR 146 (WC5+O8) - using mid-infrared fine-structure lines obtained with ISO-SWS. Stellar parameters for each star are derived using the non-local thermodynamic equilibrium model atmospheric code of Hillier & Miller, together with ultraviolet (IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In the case of gamma(2) Vel, we adopt very recent results from De Marco et al., who followed an identical approach.ISO-SWS data sets reveal the [Ne III] 15.5-mu m line in each of our targets, while [Ne II] 12.8 mu m, [S IV] 10.5 mu m and [S III] 18.7 mu m are observed solely in gamma(2) Vel. Using a method updated from Barlow et al. to account for clumped winds, we derive Ne/He = (3-4) x 10(-3) by number, plus S/He = 6 x 10(-5) for gamma(2) Vel. Neon is highly enriched, such that Ne/S in gamma(2) Vel is eight times higher than cosmic values. However, observed Ne/He ratios are a factor of 2 lower than predictions of current evolutionary models of massive stars. An imprecise mass loss and distance were responsible for the much greater discrepancy in neon content identified by Barlow et al.Our sample of WC5-8 stars span a narrow range in T-* (=55-71 kK), with no trend towards higher temperature at earlier spectral type, supporting earlier results for a larger sample by Koesterke & Hamann. Stellar luminosities range from 100 000 to 500 000 L., while 10(-5.1) less than or equal to M/(M. yr(-1)) less than or equal to 10(-4.5) adopting clumped winds, in which volume filling factors are 10 per cent. In all cases, wind performance numbers are less than 10, significantly lower than recent estimates. Carbon abundances span 0.08 less than or equal to C/He less than or equal to 0.25 by number, while oxygen abundances remain poorly constrained.

Far Ultraviolet Spectroscopic Explorer Atlas of OB Stars in the Magellanic Clouds

An atlas of the 900-1200 A region in the spectra of 47 OB stars in the Large and Small Magellanic Clouds, observed at high resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE), is presented and discussed. The systematic trends in the numerous stellar-wind features in this region, some from species (and ionizations) not represented at longer wavelengths, are charted as a function of the optical spectral types. The FUSE sample is by far the most powerful to date for that purpose. A special effort has been made to verify the spectral types of all stars included in the atlas, in a number of cases with new optical observations that are also illustrated, to avoid uncertainties from that source in the stellar-wind trends. A new O2 star has been found in the process. Most of these stars have been previously observed at longer ultraviolet wavelengths by the Hubble Space Telescope and in the optical from the ground with high-resolution, digital instruments; thus very comprehensive physical modeling of these OB atmospheres and winds now becomes possible. This atlas will serve as a guide to the FUSE Magellanic Cloud OB database for that purpose. The Magellanic Cloud sample provides a very important complement to the FUSE database of Galactic OB counterparts (Pellerin et al. 2002), both because the lower extinction and interstellar H2 absorption toward the Cloud stars allow a much clearer view of the stellar spectra below 1100 A, and because of the metallicity differences among the three galaxies. In particular, most wind features in the SMC spectra are seen to be significantly weaker than those in the LMC at the same spectral types.

Of-type stars HD 16691 and HD 190429 show WN-like spectra in infrared K band

We present 2 micrometer K-band spectra of two early-type Of stars that have infrared emission-line morphology similar to that of WN stars. Archival International Ultraviolet Explorer (IUE) spectra of these two stars indicate they appear to be Of type, rather than WN. Recently acquired optical spectra of these stars are quantitatively similar to that in the past, namely, Of attributes. We suggest that these two Of stars have stellar wind characteristics closer to WN type than other Of stars. We discuss the consequences for K-band classification of highly obscured hot stars that might not otherwise be visible in optical or UV wavelengths.

Wind inhomogeneities in Wolf-Rayet stars. IV. Using clumps to probe the wind structure in the WC8 star HD 192103

We present the most intensive, high-quality spectroscopic monitoring of optical Wolf-Rayet emission lines ever obtained. The Wolf-Rayet star HD 192103 (\WR 135; subtype WC8) was observed in the 5650¨5840 regime alternately from both the William Herschel Telescope and the Canada-France- Ae Hawaii Telescope. The —nal data consist of a series of 197 spectra spread over 64 hr, each with a resolv- ing power j/*j ^ 20,000 and a signal-to-noise ratio in the continuum ^450 per 3 pixel resolution element. We clearly and unambiguously identify stochastic, structured patterns of intrinsic variability at the 1%¨2% level of the line —ux in the broad C III j5696 emission line. The j5801/12 doublet emission is also found to be variable at the 0.2%¨0.5% level of the line —ux. We —nd a correlation between the variability patterns observed in C III and C IV, which suggests a signi—cant overlap in the emission volumes of these transitions, although C IV is known to arise somewhat closer to the star. We attempt to reproduce the observed line pro—le variation patterns using a simple phenomenological model, which assumes the wind to be fully clumped. With a minimal set of assumptions, we are able to reproduce both the shape and the variability in the C III j5696 emission pro—le. We show that the variability pattern provides constraints on the radial extent of WR 135ˇs wind where C III is produced, as well as on the local wind acceleration rate. However, our simple clump model does not reproduce the lower variability in the C IV doublet unless we assume the C IV emission to occur in a much larger volume than C III, implying that signi—cant C IV emission occurs farther out in the wind than C III. We suggest that while some C IV emission might occur farther out, possibly because of reionization from shocks, a more likely explanation is that wind clumping signi—cantly increases with distance from the star, leading to larger variability levels in C III, formed farther out than most of C IV. Alternatively, optical depth eUects and/or local ionization gradients within clumps could conspire to attenuate clumping eUects in the C IV emis- sion line while enhancing them in the C III line.

Far Ultraviolet Spectroscopic Explorer Spectroscopy of the O VI Resonance Doublet in Sand 2 (WO)

We present Far Ultraviolet Spectroscopic Explorer spectroscopy of Sand 2, an LMC WO-type Wolf-Rayet star, revealing the O VI resonance P Cygni doublet at 1032-1038 A. These data are combined with Hubble Space Telescope Faint Object Spectrograph ultraviolet and Mount Stromlo 2.3 m optical spectroscopy and analyzed using a spherical, non-LTE, line-blanketed code. Our study reveals exceptional stellar parameters: T* ~ 150,000 K, v∞ = 4100 km s-1, log(L/L☉) = 5.3, and = 1 × 10-5 M☉ yr-1, if we adopt a volume filling factor of 10%. Elemental abundances of C/He ~ 0.7 ± 0.2 and O/He ~ 0.15 by number qualitatively support previous recombination line studies. We confirm that Sand 2 is more chemically enriched in carbon than LMC WC stars and that it is expected to undergo a supernova explosion within the next 5 × 104 yr.

Ultraviolet observations of selective wind eclipses in Gamma Velorum and evidence for colliding winds

We present a study of high-resolution ultraviolet spectra of the WC8+O9I binary system γ Velorum. The data consist of Copernicus U2 (λλ=912-1450 A) and V2 (λλ=1700-3250 A) spectra secured during two separate binary cycles in 1977 and 1980, together with 40 IUE high-resolution SWP (λλ=1150-2050 A) and 30 LWR (λλ= 1850-3250 A) spectra, secured at many binary phases during six orbital cycles in 1978 and 1979. These data reveal substantial UV spectral variations confined to resonance and low-excitation lines of Si II, Si III, Si IV, C II, C III, C IV, S IV, and Fe IV. These variations are found to be strongly dependent on binary phase and highly repeatable from one binary cycle to another

Far Ultraviolet Spectroscopic Explorer observations of the stellar winds of two O7 supergiants in the magellanic clouds

We compare the stellar wind features in far-UV spectra of Sk -67°111, an O7 Ib(f) star in the LMC, with Sk 80, an O7 Iaf+ star in the SMC. The most striking differences are that Sk 80 has a substantially lower terminal velocity, much weaker O VI absorption, and stronger S IV emission. We have used line-blanketed, hydrodynamic, non-LTE atmospheric models to explore the origin of these differences. The far-UV spectra require systematically lower stellar temperatures than previous determinations for O7 supergiants derived from plane-parallel, hydrostatic models of photospheric line profiles. At these temperatures, the O VI in Sk -67°111 must be due primarily to shocks in the wind.

MEASURING THE IONIZATION OF O STAR WINDS

We present an analysis of wind line profiles from Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of two O7 supergiants in the Large and Small Magellanic Clouds (Sk -67°111 and AV 232, respectively). Model fits yield the column densities of S IV, S VI, P IV, P V, N III, and N IV, providing the first direct measurement of the ionization balance in stellar winds. The ratios of S IV/S VI and P IV/P V are consistently lower in the LMC star. IUE and Hubble Space Telescope archival spectra are also used to measure N IV and N V, but the much higher optical depth makes the N V measurements inconclusive. The velocity and optical depth distributions in the wind are qualitatively similar between the two stars, when scaled to their terminal velocities. The terminal velocities are different, with AV 232 being lower (as found previously in SMC stars and linked to lower metallicity). These are the first results from a program to investigate wind ionization and velocity structure among hot stars in local galaxies, and they demonstrate the higher accuracy in measuring column densities of less abundant ions, such as phosphorus and sulfur, observable in the FUSE range.

Wolf-Rayet stars: Physical, chemical and mass loss properties and evolutionary status

This paper reviews the current status of knowledge regarding the basic physical and chemical properties of Wolf-Rayet stars; their overall mass loss and stellar wind characteristics and current ideas about their evolutionary status. WR stars are believed to be the evolved descendents of massive O-type stars, in which extensive mass loss reveals successive stages of nuclear processed material: WN stars the products of interior CNO-cycle hydrogen burning, and WC and WO stars the products of interior helium burning. Recent stellar evolution models, particularly those incorporating internal mixing, predict results which are in good accord with the different chemical compositions observationally inferred for WN, WC and WO stars. WR stars exhibit the highest levels of mass loss amongst earlytype stars: mass loss rates, typically, lie in the range [1–10]×10−5M⊙yr−1. Radiation pressure-driven winds incorporating multi-scattering in high ionisation-stratified winds may cause these levels, but additional mechanisms may also be needed.