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NASA NTRS1997

The Structure of Active Galactic Nuclei

We are continuing our systematic investigation of the nuclear structure of nearby active galactic nuclei (AGN). Upon completion, our study will characterize hypothetical constructs such as narrow-line clouds, obscuring tori, nuclear gas disks. and central black holes with physical measurements for a complete sample of nearby AGN. The major scientific goals of our program are: (1) the morphology of the NLR; (2) the physical conditions and dynamics of individual clouds in the NLR; (3) the structure and physical conditions of the warm reflecting gas; (4) the structure of the obscuring torus; (5) the population and morphology of nuclear disks/tori in AGN; (6) the physical conditions in nuclear disks; and (7) the masses of central black holes in AGN. We will use the Hubble Space Telescope (HST) to obtain high-resolution images and spatially resolved spectra. Far-UV spectroscopy of emission and absorption in the nuclear regions using HST/FOS and the Hopkins Ultraviolet Telescope (HUT) will help establish physical conditions in the absorbing and emitting gas. By correlating the dynamics and physical conditions of the gas with the morphology revealed through our imaging program, we will be able to examine mechanisms for fueling the central engine and transporting angular momentum. The kinematics of the nuclear gas disks may enable us to measure the mass of the central black hole. Contemporaneous X-ray observations using ASCA will further constrain the ionization structure of any absorbing material. Monitoring of variability in the UV and X-ray absorption will be used to determine the location of the absorbing gas, possibly in the outflowing warm reflecting gas, or the broad-line region, or the atmosphere of the obscuring torus. Supporting ground-based observations in the optical, near-IR, imaging polarimetry, and the radio will complete our picture of the nuclear structures. With a comprehensive survey of these characteristics in a complete sample of nearby AGN, our conclusions should be more reliably extended to AGN as a class.

Kriss, Gerard A.↗
NASA NTRS1997

Simultaneous X-ray and Far-Ultraviolet Spectra of AGN with ASCA and HUT

We obtained ASCA spectra of the Seyfert 1 galaxy NGC 3516 in March 1995. Simultaneous far-UV observations were obtained with the Hopkins Ultraviolet Telescope on the Astro-2 shuttle mission. The ASCA spectrum shows a lightly absorbed power law of energy index 0.78. The low energy absorbing column is significantly less than previously seen. Prominent 0 VII and 0 VIII absorption edges are visible, but, consistent with the much lower total absorbing column, no Fe K absorption edge is detectable. A weak, narrow Fe K(alpha) emission line from cold material is present as well as a broad Fe K(alpha) line. These features are similar to those reported in other Seyfert 1 galaxies. A single warm absorber model provides only an imperfect description of the low energy absorption. In addition to a highly ionized absorber with ionization parameter U = 1.66 and a total column density of 1.4 x 10(exp 22)/sq cm, adding a lower ionization absorber with U = 0.32 and a total column of 6.9 x 10(exp 21)/sq cm significantly improves the fit. The contribution of resonant line scattering to our warm absorber models limits the Doppler parameter to less than 160 km/s at 90% confidence. Turbulence at the sound speed of the photoionized gas provides the best fit. None of the warm absorber models fit to the X-ray spectrum can match the observed equivalent widths of all the UV absorption lines. Accounting for the X-ray and UV absorption simultaneously requires an absorbing region with a broad range of ionization parameters and column densities.

Kriss, Gerard A.↗
NASA NTRS1998

Sounding Rocket

This project has resulted in 4 launches from the White Sands Missile Range of our payload in different stages of development, on: (1) 4 May 1991 Flight 36.062; (2) 16 June 1993 Flight 36.101; (3) 1 April 1995 Flight 36.104; and (5) 28 October 1996 Flight 36.149. For each flight, the following activities were accomplished: (1) prepare the experiment, including replacement and upgrading of critical components, at the University of Michigan; (2) integrate the payload either at the Wallops Flight Facility; (3) perform final far-UV calibration of instrument; (4) perform final alignment, integration, and electrical checks at WSMR (5) launch payload from WSMR (6) check condition of recovered payload (7) perform post-flight calibration, if necessary and applicable; and (8) reduce and analyze flight data. A description of the instrument is presented, along with overviews of the results of the four flights. We are presently preparing the payload for the next flight in 1998/99.

Clarke, John T.↗
NASA NTRS1998

Distinct Secondary Structures of the Leucine-Rich Repeat Proteoglycans Decorin and Biglycan: Glycosylation-Dependent Conformational Stability

Biglycan and decorin, closely related small leucine-rich repeat proteoglycans, have been overexpressed in eukaryotic cers and two major glycoforms isolated under native conditions: a proteoglycan substituted with glycosaminoglycan chains; and a core protein form secreted devoid of glycosaminoglycans. A comparative biophysical study of these glycoforms has revealed that the overall secondary structures of biglycan and decorin are different. Far-UV Circular Dichroism (CD) spectroscopy of decorin and biglycan proteoglycans indicates that, although they are predominantly Beta-sheet, biglycan has a significantly higher content of alpha-helical structure. Decorin proteoglycan and core protein are very similar, whereas the biglycan core protein exhibits closer similarity to the decorin glycoforms than to. the biglycan proteoglycan form. However, enzymatic removal of the chondroitin sulfate chains from biglycan proteoglycan does not induce a shift to the core protein structure, suggesting that the fmal form is influenced by polysaccharide addition only during biosynthesis. Fluorescence emission spectroscopy demonstrated that the single tryptophan residue, which is at a conserved position at the C-terminal domain of both biglycan and decorin, is found in similar microenvironments. This indicates that at least in this specific domain, the different glycoforms do exhibit apparent conservation of structure. Exposure of decorin and biglycan to 10 M urea resulted in an increase in fluorescent intensity, which indicates that the emission from tryptophan in the native state is quenched. Comparison of urea-induced protein unfolding curves provided further evidence that decorin and biglycan assume different structures in solution. Decorin proteoglycan and core protein unfold in a manner similar to a classic two-state model, in which there is a steep transition to an unfolded state between 1-2 M urea. The biglycan core protein also shows a similar steep transition. However, biglycan proteoglycan shows a broad unfolding transition between 1-6 M urea, probably indicating the presence of stable unfolding intermediates.

Krishnan, Priya↗
NASA NTRS1999

HST Observations of New Horizontal Branch Structures in the Globular Cluster w Centauri

The globular cluster omega Centauri contains the largest known population of very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in omega Cen. We find that over 30% of the HB objects are "extreme" HB or hot post-HB stars. A wide gap in the color distribution of the hot HB stars may correspond to gaps found earlier in several other clusters, which argues for a common mass loss mechanism. The diagram contains a significant population of hot sub-HB stars, which we interpret as the "blue-hook" objects predicted by D'Cruz (1996a). These are produced by late He-flashes in stars which have undergone unusually large giant branch mass loss. Omega Cen has a well-known spread of metal abundance, and the diagram is consistent with a giant branch mass loss efficiency which increases with metallicity. There is no evidence for a dynamical origin of the hot HB stars.

Stecher, Theodore↗
NASA NTRS1999

GEOTAIL and POLAR Observations of Auroral Kilometric Radiation and Terrestrial Low Frequency Bursts and their Relationship to Energetic Particles, Auroras, and Other Substorm Phenomena

Terrestrial low frequency (LF) bursts are plasma wave phenomena that appear to be a part of the low frequency end of the auroral kilometric radiation (AKR) spectrum and are observed during strong substorms, GEOTAIL and POLAR plasma wave observations from within the magnetosphere show that the AKR increases in intensity and its lower frequency limits decrease when LF bursts are observed. The first is expected as it is shows substorm onset and the latter indicates that the AKR source region is expanding to higher altitudes. Images from the POLAR VIS Earth Camera operating in the far-UV range and the POLAR UVI experiment usually feature an auroral brightening and an expansion of the aurora to higher latitudes at the time of the LF bursts. Enhanced fluxes of X-rays from precipitating electrons have also been observed by POLAR PIXIE. High resolution ground Abstract: magnetometer data from the CANOPUS and IMAGE networks show that the LF bursts occur when the expansive phase onset signatures are most intense. The ground magnetometer data and the CANOPUS meridian scanning photometer data sometimes show that during the LF burst events the expansive phase onset starts at unusually low latitudes and moves poleward. Large injections of energetic protons and electrons have also been detected by the GOES and LANL geosynchronous satellites during LF burst events. While most of the auroral brightenings and energetic particle injections associated with the LF bursts occur near local midnight, several have been observed as early as mid-afternoon. From these various measurements, we are achieving a better understanding of the plasma and particle motions during substorms that are associated with the generation and propagation of terrestrial LF bursts

Anderson, R . R.↗
NASA NTRS1999

A Reconnaissance of the 900-1200 A Spectra of Early O Stars in the Magellanic Clouds

Far-UV spectrograms of ten very hot stars in the Magellanic Clouds were obtained with the Hopkins Ultraviolet Telescope during the Astro-2 mission in March 1995. Very few normal OB stars were observed during Astro-1, and the only significant prior sample of such objects in this wavelength range was that obtained by Copernicus of stars near the Sun. The Magellanic Cloud sample offers the advantages of low reddening, a range in metallicity, and more extreme temperatures and luminosities than represented in the near solar neighborhood. Several interesting phenomena were found in the HUT MC sample, including very strong O VI wind profiles in O3 spectra; anomalous CNO wind features corresponding to abundances altered by stellar nucleosynthesis; weak wind features due to the metal deficiency of the Small Magellanic Cloud; and interstellar molecular hydrogen features originating in the MCs. The results were presented and discussed as a small atlas, and they were shown at the Pittsburgh AAS meeting in June 1995. A correlative atlas of Copernicus data was also prepared and published under the auspices of this grant. These publications will serve as resources for the characterization of the FUSE observations of related objects to hopefully be obtained soon. I believe that the large differences among the strong wind features in the HUT data, corresponding to CNO and systemic metallicity effects, are amenable to some relative quantitative analysis, and I shall endeavor to promote such in conjunction with HST data at longer wavelengths for the same stars (which has also not yet been completely analyzed) at a future opportunity.

Walborn, Nolan↗
NASA NTRS1997

Translimb Spectroscopy with SOHO/SUMER

We have used SUMER to obtain deep exposures of the 1300-1400 A spectrum, at the extreme limb and off-limb. Previous "translimb" studies in the thermal infrared had revealed remarkable extensions of cold material (T approximately 3000 K)--traced by carbon monoxide emission lines--into the heart of the hot chromosphere. A main objective of our program was to search for corresponding far-UV signatures of the "thermally-bifurcated" low chromosphere; for example, radiatively fluoresced emissions of the CO A-X 4th-positive system (collisional excitation would be negligible in cold gas). We conducted two separate observing programs with SUMER. Both made use of the 1"-diameter circular aperture, translated across the limb in the minimum motor step increments of 0.375", along the central meridian in the Northern polar coronal hole. The first program executed for nine hours beginning 19UT 25 Oct 1996. The full wavelength range was 1340-1400 A. It was recorded in two overlapping segments, placing key regions of the spectrum alternately on the KBr and bare parts of the detector, to help isolate 2nd-order features. Each segment was integrated for 500 s, and 32 pairs were obtained to span a 12" swath centered on the optical limb. The second program was conducted 00-09UT 01 Dec 1996. It consisted of a single wavelength setting (1300-1340 A) with exposure time 500 s, but twice the spatial coverage of the earlier series: 64 steps, for a total displacement of 24". The strong chromospheric resonance lines of atomic oxygen (1302-1305 A) and ionized carbon (1334-1335 A) were observed on the bare part of the MCP camera. We report our progress in cataloging the rich, diverse translimb emission spectrum; and our efforts to deduce fundamental properties of the thermally heterogeneous chromosphere.

Ayer, Thomas R.↗
NASA NTRS1997

CO and the Temperature Structure of the Solar Atmosphere

The surface layers of the Sun provide a crucial boundary condition for many of the processes that occur in the deep interior. The stratification of the outer solar atmosphere once was thought to be well understood. However, studies of thermally sensitive molecular absorptions in the infrared revealed puzzling anomalies. Strong lines of the CO fundamental vibration-rotation bands near 5 microns showed very cool temperatures at the extreme limb, and remarkable off-limb emissions extending well into the supposedly hot chromosphere. The conflicting pictures of the photosphere/chromosphere interface, from the widely separated wavelength regimes, has raised suspicions that those "layers" of the atmosphere are much more inhomogeneous than previously suspected. One proposal is that the low chromosphere is dominated by cool gas--the "COmosphere," if you will--which is threaded by a network of persistent small-scale hot magnetic filaments and occasionally disrupted by localized acoustic disturbances. The COmosphere is capped by the merged fields of the network elements in the chromospheric "canopy." I will describe the evidence in favor of that model, including recent work at the NSO McMath-Pierce telescope (including use of the new "Phoenix" spectrometer) and translimb far-UV spectroscopy by SOHO/SUMER.

Ayres, Thomas R.↗
NASA NTRS1998

Continuation of the University of Michigan Sounding Rocket Program

This interim report gives the activity on this grant through 31 October 1998, and the planned activities for the next fiscal year. A Project Initiation Conference (PIC) was held at the Wallops Flight Facility (WFF), with planning for the launch of flight 36.174 in Fall 1998. However, following the PIC, the WFF personnel decided that the experiment telemetry system had to be completely rewired, which required substantial design and wiring changes on the experiment side of the payload. These costs had not been anticipated in the FY98 budget, and used up considerable resources of the experimenters. In addition, the telemetry re-design was not completed until Fall 1998, so that there was no opportunity for a launch in 1998 (the Jupiter window closes in late November 1998 and it will reopen in late summer 1999). A Design Review was held by telecon with WFF personnel on 26 October 1998, with plans for the 36.174 launch now in late August 1999 with a launch window which extends into winter 2000. On a more positive note, we have been developing a vacuum UV calibration chamber in Michigan's Space Physics Research Lab, for future measurements of our experiment components, and this facility is now undergoing initial measurements of various calibration lamps in preparation for testing of the telescope UV reflective efficiency. This facility will greatly increase our ability to optimize our experiment for UV efficiency, which has been lacking in the past, and it will be made available for support of other NASA missions at Michigan and for visitors from other institutions. For the next flight 36.174, the following activities will be accomplished: (1) prepare the experiment, including replacement and upgrading of critical components, at University of Michigan; (2) pre-integrate the payload at the Wallops Flight Facility; (3) perform final far-UV calibration of instrument (4) perform final alignment, integration, and electrical checks at WSMR; (5) launch payload from WSMR and check condition of recovered payload; (6) perform post-flight calibration, if necessary and applicable (7) reduce and analyze flight data. The following pages contain information from the experimenter package for the Design Review for the next flight, 36.174, listing the plans, changes, and schedule for this flight.

Clarke, John↗
NASA NTRS2000

Solar UV Radiation and the Origin of Life on Earth

We have started a comprehensive, interdisciplinary study of the influence of solar ultraviolet radiation on the atmosphere of of the early Earth. We plan to model the chemistry of the Earth atmosphere during its evolution, using observed UV flux distributions of early solar analogs as boundary conditions in photochemical models of the Earth's atmosphere. The study has four distinct but interlinked parts: (1) Establishing the radiation of the early Sun; (2) Determining the photochemistry of the early Earth's atmosphere; (3) Estimating the rates of H2 loss from the atmosphere; and (4) Ascertaining how sensitive is the photochemistry to the metallicity of the Sun. We are currently using STIS and EUVE to obtain high-quality far-UV and extreme-UV observations of three early-solar analogs. We will perform a detailed non-LTE study of each stars, and construct theoretical model photosphere, and an empirical model chromospheres, which can be used to extrapolate the continuum to the Lyman continuum region. Given a realistic flux distribution of the early Sun, we will perform photochemical modeling of weakly reducing primitive atmospheres to determine the lifetime and photochemistry of CH4. In particular, we will make estimates of the amount of CH4 present in the prebiotic atmosphere, and estimate the atmospheric CH4 concentration during the Late Archean (2.5-3.0 b.y. ago) and determine whether it would have been sufficiently abundant to help offset reduced solar luminosity at that time. Having obtained a photochemical model, we will solve for the concentrations of greenhouse gasses and important pre-biotic molecules, and perform a detailed radiative transfer calculations to compute the UV flux reaching the surface.

Heap, Sara R.↗
NASA NTRS2001

GALSPEC: A Tool for Computing the Spectra of Star-Forming Galaxies of Low Metallicity

The far-ultraviolet spectral region is rich in diagnostics of star formation in galaxies; it gives information not only about the star-forming complex, but also about the dust and circumstellar gas. We have therefore developed a tool, called GALSPEC, for computing far-UV spectra of star-forming galaxies. The user controls the process through a graphical user interface (GUI) by specifying the input parameters for the stars, dust, and gas. GALSPEC computes the integrated spectrum of a star-forming complex, given the rate of star formation, metallicity (0.2 Z(sub \odot) or 0.5 Z(sub \odot)), and IMF upper mass limit. To compute the integrated spectrum, GALSPEC makes use of a library of stellar spectra composed of observed spectra from HST and model spectra calculated with the TLUSTY/SYNSPEC program (Hubeny & Lanz 1995). GALSPEC then applies wavelength-dependent extinction according to Calzetti's starburst extinction law. The mechanical luminosity from massive stars and supernovae sweeps up interstellar material thereby forming an expanding shell with a central cavity. GALSPEC computes the absorption spectrum of the shell using the CLOUDSPEC program (Hubeny 2000), a combination of Ferland's CLOUDY program and SYNSPEC.

Heap, S. R.↗
NASA NTRS2001

STIS Observations of the Low-z Ly-Alpha Forest

The Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope has brought a new era in studying the low-redshift Lyman-alpha Forest. The advantages of STIS over previous HST spectrographs include: (1) a high spectral resolution (7 km/s) comparable to Keck/HIRES spectra of the high-z Lyman-alpha forest; and (2) a high observing efficiency brought about by an imaging detector able to span the full far-UV wavelength range (1150-1700 A, or equivalently, z=0.0-0.4) in a single exposure. The STIS Investigation Definition Team is therefore conducting an observational program to determine the properties of the low-z intergalactic medium as manifested by the Lyman-alpha forest. The team has made observations of the quasars: 3C 273, 3C 351, and PKS 0405. We shall present preliminary results based on the sightline to 3C 273, the brightest of these sources.

Heap, Sara R.↗
NASA NTRS2003

Hot Gas in Planetary Nebulae

It was successfully obtained the FUSE spectra of all targets awarded. The analysis of the spectra has been a complex task due to the superposition of the P-Cygni profile from the wind of the central star and absorption components from low ionization and molecular species in the nebular shell. In six of the eight targets there are narrow O VI absorption components that may arise from the interface layer between the hot (l0(exp 6) K) interior gas and the surrounding warm (l0(exp 4) K) dense nebular shell. To better determine whether these narrow O VI absorption lines arise from the interface region we have obtained ground-based high-dispersion spectroscopic observations of the central star and nebula to pin-point the precise line-of-sight velocity of the nebular emission lines. The comparison between these optical spectra with the far-UV spectra obtained with FUSE is complete. The analysis shows that in most cases the narrow O VI absorption components have velocities slightly redshifted from the emission lines which arise from the approaching side of the nebular shell. Preliminary results have been published in two papers.

Gruendl, Robert A.↗
NASA NTRS2003

High-Velocity Absorption Features in FUSE Spectra of Eta Carinae

Numerous broad (200 to 1000 km/sec) features in the FUSE spectrum (905-1187 A) of eta Carinae are identified as absorption by a forest of high-velocity narrow lines formed in the expanding circumstellar envelope. These features were previously thought to be P-Cygni lines arising in the wind of the central star. The features span a heliocentric velocity range of -140 to -580 km/sec and are seen prominently in low-ionization ground-state transitions (e.g. N I 1134-35, Fe II 1145-42, 1133, 1127- 22, P II 1153, C I 1158) in addition to C III] 1176 A. The high-velocity components of the FUSE transitions have depths about 50% below the continuum. The identifications are consistent with the complex velocity structures seen in ground- and excited-state transitions of Mg I, Mg 11, Fe II, V II, etc observed in STIS/E230H spectra. The origin of other broad features of similar width and depth in the FUSE spectrum, but without low-velocity ISM absorption, are unidentified. However, they are suspected of being absorption of singly-ionized iron-peak elements (e.g. Fe II, V II, Cr II) out of excited levels 1,000 to 20,000 cmE-l above the ground state. The high-velocity features seen in Fe II 1145 are also present in Fe II 1608 (STIS/E140M), but are highly saturated in the latter. Since these transitions have nearly identical log (flambda) (1.998 vs. 2.080), the differences in the profiles are attributable to the different aperture sizes used (30 x 30 arcsec for FUSE, 0.2 x 0.2 arcsec for STIS/E140M). The high-velocity gas appears to be very patchy or has a small covering factor near the central star. Eta Carinae has been observed several times by FUSE over the past three years. The FUSE flux levels and spectral features in eta Car are essentially unchanged over the 2000 March to June 2002 period, establishing a baseline far-UV spectrum in advance of the predicted spectroscopic minimum in 2003.

Sonneborn, G.↗
NASA NTRS2003

Vesta's UV Lightcurve: Hemispheric Variation in Brightness and Spectral Reversal

Spectra of asteroid 4 Vesta obtained in October 1990 with the International Ultraviolet Explorer are reanalyzed and reinterpreted. A large portion of the eastern hemisphere (based on the prime meridian definition of Thomas et al., 1997a) is darker at UV Wavelengths than much of the western hemisphere. The UV lightcurve is in contrast with the visible lightcurve, which shows that the eastern hemisphere is brighter than the western. These IUE spectra of Vesta thus may be evidence for the "spectral reversal." first seen on the Moon by Apollo 17. where the visibly brighter lunar highlands are darker than the maria at far-UV wavelengths. This effect was linked to space weathering when it was noted (Wagner et al., 1987) that the spectral reversal appears in the laboratory spectra of lunar soils but not powdered lunar rocks. We investigate Vesta's UV lightcurve and spectral reversal, and its possible connection with space weathering. The addition to grain coatings of small amounts of submicroscopic iron (SMFe) through vapor deposition causes drastic spectral changes at UV-visible wavelengths (Hapke, 2001). while the longer wavelength spectrum remains largely unaffected. Other laboratory results (e.g., Hiroi and Pieters, 1998) indicate that the UV-visible wavelength range is affected by simulated weathering processes in a manner similar to what is seen on Vesta. It is likely that Vesta has experienced relatively minor amounts of space weathering, as indicated by the spectral reversal, along with the subtle visible-near infrared weathering effects (e.g., Binzel et al., 1997).

Hendrix, Amanda R.↗
NASA NTRS2003

Planetary Nebulae in the MW, LMC, SMC: Results from FUSE and HST data

We use FUSE and HST data to study Planetary Nebulae (PN) systems in the Milky Way, LMC and SMC. Theoretically, metallicity affects several aspects of the post-AGB evolution, including mass-loss and the yield of processed material, which are important factors in the chemical evolution of galaxies. Therefore, it is very important to study PNe in different environments. In Bianchi's FUSE programs, we observed CSPN in the Milky Way (Cycle 1), LMC (Cycle 2) and SMC (Cyde 3), representing a range of metallicities from solar to 1/10th solar. The far-UV range reveals the spectrum of the central star (CSPN), uniquely enabling a direct estimate of the ionizing source parameters. Combined with archive HST data, these spectra provide a measurement of T(sub eff), log g, L(sub bol), abundances, wind velocity and mass-loss rate for these post-AGB stars. Additionally, these spectra provide a measurement of the circumstellar H2 and HI, which added to the mass of the central star and of the ionized shell allows us to test theoretical initial-final mass relations, and to put together a complete picture of the star's evolution.

Herald, J. E.↗
NASA NTRS2004

First Detection of the Hatchett-McCray Effect in the High-Mass X-ray Binary

The orbital modulation of stellar wind UV resonance line profiles as a result of ionization of the wind by the X-ray source has been observed in the high-mass X-ray binary 4U1700-37/HD 153919 for the first time. Far-UV observations (905-1180 Angstrom, resolution 0.05 Angstroms) were made at the four quadrature points of the binary orbit with the Far Ultraviolet Spectroscopic Explorer (FUSE) in 2003 April and August. The O6.5 laf primary eclipses the X-ray source (neutron star or black hole) with a 3.41-day period. Orbital modulation of the UV resonance lines, resulting from X-ray photoionization of the dense stellar wind, the so-called Hatchett-McCray (HM) effect, was predicted for 4U1700-37/HD153919 (Hatchett 8 McCray 1977, ApJ, 211, 522) but was not seen in N V 1240, Si IV 1400, or C IV 1550 in IUE and HST spectra. The FUSE spectra show that the P V 1118-1128 and S IV 1063-1073 P-Cygni lines appear to vary as expected for the HM effect, weakest at phase 0.5 (X-ray source conjunction) and strongest at phase 0.0 (X-ray source eclipse). The phase modulation of the O VI 1032-1037 lines, however, is opposite to P V and S IV, implying that O VI may be a byproduct of the wind's ionization by the X-ray source. Such variations were not observed in N V, Si IV, and C IV because of their high optical depth. Due to their lower cosmic abundance, the P V and S IV wind lines are unsaturated, making them excellent tracers of the ionization conditions in the O star's wind.

Sonneborn, G.↗