Far Ultraviolet Spectroscopic Explorer Observations Research Articles

The search for magnetic fields in two Wolf–Rayet stars and the discovery of a variable magnetic field in WR 55

ABSTRACT Magnetic fields in Wolf–Rayet (WR) stars are not well explored, although there is indirect evidence, e.g. from spectral variability and X-ray emission, that magnetic fields should be present in these stars. Being in an advanced stage of their evolution, WR stars have lost their hydrogen envelope, but their dense winds make the stellar core almost unobservable. To substantiate the expectations on the presence of magnetic fields in the most-evolved massive stars, we selected two WR stars, WR 46 and WR 55, for the search of the presence of magnetic fields using FORS 2 spectropolarimetric observations. We achieve a formally definite detection of a variable mean longitudinal magnetic field of the order of a few hundred gauss in WR 55. The field detection in this star, which is associated with the ring nebula RCW 78 and the molecular environment, is of exceptional importance for our understanding of star formation. No field detection at a significance level of 3σ was achieved for WR 46, but the variability of the measured field strengths can be rather well phased with the rotation period of 15.5 h previously suggested by FUSE(Far Ultraviolet Spectroscopic Explorer) observations.

Analysis of far-UV data of central stars of planetary nebulae: Occurrence and variability of stellar winds

The occurrence of stellar wind in the central star of a planetary nebula (CSPN) can be revealed by the presence of P Cygni profiles of high-excitation lines overimposed on its stellar continuum. We examined the entire Far-Ultraviolet Spectroscopic Explorer (FUSE) archive and merged all useful spectroscopic observations of CSPNe to produce the highest quality spectra that can be used to assess the frequency of stellar winds. Furthermore, the individual spectra of each CSPN were compared to search for variability in the P Cygni profile. P Cygni profiles of high-excitation lines have been found in 44 CSPNe, with a clear correlation between the ionization potential of the lines and the effective temperature of the star. We introduce a prescription to derive the terminal wind velocity (v∞) from saturated and unsaturated P Cygni profiles and provide new values of v∞ for these stars. Another 23 CSPNe do not show P Cygni profiles, or else their data in the FUSE archive are not conclusive enough to determine the occurrence of P Cygni profiles. Variability in the P Cygni profile of high-excitation, far-UV lines is found for the first time in six CSPNe, namely Hen 2-131, NGC 40, NGC 1535, NGC 2392, Sp 3, and SwSt 1. This increases up to 13 the number of CSPNe with variable P Cygni profiles in the UV, including those previously reported using IUE or FUSE observations. Variability is seen primarily in the unsaturated P v and Si iv lines, but also in saturated C iii and O vi lines. The CSPNe with variable P Cygni profiles have similar stellar properties (relatively low log (g) and Teff) that suggest they are less evolved CSPNe. Some of the CSPNe with variable P Cygni profiles show O vi lines, while their effective temperature is insufficient to produce this ion. We suggest that this ion is produced by Auger ionization from X-rays associated to shocks in their stellar winds, as is the case in massive OB stars of high ionization potential ions that cannot be abundantly produced by photoionizations.

Multiwavelength campaign on Mrk 509

We present medium-resolution (λ/Δλ ~ 20 000) ultraviolet spectra covering the 1155−1760 Å spectral range of the Seyfert 1 galaxy Mrk 509 obtained using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). Our observations were obtained simultaneously with a Low Energy Transmission Grating Spectrometer observation using the Chandra X-ray Observatory, and they are part of a multiwavelength campaign in September through December 2009 which also included observations with XMM-Newton, Swift, and INTEGRAL. Our spectra are the highest signal-to-noise observations to date of the intrinsic absorption components seen in numerous prior ultraviolet observations. To take advantage of the high S/N, we describe special calibrations for wavelength, flat-field and line-spread function corrections that we applied to the COS data. We detect additional complexity in the absorption troughs compared to prior observations made with the Space Telescope Imaging Spectrograph (STIS) on HST. We attribute the UV absorption to a variety of sources in Mrk 509, including an outflow from the active nucleus, the interstellar medium and halo of the host galaxy, and possible infalling clouds or stripped gaseous material from a merger that are illuminated by the ionizing radiation of the active nucleus. Variability between the STIS and COS observation of the most blue-shifted component (#1) allows us to set an upper limit on its distance of <250 pc. Similarly, variability of component 6 between FUSE observations limits its distance to <1.5 kpc. The absorption lines in all components only partially cover the emission from the active nucleus with covering fractions that are lower than those seen in the prior STIS observations and are comparable to those seen in spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE). Given the larger apertures of COS and FUSE compared to STIS, we favor scattered light from an extended region near the active nucleus as the explanation for the partial covering. As observed in prior X-ray and UV spectra, the UV absorption has velocities comparable to the X-ray absorption,but the bulk of the ultraviolet absorption is in a lower ionization state with lower total column density than the gas responsible for the X-ray absorption. We conclude that the outflow from the active nucleus is a multiphase wind.

Characteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra

Ultraviolet (UV) spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been analyzed. Comparisons between the observed spectra and synthetic models of electron-excited H 2 have been used to determine various auroral characteristics. Far ultraviolet (FUV: 1200–1700 Å) STIS and UVIS spectra exhibit, below 1400 Å, weak absorption due to methane, with a vertical column ranging between 1.4 × 10 15 and 1.2 × 10 16 cm − 2 . Using the low-latitude Moses et al. [Moses, J.I., Bézard, B., Lellouch, E., Feuchtgruber, H., Gladstone, G.R., Allen, M., 2000. Icarus, 143, 244–298] atmospheric model of Saturn and an electron energy–H 2 column relationship, these methane columns are converted into the mean energy of the primary precipitating electrons, estimated to lie in the range 10–18 keV. This result is confirmed by the study of self-absorption with UVIS and FUSE extreme ultraviolet (EUV: 900–1200 Å) spectra. Below 1200 Å, it is seen that transitions connecting to the v ″ < 2 vibrational levels of the H 2 electronic ground state are partially self-absorbed by H 2 molecules overlying the auroral emission. Because of its low spectral resolution (∼5.5 Å), the UVIS EUV spectrum we analyzed does not allow us to unequivocally determine reasonable ranges of temperatures and H 2 columns. On the other hand, the high spectral resolution (∼0.2 Å) of the FUSE LiF1a and LiF2a EUV spectra we examined resolve the H 2 rotational lines and makes it possible to determine the H 2 temperature. The modeled spectrum best fitting the FUSE LiF1a observation reveals a temperature of 500 K and self-absorption by a H 2 vertical column of 3 × 10 19 cm − 2 . When converted to energy of precipitating electrons, this H 2 column corresponds to primary electrons of ∼10 keV. The model that best fits the LiF2a spectrum is characterized by a temperature of 400 K and is not self-absorbed, making this segment ideal to determine the H 2 temperature at the altitude of the auroral emission. The latter value is in agreement with temperatures obtained from H 3 + infrared polar spectra. Self-absorption is detectable in the LiF2a segment for H 2 columns exceeding 6 × 10 19 cm − 2 , which sets the maximum mean energy determined from the FUSE observations to ∼15 keV. The total electron energy range of 10–18 keV deduced from FUV and EUV observations places the auroral emission peak between the 0.1 and 0.3 μbar pressure levels. These values should be seen as an upper limit, since most of the Voyager UVS spectra of Saturn's aurora examined by Sandel et al. [Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R., 1982. Science 215, 548] do not exhibit methane absorption. The auroral H 2 emission is thus likely located above but close to the methane homopause. The H 2 auroral brightness in the 800–1700 Å bandwidth varies from 2.9 kR to 139 kR, comparable to values derived from FUV Faint Object Camera (FOC) and STIS images.

An ExtendedFUSESurvey of Diffuse OviEmission in the Interstellar Medium

We present a survey of diffuse O VI emission in the interstellar medium (ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). Spanning 5.5 yr of FUSE observations, from launch through 2004 December, our data set consists of 2925 exposures along 183 sight lines, including all of those with previously published O VI detections. The data were processed using an implementation of CalFUSE version 3.1 modified to optimize the signal-to-noise ratio and velocity scale of spectra from an aperture-filling source. Of our 183 sight lines, 73 show O VI λ1032 emission, 29 at >3 σ significance. Six of the 3 σ features have velocities |vLSR| > 120 km s-1, while the others have |vLSR| ≤ 50 km s-1. Measured intensities range from 1800 to 9100 LU (line unit; 1 photon cm-2 s-1 sr-1), with a median of 3300 LU. Combining our results with published O VI absorption data, we find that an O VI-bearing interface in the local ISM yields an electron density ne = 0.2-0.3 cm-3 and a path length of 0.1 pc, while O VI-emitting regions associated with high-velocity clouds in the Galactic halo have densities an order of magnitude lower and path lengths 2 orders of magnitude longer. Although the O VI intensities along these sight lines are similar, the emission is produced by gas with very different properties.

Far‐Ultraviolet and X‐Ray Observations of VV 114: Feedback in a Local Analog to Lyman Break Galaxies

We have analyzed Far Ultraviolet Spectroscopic Explorer (FUSE), XMM-Newton, and Chandra observations of VV 114, a local galaxy merger with strong similarities to typical high-redshift Lyman break galaxies (LBGs). Diffuse thermal X-ray emission encompassing VV 114 has been observed by Chandra and XMM-Newton. This region of hot (kT ~ 0.59 keV) gas has an enhanced α/Fe element ratio relative to solar abundances and follows the same relations as typical starbursts between its properties (luminosity, size, and temperature) and those of the starburst galaxy (star formation rate, dust temperature, and galaxy mass). These results are consistent with the X-ray gas having been produced by shocks driven by a galactic superwind. The FUSE observations of VV 114 show strong, broad interstellar absorption lines with a pronounced blueshifted component (similar to what is seen in LBGs). This implies an outflow of material moving at ~300-400 km s-1 relative to VV 114. The properties of the strong O VI absorption line are consistent with radiative cooling at the interface between the hot outrushing gas seen in X-rays and the cooler material seen in the other outflowing ions in the FUSE data. We show that the wind in VV 114 has not created a tunnel that enables more than a small fraction (<few percent) of the ionizing photons from VV 114 to escape into the intergalactic medium (IGM). Taken together, these data provide a more complete physical basis for understanding the outflows that seem to be generic in LBGs. This will lead to improved insight into the role that such outflows play in the evolution of galaxies and the IGM.

Far‐Ultraviolet and X‐Ray Observations of the Reverse Shock in the Small Magellanic Cloud Supernova Remnant 1E 0102.2−7219

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and X-Ray Multi-Mirror Mission (XMM-Newton) data for the reverse shock of the O-rich supernova remnant (SNR) 1E 0102.2-7219 in the Small Magellanic Cloud (SMC). The FUSE observations cover three regions with significantly different optical [O III] intensities, all associated with the relatively bright part of the X-ray ring. Emission lines of O VI λλ1032, 1038 are clearly detected in the FUSE spectra. The XMM-Newton EPIC MOS 1/2 spectra are dominated by strong emission lines of O, Ne, and Mg. By combining the O VI doublet emission with the O VII triplet and O VIII Lyα fluxes from the X-ray spectra and assuming a nonequilibrium ionization (NEI) model with a single ionization timescale for the spectra, we find an increase of the ionization timescale τ from north (τ ≈ 0.6 × 1011 s cm-3) to southeast (τ ≈ 2 × 1011 s cm-3). This is indicative of increasing density in the X-ray-bright ring, in good agreement with the optical [O III] emission, which is strongest in the southeast. However, if we assume a plane-parallel shock model with a distribution of ionization timescales, the O VI emission appears to be inconsistent with O VII and O VIII in X-rays. The analysis of the total X-ray spectra shows that there is no consistent set of values for the temperature and ionization timescale that can explain the observed line ratios for O, Ne, and Mg. This would be consistent with a structured distribution of the ejecta, as the O, Ne, and Mg would have interacted with the reverse shock at different times.

FUSE observations of molecular hydrogen on the line of sight towards HD 141569A

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of HD 141569A, a transitional object known to possess a circumstellar disk. We observe two components of gas at widely different temperatures along the line of sight. We detect cold H 2 , which is thermalized up to J = 2 at a kinetic temperature of ∼51 K. Such low temperatures are typical of the diffuse interstellar medium. Since the line of sight to HD 141569A does not pass through its disk. it appears that we are observing the cold H 2 in a low extinction envelope associated with the high Galactic latitude dark cloud complex L134N, which is in the same direction and at nearly the same distance as HD 141569A. The column densities of the higher J-levels of H 2 suggest the presence of warm gas along the line of sight. The excitation conditions do not seem to he consistent with what is generally observed in diffuse interstellar clouds. The observed radial velocity of the gas implies that the UV spectral lines we observe are likely interstellar in origin rather than circumstellar, although our absorption line study does not definitely rule out the possibility that the warm gas is close to the star. The discovery of such warm gas along the line of sight may provide evidence for turbulent phenomena in the dark cloud L134N.

Far Ultraviolet Spectroscopic ExplorerSpectroscopy of Absorption and Emission Lines from the Narrow-Line Seyfert 1 Galaxy NGC 4051

We present three Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the Narrow-Line Seyfert 1 galaxy NGC4051. The most prominent features in the far-ultraviolet (FUV) spectrum are the OVI emission and absorption lines and the HI Lyman series absorption lines which are detected up to the Lyman edge. We also identify weak emission from NIII, CIII, and HeII. The CIII line shows absorption while none is detected in the NIII and HeII lines. In HI and CIII we detect two main absorption systems at outflow velocities of -50+/-30 and -240+/-40 km/s, as well as a possible third one at ~ -450 km/s. These systems are consistent in velocity with the 10 absorption systems found previously in CIV, NV, and SiIV, though the individual systems are blended together in the FUV spectrum. We estimate column densities of the two main absorption systems and find that the HI column density is lower for systems with larger outflow velocity. We detect no flux or spectral variations of NGC4051 at FUV wavelengths during three epochs spanning one year. This is consistent with the optical light curve which shows no variations between the three epochs. It is also consistent with the X-ray light curve which shows consistent flux levels at the three epochs of the FUSE observations, although the X-ray light curve shows strong variations on much shorter timescales.

Intrinsic Absorption in the Spectrum of Markarian 279: Simultaneous Chandra , FUSE , and STIS Observations

We present a study of the intrinsic X-ray and far-ultraviolet absorption in the Seyfert 1.5 galaxy Markarian 279 using simultaneous observations from the Chandra X-ray Observatory, the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope, and the Far Ultraviolet Spectroscopic Explorer (FUSE). We also present FUSE observations made at three additional epochs. We detect the Fe K? emission line in the Chandra spectrum, and its flux is consistent with the low X-ray continuum flux level of Mrk?279 at the time of the observation. Because of low signal-to-noise ratios (S/N) in the Chandra spectrum, no O?VII or O?VIII absorption features are observable in the Chandra data, but the UV spectra reveal strong and complex absorption from H?I and high-ionization species such as O?VI, N?V, and C?IV, as well as from low-ionization species such as C?III, N?III, C?II, and N?II in some velocity components. The far-UV spectral coverage of the FUSE data provides information on high-order Lyman series absorption, which we use to calculate the optical depths and line and continuum covering fractions in the intrinsic H?I absorbing gas in a self-consistent fashion. The UV continuum flux of Mrk?279 decreases by a factor of ~7.5 over the time spanning these observations and we discuss the implications of the response of the absorption features to this change. From arguments based on the velocities, profile shapes, covering fractions and variability of the UV absorption, we conclude that some of the absorption components, particularly those showing prominent low-ionization lines, are likely associated with the host galaxy of Mrk?279, and possibly with its interaction with a close companion galaxy, while the remainder arises in a nuclear outflow.

Searching for the Intragroup Medium in Loose Groups of Galaxies

We have conducted a study with the Very Large Array (VLA), the Dominion Radio Astrophysical Observatory Synthesis Telescope, and the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to search for the intragroup medium in two loose groups of galaxies: GH 144 and GH 158. The VLA observations provide a census of the dense H I content of these groups in the form of individual galaxies and free-floating H I clouds as traced by the 21 cm H I line, while the FUSE observations trace the diffuse neutral and hot ionized gas that may fill the intragroup medium, populate the halos of individual galaxies, or reside in a skin around denser, neutral clouds. While nothing was detected in GH 158, in GH 144 we detected two previously unknown H I–rich low surface brightness group galaxies. In addition, Lyα, Lyβ, C III, and N V were detected toward GH 144. Using this suite of data, we were able to place limits on the mass of various portions of this group. If virialized, GH 144 has a mass of 2 × 1012 M⊙. Of that mass, 8% lies in the individual cataloged galaxies, and no more than that same fraction again could lie in the dense, neutral medium as constrained by our VLA observations. The absorption lines imply a diffuse gas with a volume density greater than 10-5.2 cm-3 from a layer less than 22 kpc thick, assuming a metallicity of 0.4 Z⊙. While the extent of this gas is uncertain, it seems unlikely that this diffuse gas contributes a significant fraction of the group mass. Given the depth of the absorbing material and its separation from the nearest galaxies, it seems most likely that it originates from a small clump in the intragroup medium; perhaps an ionized high-velocity cloud, but it may be associated with one of our new H I detections. This was our ambitious first attempt to search for the intergalactic medium in emission and absorption, and while it was only partially successful, we show what is possible and what more is needed for its success.

Far Ultraviolet Spectroscopic ExplorerSpectra of the Black Hole Binary LMC X-3

Far-ultraviolet spectra of LMC X-3 were taken covering photometric phases 0.47?0.74 in the 1.7 day orbital period of the black hole binary (phase zero being superior conjunction of the X-ray source). The continuum is faint and flat but appears to vary significantly during the observations. Concurrent RXTE/ASM observations show the system was in its most luminous X-ray state during the Far Ultraviolet Spectroscopic Explorer (FUSE) observations. The far-UV spectrum contains strong terrestrial airglow emission lines, while the only stellar lines clearly present are emissions from the O VI resonance doublet. Their flux does not change significantly during the FUSE observations. These lines are modeled as two asymmetric profiles, including the local ISM absorptions due to C II and possibly O VI. Velocity variations of O VI emission are consistent with the orbital velocity of the black hole and provide a new constraint on its mass.

A comparison of DA white dwarf temperatures and gravities from FUSE Lyman line and ground-based Balmer line observations

The observation of the strengths and profiles of the hydrogen Balmer absorption series is an established technique for determining the effective temperature and surface gravity of hot H-rich white dwarf stars. In principle, the Lyman series lines should be equally useful but, lying in the far-ultraviolet (FUV), are only accessible from space. Nevertheless, there are situations (for example, where the optical white dwarf spectrum is highly contaminated by the presence of a companion) in which use of the Lyman series may be essential. Therefore, it is important to establish whether or not the Lyman lines provide an equally valid means of measurement. We have already made a first attempt to study this problem, comparing Lyman line measurements from a variety of FUV instruments with ground-based Balmer line studies. Within the measurement uncertainties, we found the results from each line series to be broadly in agreement. However, we noted a number of potential systematic effects that could bias either measurement. With the availability of the Far Ultraviolet Spectroscopic Explorer (FUSE) data archive and observations from our own Guest Observer programmes, we now have an opportunity to examine the use of the Lyman series in more detail from observations of 16 DA white dwarfs. Here we have data produced by a single instrument and processed with a uniform data reduction pipeline, eliminating some of the possible systematic differences between observations of the same or different stars. We have also examined the scatter in values derived from multiple observations of the same star, which is significant. The new results partially reproduce the earlier study, showing that Balmer and Lyman line determined temperatures are in good agreement up to ∼50 000 K. However, above this value there is an increasing systematic difference between the Lyman and Balmer line results, the former yielding the higher temperature. At the moment, there is no clear explanation of this effect but we think that it is most likely associated with deficiencies in the detailed physics incorporated into the stellar model atmosphere calculations. Even so, the data do demonstrate that, for temperatures below 50 000 K, the Lyman lines give reliable results. Furthermore, for the hotter stars, a useful empirical calibration of the relationship between the Lyman and Balmer measurements has been obtained, which can be applied to other FUSE observations.

FUSE and HST ultraviolet observations of the disc wind of RWSextantis

We present Far Ultraviolet Spectroscopic Explorer (FUSE) satellite time-series data of the low-inclination, nova-like cataclysmic variable RW Sex. The data span the wavelength range between ∼905 and 1188 A, which contains the resonance lines of C iii, N iii, S iv, P v, S vi and O vi. All these spectral lines are present in the form of blueshifted absorption components, with no evidence for the lines redward of rest velocity; the FUSE data probe the fast disc-wind of RW Sex. The time-variable nature of these features is established. Substantial line profile variability is observed, confined between ∼−1000 and 0 km s−1 , and modulated on the orbital period of the system (∼0.245 d) . The fundamental characteristics of the temporal behaviour are very similar between the low and high ionization lines. The wind-dominated lines exhibit a cyclic increase in absorption optical depth which is accompanied by a shift in the maximum absorption velocity from ∼−1000 km s−1 to near zero velocity. We argue that the empirical evidence from the FUSE analysis supports an interpretation of blueshifted absorptive changes as opposed to the action of a variable (in velocity) blueshifted (perhaps disc-formed) emission component. This conclusion is further bolstered by an analysis of archival Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) data of RW Sex. We discuss a phenomenological picture of the ultraviolet (UV) line profile variability where the symmetry of a confined bipolar wind in RW Sex is broken by assuming that the outflow is oblique, perhaps because it is seated on a warped or tilted disc. The temporal UV absorption properties of non-eclipsing nova-like systems are compared.

The surprising Far-UV spectrum of the polar BY Camelopardalis

We report on the first far-UV observations of the asynchronous polar BY Cam made by the Far-Ultraviolet Spectroscopic Explorer (FUSE) . The source is known to exhibit the most extreme NV/CIV emission resonance line ratio ob- served among polars. The FUSE observations reveal a OVI resonance line weaker than in the prototype of polars, AM Her, with the absence of a detectable narrow component. The OVI broad line is detected with an equivalent width of the same order as in AM Her, the blueward doublet component is clearly present but the redward component is strongly aected by H2 absorption. The presence of a strong NIII line and weak CIII lines also confirms the peculiar CNO line flux. We compare the resonance CNO line intensities with the predictions of the CLOUDY plasma code coupled to a geometrical model of the accretion col- umn. Varying the temperature and/or intensity of the ionising spectrum is unable to reproduce the observed broad line ratios. A solution is obtained by significantly altering the element abundances with a strong depletion of C, overabundance of N and a weak underabundance of O. This confirms previous suggestions of non-solar abundances which may result from redistribution in the accreted material following nova outbursts and/or the secondary nuclear evolution. A very significant H 2 absorption is observed in front of the source, a possible indication for either the existence of a dense interstellar cloud or of circumstellar material.

Ionized Gas in the First 10 Kiloparsecs of the Interstellar Galactic Halo

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the post-asymptotic giant branch star von Zeipel 1128 (l = 425, b = +787; d = 10.2 kpc; z = 10.0 kpc; vLSR = -140 ± 8 km s-1), located in the globular cluster Messier 3. The FUSE observations cover the wavelength range 905-1187 Å at ~20 km s-1 (FWHM) resolution. These data exhibit many photospheric and interstellar absorption lines, including absorption from ions associated with the warm neutral, warm ionized, and highly ionized phases of the interstellar medium along this sight line. We derive interstellar column densities of H I, P II, Ar I, Fe II, Fe III, S III, and O VI, with lower limits for C II, C III, N I, O I, and Si II. Though the individual velocity components within the absorption profiles are unresolved by FUSE, a comparison of the velocity distribution of depleted or ionized species with the neutral species suggests that the thick-disk material along this sight line is infalling onto the Galactic plane, while material near the plane is seen closer to rest velocities. Ionized hydrogen represents ≳12%, most likely ~45%, of the total hydrogen column along this sight line, most of it associated with the warm ionized phase. The warm ionized and neutral media toward von Zeipel 1128 have very similar gas-phase abundances and kinematics: the neutral and ionized gases in this region of the thick disk are closely related. Strong O VI absorption is seen with the same central velocity as the warm ionized gas, although the O VI velocity dispersion is much higher (σ ≡ b = 32 km s-1). Virtually all of the O VI is found at velocities where lower ionization gas is seen, suggesting that the O VI and WNM/WIM probes are tracing different portions of the same structures (e.g., the O VI may reside in interfaces surrounding the WNM/WIM clouds). We see no evidence for interstellar absorption associated with the globular cluster Messier 3 itself or with the circumstellar environment of von Zeipel 1128. Neither high-velocity cloud absorption (with |vLSR|≳125 km s-1) nor high velocity dispersion gas (with σ ~ 60 km s-1) is seen toward von Zeipel 1128.

AFar Ultraviolet Spectroscopic ExplorerSurvey of Coronal Forbidden Lines in Late‐Type Stars

We present a survey of coronal forbidden lines detected in Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of nearby stars. Two strong coronal features, Fe XVIII λ974 and Fe XIX λ1118, are observed in 10 of the 26 stars in our sample. Various other coronal forbidden lines, observed in solar flares, also were sought but not detected. The Fe XVIII feature, formed at log T = 6.8 K, appears to be free of blends, whereas the Fe XIX line can be corrupted by a C I multiplet. FUSE observations of these forbidden iron lines at spectral resolution λ/Δλ ~ 15,000 provides the opportunity to study dynamics of hot coronal plasmas. We find that the velocity centroid of the Fe XVIII feature deviates little from the stellar rest frame, confirming that the hot coronal plasma is confined. The observed line widths generally are consistent with thermal broadening at the high temperatures of formation and show little indication of additional turbulent broadening. The fastest rotating stars, 31 Com, α Aur Ab, and AB Dor, show evidence for excess broadening beyond the thermal component and the photospheric v sin i. The anomalously large widths in these fast-rotating targets may be evidence for enhanced rotational broadening, consistent with emission from coronal regions extending an additional ΔR ~ 0.4-1.3 R* above the stellar photosphere, or represent the turbulent broadening caused by flows along magnetic loop structures. For the stars in which Fe XVIII is detected, there is an excellent correlation between the observed Rontgensatellit (ROSAT) 0.2-2.0 keV soft X-ray flux and the coronal forbidden line flux. As a result, Fe XVIII is a powerful new diagnostic of coronal thermal conditions and dynamics that can be utilized to study high-temperature plasma processes in late-type stars. In particular, FUSE provides the opportunity to obtain observations of important transition region lines in the far-UV, as well as simultaneous measurements of soft X-ray coronal emission, using the Fe XVIII coronal forbidden line.

The Ionized Gas and Nuclear Environment in NGC 3783. II. AveragedHubble Space Telescope/STIS andFar Ultraviolet Spectroscopic ExplorerSpectra

We present observations of the intrinsic absorption in the Seyfert 1 galaxy NGC 3783 obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer (FUSE). We have combined 18 STIS and five FUSE observations to obtain a high signal-to-noise ratio averaged spectrum spanning 905-1730 A. The averaged spectrum reveals absorption in O VI, N V, C IV, N III, C III, and the Lyman lines up to Ly in the three blueshifted kinematic components previously detected in the STIS spectrum (at radial velocities of -1320, -724, and -548 km s-1). The highest velocity component exhibits absorption in Si IV. We also detect metastable C III in this component, indicating a high density in this absorber. No lower ionization lines, i.e., C II and Si II, are detected. A weak, fourth absorption component is tentatively detected in the high-ionization lines and Lyα and Lyβ at a radial velocity of -1027 km s-1. The Lyman lines reveal a complex absorption geometry. The strength of the higher order lines indicates that Lyα and Lyβ are saturated over much of the resolved profiles in the three strongest absorption components and that, therefore, their observed profiles are determined by the covering factor. We separate the individual covering factors of the continuum and emission-line sources as a function of velocity in each kinematic component using the Lyα and Lyβ lines. The covering factor of the broad-line region is found to vary dramatically between the cores of the individual kinematic components, ranging from 0 to 0.84. In addition, we find that the continuum covering factor varies with velocity within the individual kinematic components, decreasing smoothly in the wings of the absorption by at least 60%. Comparison of the effective covering factors derived from the H I results with those determined directly from the doublets reveals that the covering factor of Si IV is less than half that of H I and N V in the high-velocity component. In addition, the FWHMs of N III and Si IV are narrower than those of the higher ionization lines in this component. These results indicate there is substructure within this absorber. We also find evidence for structure in the column density profiles of the high-ionization lines in this component. We derive a lower limit on the total column (NH ≥ 1019 cm-2) and ionization parameter (U ≥ 0.005) in the low-ionization subcomponent of this absorber. The metastable-to-total C III column density ratio implies ne ≈ 109 cm-3 and an upper limit on the distance of the absorber from the ionizing continuum of R ≤ 8 × 1017 cm. The decreasing covering factor found in the wings of the absorption and the extreme compactness of the C III* absorber are suggestive of a clumpy absorption gas with a low volume filling factor.

Far-Ultraviolet Spectra of Planetary Nebulae

The Far Ultraviolet Spectroscopic Explorer (FUSE)&gt; satellite provides a unique opportunity to obtain high-resolution far-UV spectra of a wide variety of astronomical objects, including planetary nebulae. Most FUSE observations of PNe to date have concentrated on the hot central star, providing a very effective way to study the atmosphere of the central star, the surrounding nebula through the absorption features from circumstellar gas. FUSE has found evidence of hot molecular hydrogen in several planetary nebulae, including M27 and BD+30° 3639. Central star spectra also reveal new information about stellar winds, mass loss, and photospheric abundances.

FUSE Observations of Mass Loss in Planetary Nebulae

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite observed several high and low excitation planetary nebulae in the wavelength range 905–1187 Å at a spectral resolution of λ/Δλ ~ 15,000.