Far-ultraviolet Spectra Research Articles

Far-Ultraviolet Spectra of n-Alkanes and Branched Alkanes in the Liquid Phase Observed Using an Attenuated Total Reflection—Far Ultraviolet (ATR-FUV) Spectrometer

Far-ultraviolet (FUV) spectra of n-alkanes ( n = 5–14) and branched alkanes were measured in their liquid state by using a newly developed attenuated total reflection (ATR)-FUV spectrometer to investigate spectra–structure relationship in the FUV region. The n-alkanes show a broad band near 8.3 eV and a weak shoulder near 7.7 eV. The 8.3 eV band shows a lower energy shift with a significant intensity increase with the increase in the length of alkyl chain. We have assigned the 8.3 eV band to the overlap of two bands due to the transition from the highest occupied molecular orbital (HOMO) to 3p and that from the HOMO-1 to 3s based on the observation that the peak energy of the 8.3 eV band of the n-alkanes is proportional to the first ionization energy. The 7.7 eV shoulder may be due to the transition from HOMO to 3s. The intensity of the 7.7 eV band increases markedly in the order of alkanes without branch, with tertiary, and with quaternary carbon atoms. It is very likely that the forbidden transition from HOMO to 3s becomes allowed by the large decrease in symmetry upon going from the n-alkanes to the branched ones with the quaternary carbon, respectively.

Low-n Rydberg Transitions of Liquid Ketones Studied by Attenuated Total Reflection Far-Ultraviolet Spectroscopy

Far-ultraviolet (FUV) spectra in the 8.55-6.20 eV (145-200 nm) region were measured for several kinds of ketones in the liquid phase to investigate low-n Rydberg transitions using a uniquely developed technique of attenuated total reflection (ATR) FUV spectrometry. Assignments of the transitions are attempted for absorptions in this region by comparing the spectra for the liquid phase with those for the gas phase and ab initio calculations at the equation-of-motion coupled cluster theory with single and double substitutions at the aug-cc-pVDZ level. The transition from a nonbonding electron (n) to the 3s Rydberg orbital was found at around 6.7 eV for all investigated liquid ketones. Another intense band also appeared in the higher-energy region (ca. 8.5 eV) for all the ketones. A significant shoulder was found at around 7.4 eV for branched ketones. This shoulder band near 7.4 eV was assigned to the n-3p Rydberg transition. Band broadening and higher energy shifts were observed in the spectra of the liquid phase ketones in comparison with those of the gas phase ketones.

Spectral observations of FUV auroral arcs and comparison with inverted‐V precipitating electrons

This paper presents first simultaneous observations of far ultraviolet (FUV) spectra of discrete and diffuse auroras, together with precipitating electrons measured on the same spacecraft, to emphasize the importance of high‐resolution FUV images for accurate estimation of precipitating energy flux in the auroral region. An FUV spectrograph image with ∼2 km × 3 km resolution show small‐scale features were embedded in the auroral arcs. Comparison of peak energies of inverted‐V events with the corresponding FUV spectra shows that the observed long band N2 Lyman‐Birge‐Hopfield (LBH) emission (long LBH band (LBHL): 160.0–171.5 nm) varied more sensitively to the peak energies compared to the short band. Comparison of the inverted‐V structures and their energy fluxes with the LBHL irradiance for ∼10 km × 10 km regions show they are well correlated for peak energy >2 keV. When the data are averaged over a larger area (70 km × ∼140 km), on the other hand, the LBHL irradiance becomes less bright for the corresponding electron energy flux due to the contribution from the low‐intensity background diffuse aurora produced by secondary electrons. This study demonstrates a reliable relationship between precipitating electron energy flux and LBHL intensity is obtained only if the precipitating region and FUV intensity are locally matched with a scale of less than 10 km corresponding to the size of discrete auroras.

HUBBLE/COS OBSERVATIONS OF THE Lyα FOREST TOWARD THE BL Lac OBJECT 1ES 1553+113

We present new far-ultraviolet spectra from the Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES1553+113 covering the wavelength range 1135-1795 A. The data show a smooth continuum with a wealth of narrow absorption features arising in the ISM and IGM. These features include 41 Lya absorbers at 0<z<0.43, fourteen of which are detected in multiple Lyman lines and six in one or more metal lines. We analyze a metal-rich triplet of Lya absorbers at z=0.188 in which OVI, NV, and CIII absorption is detected. Silicon ions (SiIII/IV) are not detected to fairly strong upper limits, and we use the measured SiIII/CIII upper limit to derive an abundance limit [C/Si]>0.6 for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and gamma-ray sources, 1ES1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES1553+113 has no direct redshift determination. We use intervening Lya absorbers to place a direct limit on the redshift: z_em>0.395 based on a confirmed Lya+OVI absorber and z_em>0.433 based on a single-line detection of Lya. COS/FUV data are only sensitive to Lya absorbers at z<0.47, but we present statistical arguments that z_em<0.58 based on the non-detection of any Lyb absorbers at z>0.4.

SPECTROSCOPIC ANALYSIS OF HOT, HYDROGEN-RICH WHITE DWARFS: THE PRESENCE OF METALS AND THE BALMER-LINE PROBLEM

We present an analysis of optical spectra for 29 DAO white dwarfs. First, we present our new up-to-date model atmosphere grids computed without the assumption of local thermodynamic equilibrium in which we have included carbon, nitrogen, and oxygen at solar abundances. We demonstrate that the addition of these metals in the model atmospheres is essential in overcoming the Balmer-line problem, which manifests itself as an inability to fit all the Balmer lines simultaneously with consistent atmospheric parameters. We then present the spectroscopic analysis of our sample of DAO white dwarfs for which we determine the effective temperature, surface gravity, and helium abundance. We also present 18 hot DA white dwarfs that also suffer from the Balmer-line problem. We analyze these stars with models analogous to those for the DAO white dwarfs save for the presence of helium. Systematic differences between our newly determined atmospheric parameters with respect to previous determinations are explored. Far-ultraviolet spectra from the FUSE archive are then examined to demonstrate that there exists a correlation between higher metallic abundances and instances of the Balmer-line problem. The implications of these findings for all hot, hydrogen-rich white dwarfs are discussed. Specifically, the possible evolutionary scenario for DAO white dwarfs is revised and post-extreme horizontal branch evolution is no longer needed to explain the evolution for the majority of the DAO stars. Finally, we discuss how the presence of metals might drive a weak stellar wind which in turn could explain the presence of helium in DAO white dwarfs.

SEARCHING FOR FAR-ULTRAVIOLET AURORAL/DAYGLOW EMISSION FROM HD 209458b

We present recent observations from the Hubble Space Telescope-Cosmic Origins Spectrograph aimed at characterizing the auroral emission from the extrasolar planet HD 209458b. We obtained medium-resolution (R ∼ 20,000) far-ultraviolet (1150–1700 Å) spectra at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar baseline measurement at secondary eclipse. This analysis includes a catalog of stellar emission lines and a star-subtracted spectrum of the planet. We present an emission model for planetary H2 emission, and compare this model to the planetary spectrum. No unambiguously identifiable atomic or molecular features are detected, and upper limits are presented for auroral/dayglow line strengths. An orbital velocity cross-correlation analysis finds a statistically significant (3.8σ) feature at +15(± 20) km s−1 in the rest frame of the planet, at λ1582 Å. This feature is consistent with emission from H2 B–X (2–9) P(4) (λrest = 1581.11 Å); however, the physical mechanism required to excite this transition is unclear. We compare limits on relative line strengths seen in the exoplanet spectrum with models of ultraviolet fluorescence to constrain the atmospheric column density of neutral hydrogen between the star and the planetary surface. These results support models of short-period extrasolar giant planets with weak magnetic fields and extended atomic atmospheres.

The ultraviolet reflectance of Enceladus: Implications for surface composition

The reflectance of Saturn’s moon Enceladus has been measured at far ultraviolet (FUV) wavelengths (115–190 nm) by Cassini’s Ultraviolet Imaging Spectrograph (UVIS). At visible and near infrared (VNIR) wavelengths Enceladus’ reflectance spectrum is very bright, consistent with a surface composed primarily of H 2O ice. At FUV wavelengths, however, Enceladus is surprisingly dark – darker than would be expected for pure water ice. Previous analyses have focused on the VNIR spectrum, comparing it to pure water ice (Cruikshank, D.P., Owen, T.C., Dalle Ore, C., Geballe, T.R., Roush, T.L., de Bergh, C., Sandford, S.A., Poulet, F., Benedix, G.K., Emery, J.P. [2005] Icarus, 175, 268–283) or pure water ice plus a small amount of NH 3 (Emery, J.P., Burr, D.M., Cruikshank, D.P., Brown, R.H., Dalton, J.B. [2005] Astron. Astrophys., 435, 353–362) or NH 3 hydrate (Verbiscer, A.J., Peterson, D.E., Skrutskie, M.F., Cushing, M., Helfenstein, P., Nelson, M.J., Smith, J.D., Wilson, J.C. [2006] Icarus, 182, 211–223). We compare Enceladus’ FUV spectrum to existing laboratory measurements of the reflectance spectra of candidate species, and to spectral models. We find that the low FUV reflectance of Enceladus can be explained by the presence of a small amount of NH 3 and a small amount of a tholin in addition to H 2O ice on the surface. The presence of these three species (H 2O, NH 3, and a tholin) appears to satisfy not only the low FUV reflectance and spectral shape, but also the middle-ultraviolet to visible wavelength brightness and spectral shape. We expect that ammonia in the Enceladus plume is transported across the surface to provide a global coating.

COSMIC ORIGINS SPECTROGRAPH OBSERVATIONS OF THE CHEMICAL COMPOSITION OF SNR LMC N132D

We present new far-ultraviolet spectra of an oxygen-rich knot in the Large Magellanic Cloud supernova remnant N132D, obtained with the Hubble Space Telescope/Cosmic Origins Spectrograph. Moderate resolution (v~200 km/s) spectra in the HST far-ultraviolet bandpass (1150 - 1750 A) show emission from several ionization states of oxygen as well as trace amounts of other species. We use the improvements in sensitivity and resolving power offered by COS to separate contributions from different velocity components within the remnant, as well as emission from different species within the oxygen-rich knot itself. This is the first time that compositional and velocity structure in the ultraviolet emission lines from N132D has been resolved, and we use this to assess the chemical composition of the remnant. No nitrogen is detected in N132D and multiple carbon species are found at velocities inconsistent with the main oxygen component. We find helium and silicon to be associated with the oxygen-rich knot, and use the reddening-corrected line strengths of OIII], OIV], OV, and SiIV to constrain the composition and physical characteristics of this oxygen-rich knot. We find that models with a silicon-to-oxygen abundance ratio of N(Si)/N(O) = 0.01 can reproduce the observed emission for a shock velocity of ~130 km/s, implying a mass of ~50 solar masses for the N132D progenitor star.

FUSEMEASUREMENTS OF FAR-ULTRAVIOLET EXTINCTION. III. THE DEPENDENCE ONR(V) AND DISCRETE FEATURE LIMITS FROM 75 GALACTIC SIGHTLINES

We present a sample of 75 extinction curves derived from FUSE far-ultraviolet spectra supplemented by existing IUE spectra. The extinction curves were created using the standard pair method based on a new set of dereddened FUSE+IUE comparison stars. Molecular hydrogen absorption features were removed using individualized H_2 models for each sightline. The general shape of the FUSE extinction (8.4 micron^-1 < lambda^-1 < 11 micron^-1) was found to be broadly consistent with extrapolations from the IUE extinction (3.3 micron-1 < lambda^-1 < 8.6 micron^-1) curve. Significant differences were seen in the strength of the far-UV rise and the width of the 2175 A bump. All the FUSE+IUE extinction curves had positive far-UV slopes giving no indication that the far-UV rise was turning over at the shortest wavelengths. The dependence of A(lambda)/A(V) versus R(V)^-1 in the far-UV using the sightlines in our sample was found to be stronger than tentatively indicated by previous work. We present an updated R(V) dependent relationship for the full UV wavelength range (3.3 micron^-1 <= lambda^-1 <= 11 micron^-1). Finally, we searched for discrete absorption features in the far-ultraviolet. We found a 3 sigma upper limit of ~0.12 A(V) on features with a resolution of 250 (~4 A width) and 3 sigma upper limits of ~0.15 A(V) for lambda^-1 < 9.6 micron^-1 and ~0.68 A(V) for lambda^-1 > 9.6 micron^-1 on features with a resolution of 10^4 (~0.1 A width).

FAR-ULTRAVIOLET H 2 EMISSION FROM CIRCUMSTELLAR DISKS

We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ~1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex and one, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of \h2 to study the evolution of the gas in the inner disk. We find that the \h2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting \h2 in the disk, the fact that the non-accreting sources show no \h2 emission implies that the \h2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600 {\AA}, we estimate that the column density of \h2 left in the inner regions of the debris disks in our sample is less than ~ 3x10^-6 g cm^-2, nine orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU.

Hubble Space Telescope STIS Spectroscopy of the Peculiar Nova-Like Variables BK Lyn, V751 Cygni, and V380 Oph

ABSTRACTWe obtained Hubble STIS spectra of three nova-like variables: V751 Cygni, V380 Oph, and—the only confirmed nova-like variable known to be below the period gap—BK Lyn. In all three systems, the spectra were taken during high optical brightness state, and a luminous accretion disk dominates their far-ultraviolet (FUV) light. We assessed a lower limit of the distances by applying the infrared photometric method of Knigge. Within the limitations imposed by the poorly known system parameters (such as the inclination, white dwarf mass, and the applicability of steady state accretion disks) we obtained satisfactory fits to BK Lyn using optically thick accretion disk models with an accretion rate of for a white dwarf mass of Mwd = 1.2M⊙ and for Mwd = 0.4M⊙. However, for the VY Scl-type nova-like variable V751 Cygni and for the SW Sex star V380 Oph, we are unable to obtain satisfactory synthetic spectral fits to the high state FUV spectra using optically thick steady state accretion disk models. The lack of FUV spectra information down to the Lyman limit hinders the extraction of information about the accreting white dwarf during the high states of these nova-like systems.

Potential of Far-Ultraviolet Absorption Spectroscopy as a Highly Sensitive Analysis Method for Aqueous Solutions. Part II: Monitoring the Quality of Semiconductor Wafer Cleaning Solutions Using Attenuated Total Reflection

Far-ultraviolet (FUV) spectroscopy combined with attenuated total reflection (ATR) is employed for direct measurement of the concentrations of semiconductor wafer cleaning fluids such as SC-1 (aqueous solution of NH3 and H2O2) and SC-2 (aqueous solution of HCl and H2O2). FUV spectra of these aqueous solutions in the 170-200 nm region are highly sensitive to changes in both hydrogen bonding and hydration. Although ATR measurement results in lower absorptivity compared to transmittance measurement, it is possible to increase absorption with greater evanescent wave penetration depth using a low refractive index internal reflection element (IRE). We adopt quartz as an IRE material. Since the refractive index of quartz becomes lower than that of water in the low energy side of an intense absorption band due to the n-->sigma* transition of water, the quartz IRE yields non-total reflection wavelength regions. However, near 175 nm the effective absorptivity of the tail of water's absorption band can be successfully enlarged, making the FUV-ATR technique suitable for measuring the concentrations of the components in the semiconductor wafer cleaning fluids. In the present study we prepared the same cleaning fluids as those used in actual semiconductor fabrication and measured their FUV-ATR spectra in the 150-300 nm wavelength range. It was found that even with the quartz IRE one can measure FUV-ATR spectra under total reflection conditions at 175 nm or above. We created calibration models for predicting both NH3 and H2O2 in the concentration ranges of 0-10% in SC-1 using multiple linear regression (MLR). The standard deviations of the models were 0.033% and 0.265% for NH3 and H2O2, respectively. The same procedure was repeated under the same conditions for HCl and H2O2 in SC-2, yielding corresponding values of 0.018% for HCl and 0.178% for H2O2.

FUV spectroscopy of the central star of the planetary nebula Sh 2–216

The far-ultraviolet (FUV) spectrum of LS V +46°21, the central star (CS) of the planetary nebula (PN) Sh 2−216, is strongly contaminated by interstellar absorption and thus, we have to simultaneously model both, the stellar as well as the interstellar spectrum in order to identify strategic metal lines which are only accessible in the FUV wavelength range. These lines are necessary to determine the photospheric parameters reliably. We present results of a NLTE spectral analysis of the FUV spectrum of LS V+46°21.

FUSEObservations of Heavy Elements in the Photospheres of Cool DB White Dwarfs

We present a comprehensive analysis of the far-ultraviolet spectra offive DB white dwarfs spanning the effective temperature range between 14,700 and 20,800 K. The FUSE line analysis shows that carbon features, previously observed in several hot DB stars at or above 22,000 K, are present in the two coolest (GD 408 and GD 378) and in the hottest (G270� 124) target. The observed carbon abundances range from log N(C)/N(He) �� 6: 9t o �� 8:8. In addition, fourofthefiveobjectsdisplayphotosphericlinesof silicon. Otherelementssuchasoxygen,iron,andsulfur are also observed in some objects. The variations of the abundances of heavy elements as a function of effective temperature in DB stars are discussed in terms of a competition between a stellar wind, gravitational settling, accretion from interstellar (and circumstellar) matter, and convective dredge-up. Subject headingg ultraviolet: stars — white dwarfs

A binary model for the UV-upturn of elliptical galaxies

The discovery of a flux excess in the far-ultraviolet (FUV) spectrum of elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes, rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. We have developed an evolutionary population synthesis model for the FUV excess of elliptical galaxies based on the binary model developed by Han et al. for the formation of hot subdwarfs in our Galaxy. Despite its simplicity, it successfully reproduces most of the properties of elliptical galaxies with a UV excess: the range of observed UV excesses, both in (1550 - V) and (2000 - V), and their evolution with redshift. We also present colour-colour diagrams for use as diagnostic tools in the study of elliptical galaxies. The model has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals.

Feedback in the Local Lyman‐break Galaxy Analog Haro 11 as Probed by Far‐Ultraviolet and X‐Ray Observations

We have reanalyzed FUSE data and obtained new Chandra observations of Haro 11, a local (DL = 88 Mpc) UV luminous galaxy. Haro 11 has a far-UV luminosity (1010.3 L☉), UV surface brightness (109.4 L☉ kpc-2), star formation rate (SFR), and metallicity similar to those observed in Lyman-break galaxies (LBGs). We show that Haro 11 has extended soft thermal (kT ~ 0.68 keV) X-ray emission of a luminosity and size which scales with the physical properties (e.g., SFR, stellar mass) of the host galaxy. An enhanced α/Fe ratio of ~4 relative to solar abundance suggests significant supernova enrichment. These results are consistent with the X-ray emission being produced in a shock between a supernova-driven outflow and the ambient material. The far-ultraviolet (FUV) spectra show strong absorption lines similar to those observed in LBG spectra. A blueshifted absorption component is identified as a wind outflowing at ~200-280 km s-1. O VI λλ1032,1038 emission, the dominant cooling mechanism for coronal gas at T ~ 105.5 K, is also observed. If associated with the outflow, the luminosity of the O VI emission suggests that 20% of the total mechanical energy from the supernovae and solar winds is being radiated away. This implies that radiative cooling through O VI is not significantly inhibiting the growth of the outflowing gas. In contradiction to the findings of Bergvall and coworkers, we find no convincing evidence of Lyman continuum leakage in Haro 11. We conclude that the wind has not created a tunnel allowing the escape of a significant fraction of Lyman continuum photons, and place a limit on the escape fraction of fesc 2%. Overall, both Haro 11 and a previously observed LBG analog, VV 114, provide invaluable insight into the X-ray and FUV properties of high-redshift LBGs.

The Detection of Far‐Ultraviolet Line Emission from Balmer‐Dominated Supernova Remnants in the Large Magellanic Cloud

We present the first far ultraviolet (FUV) spectra of the four known Balmer-dominated supernova remnants (SNRs) in the Large Magellanic Cloud, acquired with the Far Ultraviolet Spectroscopic Explorer. The remnants DEM L 71 (0505-67.9), 0509-67.5, 0519-69.0 and 0548-70.4 are all in the non-radiative stages of evolution and exhibit expansion speeds ranging from ~ 500 km/s to ~ 5000 km/s. We have detected broad emission lines of Ly beta, Ly gamma, C III and O VI in DEM L 71 (V(FWHM) ~ 1000 km/s) and have detected broad Ly beta and O VI emission in 0519-69.0, (V(FWHM) ~ 3000 km/s). In addition, broad Ly beta emission (V(FWHM) ~ 3700 km/s) has been observed in 0509-67.5, the first detection of broad line emission from this SNR. No emission was detected in our FUSE spectrum of 0548-70.4, allowing us to place only upper limits on the FUV line fluxes. The spectra of these SNRs are unaffected by postshock cooling, and provide valuable probes of collisionless heating efficiency in high Mach number shocks. We have used the Ly beta / O VI flux ratio and relative widths of the broad Ly beta and O VI lines to estimate the degree of electron-proton and proton-oxygen ion equilibration in DEM L 71, 0509-67.5, and 0519-69.0. Although our equilibration estimates are subject to considerable uncertainty due to the faintness of the FUV lines and contributions from bulk Doppler broadening, our results are consistent with a declining efficiency of electron- proton and proton-oxygen ion equilibration with increasing shock speed. From our shock velocity estimates we obtain ages of 295-585 years for 0509-67.5 and 520-900 years for 0519-69.0, respectively, in good agreement with the ages obtained from SN light echo studies.

Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium

We have used archival far-ultraviolet spectra from observations made by HST STIS and FUSE to determine the column densities and rotational excitation temperatures for carbon monoxide and molecular hydrogen, respectively, along 23 sight lines to Galactic O and B stars. The reddening values range from E(B - V) = 0.07 to 0.62, sampling the diffuse to translucent interstellar medium (ISM). We find that the H2 column densities range from 5 × 1018 to 8 × 1020 cm-2 and the CO from upper limits around 2 × 1012 cm-2 to detections as high as 1.4 × 1016 cm-2. CO increases with increasing H2, roughly following a power law of factor ~2. The CO/H2 column density ratio is thus not constant, ranging from 10-7 to 10-5, with a mean value of 3 × 10-6. The sample segregates into "diffuse" and "translucent" regimes, the former with molecular fraction ≲0.25 and AV/d < 1 mag kpc-1. The mean CO/H2 for these two regimes are 3.6 × 10-7 and 9.3 × 10-6, respectively, significantly lower than the canonical dark cloud value of 10-4. Six sight lines show the isotopic variant 13CO, and the isotopic ratio we observe (~50-70) is consistent with, if perhaps a little below, the average 12C/13C for the ISM at large. The average H2 rotational excitation temperature is 74 ± 24 K, agreeing well with previous studies, and the average CO temperature is 4.1 K, with some sight lines showing temperatures as high as 6.4 K. The higher excitation CO is observed with higher column densities, consistent with the effects of photon trapping in clouds with densities in the 20-100 cm-3 range. We discuss the implications for the structure of the diffuse/translucent regimes of the ISM and the estimation of molecular mass in galaxies.

OviAsymmetry and an Accelerated Outflow in an Obscured Seyfert:FUSEandHSTSTIS Spectroscopy of Mrk 533

We present far-ultraviolet spectra of the Seyfert 2 galaxy Mrk 533 obtained with FUSE. These spectra show narrow asymmetrical O VI λλ1032,1038 emission lines with stronger wings shortward of the peak wavelength, but the degree of asymmetry of these wings in velocity is much lower than that of the wings of the lines of lower ionization. In the combined O VI profile there are marginal indications of local absorptions in the outflow. The C III λ977 line is seen weakly with a similar profile, but with very low signal-to-noise ratio (S/N). These FUV spectra are among the first for a Seyfert of type 2, i.e., a purportedly obscured Seyfert. The HST STIS spectral image of Mrk 533 allows delineation of the various components of the outflow, and we infer that the outflow is accelerated. We discuss the results in terms of nuclear geometry and kinematics.

Hubble Space TelescopeSTIS Spectroscopy and Modeling of the Long‐Term Cooling of WZ Sagittae following the 2001 July Outburst

We present the latest Hubble Space Telescope Space Telescope Imaging Spectrograph (HST STIS) E140M far-ultraviolet (FUV) spectrum of the dwarf nova WZ Sge, obtained in 2004 July, 3 yr following the early superoutburst of 2001 July. With a temperature T ≈ 15,000 K, the white dwarf (WD) is still ~1500 K above its quiescent temperature, it has a FUV flux level almost twice its preoutburst value, and its spectrum does not distinctly exhibit the quasi-molecular hydrogen feature around 1400 A, which was present in the International Ultraviolet Explorer (IUE) and HST Goddard High Resolution Spectrograph (GHRS) preoutburst data. This is a clear indication that even 3 yr after outburst, the system is still showing the effect of the outburst. We model the cooling curve of WZ Sge, over a period of 3 yr, using a stellar evolution code including accretion and the effects of compressional heating. Assuming that compressional heating alone is the source of the energy released during the cooling phase, we find that (1) the mass of the WD must be quite large (≈1.0 ± 0.2 M☉), and (2) the mass accretion rate must have a time-averaged (over 52 days of outburst) value of order 10-8 M☉ yr-1 or above. The outburst mass accretion rate derived from these compressional heating models is larger than the rates estimated from optical observations (Patterson et al.) and from a FUV spectral fit (Long et al.) by up to 1 order of magnitude. This implies that during the cooling phase the energy released by the WD is not due to compressional heating alone. We suggest ongoing accretion during quiescence as an additional source of energy.