Space Telescope Imaging Spectrograph Research Articles

HST/STIS observations of the RW Aurigae bipolar jet: mapping the physical parameters close to the source

We present the results of new spectral diagnostic investigations applied to high-resolution long-slit spectra of the RW Aur bipolar jet obtained with HST/STIS. The spectra include the forbidden doublets [O I] 6300,6363 \AA, [S II] 6716,6731 \AA, and [N II] 6548, 6583 \AA that we utilized to determine electron density, electron temperature, hydrogen ionisation fraction, total hydrogen density, radial velocity and the mass outflow rate. We were able to extract the parameters as far as 3".9 in the red- and 2".1 in the blueshifted beam. The RW Aur jet appears to be the second densest outflow from a T Tauri star studied so far, but its other properties are quite similar to those found in other jets from young stars. The overall trend of the physical parameters along the first few arcseconds of the RW Aur jet is similar to that of HH 30 and DG Tau and this can reflect analogies in the mechanisms operating in that region, suggesting the same engine is accelerating the jets in the T Tauri stars with outflows. Our study of the RW Aur jet indicates for the first time that, despite the detected marked asymmetries in physical and kinematic properties between the two lobes, the mass outflow rates in the two lobes are similar. This appears to indicate that the central engine has constraining symmetries on both sides of the system, and that the observed asymmetries are probably due to different environmental conditions.

LIFETIMES AND OSCILLATOR STRENGTHS FOR ULTRAVIOLET TRANSITIONS IN SINGLY IONIZED COPPER

We present the results of lifetime measurements made using beam-foil techniques on levels of astrophysical interest in Pb ii producing lines at 1203.6 A (6s6p2 ) and 1433.9 A ( ). We also report the first detection of the Pb ii line in the interstellar medium (ISM) from an analysis of archival spectra acquired by the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The oscillator strengths derived from our experimental lifetimes for Pb ii , 1433 are generally consistent with recent theoretical results, including our own relativistic calculations. Our analysis of high-resolution HST/STIS spectra helps to confirm the relative strengths of the Pb ii , 1433 lines. However, the oscillator strength that we obtain for Pb ii (0.321 ± 0.034) is significantly smaller than earlier theoretical values, which have been used to study the abundance of Pb in the ISM. Our revised oscillator strength for yields an increase in the interstellar abundance of Pb of 0.43 dex over determinations based on the value given by Morton, indicating that the depletion of Pb onto interstellar dust grains is less severe than previously thought.

Limits on the Masses of Supermassive Black Holes in 105 Nearby Galaxies

Secure measurements of the mass of the central supermassive black hole, MBH, in external galaxies are traditionally obtained through the modeling of the stellar and/or gaseous kinematics, most often derived using Hubble Space Telescope (HST) observations in the optical domain. The modeling of the nuclear ionized-gas kinematics has led to accurate MBH measurements at a relatively cheap cost in terms of observation time compared to stellar-dynamical MBH determinations. But only a handful of the objects have turned out to have sufficiently regular gas velocity fields for the purpose of modeling. Nevertheless, the HST archive contains a yet untapped resource that can be used to better constrain the MBH budget across the different morphological types of galaxies, which consists of the vast number of the Space Telescope Imaging Spectrograph (STIS) spectra from which a central emission-line width can be measured. These data allow to put an upper limit on MBH for a large number of galaxies and promise to compensate for the lack of exact measurements when studying the MBH–host galaxy relationships.

Strong z ∼ 0.5 O vi absorption towards PKS 0405−123: implications for ionization and metallicity of the Cosmic Web★

We present observations of the intervening O VI absorption-line system at zabs = 0.495 096 towards the quasi-stellar object (QSO) PKS 0405−123 (zem = 0.5726) obtained with the Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. In addition to strong O VI, with log N(O VI) = 14.47 ± 0.02, and moderate H I, with log N (H I) = 14.29 ± 0.10, this absorber shows absorption from C III, N IV ,O IV and O V, with upper limits for another seven ions. The large number of available ions allows us to test ionization models usually adopted with far fewer constraints. We find that the observed ionic column densities cannot be matched by single-temperature collisional ionization models, in or out of equilibrium. Photoionization models can match all of the observed column densities, including O VI. If one assumes photoionization by an ultraviolet (UV) background dominated by QSOs, the metallicity of the gas is [O/H] ≈− 0.15, while if one assumes a model for the UV background with contributions from ionizing photons escaping from galaxies the metallicity is [O/H] ≈− 0.62. Both give [N/O] ∼− 0.6 and [C/H] ∼− 0.2 to ∼−0.1, though a solar C/O ratio is not ruled out. The choice of ionizing spectrum is poorly constrained and leads to systematic abundance uncertainties of ≈0.5 dex, despite the wide range of available ions. Multiphase models with a contribution from both photoionized gas (at T ∼ 10 4 K) and collisionally ionized gas [at T ∼ (1−3) × 10 5 K] can also match the observations for either assumed UV background giving very similar metallicities. We do not detect Ne VIII or Mg X absorption. The limit on Ne VIII/O VI < 0.21 (3σ )i s the lowest yet observed. Thus, this absorber shows no firm evidence of the ‘warm-hot intergalactic medium’ at T ∼ (0.5–3) × 10 6 K thought to contain a significant fraction of the baryons at low redshift. The O VI in this system is not necessarily a reliable tracer of the warm-hot intergalactic medium given the ambiguity in its origins. We present limits on the total column of warm-hot gas in this absorber as a function of temperature. This system would be unlikely to provide detectable X-ray absorption in the ions O VII or O VIII even if it resided in front of the brighter X-ray sources in the sky.

The low-mass stellar-mass functions of rich, compact clusters in the Large Magellanic Cloud

Context. We use Hubble Space Telescope photometry of six rich, compact star clusters in the Large Magellanic Cloud (LMC), with ages ranging from 0.01 to 1.0 Gyr, to derive the clusters' stellar mass functions (MFs) at their half-mass radii. Aims. The LMC is an ideal environment to study stellar MFs, because it contains a large population of compact clusters at different evolutionary stages. We aim to obtain constraints on the initial MFs (IMFs) of our sample clusters on the basis of their present-day MFs, combined with our understanding of their dynamical and photometric evolution. Methods. We derive the clusters' present-day MFs below 1.0 Msun using deep observations with the Space Telescope Imaging Spectrograph and updated stellar population synthesis models. Results. Since the relaxation timescales of low-mass stars are very long, dynamical evolution will not have affected the MFs below 1.0 Msun significantly, so that - within the uncertainties - the derived MFs are consistent with the solar-neighbourhood IMF, at least for the younger clusters. Conclusions. The IMF in the low-density, low-metallicity environment of the LMC disk is not significantly different from that in the solar neighbourhood.

THE NUCLEAR OUTFLOWS AND FEEDBACK IN THE SEYFERT 2 GALAXY MARKARIAN 573

We present a study of outflow and feedback in the well-known Seyfert 2 galaxy Markarian 573 using high angular resolution long-slit spectrophotometry obtained with the Hubble Space Telescope Imaging Spectrograph (STIS). Through analysis of the kinematics and ionization state of a biconical outflow region emanating from the nucleus, we find that the outflow does not significantly accelerate the surrounding host-galaxy interstellar gas and is too weak to be a strong ionization mechanism in the extended emission regions. Instead, the excitation of the extended regions is consistent with photoionization by the active nucleus. From energetics arguments we show that the nuclear outflow is slow and heavy and has a mechanical luminosity that is only ~1% of the estimated bolometric luminosity of the system. The energy in the outflow is able to mildly shape the gas in the extended regions but appears to be insufficient to unbind it, or even to plausibly disrupt star formation. These results are at odds with the picture of strong AGN feedback that has been invoked to explain certain aspects of galaxy evolution.

THE DUSTY NUCLEAR TORUS IN NGC 4151: CONSTRAINTS FROM GEMINI NEAR-INFRARED INTEGRAL FIELD SPECTROGRAPH OBSERVATIONS

We have used a near-infrared (near-IR) nuclear spectrum (covering the Z, J, H, and K bands) of the nucleus of NGC 4151 obtained with the Gemini Near-Infrared Integral Field Spectrograph (NIFS) and adaptive optics, to isolate and constrain the properties of a near-IR unresolved nuclear source whose spectral signature is clearly present in our data. The near-IR spectrum was combined with an optical spectrum obtained with the Space Telescope Imaging Spectrograph which was used to constrain the contribution of a power-law component. After subtraction of the power-law component, the near-IR continuum is well fitted by a blackbody function, with T = 1285 ± 50 K, which dominates the nuclear spectrum—within an aperture of radius 03—in the near-IR. We attribute the blackbody component to emission by a dusty structure, with hot dust mass MHD = (6.9 ± 1.5) × 10−4 M☉, not resolved by our observations, which provide only an upper limit for its distance from the nucleus of 4 pc. If the reddening derived for the narrow-line region also applies to the near-IR source, we obtain a temperature T = 1360 ± 50 K and a mass MHD = (3.1 ± 0.7) × 10−4 M☉ for the hot dust. This structure may be the inner wall of the dusty torus postulated by the unified model or the inner part of a dusty wind originating in the accretion disk.

The extended interacting wind structure of Eta Carinae

The highly eccentric binary system, η Car, provides clues to the transition of massive stars from hydrogen-burning via the CNO cycle to a helium-burning evolutionary state. The fastmoving wind of η Car B creates a cavity in η Car A’s slower, but more massive, stellar wind, providing an in situ probe. The Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS), with its high spatial and spectral resolutions, is well matched to follow temporal spatial and velocity variations of multiple wind features. We use observations obtained across 1998–2004 to produce a rudimentary three-dimensional model of the wind interaction in the η Car system. Broad (±500 km s −1 )[ FeII] emission line structures extend 0.7 arcsec (∼1600 au) from the stellar core. In contrast, [Fe III], [Ar III], [Ne III] and [S III] lines extend only 0.3 arcsec (700 au) from NE to SW and are blue shifted from −500 to +200 km s −1 . All observed spectral features vary with the 5.54-year orbital period. The highly ionized, forbidden emission disappears during the low state, associated with periastron passage. The high-ionization emission originates in the outer wind interaction region that is directly excited by the far-ultraviolet radiation from η Car B. The HST/STIS spectra reveal a time-varying, distorted paraboloidal structure, caused by the interaction of the massive stellar winds. The model and observations are consistent with the orbital plane aligned with the skirt of the Homunculus. However, the axis of the distorted paraboloid, relative to the major axis of the binary orbit, is shifted in a prograde rotation along the plane, which projected on the sky is from NE to NW.

MASS OUTFLOW IN THE SEYFERT 1 GALAXY NGC 5548

We present a study of the intrinsic UV absorption and emission lines in an historically low-state spectrum of the Seyfert 1 galaxy NGC 5548, which we obtained in 2004 February at high spatial and spectral resolution with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. We isolate a component of emission with a width of 680 km/s (FWHM) that arises from an "intermediate line region" (ILR), similar to the one we discovered in NGC 4151, at a distance of ~1 pc from the central continuum source. From a detailed analysis of the five intrinsic absorption components in NGC 5548 and their behavior over a span of 8 years, we present evidence that most of the UV absorbers only partially cover the ILR and do not cover an extended region of UV continuum emission, most likely from hot stars in the circumnuclear region. We also find that four of the UV absorbers are at much greater distances (>70 pc) than the ILR, and none have sufficient N V or C IV column densities to be the ILR in absorption. At least a portion of the UV absorption component 3, at a radial velocity of -530 km/s, is likely responsible for most of the X-ray absorption, at a distance < 7 pc from the central source. The fact that we see the ILR in absorption in NGC 4151 and not in NGC 5548 suggests that the ILR is located at a relatively large polar angle (~45 degrees) with respect to the narrow-line region outflow axis.

VLT/SINFONI time-resolved spectroscopy of the central, luminous, H-rich WN stars of R136

Using the Very Large Telescope's Spectrograph for INtegral Field Observation in the Near-Infrared (VLT/SINFONI), we have obtained repeated AO-assisted, NIR spectroscopy of the six central luminous, Wolf-Rayet (WR) stars in the core of the very young (~1 Myr), massive and dense cluster R136, in the Large Magellanic Cloud (LMC). We also de-archived available images that were obtained with the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS), and extracted high-quality, differential photometry of our target stars to check for any variability related to binary motion. Previous studies, relying on spatially unresolved, integrated, optical spectroscopy, had reported that one of these stars was likely to be a 4.377-day binary. Our study set out to identify the culprit and any other short-period system among our targets. However, none displays significant photometric variability, and only one star, BAT99-112 (R136c), located on the outer fringe of R136, displays a marginal variability in its radial velocities; we tentatively report an 8.2-day period. The binary status of BAT99-112 is supported by the fact that it is one of the brightest X-ray sources among all known WR stars in the LMC, consistent with it being a colliding-wind system. Follow-up observations have been proposed to confirm the orbital period of this potentially very massive system.

UNIDENTIFIED FEATURES IN THE ULTRAVIOLET SPECTRUM OF X Per

High-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) were used to search for unidentified interstellar absorption features in the well studied sightline towards X Per (HD 24534). The significance of features detected was determined from Gaussian fits to the data, as well as the features' persistence in multiple observations. Fixed pattern noise characteristics were studied in STIS echelle data to distinguish between interstellar and instrumental features. We report the detection of two unidentified features that stand out from the more common fixed pattern noise features. Both features have depths of > 3% of the continuum level making them very likely of interstellar origin. Lastly, we comment on possible carriers, and discuss future prospects for studying these and perhaps other unidentified lines in larger samples of sightlines.

A POPULATION OF WEAK METAL-LINE ABSORBERS SURROUNDING THE MILKY WAY

We report on the detection of a population of weak metal-line absorbers in the halo or nearby intergalactic environment of the Milky Way. Using high-resolution ultraviolet absorption-line spectra of bright QSOs obtained with the Space Telescope Imaging Spectrograph (STIS), along six sight lines we have observed unsaturated, narrow absorption in OI and SiII together with mildly saturated CII absorption at high radial velocities (|v_LSR|=100-320 km/s). The measured OI column densities are small, implying that these structures represent Lyman-Limit Systems and sub-Lyman-Limit System with HI column densities < 3x10^18 cm^-2, thus below the detection limits of current 21cm all-sky surveys of high-velocity clouds (HVCs). The absorbers apparently are not directly associated with any of the large high-column density HVC complexes, but rather represent isolated, partly neutral gas clumps embedded in a more tenuous, ionized gaseous medium situated in the halo or nearby intergalactic environment of the Galaxy. We speculate that this absorber population represents the local analog of weak MgII systems that are commonly observed in the circumgalactic environment of low- and high-redshift galaxies.

A QUINTET OF BLACK HOLE MASS DETERMINATIONS

We report five new measurements of central black hole masses based on Space Telescope Imaging Spectrograph and Wide Field Planetary Camera 2 observations with the Hubble Space Telescope (HST) and on axisymmetric, three-integral, Schwarzschild orbit-library kinematic models. We selected a sample of galaxies within a narrow range in velocity dispersion that cover a range of galaxy parameters (including Hubble type and core/power-law surface density profile) where we expected to be able to resolve the galaxy's sphere of influence based on the predicted value of the black hole mass from the M–σ relation. We find masses for the following galaxies: NGC 3585, MBH = 3.4+1.5−0.6 × 108 M☉; NGC 3607, MBH = 1.2+0.4−0.4 × 108 M☉; NGC 4026, MBH = 2.1+0.7−0.4 × 108 M☉; and NGC 5576, MBH = 1.8+0.3−0.4 × 108 M☉, all significantly excluding MBH = 0. For NGC 3945, MBH = 9+17−21 × 106 M☉, which is significantly below predictions from M–σ and M–L relations and consistent with MBH = 0, though the presence of a double bar in this galaxy may present problems for our axisymmetric code.

VARIABLE INTRINSIC ABSORPTION IN Mrk 279

We examine the variability in the intrinsic absorption in the Seyfert 1 galaxy Mrk 279 using three epochs of observations from the Far Ultraviolet Spectroscopic Explorer (FUSE) and two epochs of observations with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Rather than finding simple photoionization responses of the absorbing gas to changes in the underlying continuum, the observed changes in the absorption profiles can be understood more clearly if the effective covering fraction of the gas in all emission components, continuum and broad and intermediate velocity width emission lines, is accounted for. While we do not uniquely solve for all of these separate covering fractions and the ionic column densities using the spectral data, we examine the parameter space using previously well-constrained solutions for continuum and single emission component covering fractions. Assuming full coverage of the continuum, we find that of the two velocity components of the Mrk 279 absorption most likely associated with its outflow, one likely has zero coverage of the intermediate line region while the other does not. For each component, however, the broad line region is more fully covered than the intermediate line region. Changes in the O VI column densities are unconstrained due to saturation, but we show that small changes in the nonsaturated C IV and N V column densities are consistent with the outflow gas having zero or partial covering of the intermediate line region and an ionization parameter changing from ~0.01 to ~0.1 from 2002 to 2003 as the UV continuum flux increased by a factor of ~8. The absence of a change in the C III absorbing column density is attributed to this species arising outside the Mrk 279 outflow.

A NEW MINIMALLY INVASIVE TREATMENT FOR MALE STRESS URINARY INCONTINENCE: THE ADVANCE® MALE SLING, ONE YEAR DATA FROM A MULTICENTER PROSPECTIVE STUDY

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H2 band systems and H Lyman-α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2–4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5–16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325–326 (November 20–21). The global brightening events on November 20–21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ∼2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45–19:35 UT UVIS H2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6–5 kHz Galileo PWS data.

AN ULTRAVIOLET SEARCH FOR INTERSTELLAR CS

High- and medium-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) and the Goddard High Resolution Spectrograph (GHRS) were used to study the diatomic molecule CS through the C-X(0,0) band at 1401 A. The band was modeled to verify profile shape. The rest wavelength of the C-X band is refined to a value of 1400.88 A and a 3σ lower limit is set on the oscillator strength at 0.14 based on equivalent width upper limits of the A-X(0,0) CS Band at 2577 A. The strength of the 1401 A band is compared to other interstellar parameters and implications for CS formation and destruction are briefly discussed.

MASSIVE STAR MULTIPLICITY: THE CEPHEID W Sgr

We have obtained spectra of the W Sgr system with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST). The spectra resolve the system into a distant companion B which is the hottest star in the system and the spectroscopic binary (A = Aa + Ab). A and B are separated by 016. We have extracted the spectra of both of these. We see no flux in the Aa + Ab spectrum which cannot be accounted for by the Cepheid, and put an upper limit on the spectral type and mass of the companion Ab of F5 V and ≤1.4 M ☉. Using the orbit from HST fine guidance sensor measurements from Benedict et al., this results in an upper limit to the mass of the Cepheid of ≤5.4 M ☉. We also discuss two possible distant companions. Based on the photometry from the Two Micron All Sky Survey Point Source Catalog, they are not physical companions of the W Sgr system.

Io's dayside SO 2 atmosphere

An extensive set of HI Lyman- α images obtained with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) from 1997–2001 has been analyzed to provide information about the spatial and temporal character of Io's SO 2 atmosphere. An atmospheric distribution map derived from the observations reveals that the sunlit SO 2 atmosphere is temporally stable on a global scale, with only small local changes. An anti-/sub-jovian asymmetry in the SO 2 distribution is present in all 5 years of the observations. The average daytime atmosphere is densest on the anti-jovian hemisphere in the equatorial regions, with a maximum equatorial column density of 5.0 × 10 16 cm − 2 at 140° longitude. The SO 2 atmosphere also has greater latitudinal extent on the anti-jovian hemisphere as compared to the sub-jovian. The atmospheric distribution appears to be best correlated with the location of hot spots and known volcanic plumes, although small number statistics for the plumes limits the correlation.

T Tauri Jet Physics Resolved Near the Launching Region with theHubble Space Telescope

We present an analysis of the gas physics at the base of jets from five T Tauri stars based on high angular resolution optical spectra, using the Hubble Space Telescope Imaging Spectrograph (HST STIS). The spectra refer to a region within 100 AU of the star, i.e., where the collimation of the jet has just taken place. We form position-velocity (PV) images of the line ratios to get a global picture of the flow excitation. We then apply a specialized diagnostic technique to find the electron density, ionization fraction, electron temperature, and total density. Our results are in the form of PV maps of the obtained quantities, in which the gas behavior is resolved as a function of both radial velocity and distance from the jet axis. They highlight a number of interesting physical features of the jet collimation region, including regions of extremely high density, asymmetries with respect to the axis, and possible shock signatures. Finally, we estimate the jet mass and angular momentum outflow rates, both of which are fundamental parameters in constraining models of accretion-ejection structures, particularly if the parameters can be determined close to the jet footpoint. Comparing mass flow rates for cases where the mass accretion rate is available in the literature (i.e., for DG Tau, RW Aur, and CW Tau) reveals a mass ejection-to-accretion ratio of 0.01-0.07. Finally, where possible (i.e., for DG Tau and CW Tau), both mass and angular momentum outflow rates are resolved into higher and lower velocity jet material. For the clearer case of DG Tau, this reveals that the more collimated higher velocity component plays a dominant role in mass and angular momentum transport.

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.