Ultraviolet Spectrograph Research Articles

Spectrum and energy levels of the Yb4+ free ion (Yb V)

The spectrum of ionized ytterbium produced by a sliding spark source was recorded on the 10 m high resolution vacuum ultraviolet normal-incidence spectrograph of the Meudon Observatory. About 1080 lines attributed to Yb V, hitherto unknown, have been identified. The analysis of this spectrum established all the energy levels of the ground configuration 4f12 and, respectively 174, 12 and 43 levels of the excited configurations 4f115d,4f116s and 4f116p. The theoretical calculations by means of the Cowan codes included a least-squares optimization of the relevant radial parameters by minimizing the differences between calculated and experimental level energies, which led to mean errors of 55 cm−1 for the 56 even parity levels and 51 cm−1 for the 186 odd parity ones. Interactions with the unknown core-excited configurations 5p54f13, 5p54f126p, 5p54f125d and 5p54f126s were taken into account.

An investigation of the line of sight towards QSO PKS 0237−233★

We present a detailed analysis of absorption systems along the line of sight towards QSO PKS 0237−233 using a high-resolution spectrum of signal-to-noise ratio ∼60–80 obtained with the Ultraviolet and Visual Echelle Spectrograph mounted on the Very Large Telescope (VLT/UVES). This line of sight is known to show a remarkable overdensity of C iv systems that has been interpreted as revealing the presence of a supercluster of galaxies. A detailed analysis of each of these absorption systems is presented. In particular, for the zabs = 1.6359 (with two components of log NHI [cm−2] = 18.45, 19.05) and zabs = 1.6720 (log NHI = 19.78) sub-damped Lyα systems (sub-DLAs), we measure accurate abundances (resp. [O/H] = −1.63 ± 0.07 and [Zn/H] = −0.57 ± 0.05 relative to solar). While the depletion of refractory elements on to dust grains in both sub-DLAs is not noteworthy, photoionization models show that ionization effects are important in a part of the absorbing gas of the sub-DLA at zabs = 1.6359 (H i is 95 per cent ionized) and in a part of the gas of the sub-DLA at zabs = 1.6720. The C iv clustering properties along the line of sight is studied in order to investigate the nature of the observed overdensity. We conclude that despite the unusually high number of C iv systems detected along the line of sight, there is no compelling evidence for the presence of a single unusual overdensity and that the situation is consistent with chance coincidence.

The UVES large program for testing fundamental physics – II. Constraints on a change in μ towards quasar HE 0027−1836★

We present an accurate analysis of the H2 absorption lines from the zabs ~ 2.4018 damped Ly{\alpha} system towards HE 0027-1836 observed with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (VLT/UVES) as a part of the European Southern Observatory Large Programme "The UVES large programme for testing fundamental physics" to constrain the variation of proton-to-electron mass ratio, {\mu} = mp/me. We perform cross-correlation analysis between 19 individual exposures taken over three years and the combined spectrum to check the wavelength calibration stability. We notice the presence of a possible wavelength dependent velocity drift especially in the data taken in 2012. We use available asteroids spectra taken with UVES close to our observations to confirm and quantify this effect. We consider single and two component Voigt profiles to model the observed H2 absorption profiles. We use both linear regression analysis and Voigt profile fitting where {\Delta}{\mu}/{\mu} is explicitly considered as an additional fitting parameter. The two component model is marginally favored by the statistical indicators and we get {\Delta}{\mu}/{\mu} = (-2.5 +/- 8.1(stat) +/- 6.2(sys)) ppm. When we apply the correction to the wavelength dependent velocity drift we find {\Delta}{\mu}/{\mu} = (-7.6 +/- 8.1(stat) +/- 6.3(sys)) ppm. It will be important to check the extent to which the velocity drift we notice in this study is present in UVES data used for previous {\Delta}{\mu}/{\mu} measurements.

High-Throughput, High-Resolution Echelle Deep-UV Raman Spectrometer

We constructed an ultrahigh-throughput, high-resolution ultraviolet (UV) Raman spectrograph that utilizes a high-efficiency filter-stage monochromator and a high-dispersion Echelle spectrograph. The spectrograph utilizes a total of six mirrors and two gratings, with an overall efficiency at 229 nm of ~18%. The limiting resolution of our spectrometer is 0.6 cm⁻¹ full width half-maximum (FWHM), as measured for 229 nm Rayleigh scattering. Use of a 1 mm-wide entrance slit gives rise to an approximately 10 cm⁻¹ FWHM resolution at 229 nm. The ultrahigh spectrograph throughput enables ultrahigh signal-to-noise ratio, deep UV Raman spectra that allow us to monitor <1% changes in peptide bond composition. The throughput is measured to be 35-fold greater than conventional deep UV Raman spectrometers.

Spatio-kinematic modelling of Abell 65, a double-shelled planetary nebula with a binary central star★

We present the first detailed spatio-kinematical analysis and modelling of the planetary nebula Abell 65, which is known to host a post-common envelope, binary, central star system. As such, this object is of great interest in studying the link between nebular morphology and central star binarity. [OIII]5007A and H-alpha+[NII]6584A longslit spectra and imagery of Abell 65 were obtained with the Manchester Echelle Spectrometer on the 2.1-m telescope at the San Pedro Martir Observatory (MES-SPM). Further [OIII]5007A longslit spectra were obtained with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope (VLT-UVES). These data were used to develop a spatio-kinematical model for the [OIII]5007A emission from Abell 65. A "best-fit" model was found by comparing synthetic spectra and images rendered from the model to the data. The model comprises an outer shell and an inner shell, with kinematical ages of 15000 +/- 5000 yr kpc^-1 and 8000 +/- 3000 yr kpc^-1, respectively. Both shells have peanut-shaped bipolar structures with symmetry axes at inclinations of 55 +/- 10 deg (to the line-of-sight) for the outer shell and 68 +/- 10 deg for the inner shell. The near-alignment between the nebular shells and the binary orbital inclination (of 68 +/- 2 deg) is strongly indicative that the binary is responsible for shaping the nebula. Abell 65 is one of a growing number of planetary nebulae (seven to date, including Abell 65 itself) for which observations and modelling support the shaping influence of a central binary.

Near-Ultraviolet Emission Spectroscopy of the Hayabusa Reentry

Quantitative time-resolved irradiance measurements were obtained of the Hayabusa Sample Return Capsule’s entry on 13 June 2010, as measured from a 12.6 km altitude with the Australian Ultraviolet Spectrograph instrument on board the NASA DC-8 Airborne Science Laboratory, jointly managed by the NASA Airborne Science Program and the National Suborbital Education and Research Center. The NASA DC-8 Airborne Science Laboratory was stationed just outside of the landing site at the Woomera Test Range in Australia. The measurements were calibrated against National Institute of Standards and Technology traceable standard calibration lamps on the tarmac at NASA Dryden Aircraft Operations Facility, operated by NASA Dryden Flight Research Center, and at NASA Ames Research Center. The recorded spectra cover the wavelength range from 300 to 470 nm over the duration from 13:52:02.5 until 13:52:33.5 coordinated universal time. The spectra show shock emission from singly ionised molecular nitrogen (), as well as several ablation products from the heat shield material: cyanogen radical, calcium, and aluminium. In addition, the high-temperature tail of gray-body emission is observed to enter the wavelength range after 13:52:13 coordinated universal time. These data were evaluated in terms of the temporal evolution of the capsule’s stagnation temperature and emissions within the cyanogen radical and manifolds. A peak in the irradiance within the wavelength band from 365 to 392 nm was registered at approximately 13:52:07 coordinated universal time and elevated levels of irradiance within the combined cyanogen radical and manifolds were subsequently registered at about 13:52:20 coordinated universal time. An apparent maximum capsule temperature close to 3050 K was found at around 13:52:23 coordinated universal time, consistent with apparent capsule temperature values deduced using other instruments.

The formation of molecular hydrogen from water ice in the lunar regolith by energetic charged particles

On 9 October 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) mission impacted a spent Centaur rocket into the permanently shadowed region (PSR) within Cabeus crater and detected water vapor and ice, as well as other volatiles, in the ejecta plume. The Lyman Alpha Mapping Project (LAMP), a far ultraviolet (FUV) imaging spectrograph on board the Lunar Reconnaissance Orbiter (LRO), observed this plume as FUV emissions from the fluorescence of sunlight by molecular hydrogen (H2) and other constituents. Energetic charged particles, such as galactic cosmic rays (GCRs) and solar energetic particles (SEPs), can dissociate the molecules in water ice to form H2. We examine how much H2can be formed by these types of particle radiation interacting with water ice sequestered in the regolith within PSRs, and we assess whether it can account for the H2 observed by LAMP. To estimate H2formation, we use the GCR and SEP radiation dose rates measured by the LRO Cosmic Ray Telescope for the Effects of Radiation (CRaTER). The exposure time of the ice is calculated by considering meteoritic gardening and the penetration depth of the energetic particles. We find that GCRs and SEPs could convert at least 1–7% of the original water molecules into H2. Therefore, given the amount of water detected by LCROSS, such particle radiation‒induced dissociation of water ice could likely account for a significant percentage (10–100%) of the H2measured by LAMP.

Hot DAVs: a probable new class of pulsating white dwarf stars

We have discovered a pulsating DA white dwarf at the lower end of the temperature range 45 000–30 000 K where a few helium atmosphere white dwarfs are known. There are now three such pulsators known, suggesting that a new class of theoretically predicted pulsating white dwarf stars exists. We name them the hot DAV stars. From high-speed photometric observations with the ULTRACAM photometer on the 4.2-m William Herschel Telescope, we show that the hydrogen atmosphere white dwarf star WD1017−138 pulsates in at least one mode with a frequency of 1.62 mHz (a period of 624 s). The amplitude of that mode was near 1 mmag at a 10σ confidence level on one night of observation and an 8.4σ confidence level on a second night. The combined data have a confidence level of 11.8σ. This supports the two other detections of hot DAV stars previously reported. From three Very Large Telescope Ultraviolet and Visual Echelle Spectrograph spectra we confirm also that WD1017−138 is a hydrogen atmosphere white dwarf with no trace of helium or metals with Teff = 32 600 K, log g = 7.8 (cgs) and M = 0.55 M⊙. The existence of pulsations in these DA white dwarfs at the cool edge of the 45 000–30 000 K temperature range supports the thin hydrogen layer model for the deficit of helium atmosphere white dwarfs in this range. DA white dwarfs with thick hydrogen layers do not have the superadiabatic, chemically inhomogeneous (μ-gradient) zone that drives pulsation in this temperature range. The potential for higher amplitude hot DAV stars exists; their discovery would open the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars. A search for pulsation in a further 22 candidates with ULTRACAM on the European Southern Observatory New Technology Telescope gave null results for pulsation at precisions in the range 0.5–3 mmag, suggesting that the pulsation amplitudes in such stars are relatively low, hence near the detection limit with the ground-based telescopes used in the survey.

SOLAR OCCULTATION BY TITAN MEASURED BY CASSINI /UVIS

We present the first published analysis of a solar occultation by Titan's atmosphere measured by the Ultraviolet Imaging Spectrograph on board Cassini. The data were measured during flyby T53 in 2009 April and correspond to latitudes between 21 Degree-Sign and 28 Degree-Sign south. The analysis utilizes the absorption of two solar emission lines (584 A and 630 A) in the ionization continuum of the N{sub 2} absorption cross section and solar emission lines around 1085 A where absorption is due to CH{sub 4}. The measured transmission at these wavelengths provides a direct estimate of the N{sub 2} and CH{sub 4} column densities along the line of sight from the spacecraft to the Sun, which we inverted to obtain the number densities. The high signal-to-noise ratio of the data allowed us to retrieve density profiles in the altitude range 1120-1400 km for nitrogen and 850-1300 km for methane. We find an N{sub 2} scale height of {approx}76 km and a temperature of {approx}153 K. Our results are in general agreement with those from previous work, although there are some differences. Particularly, our profiles agree, considering uncertainties, with the density profiles derived from the Voyager 1 Ultraviolet Spectrograph data, and with inmore » situ measurements by the Ion Neutral Mass Spectrometer with revised calibration.« less

An empirical determination of proton auroral far ultraviolet emission efficiencies using a new nonclimatological proton flux extrapolation method

Model‐derived electron and proton auroral FUV emission efficiencies of relevance to auroral FUV remote sensing methods are evaluated using coincident observations by Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor J/5 (SSJ/5), both on board the Defense Meteorological Satellite Program (DMSP) satellite F16. This follows earlier work by Knight et al. (2008), which reported higher than expected proton Lyman‐Birge‐Hopfield (LBH) emission efficiencies based on F16 SSUSI and SSJ/5 comparisons, and Correira et al. (2011), which suggested a downward revision in the proton LBH efficiencies from Knight et al. (2008). These proton efficiency results rely on proton extrapolation methods to supply the unmeasured proton flux above 30 keV (the upper limit of SSJ/5). Correira et al. (2011) determined that there was a bias in the proton extrapolation method used by Knight et al. (2008) that was caused by column emission rate (CER) thresholding in the coincident SSUSI and SSJ/5 sets. In the latest work, a more robust proton flux extrapolation method is introduced which does not have the problem of CER threshold dependence. The new extrapolation method uses coincident SSUSI Lyman alpha observations to constrain the extrapolated proton flux above 30 keV in such a way that unknown Lyman alpha model yield errors and SSUSI and SSJ/5 calibration errors drop out without biasing the extrapolation. With the latest extrapolation method, SSUSI‐SSJ/5 comparisons indicate that proton aurora is typically a factor of ∼4.5 more efficient per unit of energy flux in producing LBH than electron aurora.

Temporal variability of lunar exospheric helium during January 2012 from LRO/LAMP

We report observations of the lunar helium exosphere made between December 29, 2011, and January 26, 2012, with the Lyman Alpha Mapping Project (LAMP) ultraviolet spectrograph on NASA’s Lunar Reconnaissance Orbiter Mission (LRO). The observations were made of resonantly scattered He iλ584 from illuminated atmosphere against the dark lunar surface on the dawn side of the terminator. We find no or little variation of the derived surface He density with latitude but day-to-day variations that likely reflect variations in the solar wind alpha flux. The five-day passage of the Moon through the Earth’s magnetotail results in a factor of two decrease in surface density, which is well explained by model simulations.

MeV electrons detected by the Alice UV spectrograph during the New Horizons flyby of Jupiter

In early 2007, the New Horizons spacecraft flew through the Jovian magnetosphere on the dusk side. Here, we present results from a novel means of detecting energetic electrons along New Horizons' trajectory: the background count rate of the Alice ultraviolet spectrograph. Electrons with energies &gt;1 MeV can penetrate the thin aluminum housing of Alice, interact with the microchannel plate detector, and produce a count that is indistinguishable from an FUV photon. We present Alice data, proportional to the MeV electron flux, from an 11‐day period centered on the spacecraft's closest approach to Jupiter, and compare it to electron data from the PEPSSI instrument. We find that a solar wind compression event passed over the spacecraft just prior to it entering the Jovian magnetosphere. Subsequently, the magnetopause boundary was detected at a distance of 67 RJ suggesting a compressed magnetospheric configuration. Three days later, when the spacecraft was 35–90 RJ downstream of Jupiter, New Horizons observed a series of 15 current sheet crossings, all of which occurred significantly northward of model predictions implying solar wind influence over the middle and outer Jovian magnetosphere, even to radial distances as small as ∼35 RJ. In addition, we find the Jovian current sheet, which had a half‐thickness of at least 7.4 RJ between 1930 and 2100 LT abruptly thinned to a thickness of ∼3.4 RJ around 2200 LT.

INVESTIGATING THE RELIABILITY OF CORONAL EMISSION MEASURE DISTRIBUTION DIAGNOSTICS USING THREE-DIMENSIONAL RADIATIVE MAGNETOHYDRODYNAMIC SIMULATIONS

Determining the temperature distribution of coronal plasmas can provide stringent constraints on coronal heating. Current observations with the Extreme ultraviolet Imaging Spectrograph onboard Hinode and the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory provide diagnostics of the emission measure distribution (EMD) of the coronal plasma. Here we test the reliability of temperature diagnostics using 3D radiative MHD simulations. We produce synthetic observables from the models, and apply the Monte Carlo Markov chain EMD diagnostic. By comparing the derived EMDs with the "true" distributions from the model we assess the limitations of the diagnostics, as a function of the plasma parameters and of the signal-to-noise of the data. We find that EMDs derived from EIS synthetic data reproduce some general characteristics of the true distributions, but usually show differences from the true EMDs that are much larger than the estimated uncertainties suggest, especially when structures with significantly different density overlap along the line-of-sight. When using AIA synthetic data the derived EMDs reproduce the true EMDs much less accurately, especially for broad EMDs. The differences between the two instruments are due to the: (1) smaller number of constraints provided by AIA data, (2) broad temperature response function of the AIA channels which provide looser constraints to the temperature distribution. Our results suggest that EMDs derived from current observatories may often show significant discrepancies from the true EMDs, rendering their interpretation fraught with uncertainty. These inherent limitations to the method should be carefully considered when using these distributions to constrain coronal heating.

Rapid-response mode VLT/UVES spectroscopy of super iron-rich gas exposed to GRB 080310

We analyse high-resolution near-UV and optical spectra of the afterglow of GRB 080310, obtained with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (VLT/UVES), to investigate the circumburst environment and the interstellar medium of the gamma-ray burst (GRB) host galaxy. The VLT rapid-response mode (RRM) enabled the observations to start only 13 minutes after the Swift trigger and a series of four exposures to be collected before dawn. A low neutral-hydrogen column-density (log N (HI) = 18.7) is measured at the host-galaxy redshift of z = 2.42743. At this redshift, we also detect a large number of resonance ground-state absorption lines (e.g., CII, MgII, AlII, SiII, CrII, CIV, SiIV), as well as time-varying absorption from the fine-structure levels of FeII. Time-varying absorption from a highly excited FeIII energy level (7S3), giving rise to the so-called UV34 line triplet, is also detected, for the first time in a GRB afterglow. The CrII ground-state and all observed FeII energy levels are found to depopulate with time, whilst the FeIII 7S3 level is increasingly populated. This absorption-line variability is clear evidence of ionization by the GRB, which is for the first time conclusively observed in a GRB afterglow spectrum. We derive ionic column densities at each epoch of observations by fitting absorption lines with a four-component Voigt-profile model. We perform CLOUDY photo-ionization modelling of the expected pre-burst ionic column densities, to estimate that, before the onset of the burst, [C/H] = -1.3 \pm 0.2, [O/H] < -0.8, [Si/H] = -1.2 \pm 0.2, [Cr/H] = +0.7 \pm 0.2, and [Fe/H] = +0.2 \pm 0.2 for the integrated line profile, indicating strong overabundances of iron and chromium. For one of the components, we observe even more extreme ratios of [Si/Fe] \leq -1.47 and [C/Fe] \leq -1.74. [abridged]

The12C2/12C13C isotopic ratio in comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR)

Context. Measuring the carbon isotope abundance ratio in comets allows one to constrain the conditions in the outer protosolar nebula. Different measurements of the 12 C/ 13 C ratio, using various molecules, have already been published for different solar system objects, such as the Sun, the Earth, the Moon, asteroids, planets, or comets. So far, all these measurements are consistent with 12 C/ 13 C ∼ 90, but significant differences have been observed. This ratio is remarkably constant in comets (91.0 ± 3.6) for studies based on the CN radical, but it presents stronger variations in studies based on other radicals. Aims. This paper aims at measuring the 12 C/ 13 C ratio in two bright Oort cloud comets using the 12 C 2 and 12 C 13 C emission lines and an improved method. The ratios will be compared to those obtained for the same comets with another radical, CN. Methods. We used the (2, 1) and (1, 0) bandheads of the 12 C 13 C, near 4723 and 4745 A to measure the 12 C/ 13 C ratio and compared their intensity to the 12 C 2 lines of the same bands. We developed a model for interpreting observational data obtained at high resolution (∼70000) using the 8.2-m Kueyen telescope (UT2) of the Very Large Telescope (VLT) with the Ultraviolet and Visual Echelle Spectrograph (UVES) in two comets: C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR). Results. Our modeling has provided 12 C/ 13 C = 85 ± 20 for C/2002 T7 (LINEAR) and 80 ± 20 for C/2001 Q4 (NEAT). These values are compatible with previous measurements performed with the CN radical.

New findings expand Apollo observations of lunar atmosphere

In December 1972 the astronauts of Apollo 17—the last manned mission to the moon—deployed the Lunar Atmospheric Composition Experiment (LACE), a spectrometer designed to measure and characterize the thin lunar atmosphere. Forty years later, Stern et al built upon those initial measurements, providing the first remotely sensed measurement of the Moon's gaseous environment from lunar orbit. Using the Lyman Alpha Mapping Project's (LAMP) far ultraviolet spectrograph aboard the Lunar Reconnaissance Orbiter, the authors determined the atmospheric concentration of helium.

Absorption signatures of warm-hot gas at low redshift: broad H i Lyα absorbers

We investigate the physical state of HI absorbing gas at low redshift (z = 0.25) using a subset of cosmological, hydrodynamic simulations from the OWLS project, focusing in particular on broad (b_HI > 40 km/s) HI Lyman-Alpha absorbers (BLAs), which are believed to originate in shock-heated gas in the warm-hot intergalactic medium (WHIM). Our fiducial model, which includes radiative cooling by heavy elements and feedback by supernovae and active galactic nuclei, predicts that by z = 0.25 nearly 60 per cent of the gas mass ends up at densities and temperatures characteristic of the WHIM and we find that half of this fraction is due to outflows. The standard HI observables (distribution of HI column densities N_HI, distribution of Doppler parameters b_HI, b_HI - N_HI correlation) and the BLA line number density predicted by our simulations are in remarkably good agreement with observations. BLAs arise in gas that is hotter, more highly ionised and more enriched than the gas giving rise to typical Lyman-Alpha forest absorbers. The majority of the BLAs arise in warm-hot (log (T/K) ~ 5) gas at low (log Delta < 1.5) overdensities. On average, thermal broadening accounts for at least 60 per cent of the BLA line width, which in turn can be used as a rough indicator of the thermal state of the gas. Detectable BLAs account for only a small fraction of the true baryon content of the WHIM at low redshift. In order to detect the bulk of the mass in this gas phase, a sensitivity at least one order of magnitude better than achieved by current ultraviolet spectrographs is required. We argue that BLAs mostly trace gas that has been shock-heated and enriched by outflows and that they therefore provide an important window on a poorly understood feedback process.

Constraining the variation of fundamental constants at z ∼ 1.3 using 21-cm absorbers

We present high resolution optical spectra obtained with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) and 21-cm absorption spectra obtained with the Giant Metrewave Radio Telescope (GMRT) and the Green Bank Telescope (GBT) of five quasars along the line of sight of which 21-cm absorption systems at 1.17 < z < 1.56 have been detected previously. We also present milliarcsec scale radio images of these quasars obtained with the Very Large Baseline Array (VLBA). We use the data on four of these systems to constrain the time variation of x = g_p*alpha^2/mu where g_p is the proton gyromagnetic factor, alpha is the fine structure constant, and mu is the proton-to-electron mass ratio. We carefully evaluate the systematic uncertainties in redshift measurements using cross-correlation analysis and repeated Voigt profile fitting. In two cases we also confirm our results by analysing optical spectra obtained with the Keck telescope. We find the weighted and the simple means of Delta_x / x to be respectively -(0.1 +/- 1.3)x10^-6 and (0.0 +/- 1.5)x10^-6 at the mean redshift of <z> = 1.36 corresponding to a look back time of ~ 9 Gyr. This is the most stringent constraint ever obtained on Delta_x / x. If we only use the two systems towards quasars unresolved at milliarcsec scales, we get the simple mean of Delta_x / x = + (0.2 +/- 1.6)x10^-6. Assuming constancy of other constants we get Delta_alpha / alpha = (0.0 +/- 0.8)x10^-6 which is a factor of two better than the best constraints obtained so far using the Many Multiplet Method. On the other hand assuming alpha and g_p have not varied we derive Delta_mmu / mu = (0.0 +/- 1.5)x10^-6 which is again the best limit ever obtained on the variation of mu over this redshift range. [Abridged]

Saturn upper atmospheric structure from Cassini EUV and FUV occultations 1This article is part of a Special Issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career.

Stellar occultations of the Saturn atmosphere using the Cassini ultraviolet imaging spectrograph (UVIS) experiment have provided vertical structure at a range of latitudes. The transmission spectra in the extreme–far ultraviolet (EUV–FUV) range allow extraction of vertical profiles of H2 and hydrocarbon abundances from the top of the atmosphere to about 300 km above the 1 bar (1 bar = 100 kPa) pressure level. A reanalysis of the Voyager 2 δSco occultation in 1981 is consistent with the original report. The hydrocarbon homopause is near a pressure of 0.2 μbar in the UVIS analysis, compared to ∼0.01 μbar obtained from the Voyager occultation. Measured hydrocarbon abundances are obtained in the pressure range 600–0.1 μbar in the Cassini UVIS experiment. The combined UVIS results provide evidence for significant latitudinal dependence of vertical temperature profile. The confinement of the hydrocarbons in the current observations compared to published models and the Voyager ultraviolet spectrograph (UVS) results at solar maximum, infer smaller eddy diffusion coefficients in this epoch. Model calculations indicate that the latitudinal dependence of H2 vertical displacements is caused primarily by the combined effects of gravitational potential and evident differences in electron energy deposition at the top of the atmosphere affecting the temperature profile. The derived H2 density profiles from ~–40° latitude and others close to the equator, are found to be nearly identical on a pressure scale below the exobase. The inference is that that the pressure profile of H2 density at Saturn is unchanged over a broad range of latitudes.

The intergalactic medium thermal history at redshiftz= 1.7-3.2 from the Lyα forest: a comparison of measurements using wavelets and the flux distribution

We investigate the thermal history of the intergalactic medium (IGM) in the redshift interval z=1.7--3.2 by studying the small-scale fluctuations in the Lyman alpha forest transmitted flux. We apply a wavelet filtering technique to eighteen high resolution quasar spectra obtained with the Ultraviolet and Visual Echelle Spectrograph (UVES), and compare these data to synthetic spectra drawn from a suite of hydrodynamical simulations in which the IGM thermal state and cosmological parameters are varied. From the wavelet analysis we obtain estimates of the IGM thermal state that are in good agreement with other recent, independent wavelet-based measurements. We also perform a reanalysis of the same data set using the Lyman alpha forest flux probability distribution function (PDF), which has previously been used to measure the IGM temperature-density relation. This provides an important consistency test for measurements of the IGM thermal state, as it enables a direct comparison of the constraints obtained using these two different methodologies. We find the constraints obtained from wavelets and the flux PDF are formally consistent with each other, although in agreement with previous studies, the flux PDF constraints favour an isothermal or inverted IGM temperature-density relation. We also perform a joint analysis by combining our wavelet and flux PDF measurements, constraining the IGM thermal state at z=2.1 to have a temperature at mean density of T0/[10^3 K]=17.3 +/- 1.9 and a power-law temperature-density relation exponent gamma=1.1 +/- 0.1 (1 sigma). Our results are consistent with previous observations that indicate there may be additional sources of heating in the IGM at z<4.