Opacity Variations Research Articles

Correlated spectral variability in brown dwarfs

Abstract Models of brown dwarf atmospheres suggest they exhibit complex physical behaviour. Observations have shown that they are indeed dynamic, displaying small photometric variations over time-scales of hours. Here, I report results of infrared (0.95–1.64 μm) spectrophotometric monitoring of four field L and T dwarfs spanning time-scales of 0.1–5.5 h, the goal being to learn more about the physical nature of this variability. Spectra are analysed differentially with respect to a simultaneously observed reference source in order to remove Earth-atmospheric variations. The variability amplitude detected is typically 2–10 per cent, depending on the source and wavelength. I analyse the data for correlated variations between spectral indices. This approach is more robust than single band or χ2 analyses, because it does not assume an amplitude for the (often uncertain) noise level (although the significance test still assumes a shape for the noise power spectrum). Three of the four targets show significant evidence for correlated variability. Some of this can be associated with specific features including Fe, FeH, VO and K i, and there is good evidence for intrinsic variability in H2O and possibly also CH4. Yet some of this variability covers a broader spectral range which would be consistent with dust opacity variations. The underlying common cause is plausibly localized temperature or composition fluctuations caused by convection. Looking at the high signal-to-noise ratio stacked spectra, we see many previously identified spectral features of L and T dwarfs, such as K i, Na i, FeH, H2O and CH4. In particular, we may have detected methane absorption at 1.3–1.4 μm in the L5 dwarf SDSS 0539−0059.

Modeling hydrogen peroxide bleaching to predict optical properties of bleached hardwood CMP

Due to the variations in opacity and brightness of peroxide bleached pulp at Mazandaran Wood and Paper Industries Company (MWPI), empirical models were developed to predict chemimechanical pulp (CMP) brightness and opacity from peroxide bleaching conditions and to drive the optimum operating conditions. To overcome the inconsistency problem, a multi-variate regression analysis method was used for model building. The models were then validated using a new data set from the bleach plant at MWPI, assessing the models’ predictive ability and performance. The results show that there is a relationship between bleaching variables and such dependent variables as pulp brightness and opacity. In addition to the hydrogen peroxide charge and pulp initial brightness, the initial opacity had a significant reverse effect on the final CMP brightness. It was also found that the concentration of total Na+ in the CMP tower was the most important variable affecting the final pulp opacity. The validation results demonstrated that these models can be employed as useful tools for process optimization purposes.

The effect of the intergalactic environment on the observability of Lyα emitters during reionization

Observations of high-redshift Lya sources are a major tool for studying the high-redshift universe and are one of the most promising ways to constrain the later stages of reionization. The understanding and interpretation of the data is far from straightforward, however. We discuss the effect of the reionizing intergalactic medium (IGM) on the observability of Lya sources based on large simulations of early structure formation with radiative transfer. This takes into account self-consistently the reionization history, density, velocity and ionization structures and non-linear source clustering. We find that all fields are highly anisotropic and as a consequence there are very large variations in opacity among the different lines of sight. The velocity effects, from both infall and source peculiar velocity are most important for the luminous sources, affecting the line profile and depressing the bright end of the luminosity function. The line profiles are generally asymmetric and the line centres of the luminous sources are always absorbed due to the high density of the local IGM. For both luminous and average sources the damping wing effects are of similar magnitude and remain significant until fairly late, when the IGM is ionized between 30 and 70 per cent by mass.

Correlation between Infrared Colors and Intensity Ratios of SiO Maser Lines

We present the results of SiO millimeter line observations of a sample of known SiO maser sources covering a wide dust temperature range. A cold part of the sample was selected from the SiO maser sources found in our recent SiO maser survey of cold dusty objects. The aim of the present research is to investigate the causes of the correlation between infrared colors and SiO maser intensity ratios among different transition lines. In particular, the correlation between infrared colors and SiO maser intensity ratio among the J = 1-0 v = 1, 2, and 3 lines are our main concern in this paper. We observed in total 75 SiO maser sources with the Nobeyama 45 m telescope quasi-simultaneously in the SiO J = 1-0 v = 0, 1, 2, 3, 4 and J = 2-1 v = 1, 2 lines. We also observed the sample in the 29SiO J = 1-0 v = 0 and J = 2-1 v = 0 lines, 30SiO J = 1-0 v = 0 line, and the H2O 61,6-52,3 line. As reported in previous papers, we confirmed that the intensity ratios of the SiO J = 1-0 v = 2 to v = 1 lines clearly correlate with infrared colors. In addition, we found possible correlation between infrared colors and the intensity ratios of the SiO J = 1-0 v = 3 to v = 1 and 2 lines. Two overlap lines of H2O (i.e., 116,6 ν2 = 1 → 127,5 ν2 = 0 and 50,5 ν2 = 2 → 63,4 ν2 = 1) might explain this correlation if these overlap lines become stronger with an increase of infrared colors, although the phenomena also might be explained in more fundamental ways if we take into account the variation of opacity from object to object.

Restricted fish feeding reduces cod otolith opacity

Summary The purpose of this work was to examine the effect of reduced feeding and constant temperature on cod otolith opacity. Three groups of juvenile cod were given restricted food rations at different times for 4 months, resulting in depressed somatic growth. Otolith opacity was measured on pictures of the otolith sections. The otolith carbonate deposited during the experimental period was generally opaque compared to the more translucent otolith material deposited prior to and after the experimental period, when the fish were kept in a pond and in sea-cages at higher temperatures. Large variations in otolith opacity were found between individual fish both within groups and between groups. In two of the three groups significantly more translucent otolith material was deposited in response to reduced feeding. Our results show that variations in feeding and hence fish growth resulted in variation in otolith opacity, but the effect was minor compared to that of variations in ambient temperature. The combined influence of these effects, which both act on fish metabolism, are most likely controlling the seasonal opacity changes observed in wild fish. Our results help explain the variations seen in fish at constant temperatures.

Spectral information retrieval from integrated broadband photodiode Martian ultraviolet measurements

We propose an algorithm to retrieve the global features of the spectral dependence of the ultraviolet (UV) irradiance from integrated, broadband UV measurements performed with a set of photodiodes with different UV filters. This fit, when applied to ground-based measurements and compared with the incident Solar spectral irradiance on the top of the atmosphere, may be used to extract the spectral dependence of the UV opacity and the most relevant parameters characterizing the scattering with atmospheric aerosol (Angstrom exponent, etc.) as well as the biological effective doses. In this way, using a set of photodiodes instead of a spectrophotometer, one may get spectral information within very low mass, package, and weight constraints, which is particularly useful for space missions. We consider its application for the rover-based exploration of the Martian ground, which is subject to daily and seasonal opacity variations.

The transverse proximity effect in spectral hardness on the line of sight towards HE 2347–4342

We report the discovery of 14 quasars in the vicinity of HE2347-4342, one of the two quasars whose intergalactic HeII forest has been resolved with FUSE. By analysing the HI and the HeII opacity variations separately, no transverse proximity effect is detected near three foreground quasars of HE2347-4342: QSOJ23503-4328 (z=2.282, $\vartheta=3.59$ arcmin), QSOJ23500-4319 (z=2.302, $\vartheta=8.77$ arcmin) and QSOJ23495-4338 (z=2.690, $\vartheta=16.28$ arcmin). This is primarily due to line contamination and overdensities probably created by large-scale structure. By comparing the HI absorption and the corresponding HeII absorption, we estimated the fluctuating spectral shape of the extragalactic UV radiation field along this line of sight. We find that the UV spectral shape near HE2347-4342 and in the projected vicinity of the three foreground quasars is statistically harder than expected from UV background models dominated by quasars. In addition, we find three highly ionised metal line systems near the quasars. However, they do not yield further constraints on the shape of the ionising field. We conclude that the foreground quasars show a transverse proximity effect that is detectable as a local hardening of the UV radiation field, although the evidence is strongest for QSOJ23495-4338. Thus, the relative spectral hardness traces the proximity effect also in overdense regions prohibiting the traditional detection in the HI forest. Furthermore, we emphasise that softening of quasar radiation by radiative transfer in the intergalactic medium is important to understand the observed spectral shape variations. From the transverse proximity effect of QSOJ23495-4338 we obtain a lower limit on the quasar lifetime of ~25 Myr.

Stellar occultations at UV wavelengths by the SPICAM instrument: Retrieval and analysis of Martian haze profiles

Observations made by the SPICAM ultraviolet spectrometer on board the Mars Express orbiter are presented. We focus on several hundreds of atmospheric profiles which have been collected over 3/4 of a Martian year by making use of the stellar occultation technique. The typical structure of the Martian haze possesses at least one discrete layer (60% of all cases) standing over an extended portion wherein opacity continuously increases down to the surface. Differences of morphology are, however, noted between profiles observed near the equator and profiles collected elsewhere. The Martian haze exhibits a pronounced seasonal signal manifested by variations of the maximum elevation at which particles are observed. For reasons related to both convective activity and changes in the hygropause level, cold regions display a much lower hazetop than warm regions. Using UV spectrometry data, we put constraints on haze microphysical properties. Vertical variations of particle size are keyed to variations of opacity; e.g., an increase of particle size is systematically observed near extinction peaks. This is the likely consequence of cloud formation which results into a local increase of particle cross section. Despite marked differences of aerosol profiles between low and high latitudes, haze properties above 60 km remain invariant, possibly reflecting the long‐term presence of a background submicronic particle population. Several profiles have been analyzed in more detail to extract properties of detached cloud layers lofted above 40 km. Their optical depth ranges between 0.01 and 0.1 in the visible. Estimation of cloud particle size is technically restricted because of SPICAM wavelength sampling, but it generally yields a minimum radius value of about 0.3 μm, while several estimates are consistent with a robust 0.1–0.2 μm. This crystal size, significantly smaller than the 1 to 4 μm associated with recently classified type I and II clouds, suggests that a different class of clouds, henceforth type III clouds, can be extracted from our data. Observations made in the southern winter polar night indicate a very distinct aerosol behavior where particles are less abundant (τ < 0.1), confined to lower heights (vertical profile consistent with a Conrath parameter exceeding 0.04) and made of particles having a radius on the order of 0.1 μm. This shows that the Martian polar night is a region with a very clean atmosphere and with a distinct type of aerosols.

Remote sensing of Venus’ lower atmosphere from ground-based IR spectroscopy: Latitudinal and vertical distribution of minor species

We present an investigation of the lower atmosphere in the 25–40 km altitude range from data of Venus’ night side in the 2.3- μ m region. We used observations recorded at the NASA/IRTF in August 2004 with the SpeX spectro-imager. The previous conclusions of Marcq et al. [2005, Latitudinal variations of CO and OCS in the lower atmosphere of Venus from near-infrared nightside spectro-imaging, Icarus 179, 375–386] about CO and OCS abundance latitudinal variations are confirmed with a better accounting for cloud opacity variations; in particular, an enhancement of 12 ± 4 % in CO at 36 km is found at 40 ∘ S with respect to the equator. There is also a depletion of OCS southward of 20 ∘ S, that may be correlated with the CO increase. Information on the vertical gradients of the CO and OCS profiles was derived, yielding the following mixing ratios in the probed altitude range: CO ( 30 km ) = 22.5 ± 3.5 ppm , CO ( 36 km ) = 24 ± 2 ppm ; OCS ( 30 km ) = 5 – 20 ppm , OCS ( 36 km ) = 0.55 ± 0.15 ppm . The study was also extended to H 2 O and no latitudinal variations are found beyond our error bars ( 26 ± 4 ppm at 35 km ).

Quasars near the line of sight towards Q 0302-003 and the transverse proximity effect

We report the discovery of the faint (V=21.7) quasar QSO03027-0010 at z=2.808 in the vicinity of Q0302-003, one of the few quasars observed with STIS to study intergalactic HeII absorption. Together with another newly discovered QSO at z=2.29, there are now 6 QSOs known near the line of sight towards Q0302-003, of which 4 are located within the redshift region 2.76<=z<=3.28 covered by the STIS spectrum. We correlated the opacity variations in the HI and HeII Lyman forest spectra with the locations of known quasars. There is no significant proximity effect in the HI Lyman alpha forest for any of the QSOs, except for the well-known line of sight effect for Q0302-003 itself. By comparing the absorption properties in HI and HeII, we estimated the fluctuating hardness of the extragalactic UV radiation field along this line of sight. We find that close to each foreground quasar, the ionizing background is considerably harder than on average. In particular, our newly discovered QSO03027-0010 shows such a hardness increase despite being associated with an overdensity in the HI Lyman forest. We argue that the spectral hardness is a sensitive physical measure to reveal the influence of QSOs onto the UV background even over scales of several Mpc, and that it breaks the density degeneracy hampering the traditional transverse proximity effect analysis. We infer from our sample that there is no need for significantly anisotropic UV radiation from the QSOs. From the transverse proximity effect detected in the sample we obtain minimum quasar lifetimes in the range ~10-30 Myr.

A High‐Resolution Comparative Study of the Slowly Contracting Starless Cores L694‐2 and L1544

We present interferometric observations of N2H+(1-0) in the starless, dense core L694-2 and compare them to previously published maps of L1544. Both cores are starless, centrally condensed, and show spectral signatures of rotation and collapse. We fit radially averaged spectra using a two-layer infall model and measure the variation of opacity and infall speed in each core. Both functions increase toward the center of each core, but the radial gradients are shallower, and the central values lower, in L694-2. This general behavior is predicted in models of gravitational collapse with thermal plus magnetic support, and the lower values in L694-2 may be due to its lower mass or a slightly earlier evolutionary state. In either case, it appears that both cores will form stars within a few 104 yr.

Taxing the rich: recombinations and bubble growth during reionization

Reionization is inhomogeneous for two reasons: the clumpiness of the intergalactic medium (IGM), and clustering of the discrete ionizing sources. While numerical simulations can in principle take both into account, they are at present limited by small box sizes. On the other hand, analytic models have only examined the limiting cases of a clumpy IGM (with uniform ionizing emissivity) and clustered sources (embedded in a uniform IGM). Here, we present the first analytic model that includes both factors. At first, recombinations can be ignored and ionized bubbles grow primarily through major mergers, because at any given moment the bubbles have a well-defined characteristic size. As a result, reionization resembles ‘punctuated equilibrium,’ with a series of well-separated sharp jumps in the ionizing background. These features are local effects and do not reflect similar jumps in the global ionized fraction. We then combine our bubble model with a simple description of recombinations in the IGM. We show that the bubbles grow until recombinations balance ionizations, when their expansion abruptly halts. If the IGM density structure is similar to that at moderate redshifts, this limits the bubble radii to ∼20 comoving Mpc; however, if the IGM is significantly clumpier at higher redshifts (because of minihalo formation, for example), the limit could be much smaller. Once a bubble reaches saturation, that region of the Universe has for all intents and purposes entered the ‘post-overlap’ stage. Because different H ii regions saturate over a finite time interval, the overlap epoch actually has a finite width. Our model also predicts a mean recombination rate several times larger than expected for a uniformly illuminated IGM. This picture naturally explains the substantial large-scale variation in Lyman-series opacity along the lines of sight to the known z > 6 quasars. More quasar spectra will shed light on the transition between the ‘bubble-dominated’ topology characteristic of reionization and the ‘web-dominated’ topology characteristic of the later Universe.

Spatial and Temporal Variations in Small-Scale Galactic HiStructure toward 3C 138

Several recent studies of Galactic H I absorption toward background quasars and pulsars have provided evidence that there are opacity changes in the neutral Galactic interstellar medium on size scales as small as a few AU. The nature of these opacity variations has remained a matter of debate but could reflect a variety of physical processes, including changes in the H I spin temperature or gas density. We present three epochs of Very Long Baseline Array (VLBA) observations of Galactic H I absorption toward the quasar 3C 138 with resolutions of 20 mas (~10 AU). This analysis includes VLBA data from observations in 1999 and 2002 along with a reexamination of the 1995 VLBA data, reported by Faison and coworkers. Improved data reduction and imaging techniques have led to an order of magnitude improvement in sensitivity compared to previous work. With these new data we confirm the previously detected milliarcsecond-scale spatial variations in the H I opacity at the level of Δτmax = 0.50 ± 0.05. The typical size scale of the optical depth variations is ~50 mas or 25 AU. In addition, for the first time we see clear evidence for temporal variations in the H I opacity over the 7 yr time span of our three epochs of data. We also attempt to detect the magnetic field strength in the H I gas using the Zeeman effect. From this analysis we have been able to place a 3 σ upper limit on the magnetic field strength per pixel of ~45 μG. We have also been able to calculate for the first time the plane-of-sky covering fraction of the small-scale H I gas of ~10%. This small covering fraction suggests that the filling factor of such gas is quite low, in agreement with recent optical observations. We also find that the line widths of the milliarcsecond-scale H I features are comparable to those determined from previous single-dish measurements toward 3C 138, suggesting that the opacity variations cannot be due to changes in the H I spin temperature. From these results we favor a density enhancement interpretation for the small-scale H I structures, although these enhancements appear to be of short duration and are unlikely to be in equilibrium.

Methane absorption in the atmosphere of Jupiter from 1800 to 9500 cm −1 and implications for vertical cloud structure

New measurements of the low-temperature near-infrared absorption of methane (Sihra, 1998, Laboratory measurements of near-infrared methane bands for remote sensing of the jovian atmosphere, Ph.D. thesis, University of Oxford) have been combined with existing, longer path-length, higher-temperature data of Strong et al. (1993, Spectral parameters of self- and hydrogen-broadened methane from 2000 to 9500 cm −1 for remote sounding of the atmosphere of Jupiter, J. Quant. Spectrosc. Radiat. Trans. 50, 309–325) and fitted with band models. The combined data set is found to be more consistent with previous low-temperature methane absorption measurements than that of Strong et al. (1993, J. Quant. Spectrosc. Radiat. Trans. 50, 309–325) but covers the same wider wavelength range and accounts for both self- and hydrogen-broadening conditions. These data have been fitted with k-coefficients in the manner described by Irwin et al. (1996, Calculated k-distribution coefficients for hydrogen- and self-broadened methane in the range 2000–9500 cm −1 from exponential sum fitting to band modelled spectra, J. Geophys. Res. 101, 26,137–26,154) and have been used in multiple-scattering radiative transfer models to assess their impact on our previous estimates of the jovian cloud structure obtained from Galileo Near-Infrared Mapping Spectrometer (NIMS) observations (Irwin et al., 1998, Cloud structure and atmospheric composition of Jupiter retrieved from Galileo NIMS real-time spectra, J. Geophys. Res. 103, 23,001–23,021; Irwin et al., 2001, The origin of belt/zone contrasts in the atmosphere of Jupiter and their correlation with 5-μm opacity, Icarus 149, 397–415; Irwin and Dyudina, 2002, The retrieval of cloud structure maps in the equatorial region of Jupiter using a principal component analysis of Galileo/NIMS data, Icarus 156, 52–63). Although significant differences in methane opacity are found at cooler temperatures, the difference in the optical depth of the atmosphere due to methane is found to diminish rapidly with increasing pressure and temperature and thus has negligible effect on the cloud structure inferred at deeper levels. Hence the main cloud opacity variation is still found to peak at around 1–2 bar using our previous analytical approach, and is thus still in disagreement with Galileo Solid State Imager (SSI) determinations (Banfield et al., 1998, Jupiter's cloud structure from Galileo imaging data, Icarus 135, 230–250; Simon-Miller et al., 2001, Color and the vertical structure in Jupiter's belts, zones and weather systems, Icarus 154, 459–474) which place the main cloud deck near 0.9 bar. Further analysis of our retrievals reveals that this discrepancy is probably due to the different assumptions of the two analyses. Our retrievals use a smooth vertically extended cloud profile while the SSI determinations assume a thin NH 3 cloud below an extended haze. When the main opacity in our model is similarly assumed to be due to a thin cloud below an extended haze, we find the main level of cloud opacity variation to be near the 1 bar level—close to that determined by SSI and moderately close to the expected condensation level of ammonia ice of 0.85 bar, assuming that the abundance of ammonia on Jupiter is ( 7 ± 1 ) × 10 −4 (Folkner et al., 1998, Ammonia abundance in Jupiter's atmosphere derived from the attenuation of the Galileo probe's radio signal, J. Geophys. Res. 103, 22,847–22,855; Atreya et al., 1999, A comparison of the atmospheres of Jupiter and Saturn: deep atmospheric composition, cloud structure, vertical mixing, and origin, Planet. Space Sci. 47, 1243–1262). However our data in the 1–2.5 μm range have good height discrimination and our lowest estimate of the cloud base pressure of 1 bar is still too great to be consistent with the most recent estimates of the ammonia abundance of 3.5 × solar. Furthermore the observed limited spatial distribution of ammonia ice absorption features on Jupiter suggests that pure ammonia ice is only present in regions of localised vigorous uplift (Baines et al., 2002, Fresh ammonia ice clouds in Jupiter: spectroscopic identification, spatial distribution, and dynamical implications, Icarus 159, 74–94) and is subsequently rapidly modified in some way which masks its pure absorption features. Hence we conclude that the main cloud deck on Jupiter is unlikely to be composed of pure ammonia ice and instead find that it must be composed of either NH 4SH or some other unknown combination of ammonia, water, and hydrogen sulphide and exists at pressures of between 1 and 2 bar.

On the latitudinal distribution of Titan's haze at the Voyager epoch

We have analysed all the available high phase angle images of Titan limb taken by Voyager 1 and 2, in early 1980. For several different phase angles and wavelengths, we seek for a consistent set of haze parameters able to fit all data simultaneously. Our main purpose is to obtain an accurate estimate of the latitudinal variation of haze opacity at 200 km altitude at the time of the Voyager flyby's. We find that haze opacity at 200 km is about constant in the southern hemisphere and drops between equator and 60 ∘ N by about 30–50%, sharply increasing again beyond 60 ∘ N . The latter feature is clearly due to the north polarhood. This behaviour is opposite to total optical depth variations retrieved from IRIS observation, at the same epoch. The IRIS data refer to levels below 100 km altitude. A comparison of our results with calculations from a general circulation model, shows that (1) our results are realistic and can be considered as robust (2) the opacity variations at 200 km (this work) and at ground (IRIS data), although opposite, are not inconsistent with each other.

A Comparative Study of the Spatial Distribution of Ultraviolet and Far-Infrared Fluxes from M101

The total ultraviolet (UV) flux (from 1412 to 2718 A) of M101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 μm) using the maps of the galaxy taken by the Galaxy Evolution Explorer (GALEX) in the near-UV and far-UV and by the Infrared Space Observatory (ISO) at 60, 100, and 170 μm. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from ~4 in the nuclear region to nearly zero toward the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth that decreases with radius and that dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio of taking on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

Image of Fomalhaut Dust Ring at 350 Microns: The Relative Column Density Map Shows Pericenter-Apocenter Asymmetry

We have imaged the circumstellar disk of Fomalhaut at 350 mm wavelength, using SHARC II (Submillimeter High Angular Resolution Camera II) at the Caltech Submillimeter Observatory. The spatial resolution of the raw images (9) has been enhanced by a factor of 3 using the HiRes deconvolution procedure. We find that at this wavelength and signal-to-noise ratio (approx.12), the observed morphology is that of a simple inclined ring (i approx. 70 deg), with little or no other apparent structure--this is the first observation that shows clearly the ring morphology of the disk. We have combined our 350 mm data with Spitzer Space Telescope images at 24, 70, and 160 mm in order to estimate the two-dimensional spatial variation of relative column density (tau map) using our DISKFIT procedure. The tau map is based on the following physical assumptions: (1) the wavelength variation of opacity is the same throughout the disk, (2) the radial variation of dust temperature is dictated by the energy balance of individual grains in the stellar radiation field, and (3) the vertical scale height of the disk follows a power-law radial variation. The results confirm the ringlike morphology but also show that the geometric center is displaced from the star by about 8 AU and that the ring has an apocentric enhancement of approximately 14% in integrated column density. If we interpret the displacement in terms of elliptical orbital motion due to gravitational perturbation by an unseen planet, then the implied forced eccentricity is ~0.06; dynamical modeling then predicts an apocentric density enhancement consistent with that inferred from the tau map.

Opacity Variations in the Ionized Absorption in NGC 3783: A Compact Absorber

We show that the Fe VII-Fe XII M-shell unresolved transition array (UTA) in the Chandra HETGS observation of NGC 3783 (900 ks) clearly changes in opacity on a timescale of 31 days, responding to a factor of ~2 change in the ionizing continuum. The opacity variation is observed at a level >10 σ. There is also evidence for variability in the O VI K edge (at ~3 σ). The observed changes are consistent with the gas producing these absorption features (i.e., the low-ionization component) being close to photoionization equilibrium. The gas responsible for the Fe XVII-Fe XXII L-shell absorption (i.e., the high-ionization component) does not seem to be responding as expected in photoionization equilibrium. The observed change in opacity for the UTA implies a density >1 × 104 cm-3, thus locating the gas within 6 pc of the X-ray source. The scenario in which the gas is composed of a continuous radial range of ionization structures is ruled out, as in such scenario, no opacity variations are expected. Rather, the structure of the absorber is likely composed of heavily clumped gas.

Differences in membrane fluidity and fatty acid composition between phenotypic variants of Streptococcus pneumoniae.

Phase variation in the colonial opacity of Streptococcus pneumoniae has been implicated as a factor in the pathogenesis of pneumococcal disease. This study examined the relationship between membrane characteristics and colony morphology in a few selected opaque-transparent couples of S. pneumoniae strains carrying different capsular types. Membrane fluidity was determined on the basis of intermolecular excimerization of pyrene and fluorescence polarization of 1,6-diphenyl 1,3,5-hexatriene (DPH). A significant decrease, 16 to 26% (P < or = 0.05), in the excimerization rate constant of the opaque variants compared with that of the transparent variants was observed, indicating higher microviscosity of the membrane of bacterial cells in the opaque variants. Liposomes prepared from phospholipids of the opaque phenotype showed an even greater decrease, 27 to 38% (P < or = 0.05), in the pyrene excimerization rate constant compared with that of liposomes prepared from phospholipids of bacteria with the transparent phenotype. These findings agree with the results obtained with DPH fluorescence anisotropy, which showed a 9 to 21% increase (P < or = 0.001) in the opaque variants compared with the transparent variants. Membrane fatty acid composition, determined by gas chromatography, revealed that the two variants carry the same types of fatty acids but in different proportions. The trend of modification points to the presence of a lower degree of unsaturated fatty acids in the opaque variants compared with their transparent counterparts. The data presented here show a distinct correlation between phase variation and membrane fluidity in S. pneumoniae. The changes in membrane fluidity most probably stem from the observed differences in fatty acid composition.

The Fluctuating Intergalactic Radiation Field at Redshiftsz= 2.3–2.9 from Heiiand HiAbsorption toward HE 2347−4342

We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer and the Ultraviolet-Visual Echelle Spectrograph on the Very Large Telescope. Following up on our earlier study, we focus here on two major topics: (1) small-scale variability (Δz ≈ 10-3) in the ratio η = N(He )/N(H ) and (2) an observed correlation of high-η absorbers (soft radiation fields) with voids in the (H I) Lyα distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Given the photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with η 100. These voids may be ionized by local soft sources (dwarf starbursts) or by QSO radiation softened by escape from the active galactic nucleus cores or transfer through the cosmic web. The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs of H I (z ~ 6.2 ± 0.2) and He II (z ~ 2.8 ± 0.2).