Opacity Variations Research Articles

A numerical model for multigroup radiation hydrodynamics

We present in this paper a multigroup model for radiation hydrodynamics to account for variations of the gas opacity as a function of frequency. The entropy closure model ( M 1) is applied to multigroup radiation transfer in a radiation hydrodynamics code. In difference from the previous grey model, we are able to reproduce the crucial effects of frequency-variable gas opacities, a situation omnipresent in physics and astrophysics. We also account for the energy exchange between neighbouring groups which is important in flows with strong velocity divergence. These terms were computed using a finite volume method in the frequency domain. The radiative transfer aspect of the method was first tested separately for global consistency (reversion to grey model) and against a well-established kinetic model through Marshak wave tests with frequency-dependent opacities. Very good agreement between the multigroup M 1 and kinetic models was observed in all tests. The successful coupling of the multigroup radiative transfer to the hydrodynamics was then confirmed through a second series of tests. Finally, the model was linked to a database of opacities for a Xe gas in order to simulate realistic multigroup radiative shocks in Xe. The differences with the previous grey models are discussed.

A New Flat-Fielding Technique for X-Ray Radiography of Low-Z Samples Using Xradia MicroXCT Microscope

High image resolution (˜1.3 μm/pixel) and precision positioning capability make the Xradia X-ray microscopy an attractive platform on which to study X-ray opacity variations. It can complement precision radiography (PR) as an instrument with much higher spatial resolution. PR measures X-ray transmission intensity variations down to 0.01% at 100-μm resolution. Since the requirement to differentiate minute lateral variations in X-ray transmission intensity scales inversely with the spatial resolution, an X-ray imaging microscope such as the Xradia MicroXCT can be useful if it measures the transmission intensity variations to <1%. In normal practice, a number of imaging artifacts limit the intensity measurement to only ˜2% precision. Such artifacts include the thermal drift and the illumination uniformity of the X-ray source, as well as thickness variations in the scintillator plate and the beryllium X-ray tube window. The conventional flat-fielding technique is not effective against the dynamic interaction between the beryllium window texture and the moving shadow cast by a moving X-ray spot. We have modified the image processing routine so that the lateral variations in the transmitted intensity can be measured to ˜0.3% precision on low-Z samples. This technique can be used to record microstructure variations in beryllium samples. Currently, the beryllium microstructures are characterized by ultrasmall angle X-ray scattering on a synchrotron source, which is not commonly accessible, is expensive, and has a long turnaround time. This Xradia-based method has the potential to make it a routine measurement.

Color stability, opacity and degree of conversion of pre-heated composites

Objectives To assess color stability and opacity associated with the degree of conversion of a pre-heated nanohybrid composite (Tetric N-Ceram, Ivoclar/Vivadent, Schaan, Liechtenstein). Methods Twenty-seven specimens were prepared ( n = 9) using a Teflon matrix following storage of compules containing the composite at temperatures of 8 °C, 25 °C or 60 °C. After photoactivation and polishing, baseline readings of six specimens were taken regarding their color and opacity (Spectrophotometer PCB 6807, Byk Gardner, Geretsried, Germany). Then, the specimens were submitted to artificial ageing for 384 h (C-UV, Adexim Comexim, São Paulo, SP, Brazil), after which the final readings were taken. Three specimens for each temperature were submitted to analysis of degree of conversion (Nicolet 380, Thermo Scientific, Waltham, MA, USA). The results were analysed by 1-way ANOVA/Tukey ( p < 0.05). Results There was no significant difference in color stability and opacity variation amongst the temperatures evaluated. The composite pre-heated at 60 °C had a higher degree of conversion (65.13%), with statistically significant difference compared to the other temperatures ( p < 0.05). Conclusions Composite pre-heating does not promote changes in the optical properties, despite the increase in the degree of conversion.

CONSTRAINTS ON THE OPACITY PROFILE OF THE SUN FROM HELIOSEISMIC OBSERVABLES AND SOLAR NEUTRINO FLUX MEASUREMENTS

Motivated by the solar composition problem and by using the recently developed Linear Solar Model approach, we analyze the role of opacity and metals in the sun. After a brief discussion of the relation between the effects produced by a variation of composition and those produced by a modification of the radiative opacity, we calculate numerically the opacity kernels that, in a linear approximation, relate an arbitrary opacity variation to the corresponding modification of the solar observable properties. We use these opacity kernels to discuss the present constraints on opacity (and composition) provided by helioseismic and solar neutrino data.

On the radiative equilibrium of irradiated planetary atmospheres

<i>Context. <i/>The evolution of stars and planets is mostly controlled by the properties of their atmosphere. This is particularly true in the case of exoplanets close to their stars, for which one has to account both for an (often intense) irradiation flux, and from an intrinsic flux responsible for the progressive loss of the inner planetary heat.<i>Aims. <i/>The goals of the present work are to help understanding the coupling between radiative transfer and advection in exoplanetary atmospheres and to provide constraints on the temperatures of the deep atmospheres. This is crucial in assessing whether modifying assumed opacity sources and/or heat transport may explain the inflated sizes of a significant number of giant exoplanets found so far.<i>Methods. <i/>I use a simple analytical approach inspired by Eddington's approximation for stellar atmospheres to derive a relation between temperature and optical depth valid for plane-parallel static grey atmospheres which are both transporting an intrinsic heat flux and receiving an outer radiation flux. The model is parameterized as a function of mean visible and thermal opacities, respectively.<i>Results. <i/>The model is shown to reproduce relatively well temperature profiles obtained from more sophisticated radiative transfer calculations of exoplanetary atmospheres. It naturally explains why a temperature inversion (stratosphere) appears when the opacity in the optical becomes significant compared to that in the infrared. I further show that the mean equivalent flux (proportional to <i>T<i/><sup>4<sup/>) is conserved in the presence of horizontal advection on constant optical depth levels. This implies with these hypotheses that the deep atmospheric temperature used as outer boundary for the evolution models should be calculated from models pertaining to the entire planetary atmosphere, not from ones that are relevant to the day side or to the substellar point. In these conditions, present-day models yield deep temperatures that are ~1000 K too cold to explain the present size of planet HD 209458b. An tenfold increase in the infrared to visible opacity ratio would be required to slow the planetary cooling and contraction sufficiently to explain its size. However, the mean equivalent flux is not conserved anymore in the presence of opacity variations, or in the case of non-radiative vertical transport of energy: The presence of clouds on the night side or a downward transport of kinetic energy and its dissipation at deep levels would help making the deep atmosphere hotter and may explain the inflated sizes of giant exoplanets.

ARECIBO MULTI-EPOCH H I ABSORPTION MEASUREMENTS AGAINST PULSARS: TINY-SCALE ATOMIC STRUCTURE

We present results from multi-epoch neutral hydrogen (HI) absorption observations of six bright pulsars with the Arecibo telescope. Moving through the interstellar medium (ISM) with transverse velocities of 10--150 AU/yr, these pulsars have swept across 1--200 AU over the course of our experiment, allowing us to probe the existence and properties of the tiny scale atomic structure (TSAS) in the cold neutral medium (CNM). While most of the observed pulsars show no significant change in their HI absorption spectra, we have identified at least two clear TSAS-induced opacity variations in the direction of B1929+10. These observations require strong spatial inhomogeneities in either the TSAS clouds' physical properties themselves or else in the clouds' galactic distribution. While TSAS is occasionally detected on spatial scales down to 10 AU, it is too rare to be characterized by a spectrum of turbulent CNM fluctuations on scales of 10-1000 AU, as previously suggested by some work. In the direction of B1929+10, an apparent correlation between TSAS and interstellar clouds inside the warm Local Bubble (LB) indicates that TSAS may be tracing the fragmentation of the LB wall via hydrodynamic instabilities. While similar fragmentation events occur frequently throughout the ISM, the warm medium surrounding these cold cloudlets induces a natural selection effect wherein small TSAS clouds evaporate quickly and are rare, while large clouds survive longer and become a general property of the ISM.

Magnetohydrostatic equilibrium in starspots: dependences on color (Teff) and surface gravity (g)

AbstractTemperature contrasts and magnetic field strengths of sunspot umbrae broadly follow the thermal-magnetic relationship obtained from magnetohydrostatic equilibrium. Using a compilation of recent observations, especially in molecular bands, of temperature contrasts of starspots in cool stars, and a grid of Kurucz stellar model atmospheres constructed to cover layers of sub-surface convection zone, we examine how the above relationship scales with effective temperature (Teff), surface gravity g and the associated changes in opacity of stellar photospheric gas. We calculate expected field strengths in starpots and find that a given relative reduction in temperatures (or the same darkness contrasts) yield increasing field strengths against decreasing Teff due to a combination of pressure and opacity variations against Teff.

Martian atmosphere as observed by VIRTIS‐M on Rosetta spacecraft

The Rosetta spacecraft accomplished a flyby of Mars on its way to 67P/Churyumov‐Gerasimenko on 25 February 2007. In this paper we describe the measurements obtained by the M channel of the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS‐M) and the first scientific results derived from their analysis. The broad spectral coverage of the VIRTIS‐M in the IR permitted the study of various phenomena occurring in the Martian atmosphere; observations were further exploited to achieve accurate absolute radiometric calibration. Nighttime data from the VIRTIS‐M constrain the air temperature profile in the lower atmosphere (5–30 km), using variations in CO2 opacity at 4.3 μm. A comparison of this data with the global circulation model (GCM) by Forget et al. (1999) shows a trend of slightly higher air temperature in the VIRTIS‐M retrievals; this is accompanied by the presence of moderate decreases (∼5 K) in large sections of the equatorial region. This is potentially related to the occurrence of water ice clouds. Daytime data from the VIRTIS‐M reveal CO2 non–local thermodynamic equilibrium emission in the high atmosphere. A mapping of emission intensity confirms its strict dependence on solar zenith angle. Additionally, devoted limb observations allowed the retrieval of vertical emission intensity profiles, indicating a peak around 105 km in southern tropical regions. Ozone content can be effectively monitored by the emission of O2 (a1Δg) at 1.27 μm. Retrieved emission intensity shows that polar regions are particularly rich in ozone. Aerosol scattering was observed in the 1–2.5 μm region above the night region above the night disk, suggesting the occurrence of very high noctilucent clouds.

Dust properties of protoplanetary disks in the Taurus-Auriga star forming region from millimeter wavelengths

We present the most sensitive 3 mm-survey to date of protoplanetary disks carried in the Taurus-Auriga star forming region (average rms of about 0.3 mJy), using the IRAM PdBI. With our high detection rate of 17/19, we provide the first detections at wavelengths longer than about 1 mm for 12 sources. This enables us to study statistically the mm SED slopes and dust properties of faint disks and compare them to brighter disks using a uniform analysis method. With these new data and literature measurements at sub-millimeter and millimeter wavelengths, we analyze the dust properties of a sample of 21 isolated disks around T Tauri stars in the Taurus-Auriga star forming region. Together with the information about the disks spatial extension from sub/mm-mm interferometric studies, we derive from the observed sub-mm/mm spectral energy distribution constraints on the dust opacity law at these wavelengths, using two-layer flared disk models and a self-consistent dust model that takes properly into account the variation of the dust opacity with grain growth. We find evidence for the presence in the disk midplane of dust particles that have grown to sizes as large as at least 1 millimeter in all the disks of our sample, confirming what was previously observed on smaller brighter objects. This indicates that the dust coagulation from ISM dust to mm-sized grains is a very fast process in protoplanetary disks, that appears to occur before a young stellar object enters the Class II evolutionary stage. Also, the amount of these large grains in the disk outer regions is stationary throughout all the Class II evolutionary stage, indicating that mechanisms slowing down the dust inward migration are playing an important role in the Taurus-Auriga protoplanetary disks.

SUZAKUMONITORING OF THE SEYFERT 1 GALAXY NGC 5548: WARM ABSORBER LOCATION AND ITS IMPLICATION FOR COSMIC FEEDBACK

(Abridged) We present a two month Suzaku X-ray monitoring of the Seyfert 1 galaxy NGC 5548. The campaign consists of 7 observations. We analyze the response in the opacity of the gas that forms the ionized absorber to ionizing flux variations. Despite variations by a factor of 4 in the impinging continuum, the soft X-ray spectra of the source show little spectral variations, suggesting no response from the ionized absorber. A detailed time modeling confirms the lack of opacity variations for an absorbing component with high ionization. Instead, the models tentatively suggest that the ionization parameter of a low ionization absorbing component might be changing with the ionizing flux, as expected for gas in photoionization equilibrium. Using the lack of variations, we set an upper limit of n_e <2.0E7 cm-3 for the electron density of the gas forming the high ionization, high velocity component. This implies a large distance from the continuum source (R > 0.033 pc). If the variations in the low ionization component are real, they imply n_e >9.8E4 cm-3 and R < 3 pc. We discuss our results in terms of two different scenarios: a large scale outflow originating in the inner parts of the accretion disk, or a thermally driven wind originating much farther out. Given the large distance of the wind, the implied mass outflow rate is also large (Mw > 0.08 Maccr). The associated total kinetic energy deployed by the wind in the host galaxy (>1.2E56 erg) can be enough to disrupt the interstellar medium, possibly regulating large scale star formation. The total mass and energy ejected by the wind is still lower than the one required for cosmic feedback, even when extrapolated to quasar luminosities. Such feedback would require that we are observing the wind before it is fully accelerated.

Quantitative Data Analysis Method for Precision Radiography

Rayleigh-Taylor instabilities are caused by features that affect shock velocity. These features can be statistically measured by radiography. We designed a precision radiography (PR) system that measures X-ray opacity variations in National Ignition Facility (NIF) ablator capsules to 10-4. Quantitative interpretation of the PR data is challenging and is the subject of this paper. The PR opacity power spectrum (PS) must be related to the NIF surface PS requirements (commonly known as the “NIF curves”). This relationship must be calculated for each specific shell. The compounding factors include X-ray spectra and spot size, detector resolution, shell diameter, coating thickness, dopant and impurity levels, and the coherency status of interface roughness between different layers. In this work, we developed a useful tool to quickly compute the NIF opacity curve (more precisely referred to as NIF “OD [optical depth] PS reference curve” in this paper) for any partially coated NIF shells or nonstandard developmental shells. This allows more rapid feedback on the quality of shells using only partially coated shells and enables benchmarking between the opacity (measured by a radiographic instrument) and surface roughness (measured by an atomic force microscope).

SPATIAL VARIATIONS IN GALACTIC H I STRUCTURE ON AU-SCALES TOWARD 3C 147 OBSERVED WITH THE VERY LONG BASELINE ARRAY

This paper reports dual-epoch, Very Long Baseline Array observations of H i absorption toward 3C 147. One of these epochs (2005) represents new observations, while one (1998) represents the reprocessing of previous observations to obtain higher signal-to-noise results. Significant H i opacity and column density variations, both spatially and temporally, are observed with typical variations at the level of Δτ ≈ 0.20 and in some cases as large as Δτ ≈ 0.70, corresponding to column density fluctuations of order 5 × 1019 cm−2 for an assumed 50 K spin temperature. The typical angular scale is 15 mas; while the distance to the absorbing gas is highly uncertain, the equivalent linear scale is likely to be about 10 AU. Approximately 10% of the face of the source is covered by these opacity variations, probably implying a volume filling factor for the small-scale absorbing gas of no more than about 1%. Comparing our results with earlier results toward 3C 138 (Brogan et al.), we find numerous similarities, and we conclude that small-scale absorbing gas is a ubiquitous phenomenon, albeit with a low probability of intercept on any given line of sight. Further, we compare the volumes sampled by the line of sight through the Galaxy between our two epochs and conclude that, on the basis of the motion of the Sun alone, these two volumes are likely to be substantially different. In order to place more significant constraints on the various models for the origin of these small-scale structures, more frequent sampling is required in any future observations.

The haze and methane distributions on Uranus from HST-STIS spectroscopy

We analyzed a unique, three-dimensional data set of Uranus acquired with the STIS Hubble spectrograph on August 19, 2002. The data covered a full afternoon hemisphere at 0.1 arc-sec spatial resolution between 300 and 1000 nm wavelength at 1 nm resolution. Navigation was accurate to 0.002 arc-sec and 0.02 nm. We tested our calibration with WFPC2 images of Uranus and found good agreement. We constrained the vertical aerosol structure with radiative transfer calculations. The standard types of models for Uranus with condensation cloud layers did not fit our data as well as models with an extended haze layer. The dark albedo of Uranus at near-infrared methane windows could be explained by methane absorption alone using conservatively scattering aerosols. Ultraviolet absorption from small aerosols in the stratosphere was strongest at high southern latitudes. The uppermost troposphere was almost clear, but showed a remarkable narrow spike of opacity centered on the equator to 0.2° accuracy. This feature may have been related to influx from ring material. At lower altitudes, the feature was centered at 1–2° latitude, suggesting an equatorial circulation toward the north. Below the 1.2 bar level, the aerosol opacity increased some 100 fold. A comparison of methane and hydrogen absorptions contradicted the standard interpretation of methane band images, which assumes that the methane mixing ratio is independent of latitude and attributes reflectivity variations to variations in the aerosol opacity. The opposite was true for the main contrast between brighter high latitudes and darker low latitudes, probing the 1–3 bar region. The methane mixing ratio varied between 0.014 and 0.032 from high to low southern latitudes, while the aerosol opacity varied only moderately with latitude, except for an enhancement at −45° latitude and a decrease north of the equator. The latitudinal variation of methane had a similar shape as that of ammonia probed by microwave observations at deeper levels. The variability of methane challenges our understanding of Uranus and requires reconsideration of previous investigations based on a faulty assumption. Below the 2 bar level, the haze was thinning somewhat. Our global radiative transfer models with 1° latitude sampling fit the observed reflectivities to 2% rms. The observed spectra of two discrete clouds could be modeled by using the background model of the appropriate latitude and adding small amounts of additional opacity at levels near 1.2 bar (southern cloud) and levels as high as 0.1 bar (northern cloud). These clouds may have been methane condensation clouds of low optical depth (∼0.2).

Multivariate analysis of Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) Venus Express nightside and limb observations

Spectral signatures measured by Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the nightside and at the limb of Venus are analyzed with Independent Component Analysis. A methodology has been set up to minimize instrumental effects and to interpret the results on the basis of studies of the most common situations in the data set. The main spectral components commonly retrieved on the nightside include the bulk signal modulated by atmospheric opacity variations, photometric variations in the long‐wavelength atmospheric windows, a branching parameter describing particle size variations, and O2 emission at 1.26 and 1.58 μm. Faint atmospheric windows are detected at 1.51, 1.55, 1.78, and 1.82 μm for the first time. The polar vortex structure is outlined, with two main circular areas made of many concentric rings with alternating particle sizes. Discrete clouds about 100 km across are observed in low opacity conditions. High‐altitude, warm clouds are tentatively observed from the polar vortex down to 55°S. At the limb, the two signatures of CO2 nonlocal thermodynamic equilibrium emission are directly mapped, and the thermal structure of the cloud layers and upper atmosphere is apparent. Surface emission is detected with a spatial resolution limited by atmosphere blurring, reaching ∼35 km in exceptional conditions. Horizontal offsets indicate that the radiation propagates mostly vertically, consistent with the large optical depth and vertical extent of the cloud layer. Intense scattering is suspected to take place at the bottom of the atmosphere, at least in the southern plains.

METHOD FOR CALCULATING THE OPACITY OF THE ATOMIC LINES IN STELLAR ATMOSPHERES

A new approximate method for calculating the opacity of the atomic lines in the computation of stellar model atmospheres is presented. Transforming the sums in the equation of the opacity of the lines into integrals and considering the Kramers equation for the oscillator strengths for level transitions in the hydrogenic atoms as a continuous function of frequency, we can apply the mean value theorem for integrals in order to describe the variation of the global opacity of the lines of all the chemical elements. The high-lying levels in nonhydrogenic atoms as well as their hydrogen-like transitions can also benefit from this method. For the case of stellar atmospheres not in local thermodynamic equilibrium, we can apply our method directly. The compact analytic expressions obtained are easy to use. The application of the method to the hydrogen lines shows the classical line-blanketing effects.

Latitudinal differences in the timing of otolith growth: A comparison between the Barents Sea and southern North Sea

Seasonality of otolith zone formation can vary between fish species, and between populations within the same species. Although information on otolith zone formation is important for fish age estimation, there is incomplete knowledge of factors influencing variation in otolith opacity. In this study, marginal increment types (opaque or translucent) of two spatially distinct Atlantic cod stocks were assessed. Cod otoliths from the southern North Sea, where the fish experience high temperatures were compared with cod otoliths from the Barents Sea, where the fish experience much lower temperatures. There was approximately a shift of 5 months for deposition of translucent otolith material between the two areas. Cod from the southern North Sea deposited translucent otolith material in the period July–November, while it occurred in the period December–April in the Barents Sea. Inter-annual differences within areas were minor compared to differences between the two areas.

Low temperature Rosseland opacities with varied abundances of carbon and nitrogen

Context. With certain assumptions, radiative energy transport can be modelled by the diffusion approximation. In this case, the Rosseland mean opacity coefficient characterises the interaction between radiation and matter. The opacity data are usually available in pre-tabulated form, and in the generation of the data one assumes a distinct heavy element mixture, which is usually a scaled solar one. Therefore, presently available data is unable to cover the full parameter range of some astrophysical problems, in which the chemical composition of the medium being considered varies. Aims. We attempt to produce low temperature opacity data incorporating the effects of varied abundances of the elements carbon and nitrogen. For our temperature range of interest, molecules represent the dominant opacity source. Our dataset covers a wide metallicity range and is meant to provide important input data for stellar evolution models and other applications. Methods. We conduct chemical equilibrium calculations to evaluate the partial pressures of neutral atoms, ions, and molecules. Based on a large dataset containing atomic line and continuum data and, most importantly, a plethora of molecular lines, we calculate Rosseland mean opacity coefficients not only for a number of different metallicities, but also for varied abundances of the isotopes 12 C and 14 N at each metallicity. The molecular data comprise the main opacity sources for either an oxygen-rich or carbon-rich chemistry. We tabulate the opacity coefficients as a function of temperature and, basically, density. Results. Due to the special role of the CO molecule, within a certain chemistry regime an alteration to the carbon abundance causes considerable changes in the Rosseland opacity. The transition from a scaled solar (i.e. oxygen-rich) mixture to a carbon-rich regime results in opacities that can, at low temperatures, differ by orders of magnitude from to the initial situation. The reason is that the mean opacity in either case is due to different molecular absorbers. Variations in the abundance of nitrogen have less pronounced effects but, nevertheless, cannot be neglected. Conclusions. In typical astrophysical applications, it is indispensable to take into account opacity variations due to chemistry changes. In this respect, the new data is superior to previous compilations, but is, however, still subject to uncertainties.

Theoretical Radii of Extrasolar Giant Planets: The Cases of TrES‐4, XO‐3b, and HAT‐P‐1b

To explain their observed radii, we present theoretical radius-age trajectories for the extrasolar giant planets (EGPs) TrES-4, XO-3b, and HAT-P-1b. We factor in variations in atmospheric opacity, the presence of an inner heavy-element core, and possible heating due to orbital tidal dissipation. A small, yet nonzero, degree of core heating is needed to explain the observed radius of TrES-4, unless its atmospheric opacity is significantly larger than a value equivalent to that at -->10 × solar metallicity with equilibrium molecular abundances. This heating rate is reasonable, and corresponds for an energy dissipation parameter (Qp) of ~103.8 to an eccentricity of ~0.01, assuming -->3 × solar atmospheric opacity and a heavy-element core of -->Mc = 30 M⊕. For XO-3b, which has an observed orbital eccentricity of 0.26, we show that tidal heating needs to be taken into account to explain its observed radius. Furthermore, we reexamine the core mass needed for HAT-P-1b in light of new measurements and find that it now generally follows the correlation between stellar metallicity and core mass suggested recently. Given various core heating rates, theoretical grids and fitting formulae for a giant planet's equilibrium radius and equilibration timescale are provided for planet masses -->Mp = 0.5, 1.0, and 1.5 MJ with -->a = 0.02–0.06 AU, orbiting a G2 V star. When the equilibration timescale is much shorter than that of tidal heating variation, the effective age of the planet is shortened, resulting in evolutionary trajectories more like those of younger EGPs. Motivated by the work of B. Jackson et al., we suggest that this effect could indeed be important in better explaining some observed transit radii.

Radiative Hydrodynamical Studies of Irradiated Atmospheres

AbstractTransiting planets provide a unique opportunity to study the atmospheres of extra-solar planets. Radiative hydrodynamical models of the atmosphere provide a crucial link between the physical characteristics of the atmosphere and the observed properties. Here I present results from 3D simulations which solve the full Navier-Stokes equations coupled to a flux-limited diffusion treatment of radiation transfer. Variations in opacity amongst models leads to a variation in the temperature differential across the planet, while atmospheric dynamics becomes much more variable at longer orbital periods. I also present 3D radiative simulations illustrating the importance of distinguishing between optical and infrared opacities.

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.