Gravity Range Research Articles

Airborne Gravity Surveys Over Taiwan Island and Strait, Kuroshio Current and South China Sea: Comparison of GPS and Gravity Accuracies at Different Flight Altitudes

We compare the accuracies of GPS and gravity observations obtained from three airborne gravity surveys over Taiwan Island at altitude 5000 m and over Kuroshio Current, Taiwan Strait, and Dongsha Atoll at altitude 1500 m. A kinematic network adjustment was used to determine the positions of the aircrafts. GPS positioning errors are at the decimeter-level, which are not entirely propagated to velocity and acceleration errors due to cancellation of long wavelength errors. Outliers are downweighted in the Gaussian filtering to improve the gravity accuracy, especially at altitude 1500 m. Compared with the upward-continued gravity, the gravity anomalies from the 1500-m surveys show a consistent accuracy of about 3 mgal; the accuracy from the 5000-m survey is degraded, especially over high mountains. The RMS crossover differences at 1500 m and 5000 m are all below 3 mgal, suggesting flight altitudes do not affect the crossover difference. Coherence analysis suggests that the spatial resolvable wavelengths of the airborne gravity range from 4 km (altitude 1500 m) to 6 km (altitude 5000 m).

A NEW REDDENING LAW FOR M4

We have used a combination of broad-band near-infrared and optical Johnson-Cousins photometry to study the dust properties in the line of sight to the Galactic globular cluster M4. We have investigated the reddening effects in terms of absolute strength and variation across the cluster field, as well as the form of the reddening law defined by the type of dust. Here, we determine the ratio of absolute to selective extinction (R_V) in the line of sight towards M4, which is known to be a useful indicator for the type of dust and therefore characterizes the applicable reddening law. Our method is independent of age assumptions and appears to be significantly more precise and accurate than previous approaches. We obtain A_V/E(B-V)=3.76\pm0.07 (random error) for the dust in the line of sight to M4 for our set of filters. With this value, the distance to M4 is found to be 1.80\pm0.05 kpc (random error). A reddening map for M4 has been created, which reveals a spatial differential reddening of delta E(B-V)>0.2 mag across a field within 10' around the cluster centre and a total mean reddening of E(B-V)=0.37\pm0.01. In order to provide accurate zero points for the extinction coefficients of our photometric filters, we investigated the impact of stellar parameters such as temperature, surface gravity and metallicity on the extinction properties in different bandpasses. Using both synthetic ATLAS9 spectra and observed spectral energy distributions, we found similar sized effects for the range of temperature, surface gravity, and metallicity typical of globular cluster stars: each causes a change of about 3% in the necessary correction factor for each filter combination. From our calculations, we provide extinction zero points for Johnson-Cousins and 2MASS filters, spanning a wide range of stellar parameters and dust types, suited for accurate, object-specific extinction corrections.

CONSTRAINTS ON NEUTRON STAR MASS AND RADIUS IN GS 1826–24 FROM SUB-EDDINGTON X-RAY BURSTS

We investigate the constraints on neutron star mass and radius in GS 1826-24 from models of lightcurves and spectral evolution of type I X-ray bursts. This source shows remarkable agreement with theoretical calculations of burst energies, recurrence times, and lightcurves. We first exploit this agreement to set the overall luminosity scale of the observed bursts. When combined with a measured blackbody normalization, this leads to a distance and anisotropy independent measurement of the ratio between the redshift 1+z and color correction factor f_c. We find 1+z=1.19-1.28 for f_c=1.4-1.5. We then compare the evolution of the blackbody normalization with flux in the cooling tail of bursts with predictions from spectral models of Suleimanov et al. (2011b). The observations are well described by the models at luminosities greater than about one third of the peak luminosity, with deviations emerging at luminosities below that. We show that this comparison leads to distance independent upper limits on R_\infty and neutron star mass of R_\infty\lesssim 9.0-13.2 km and M<1.2-1.7 M_\odot, respectively, for solar abundance of hydrogen at the photosphere and a range of metallicity and surface gravity. The radius limits are low in comparison to previous measurements. This may be indicative of a subsolar hydrogen fraction in the GS 1826-24 photosphere, or of larger color corrections than that predicted by spetral models. Our analysis also gives an upper limit on the distance to GS 1826-24 of d<4.0-5.5 kpc \xi_b^{-1/2}, where \xi_b is the degree of anisotropy of the burst emission.

Application of the Bergman-Sutton Method for Determining Blend Viscosity

Summary As the petroleum industry accesses more low-gravity-oil resources, modification of viscosity by blending lighter hydrocarbons has become a necessity in order to attain bulk properties that will flow though a pipeline. In the more conventional oil reservoirs, the need to estimate the viscosity of oil blends occurs when reservoir fluids are contaminated with oil-based muds or when production streams from different reservoirs or fields are commingled in a single pipeline. Several methods have appeared in the literature for estimating blend viscosity. All of these methods require a measured viscosity for each component of the blend. The number of viscosity measurements is compounded when the viscosity of the blended mixture is required at several temperatures. Of the viscosity correlations published, the Bergman and Sutton method has the widest range of temperature and oil API gravity and has been consistently demonstrated to provide accurate results over these conditions. This method requires the component specific gravity, the Watson characterization factor (Watson K factor), and temperature to estimate viscosity. By using the proper mixing rules, an estimate of blend viscosity can be made with comparable or improved accuracy over the "best" published methods without the need for individual-component viscosity measurements. A database of 2,059 blend-viscosity measurements from more than 800 mixtures was created to compare the accuracy of the various methods. Viscosity measurements of the individual components in the blends studied exceeded 7,600 data points. A diverse group of mixtures, ranging from light alkane or aromatic pure components to bitumen, diesel, biodiesel, condensate, crude, and assay fractions, was included in this database. Blends comprised the typical binary mixtures but ranged up to a maximum of eight components in the mixture.

Preparation of Lightweight Bricks Using Sewage Sludge

8,285 tons/day of sewage sludge was discharged in 2009. Our study investigated improving sewage sludge’s recyclability by making lightweight bricks from it. The bricks’ compressive strength, specific gravity, water absorption and so on were measured to evaluate its characteristics. Its compressive strength ranged from 1.11 MPa to 10.92 MPa (3-class clay brick, the Korean standard, is 10.78 MPa or more). Its highest compressive strength was obtained at the mixing ratio of SS:MBA:BFS:WA = 6:1:1:2, and its range of specific gravity was 0.73–1.02 g/cm3. Heavy metal did not leach from it. Therefore, sewage sludge can be prepared and used to make lightweight brick.

Comparison of Limb-Darkening Laws from Plane-Parallel and Spherically-Symmetric Model Stellar Atmospheres

AbstractLimb-darkening is a fundamental constraint for modeling eclipsing binary and planetary transit light curves. As observations, for example from Kepler, CoRot, and Most, become more precise then a greater understanding of limb-darkening is necessary. However, limb-darkening is typically modeled as simple parameterizations fit to plane-parallel model stellar atmospheres that ignores stellar atmospheric extension. In this work, I compute linear, quadratic and four-parameter limb-darkening laws from grids of plane-parallel and spherically-symmetric model stellar atmospheres in a temperature and gravity range representing stars evolving on the Red Giant branch. The limb-darkening relations for each geometry are compared and are found to fit plane-parallel models much better than the spherically-symmetric models. Assuming that limb-darkening from spherically-symmetry model atmospheres are more physically representative of actual stellar limb-darkening than plane-parallel models, then these limb-darkening laws will not fit the limb of a stellar disk leading to errors in a light curve fit. This error will increase with a star's atmospheric extension.

The basal chromospheric Mg ii h+k flux of evolved stars: probing the energy dissipation of giant chromospheres

Of a total of 177 cool G, K, and M giants and supergiants, we measured the Mg II h+k line emission of extended chromospheres in high-resolution (LWR) IUE spectra by using the IUE final data archive at STScI, and derived the respective stellar surface fluxes. They represent the chromospheric radiative energy losses presumably related to basal heating by the dissipation of acoustic waves, plus a highly variable contribution due to magnetic activity. Thanks to the large sample size, we find a very well defined lower limit, the basal chromospheric Mg II h+k line flux of cool giant chromospheres, as a function of T_eff. A total of 16 giants were observed several times, over a period of up to 20 years. Their respective minimal Mg II h+k line fluxes confirm the basal flux limit very well because none of their emissions dip beneath the empirically deduced basal flux line representative for the overall sample. Based on a total of 15 to 22 objects with very low Mg II h+k emission, we find as limit: log F_Mg_h+k = 7.33 log T_eff - 21.75 (cgs units; based on B-V). Within its uncertainties, this is almost the same relation as has been found in the past for the geometrically much thinner chromospheres of main sequence stars. Concerning any residual dependence of the basal flux on the surface gravity it can be stated that over a gravity range of more than four orders of magnitude (main-sequence stars to supergiants), the basal flux does not appear to vary by more than a factor of 2. These findings are in good agreement with the predictions by previous hydrodynamic models of acoustic wave propagation and energy dissipation, as well as with earlier empirical determinations. Finally, we discuss the idea that the ample energy flux of the chromospheric acoustic waves in a cool giant may yield, as a by-product, the energy flux required by its cool wind (i.e., non-dust-driven, "Reimers-type" mass-loss).

Assessment of Urine Solute and Matrix Effects on the Performance of an Enzyme-Linked Immunosorbent Assay for Measurement of Interleukin-6 in Dog Urine

Measurement of cytokine concentrations within body fluids is a means of recognizing subclinical and/or unresolved, infectious and inflammatory states in patients. In the urinary tract, such information may be useful for identifying patients with pyelonephritis, asymptomatic bacteriuria, recurrent infections, and cystitis. One such cytokine, interleukin-6 (IL-6), is recognized as a primary cytokine that is produced following exposure of the urothelium to bacterial virulence factors. Complicating reliable testing for this and other cytokines is the nature of urine itself. Urine varies widely in its composition as indicated by the range of pH and urine specific gravity (USG) observed in healthy patients. An additional variable is the protein and carbohydrate matrix capable of hindering immunologic testing modalities, such as enzyme-linked immunosorbent assays (ELISAs). The purpose of the current study was to examine the role of urine pH, USG, and matrix while optimizing a canine-specific chemiluminescent ELISA for the measurement of IL-6 in the urine of dogs. Urine spiked with IL-6 obtained maximal IL-6 quantitative recoveries of only 55 ± 10% (mean ± 1 standard deviation) when an ELISA optimized for cell culture supernatants was used. The urine matrix and variations in USG were determined to by contributing to this poor IL-6 recovery. Using specific matrix inhibitors and optimal dilutions improved the IL-6 quantitative recovery to 91 ± 5%. Urine pH (5.5-9.5) had no effect. The current work underscores the importance of critically optimizing testing modalities for biomarkers, particularly if they are immunologic in origin.

THINGS about MOND

We present an analysis of 12 high-resolution galactic rotation curves from The HI Nearby Galaxy Survey (THINGS) in the context of modified Newtonian dynamics (MOND). These rotation curves were selected to be the most reliable for mass modelling, and they are the highest quality rotation curves currently available for a sample of galaxies spanning a wide range of luminosities. We fit the rotation curves with the “simple” and “standard” interpolating functions of MOND, and we find that the “simple” function yields better results. We also redetermine the value of a0, and find a median value very close to the one determined in previous studies, a0 = (1.22 ± 0.33) × 10-8 cm s-2. Leaving the distance as a free parameter within the uncertainty of its best independently determined value leads to excellent quality fits for 75% of the sample. Among the three exceptions, two are also known to give relatively poor fits in Newtonian dynamics plus dark matter. The remaining case (NGC 3198) presents some tension between the observations and the MOND fit, which might, however, be explained by the presence of non-circular motions, by a small distance, or by a value of a0 at the lower end of our best-fit interval, 0.9 × 10-8 cm s-2. The best-fit stellar M/L ratios are generally in remarkable agreement with the predictions of stellar population synthesis models. We also show that the narrow range of gravitational accelerations found to be generated by dark matter in galaxies is consistent with the narrow range of additional gravity predicted by MOND.

非均质泥石流固液两相运动特征探讨

Non-homogeneous debris flow is a typical two-phase flow with a large range of solid grain size and specific gravity and no-uniform velocity distribution. Based on the principle of the minimum energy dissipation, the critical diameter between solid phase of the coarse particles and liquid phase composed by both the fine particles and water in non-homogeneous debris flow was established. Furthermore, based on the modified Darcy’s formula and mass conservation law, two-phase flowing velocities for non-homogeneous debris flow were developed preliminarily. Correspondingly, it has been verified that calculating value using the velocity formulas of solid and liquid two-phase flow is corresponding to the measured data from Jiangjia gully, Yunnan Province, China. There comes to some meaningful conclusions, such as the velocity of solid phase is more than one of liquid phase due to the specific gravity higher than 2.234 of debris flow, the corresponding deposition shows the inverse property; whereas velocity of solid phase is usually less than one of liquid phase in the general situations, the corresponding deposition behaves the straight property. If the velocity of two phases become much closer, the corresponding deposition possesses intermix property. These results are very significant for both developing dynamical model for non-homogeneous debris flow and designing of control engineering for debris flow disasters.

Equation-of-State-Based Downhole Fluid Characterization

Summary Downhole fluid analysis (DFA), together with focused-sampling techniques and wireline-formation-tester (WFT) tools, provides real-time measurements of reservoir-fluid properties such as the compositions of four or five hydrocarbon components/groups and gas/oil ratio (GOR). With the introduction of a new generation of DFA tools that analyze fluids at downhole conditions, the accuracy and reliability of the DFA measurements are improved significantly. Furthermore, downhole measurements of live-fluid densities are integrated into the new tools. Direct pressure and temperature measurements of the flowline ensure capture of accurate fluid conditions. To enhance these advanced features further, a new method of downhole fluid characterization based on the equation-of-state (EOS) approach is proposed in this work. The motivation for this work is to develop a new approach to maximize the value of DFA data, perform quality assurance or quality control of DFA data, and establish a fluid model for DFA log predictions along with DFA fluid profiling. The basic inputs from DFA measurements are weight percentages of CO2, C1, C2, C3–C5 and C6+, along with live-fluid density and viscosity. A new method was developed in this work to delump and characterize the DFA measurements of C3–C5 (or C2–C5) and C6+ into full-length compositional data. The full-length compositional data predicted by the new method were compared with the laboratory-measured gas chromatograph data up to C30+ for more than 1,000 fluids, including heavy oil, conventional black oil, volatile oil, rich gas condensate, lean gas condensate, and wet gas. These fluids have a GOR range of 8–140,000 scf/STB and a gravity range from 9 to 50°API. A good agreement was achieved between the delumped and gas-chromatograph compositions. In addition, on the basis of the delumped and characterized full-length compositional data, EOS models were established that can be applied to predict fluid-phase behavior and physical properties by virtue of DFA data as inputs. The EOS predictions were validated and compared with the laboratory-measured pressure/volume/temperature (PVT) properties for more than 1,000 fluids. The GOR, formation-volume factor, density, and viscosity predictions were in good agreement with the laboratory measurements. The established EOS model then was able to predict other PVT properties, and the results were compared with the laboratory measurements in good agreement. Consequently, the established EOS models have laid a solid foundation for DFA log predictions in DFA fluid profiling, which has been integrated successfully with DFA measurements in real time to delineate compositional and asphaltene gradients in oil columns and to determine reservoir connectivity. The latter results are beyond the scope of this work and have been given in separate technical papers.

Multidisciplinary characterisation of sedimentary processes in a recent maar lake (Lake Pavin, French Massif Central) and implication for natural hazards

Abstract. Sedimentation processes occurring in the most recent maar lake of the French Massif Central (Lake Pavin) are documented for the first time based on high resolution seismic reflection and multibeam bathymetric surveys and by piston coring and radiocarbon dating on a sediment depocentre developed on a narrow sub aquatic plateau. This new data set confirms the mid Holocene age of maar lake Pavin formation at 6970±60 yrs cal BP and highlights a wide range of gravity reworking phenomena affecting the basin. In particular, a slump deposit dated between AD 580–640 remoulded both mid-Holocene lacustrine sediments, terrestrial plant debris and some volcanic material from the northern crater inner walls. Between AD 1200 and AD 1300, a large slide scar mapped at 50 m depth also affected the southern edge of the sub aquatic plateau, suggesting that these gas-rich biogenic sediments (laminated diatomite) are poorly stable. Although several triggering mechanisms can be proposed for these prehistoric sub-aquatic mass wasting deposits in Lake Pavin, we argue that such large remobilisation of gas-rich sediments may affect the gas stability in deep waters of meromictic maar lakes. This study highlights the need to further document mass wasting processes in maar lakes and their impacts on the generation of waves, favouring the development of dangerous (and potentially deadly) limnic eruptions.

Galactic evolution of oxygen

The measurement of oxygen lines in metal-poor unevolved stars, in particular near-UV OH lines, can provide invaluable information on the properties of the Early Galaxy. Near-UV OH lines constitute an important tool to derive oxygen abundances in metal-poor dwarf stars. Therefore, it is important to correctly model the line formation of OH lines, especially in metal-poor stars, where 3D hydrodynamical models commonly predict cooler temperatures than plane-parallel hydrostatic models in the upper photosphere. We have made use of a grid of 52 3D hydrodynamical model atmospheres for dwarf stars computed using the code CO5BOLD, extracted from the more extended CIFIST grid. The 52 models cover the effective temperature range 5000-6500K, the surface gravity range 3.5-4.5 and the metallicity range -3<[Fe/H]<0. We determine 3D-LTE abundance corrections in all the 52 3D models for several OH lines and FeI lines of different excitation potentials. These 3D-LTE corrections are generally negative reaching values of roughly -1 dex (for the OH 3167 with excitation potential of approximately 1 eV) for the higher temperatures and surface gravities. We apply these 3D-LTE corrections to the individual O abundances, derived from OH lines, of a sample the metal-poor dwarf stars reported in Israelian et al.(1998, 2001) and Boesgaard et al.(1999), by interpolating the stellar parameters of the dwarfs in the grid of 3D-LTE corrections. The new 3D-LTE [O/Fe] ratio still keeps a similar trend as the 1D-LTE, i.e, increasing towards lower [Fe/H] values. We applied 1D-NLTE corrections to 3D FeI abundances and we still see an increasing [O/Fe] ratio towards lower metallicites. However, the Galactic [O/Fe] ratio must be revisited once 3D-NLTE corrections become available for OH and Fe lines for a grid of 3D hydrodynamical model atmospheres.

Extraterrestrial dunes: An introduction to the special issue on planetary dune systems

Aeolian dune fields have been described on Earth, Mars, Venus and Titan. The plethora of data returned from recent planetary missions has enabled a new era in planetary geomorphic studies. Much of our understanding of planetary dune systems comes from the application of Earth analogs, wind tunnel experiments and modeling studies. Despite the range of atmospheric pressures, composition and gravity, many of the dune forms on extraterrestrial surfaces are similar to those on Earth, although some have notable differences in bedform scale and composition. As an introduction to the special issue on planetary dune systems this paper summarizes the current state of knowledge of planetary dune studies and highlights outstanding questions that require further investigation.

The ilium forward modelling algorithm for multivariate parameter estimation and its application to derive stellar parameters fromGaiaspectrophotometry

I introduce an algorithm for estimating parameters from multidimensional data based on forward modelling. In contrast to many machine learning approaches it avoids fitting an inverse model and the problems associated with this. The algorithm makes explicit use of the sensitivities of the data to the parameters, with the goal of better treating parameters which only have a weak impact on the data. The forward modelling approach provides uncertainty (full covariance) estimates in the predicted parameters as well as a goodness-of-fit for observations. I demonstrate the algorithm, ILIUM, with the estimation of stellar astrophysical parameters (APs) from simulations of the low resolution spectrophotometry to be obtained by Gaia. The AP accuracy is competitive with that obtained by a support vector machine. For example, for zero extinction stars covering a wide range of metallicity, surface gravity and temperature, ILIUM can estimate Teff to an accuracy of 0.3% at G=15 and to 4% for (lower signal-to-noise ratio) spectra at G=20. [Fe/H] and logg can be estimated to accuracies of 0.1-0.4dex for stars with G<=18.5. If extinction varies a priori over a wide range (Av=0-10mag), then Teff and Av can be estimated quite accurately (3-4% and 0.1-0.2mag respectively at G=15), but there is a strong and ubiquitous degeneracy in these parameters which limits our ability to estimate either accurately at faint magnitudes. Using the forward model we can map these degeneracies (in advance), and thus provide a complete probability distribution over solutions. (Abridged)

THE CAPACITY OF THE SCINTREX AUTOGRAV CG-3M NO. 4492 GRAVIMETER FOR "ABSOLUTE-SCALE" SURVEYS

In autumn 2001, the Institut für Erdmessung (IfE), received the new Scintrex Autograv CG-3M spring gravimeter no. 4492 (SC-4492). Compared with LaCoste-Romberg gravimeters, the Scintrex instrument offers the advantage of a more simple calibration function. Due to the straightforward sensor design (no micrometer screws and gearboxes, no astatisation), a modelling of periodical calibration terms and of a higher order polynomial calibration function is not required. The costly calibration measurements can be reduced to some relative observations between two reliable absolute gravity stations. No complex calibration system should be necessary. A reliable evaluation of the SC-4492 was done with respect to the stability of the calibration, measurement accuracy and precision, drift behaviour, and gravity range dependency of the calibration factor. Measurements were performed on the Hannover vertical calibration line (200 μm/s2 (20 mGal) range, 20-storied building, 10 μm/s2 interval), on the Cuxhaven-Harz north-south line (3000 μm/s2 range, 34 stations, 90 μm/s2 interval) of the gravimeter calibration system Hannover, and in the absolute gravimetry net of the Fennoscandian land uplift area (6200 μm/s2 range). A total gravity range of almost 0.015 m/s2 has been covered. In addition, the gravimeter was employed in the determination of vertical gravity gradients and centring to safety points for absolute gravimetry, and the observation of the horizontal gradient field at the IfE absolute gravity reference station in Clausthal. This paper presents the results as obtained in regional, local and microgravimetric surveys. The achieved accuracies are in the order of ±10 to 100 nm/s2 (±1 to 10 μGal). No instability of the calibration and no gravity range dependency could be proven within the order of 1·10-4. Overall, the gravity meter SC-4492 meets fully the expectations of IfE.

Convection and 6Li in the atmospheres of metal-poor halo stars

AbstractBased on 3D hydrodynamical model atmospheres computed with the CO5BOLD code and 3D non-LTE (NLTE) line formation calculations, we study the effect of the convection-induced line asymmetry on the derived 6Li abundance for a range in effective temperature, gravity, and metallicity covering the stars of the Asplund et al. (2006) sample. When the asymmetry effect is taken into account for this sample of stars, the resulting 6Li/7Li ratios are reduced by about 1.5% on average with respect to the isotopic ratios determined by Asplund et al. (2006). This purely theoretical correction diminishes the number of significant 6Li detections from 9 to 4 (2σ criterion), or from 5 to 2 (3σ criterion). In view of this result the existence of a 6Li plateau appears questionable. A careful reanalysis of individual objects by fitting the observed lithium 6707 Å doublet both with 3D NLTE and 1D LTE synthetic line profiles confirms that the inferred 6Li abundance is systematically lower when using 3D NLTE instead of 1D LTE line fitting. Nevertheless, halo stars with unquestionable 6Li detection do exist even if analyzed in 3D-NLTE, the most prominent example being HD 84937.

KECK LASER GUIDE STAR ADAPTIVE OPTICS MONITORING OF THE M8+L7 BINARY LHS 2397aAB: FIRST DYNAMICAL MASS BENCHMARK AT THE L/T TRANSITION

We present Keck LGS AO imaging and aperture masking observations of the M8+L7 binary LHS 2397aAB. Together with archival HST, Gemini-North, and VLT data, our observations span 11.8 years of the binary's 14.2-year orbital period. We determine a total dynamical mass of 0.146+/-0.014 MSun (153+/-15 MJup). Using the combined observational constraints of the total mass and individual luminosities, the Tucson (Lyon) evolutionary models give an age for the system of 1.6 Gyr (1.8 Gyr), consistent with its space motion. We also use these models to determine the mass ratio, giving individual masses of 0.0839+/-0.0011 MSun (0.0848+/-0.0011 MSun) for LHS 2397aA and 0.061+/-0.013 MSun (0.060+/-0.010 MSun) for LHS 2397aB. Because LHS 2397aB is very close to the theoretical mass-limit of lithium burning, measuring its lithium depletion would uniquely test substellar models. We estimate a spectral type of L7+/-1 for LHS 2397aB, making it the first L/T transition object with a dynamical mass determination. This enables a precise estimate of its effective temperature from Tucson (Lyon) models of 1460+/-40 K (1440+/-40 K), which is 200 K higher than estimates for young late-L companions but consistent with older late-L field dwarfs, supporting the idea that the temperature of the L/T transition is surface gravity dependent. Comparing our temperature estimate for LHS 2397aB to those derived from spectral synthesis modeling for similar objects reveals consistency between evolutionary and atmospheric models at the L/T transition, despite the currently limited understanding of this phase of substellar evolution. Future dynamical masses for L/T binaries spanning a range of surface gravity and age will provide the next critical tests of substellar models at the L/T transition.

On the selection of optimal global geopotential model for geoid modeling: A case study in Pakistan

A statistical comparison has been made between gravity field parameters derived from different global geopotential models (GGMs) and observed gravity anomalies, gravimetric geoid and GPS-Leveling data. The motivation behind this study is the selection of best possible global geopotential model that best matches statistically with the local observed data in Pakistan. This will facilitate in decreasing the load on observed data for the development of regional gravimetric geoid in remove-compute-restore technique when used in the Stokes’s integral for computation of the residual part. It is observed that combined geopotential models such as EGM96 and PGM200A, EIGEN-GL04C and EIGEN-CG03C reflect the better match in the total spectral range of gravity and GPS-Leveling data. Results of the precise local geoid model also indicate similar characteristics. A very-high-degree model “EGM2008” (degree/order 2160) exhibits relatively superior statistical fit with observed ground data in Pakistan region. For satellite-only models an increasing trend in the standard deviation can be seen with maximum of about ∼4 m in difference between GPS-Leveling and corresponding GGM’s geoid with increase in the order from 50 to 120 and then it decreases afterwards. However, for the EIGEN-CHAMP03SP, standard deviation saturates to a value of 3.4 m. This is an indication of contamination in the long to medium wavelength part, i.e. 50–100° for the satellite-only models. Moreover, the models DEOS-CHAMP-01C, GGM02C and then ITG-GRACE03 appear to have better fit for medium to long wavelength and can possibly be recommended for use as long wavelength part with the local observed data. While a hybrid geopotential model selection can be achieved through the selection from either of DEOS-CHAMP-01C, GGM02C, GGM02S, EIGEN-GRACE02S or ITG-GRACE03 in the long wavelength (to degree and order 40) and EGM96, PGM200A, EIGEN-GL04C, EIGEN-CG03C or even EGM2008 in medium to short wavelength, i.e. from degree 41 to maximum degree and order.

Chameleon vector bosons

We show that for a force mediated by a vector particle coupled to a conserved $U(1)$ charge, the apparent range and strength can depend on the size and density of the source, and the proximity to other sources. This chameleon effect is due to screening from a light charged scalar. Such screening can weaken astrophysical constraints on new gauge bosons. As an example we consider the constraints on chameleonic gauged $B\ensuremath{-}L$. We show that although Casimir measurements greatly constrain any $B\ensuremath{-}L$ force much stronger than gravity with range longer than $0.1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, there remains an experimental window for a long-range chameleonic $B\ensuremath{-}L$ force. Such a force could be much stronger than gravity, and long or infinite range in vacuum, but have an effective range near the surface of the earth which is less than a micron.