A Long-Lived Accretion Disk Around a Lithium-Depleted Binary T Tauri Star

We present a high dispersion optical spectrum of St 34 and identify the system as a spectroscopic binary with components of similar luminosity and temperature (both M3+/-0.5). Based on kinematics, signatures of accretion, and location on an H-R diagram, we conclude that St 34 is a classical T Tauri star belonging to the Taurus-Auriga T Association. Surprisingly, however, neither component of the binary shows LiI 6708 A, absorption, the most universally accepted criterion for establishing stellar youth. In this uniquely known instance, the accretion disk appears to have survived longer than the lithium depletion timescale. We speculate that the long-lived accretion disk is a consequence of the sub-AU separation companion tidally inhibiting, though not preventing, circumstellar accretion. Comparisons with pre-main sequence evolutionary models imply, for each component of St 34, a mass of 0.37+/-0.08 Msun and an isochronal age of 8+/-3 Myr, which is much younger than the predicted lithium depletion timescale of ~ 25 Myr. Although a distance 38% closer than that of Taurus-Auriga or a hotter temperature scale could reconcile this discrepancy at 21-25 Myr, similar discrepancies in other systems and the implications of an extremely old accreting Taurus-Auriga member suggest instead a possible problem with evolutionary models. Regardless, the older age implied by St 34's depleted lithium abundance is the first compelling evidence for a substantial age spread in this region. Additionally, since St 34's coeval co-members with early M spectral types would likewise fail the lithium test for youth, current membership lists may be incomplete.Comment: 4 pages, including 2 figures. Accepted for publication in ApJ Let

Direct numerical simulations for non-Newtonian rheology of concentrated particle dispersions

The non-Newtonian behavior of a monodisperse concentrated dispersion of spherical particles was investigated using a direct numerical simulation method, that takes into account hydrodynamic interactions and thermal fluctuations accurately. Simulations were performed under steady shear flow with periodic boundary conditions in the three directions. The apparent shear viscosity of the dispersions was calculated at volume fractions ranging from 0.31 to 0.56. Shear-thinning behavior was clearly observed at high volume fractions. The low- and high-limiting viscosities were then estimated from the apparent viscosity by fitting these data into a semi-empirical formula. Furthermore, the short-time motions were examined for Brownian particles fluctuating in concentrated dispersions, for which the fluid inertia plays an important role. The mean square displacement was monitored in the vorticity direction at several different Peclet numbers and volume fractions so that the particle diffusion coefficient is determined from the long-time behavior of the mean square displacement. Finally, the relationship between the non-Newtonian viscosity of the dispersions and the structural relaxation of the dispersed Brownian particles is examined

Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius

We present the results of a high-resolution imaging survey of 12 brown dwarfs and very low mass stars in the closest (~145 pc) young (~5 Myr) OB association, Upper Scorpius. We obtained images with the Advanced Camera for Surveys/High Resolution Camera on HST through the F555W (V), F775W (i'), and F850LP (z') filters. This survey discovered three new binary systems, including one marginally resolved pair with a projected separation of only 4.9 AU, resulting in an observed binary fraction of 25+/-14% at separations >4 AU. After correcting for detection biases assuming a uniform distribution of mass ratios for q>0.6, the estimated binary fraction is 33+/-17%. The binary fraction is consistent with that inferred for higher-mass stars in Upper Sco, but the separation and mass ratio distributions appear to be different. All three low-mass binary systems in Upper Sco are tight (<18 AU) and of similar mass (q>0.6), consistent with expectations based on previous multiplicity studies of brown dwarfs and very low mass stars in the field and in open clusters. The implication is that the distinct separation and mass ratio distributions of low-mass systems are set in the formation process or at very young ages, rather than by dynamical disruption of wide systems at ages >5 Myr. Finally, we combine the survey detection limits with the models of Burrows et al. (1997) to show that there are no planets or very low-mass brown dwarfs with masses >10 M_J at projected separations >20 AU, or masses >5 M_J at projected separations >40 AU orbiting any of the low-mass (0.04-0.10 M_sun) objects in our sample.Comment: Accepted for publication in ApJ; 10 pages, 4 figures in emulateapj forma

Remote sensing applications in forestry. The development of an earth resources information system using aerial photographs and digital computers photographs and digital computers

Remote aerial sensing and automatic mapping for forest resources information syste

Validation of Slosh Model Parameters and Anti-Slosh Baffle Designs of Propellant Tanks by Using Lateral Slosh Testing

The slosh dynamics of propellant tanks can be represented by an equivalent mass-pendulum-dashpot mechanical model. The parameters of this equivalent model, identified as slosh mechanical model parameters, are slosh frequency, slosh mass, and pendulum hinge point location. They can be obtained by both analysis and testing for discrete fill levels. Anti-slosh baffles are usually needed in propellant tanks to control the movement of the fluid inside the tank. Lateral slosh testing, involving both random excitation testing and free-decay testing, are performed to validate the slosh mechanical model parameters and the damping added to the fluid by the anti-slosh baffles. Traditional modal analysis procedures were used to extract the parameters from the experimental data. Test setup of sub-scale tanks will be described. A comparison between experimental results and analysis will be presented

LONTalk as a Standard Protocol For Underwater Sensor Platforms

Proceedings IEEE, Oceans 97, Halifax, Oct. 1997 IEEE CD-ROM 0-7803-4111-

Stability of Colloidal Quasicrystals

Freezing of charge-stabilized colloidal suspensions and relative stabilities of crystals and quasicrystals are studied using thermodynamic perturbation theory. Macroion interactions are modelled by effective pair potentials combining electrostatic repulsion with polymer-depletion or van der Waals attraction. Comparing free energies -- counterion terms included -- for elementary crystals and rational approximants to icosahedral quasicrystals, parameters are identified for which one-component quasicrystals are stabilized by a compromise between packing entropy and cohesive energy.Comment: 6 pages, 4 figure

Effective Interactions and Volume Energies in Charge-Stabilized Colloidal Suspensions

Charge-stabilized colloidal suspensions can be conveniently described by formally reducing the macroion-microion mixture to an equivalent one-component system of pseudo-particles. Within this scheme, the utility of a linear response approximation for deriving effective interparticle interactions has been demonstrated [M. J. Grimson and M. Silbert, Mol. Phys. 74, 397 (1991)]. Here the response approach is extended to suspensions of finite-sized macroions and used to derive explicit expressions for (1) an effective electrostatic pair interaction between pseudo-macroions and (2) an associated volume energy that contributes to the total free energy. The derivation recovers precisely the form of the DLVO screened-Coulomb effective pair interaction for spherical macroions and makes manifest the important influence of the volume energy on thermodynamic properties of deionized suspensions. Excluded volume corrections are implicitly incorporated through a natural modification of the inverse screening length. By including nonlinear response of counterions to macroions, the theory may be generalized to systematically investigate effective many-body interactions.Comment: 13 pages (J. Phys.: Condensed Matter, in press

Multiplicity and Optical Excess Across the Substellar Boundary in Taurus

We present the results of a high-resolution imaging survey of 22 brown dwarfs and very low mass stars in the nearby (~145 pc) young (~1-2 Myr) low-density star-forming region Taurus-Auriga. We obtained images with the Advanced Camera for Surveys/High Resolution Channel on HST through the F555W (V), F775W (i'), and F850LP (z') filters. This survey confirmed the binarity of MHO-Tau-8 and discovered a new candidate binary system, V410-Xray3, resulting in a binary fraction of 9+/-5% at separations >4 AU. Both binary systems are tight (<10 AU) and they possess mass ratios of 0.75 and 0.46, respectively. The binary frequency and separations are consistent with low-mass binary properties in the field, but the mass ratio of V410-Xray3 is among the lowest known. We find that the binary frequency is higher for very low mass stars and high-mass brown dwarfs than for lower-mass brown dwarfs, implying either a decline in frequency or a shift to smaller separations for the lowest mass binaries. Combining these results with multiplicity statistics for higher-mass Taurus members suggests a gradual decline in binary frequency and separation toward low masses. The implication is that the distinct binary properties of very low-mass systems are set during formation and that the formation process is similar to the process which creates higher-mass stellar binaries, but occurs on a smaller scale. We show that there are no planets or very low-mass brown dwarfs with mass >3 M_J at projected separation >40 AU orbiting any of the Taurus members in our sample. We identify several BDs with significant (>1 mag) V-band excesses. The excesses appear to be correlated with signatures of accretion, and if attributed to accretion luminosity, may imply mass accretion rates several orders of magnitude above those inferred from line-profile analyses. (abridged)Comment: Accepted for publication in ApJ; 15 pages, 8 figures in emulateapj forma

Embedding the concept of ecosystems services:The utilisation of ecological knowledge in different policy venues

The concept of ecosystem services is increasingly being promoted by academics and policy makers as a means to protect ecological systems through more informed decision making. A basic premise of this approach is that strengthening the ecological knowledge base will significantly enhance ecosystem health through more sensitive decision making. However, the existing literature on knowledge utilisation, and many previous attempts to improve decision making through better knowledge integration, suggest that producing ‘more knowledge’ is only ever a necessary but insufficient condition for greater policy success. We begin this paper by reviewing what is already known about the relationship between ecological knowledge development and utilisation, before introducing a set of theme issue papers that examine—for the very first time—how this politically and scientifically salient relationship plays out across a number of vital policy venues such as land-use planning, policy-level impact assessment, and cost–benefit analysis. Following a detailed synthesis of the key findings of all the papers, this paper identifies and explores new research and policy challenges in this important and dynamic area of environmental governance