Mutations in human dynamin block an intermediate stage in coated vesicle formation

The role of human dynamin in receptor-mediated endocytosis was investigated by transient expression of GTP-binding domain mutants in mammalian cells. Using assays which detect intermediates in coated vesicle formation, the dynamin mutants were found to block endocytosis at a stage after the initiation of coat assembly and preceding the sequestration of ligands into deeply invaginated coated pits. Membrane transport from the ER to the Golgi complex was unaffected indicating that dynamin mutants specifically block early events in endocytosis. These results demonstrate that mutations in the GTP-binding domain of dynamin block Tfn-endocytosis in mammalian cells and suggest that a functional dynamin GTPase is required for receptor-mediated endocytosis via clathrin-coated pits

A Model for the Voltage Steps in the Breakdown of the Integer Quantum Hall Effect

In samples used to maintain the US resistance standard the breakdown of the dissipationless integer quantum Hall effect occurs as a series of dissipative voltage steps. A mechanism for this type of breakdown is proposed, based on the generation of magneto-excitons when the quantum Hall fluid flows past an ionised impurity above a critical velocity. The calculated generation rate gives a voltage step height in good agreement with measurements on both electron and hole gases. We also compare this model to a hydrodynamic description of breakdown.Comment: 4 pages including 3 figure

The Brown Dwarf Kinematics Project (BDKP). III. Parallaxes for 70 Ultracool Dwarfs

We report parallax measurements for 70 ultracool dwarfs (UCDs). Using both literature values and our sample, we report new polynomial relations between spectral type and M$_{JHK}$. Including resolved L/T transition binaries in the relations, we find no reason to differentiate between a "bright" (unresolved binary) and "faint" (single source) sample across the L/T boundary. Isolating early T dwarfs, we find that the brightening of T0-T4 sources is prominent in M$_{J}$ where there is a [1.2 - 1.4] magnitude difference. A similar yet dampened brightening of [0.3 - 0.5] magnitude happens at M$_{H}$ and a plateau or dimming of [-0.2 - -0.3] magnitude is seen in M$_{K}$. Comparing with evolutionary models that vary gravity, metallicity, and cloud thickness we find that a near constant temperature of 1200 $\pm$100 K along a narrow spectral subtype of T0-T4 is required to account for the brightening and color magnitude diagram of the L-dwarf/T-dwarf transition. Furthermore, there is a significant population of both L and T dwarfs which are red or potentially "ultra-cloudy" compared to the models, many of which are known to be young indicating a correlation between enhanced photospheric dust and youth. For the low surface-gravity or young companion L dwarfs we find that 8 out of 10 are at least [0.2-1.0] magnitude underluminous in M$_{JH}$ and/or M$_{K}$ compared to equivalent spectral type objects. We speculate that this is a consequence of increased dust opacity and conclude that low-surface gravity L dwarfs require a completely new spectral-type/absolute magnitude polynomial for analysis.Comment: 65 pages, Accepted for publication to Ap

Color Transformations for the 2MASS Second Incremental Data Release

Transformation equations are presented to convert colors and magnitudes measured in the AAO, ARNICA, CIT, DENIS, ESO, LCO (Persson standards), MSSSO, SAAO, and UKIRT photometric systems to the photometric system inherent to the 2MASS Second Incremental Data Release. The transformations have been derived by comparing 2MASS photometry with published magnitudes and colors for stars observed in these systems. Transformation equations have also been derived indirectly for the Bessell & Brett (1988) and Koornneef (1983) homogenized photometric systems.Comment: To appear in AJ, May 200

When Interval Analysis Helps Inter-Block Backtracking

International audienceInter-block backtracking (IBB) computes all the solutions of sparse systems of non-linear equations over the reals. This algorithm, introduced in 1998 by Bliek et al., handles a system of equations previously decomposed into a set of (small) k × k sub-systems, called blocks. Partial solutions are computed in the different blocks and combined together to obtain the set of global solutions. When solutions inside blocks are computed with interval-based techniques, IBB can be viewed as a new interval-based algorithm for solving decomposed equation systems. Previous implementations used Ilog Solver and its IlcInterval library. The fact that this interval-based solver was more or less a black box implied several strong limitations. The new results described in this paper come from the integration of IBB with the interval-based library developed by the second author. This new library allows IBB to become reliable (no solution is lost) while still gaining several orders of magnitude w.r.t. solving the whole system. We compare several variants of IBB on a sample of benchmarks, which allows us to better understand the behavior of IBB. The main conclusion is that the use of an interval Newton operator inside blocks has the most positive impact on the robustness and performance of IBB. This modifies the influence of other features, such as intelligent backtracking and filtering strategies

Strong, Ultra-narrow Peaks of Longitudinal and Hall Resistances in the Regime of Breakdown of the Quantum Hall Effect

With unusually slow and high-resolution sweeps of magnetic field, strong, ultra-narrow (width down to $100 {\rm \mu T}$) resistance peaks are observed in the regime of breakdown of the quantum Hall effect. The peaks are dependent on the directions and even the history of magnetic field sweeps, indicating the involvement of a very slow physical process. Such a process and the sharp peaks are, however, not predicted by existing theories. We also find a clear connection between the resistance peaks and nuclear spin polarization.Comment: 5 pages with 3 figures. To appear in PR

The mineralogy, geometry and mass-loss history of IRAS 16342-3814

We present the 2-200 um Infrared Space Observatory (ISO) spectrum and 3.8-20 um ISAAC and TIMMI2 images of the extreme OH/IR star IRAS 16342-3814. Amorphous silicate absorption features are seen, together with crystalline silicate absorption features up to almost 45 um. No other OH/IR star is known to have crystalline silicate features in absorption up to these wavelengths. This suggests that IRAS 16342-3814 must have, or recently had, an extremely high mass-loss rate. Preliminary radiative transfer calculations suggest that the mass-loss rate may be as large as 10^{-3} Msun/yr. The 3.8 um ISAAC image shows a bipolar reflection nebula with a dark equatorial waist or torus, similar to that seen in optical Hubble Space Telescope (HST) images. The position angle of the nebula decreases significantly with increasing wavelength, suggesting that the dominant source of emission changes from scattering to thermal emission. Still, even up to 20 um the nebula is oriented approximately along the major axis of the nebula seen in the HST and ISAAC images, suggesting that the torus must be very cold, in agreement with the very red ISO spectrum. The 20 um image shows a roughly spherically symmetric extended halo, approximately 6'' in diameter, which is probably due to a previous phase of mass-loss on the AGB, suggesting a transition from a (more) spherically symmetric to a (more) axial symmetric form of mass-loss at the end of the AGB. We estimate the maximum dust particle sizes in the torus and in the reflection nebula to be 1.3 and 0.09 um respectively. The size of the particles in the torus is large compared to typical ISM values, but in agreement with high mass-loss rate objects like AFGL 4106 and HD161796. We discuss the possible reason for the difference in particle size between the torus and the reflection nebula.Comment: Accepted for publication by A&

Electron propagation in crossed magnetic and electric fields

Laser-atom interaction can be an efficient mechanism for the production of coherent electrons. We analyze the dynamics of monoenergetic electrons in the presence of uniform, perpendicular magnetic and electric fields. The Green function technique is used to derive analytic results for the field--induced quantum mechanical drift motion of i) single electrons and ii) a dilute Fermi gas of electrons. The method yields the drift current and, at the same time it allows us to quantitatively establish the broadening of the (magnetic) Landau levels due to the electric field: Level number k is split into k+1 sublevels that render the $k$th oscillator eigenstate in energy space. Adjacent Landau levels will overlap if the electric field exceeds a critical strength. Our observations are relevant for quantum Hall configurations whenever electric field effects should be taken into account.Comment: 11 pages, 2 figures, submitte

The Vega Debris Disk -- A Surprise from Spitzer

We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved and its angular size is much larger than found previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and 105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are circular, smooth and without clumpiness at all three wavelengths. The radial surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an r^-1 power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger than 180 um and at radii of 86-200 AU from the star, can reproduce the observed 850 um flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters amongthese large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances where we detect its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS results. This rate would require a very massive asteroidal reservoir for the dust to be produced in a steady state throughout Vega's life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and subsequent collisional cascade.Comment: 13 pages, 17 figures, accepted for publication in ApJ. (Figures 2, 3a, 3b and 4 have been degraded to lower resolutions.