398 research outputs found
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 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&
Grain growth and dust settling in a brown dwarf disk: Gemini/T-ReCS observations of CFHT-BD-Tau 4
We present accurate mid-infrared observations of the disk around the young,
bona-fide brown dwarf CFHT-BD-Tau 4. We report GEMINI/T-ReCS measurements in
the 7.9, 10.4 and 12.3 micron filters, from which we infer the presence of a
prominent, broad silicate emission feature. The shape of the silicate feature
is dominated by emission from 2 micron amorphous olivine grains. Such grains,
being an order of magnitude larger than those in the interstellar medium, are a
first proof of dust processing and grain growth in disks around brown dwarfs.
The object's spectral energy distribution is below the prediction of the
classical flared disk model but higher than that of the two-layer flat disk. A
good match can be achieved by using an intermediate disk model with strongly
reduced but non-zero flaring. Grain growth and dust settling processes provide
a natural explanation for this disk geometry and we argue that such
intermediate flaring might explain the observations of several other brown
dwarf disks as well.Comment: Accepted for publication in Astronomy & Astrophysics Letters, 4.5
pages with 1 figur
Evolution of Young Brown Dwarf Disks in the Mid-Infrared
We have imaged two bona-fide brown dwarfs with TReCS/GEMINI-S and find
mid-infrared excess emission that can be explained by optically thick dust disk
models. In the case of the young (2Myr) Cha H1 we measure
fluxes at 10.4m and 12.3m that are fully consistent with a standard
flared disk model and prominent silicate emission. For the 10Myr old
brown dwarf 2MASS1207-3932 located in the TW Hydrae association we find excess
emission at 8.7m and 10.4m with respect to its photosphere, and
confirm disk accretion as likely cause of its strong activity. Disks around
brown dwarfs likely last at least as long as their low-mass stellar
counterparts in the T-Tauri phase. Grain growth, dust settling, and evolution
of the geometry of brown dwarfs disks may appear on a timescale of 10Myr and
can be witnessed by observations in the mid-infrared.Comment: 6 pages, 4 figure
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 ) 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
Modifying Anthocyanins Biosynthesis in Tomato Hairy Roots:A Test Bed for Plant Resistance to Ionizing Radiation and Antioxidant Properties in Space
Gene expression manipulation of specific metabolic pathways can be used to obtain bioaccumulation of valuable molecules and desired quality traits in plants. A single-gene approach to impact different traits would be greatly desirable in agrospace applications, where several aspects of plant physiology can be affected, influencing growth. In this work, MicroTom hairy root cultures expressing a MYB-like transcription factor that regulates the biosynthesis of anthocyanins in Petunia hybrida (PhAN4), were considered as a testbed for bio-fortified tomato whole plants aimed at agrospace applications. Ectopic expression of PhAN4 promoted biosynthesis of anthocyanins, allowing to profile 5 major derivatives of delphinidin and petunidin together with pelargonidin and malvidin-based anthocyanins, unusual in tomato. Consistent with PhAN4 features, transcriptomic profiling indicated upregulation of genes correlated to anthocyanin biosynthesis. Interestingly, a transcriptome reprogramming oriented to positive regulation of cell response to biotic, abiotic, and redox stimuli was evidenced. PhAN4 hairy root cultures showed the significant capability to counteract reactive oxygen species (ROS) accumulation and protein misfolding upon high-dose gamma irradiation, which is among the most potent pro-oxidant stress that can be encountered in space. These results may have significance in the engineering of whole tomato plants that can benefit space agriculture
Sub-mm observations and modelling of Vega type stars
We present new sub-mm observations and modelling of Vega excess stars, using
realistic dust grain models. For resolved disks, we find that different objects
require very different dust grain properties in order to simultaneously fit the
image data and SED. Fomalhaut and Vega require solid dust grains, whilst HR4796
and HD141569 can only be fitted using porous grains. The older stars tend to
have less porous grains than younger stars, which may indicate that collisions
have compacted the dust grains. Eps Eri appears to be deficient in small dust
grains compared to our best fitting model. This may be due to factors which
affect the size distribution of grains close to the radiation pressure blowout
limit. Alternatively, this discrepancy may be due to some external influence on
the disk (e.g. a planet). When the model is applied to unresolved targets, an
estimate of the disk size can be made. However, the large diversity in dust
composition for the resolved disks means that we cannot make a reliable
assumption as to the composition of the grains in an unresolved disk, and there
is corresponding uncertainty in the disk size. In addition, the poor fit for
Eps Eri shows that the model cannot always account for the SED even if the disk
size is known. These two factors mean that it may not be possible to determine
a disk's size without actually resolving it.Comment: 15 pages, 15 figures, accepted by MNRAS. Revised Eps Eri modelling to
show larger range of minimum size cutoffs with porous grains, Figure
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