445 research outputs found
Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps
We have identified four circumstellar disks with a deficit of dust emission
from their inner 15-50 AU. All four stars have F-G spectral type, and were
uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy
Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence
stars. Modeling of the spectral energy distributions indicates a reduction in
dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU,
but with massive gas-rich disks at larger radii. This large contrast between
the inner and outer disk has led us to use the term `cold disks' to distinguish
these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present
close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux
density ratio as a new diagnostic for identifying cold disks. The mechanisms
for dust clearing over such large gaps are discussed. Though rare, cold disks
are likely in transition from an optically thick to an optically thin state,
and so offer excellent laboratories for the study of planet formation.Comment: 13 pages, 3 figures, accepted to ApJ
Level set based eXtended finite element modelling of the response of fibrous networks under hygroscopic swelling
Materials like paper, consisting of a network of natural fibres, exposed to
variations in moisture, undergo changes in geometrical and mechanical
properties. This behaviour is particularly important for understanding the
hygro-mechanical response of sheets of paper in applications like digital
printing. A two-dimensional microstructural model of a fibrous network is
therefore developed to upscale the hygro-expansion of individual fibres,
through their interaction, to the resulting overall expansion of the network.
The fibres are modelled with rectangular shapes and are assumed to be perfectly
bonded where they overlap. For realistic networks the number of bonds is large
and the network is geometrically so complex that discretizing it by
conventional, geometry-conforming, finite elements is cumbersome. The
combination of a level-set and XFEM formalism enables the use of regular,
structured grids in order to model the complex microstructural geometry. In
this approach, the fibres are described implicitly by a level-set function. In
order to represent the fibre boundaries in the fibrous network, an XFEM
discretization is used together with a Heaviside enrichment function. Numerical
results demonstrate that the proposed approach successfully captures the
hygro-expansive properties of the network with fewer degrees of freedom
compared to classical FEM, preserving desired accuracy.Comment: 27 pages, 22 figures, 4 tables, J. Appl. Mech. June 19, 202
A Newton Solver for Micromorphic Computational Homogenization Enabling Multiscale Buckling Analysis of Pattern-Transforming Metamaterials
Mechanical metamaterials feature engineered microstructures designed to
exhibit exotic, and often counter-intuitive, effective behaviour. Such a
behaviour is often achieved through instability-induced transformations of the
underlying periodic microstructure into one or multiple patterning modes. Due
to a strong kinematic coupling of individual repeating microstructural cells,
non-local behaviour and size effects emerge, which cannot easily be captured by
classical homogenization schemes. In addition, the individual patterning modes
can mutually interact in space as well as in time, while at the engineering
scale the entire structure can buckle globally. For efficient numerical
macroscale predictions, a micromorphic computational homogenization scheme has
recently been developed. Although this framework is in principle capable of
accounting for spatial and temporal interactions between individual patterning
modes, its implementation relied on a gradient-based quasi-Newton solution
technique. This solver is suboptimal because (i) it has sub-quadratic
convergence, and (ii) the absence of Hessians does not allow for proper
bifurcation analyses. Given that mechanical metamaterials often rely on
controlled instabilities, these limitations are serious. To address them, a
full Newton method is provided in detail in this paper. The construction of the
macroscopic tangent operator is not straightforward due to specific model
assumptions on the decomposition of the underlying displacement field pertinent
to the micromorphic framework, involving orthogonality constraints. Analytical
expressions for the first and second variation of the total potential energy
are given, and the complete algorithm is listed. The developed methodology is
demonstrated with two examples in which a competition between local and global
buckling exists and where multiple patterning modes emerge.Comment: 34 pages, 17 figures, 1 table, 1 algorithm, abstract shortened to
fulfill 1920 character limi
C2D Spitzer-IRS spectra of disks around T Tauri stars: IV. Crystalline silicates
Aims. Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96).
Methods. As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5-35 ΞΌm where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object.
Results. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at Ξ» > 20 ΞΌm correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 ΞΌm feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (β€1 AU) emitting at ~ 10 ΞΌm and a much colder region emitting at Ξ» > 20 ΞΌm (β€10 AU). We identify a crystallinity paradox, as the long-wavelength (Ξ» > 20 m) crystalline silicate features are detected 3.5 times more frequently (~55% vs. ~15%) than the crystalline features arising from much warmer disk regions (Ξ» ~ 10 ΞΌm). This suggests that the disk has an inhomogeneous dust composition within ~10 AU. The analysis of the shape and strength of both the amorphous 10 ΞΌm feature and the crystalline feature around 23 ΞΌm provides evidence for the prevalence of ΞΌm-sized (amorphous and crystalline) grains in upper layers of disks.
Conclusions. The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of ΞΌm-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure
Depairing currents in the superconductor/ferromagnet proximity system Nb/Fe
We have investigated the behaviour of the depairing current J_{dp} in
ferromagnet/superconductor/ferromagnet (F/S/F) trilayers as function of the
thickness d_s of the superconducting layers. Theoretically, J_{dp} depends on
the superconducting order parameter or the pair density function, which is not
homogeneous across the film due to the proximity effect. We use a proximity
effect model with two parameters (proximity strength and interface
transparency), which can also describe the dependence of the superconducting
transition temperature T_c on d_s. We compare the computations with the
experimentally determined zero-field critical current J_{c0} of small strips
(typically 5~ \mu m wide) of Fe/Nb/Fe trilayers with varying thickness d_{Nb}
of the Nb layer. Near T_c the temperature dependence J_{c0}(T) is in good
agreement with the expected behaviour, which allows extrapolation to T = 0.
Both the absolute values of J_{c0}(0) and the dependence on d_{Nb} agree with
the expectations for the depairing current. We conclude that J_{dp} is
correctly determined, notwithstanding the fact that the strip width is larger
than both the superconducting penetration depth and the superconducting
coherence length, and that J_{dp}(d_s) is correctly described by the model.Comment: 10 pages, 5 figures, submitted to PR
Spatially extended PAHs in circumstellar disks around T Tauri and Herbig Ae stars
Our aim is to determine the presence and location of the emission from
polycyclic aromatic hydrocarbons (PAHs) towards low and intermediate mass young
stars with disks using large aperture telescopes.
VLT-VISIR N-band spectra and VLT-ISAAC and VLT-NACO L-band spectra of 29
sources are presented, spectrally resolving the 3.3, 8.6, 11.2, and 12.6 micron
PAH features. Spatial-extent profiles of the features and the continuum
emission are derived and used to associate the PAH emission with the disks. The
results are discussed in the context of recent PAH-emission disk models.
The 3.3, 8.6, and 11.2 micron PAH features are detected toward a small
fraction of the T Tauri stars, with typical upper limits between 1E-15 and
5E-17 W/m^2. All 11.2 micron detections from a previous Spitzer survey are
confirmed with (tentative) 3.3 micron detections, and both the 8.6 and the 11.2
micron features are detected in all PAH sources. For 6 detections, the spatial
extent of the PAH features is confined to scales typically smaller than
0.12-0.34'', consistent with the radii of 12-60 AU disks at their distances
(typically 150 pc). For 3 additional sources, WL 16, HD 100546, and TY CrA, one
or more of the PAH features are more extended than the hot dust continuum of
the disk, whereas for Oph IRS 48, the size of the resolved PAH emission is
confirmed as smaller than for the large grains. For HD 100546, the 3.3 micron
emission is confined to a small radial extent of 12 +- 3 AU, most likely
associated with the outer rim of the gap in this disk. Gaps with radii out to
10-30 AU may also affect the observed PAH extent for other sources. For both
Herbig Ae and T Tauri stars, the small measured extents of the 8.6 and 11.2
micron features are consistent with larger (>= 100 carbon atoms) PAHs.Comment: 14 pages, 17 figures, accepted for publication in A&
Vortex Motion Noise in Micrometre-Sized Thin Films of the Amorphous Nb0.7Ge0.3 Weak-Pinning Superconductor
We report high-resolution measurements of voltage (V) noise in the mixed
state of micrometre-sized thin films of amorphous Nb0.7Ge0.3, which is a good
representative of weak-pinning superconductors. There is a remarkable
difference between the noise below and above the irreversibility field Birr.
Below Birr, in the presence of measurable pinning, the noise at small applied
currents resembles shot noise, and in the regime of flux flow at larger
currents decreases with increasing voltage due to a progressive ordering of the
vortex motion. At magnetic fields B between Birr and the upper critical field
Bc2 flux flow is present already at vanishingly small currents. In this regime
the noise scales with (1-B/Bc2)^2 V^2 and has a frequency (f) spectrum of 1/f
type. We interpret this noise in terms of the properties of strongly driven
depinned vortex systems at high vortex density.Comment: 8 pages, 5 figures, version accepted for publication in PR
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