Rings are the most frequently revealed substructure in Atacama Large Millimeter/submillimeter Array (ALMA) dust observations of protoplanetary disks, but their origin is still hotly debated. In this ...paper, we identify dust substructures in 12 disks and measure their properties to investigate how they form. This subsample of disks is selected from a high-resolution (∼0 12) ALMA 1.33 mm survey of 32 disks in the Taurus star-forming region, which was designed to cover a wide range of brightness and to be unbiased to previously known substructures. While axisymmetric rings and gaps are common within our sample, spiral patterns and high-contrast azimuthal asymmetries are not detected. Fits of disk models to the visibilities lead to estimates of the location and shape of gaps and rings, the flux in each disk component, and the size of the disk. The dust substructures occur across a wide range of stellar mass and disk brightness. Disks with multiple rings tend to be more massive and more extended. The correlation between gap locations and widths, the intensity contrast between rings and gaps, and the separations of rings and gaps could all be explained if most gaps are opened by low-mass planets (super-Earths and Neptunes) in the condition of low disk turbulence ( = 10−4). The gap locations are not well correlated with the expected locations of CO and N2 ice lines, so condensation fronts are unlikely to be a universal mechanism to create gaps and rings, though they may play a role in some cases.
We present a high-resolution (∼0 12, ∼16 au, mean sensitivity of 50 Jy beam−1 at 225 GHz) snapshot survey of 32 protoplanetary disks around young stars with spectral type earlier than M3 in the ...Taurus star-forming region using the Atacama Large Millimeter Array. This sample includes most mid-infrared excess members that were not previously imaged at high spatial resolution, excluding close binaries and objects with high extinction, thereby providing a more representative look at disk properties at 1-2 Myr. Our 1.3 mm continuum maps reveal 12 disks with prominent dust gaps and rings, 2 of which are around primary stars in wide binaries, and 20 disks with no resolved features at the observed resolution (hereafter smooth disks), 8 of which are around the primary star in wide binaries. The smooth disks were classified based on their lack of resolved substructures, but their most prominent property is that they are all compact with small effective emission radii (Reff,95% 50 au). In contrast, all disks with Reff,95% of at least 55 au in our sample show detectable substructures. Nevertheless, their inner emission cores (inside the resolved gaps) have similar peak brightness, power-law profiles, and transition radii to the compact smooth disks, so the primary difference between these two categories is the lack of outer substructures in the latter. These compact disks may lose their outer disk through fast radial drift without dust trapping, or they might be born with small sizes. The compact dust disks, as well as the inner disk cores of extended ring disks, that look smooth at the current resolution will likely show small-scale or low-contrast substructures at higher resolution. The correlation between disk size and disk luminosity correlation demonstrates that some of the compact disks are optically thick at millimeter wavelengths.
Abstract
Stars collect most of their mass during the protostellar stage, yet the accretion luminosity and stellar parameters, which are needed to compute the mass accretion rate, are poorly ...constrained for the youngest sources. The aim of this work is to fill this gap, computing the stellar properties and the accretion rates for a large sample of Class I protostars located in nearby (<500 pc) star-forming regions and analyzing their interplay. We used a self-consistent method to provide accretion and stellar parameters by modeling the spectral energy distribution and using veiling information from near-IR observations when possible. We calculated accretion and stellar properties for the first time for 50 young stars. We focused our analysis on the 39 confirmed protostars, finding that their mass accretion rate varies between ∼10
−8
and ∼10
−4
M
⊙
yr
−1
in a stellar mass range between ∼0.1 and 3
M
⊙
. We find systematically larger mass accretion rates for our Class I sample than for Class II objects. Although the mass accretion rate we found is high, it still suggests that either stars collect most of their mass before the Class I stage, or eruptive accretion is needed during the overall protostellar phase. Indeed, our results suggest that for a large number of protostars the disk can be unstable, which can result in accretion bursts and disk fragmentation in the past or in the future.
This contribution outlines Synchrotron-based X-ray micro-tomography and its potential use in structural geology and rock mechanics. The paper complements several recent reviews of X-ray ...microtomography. We summarize the general approach to data acquisition, post-processing as well as analysis and thereby aim to provide an entry point for the interested reader. The paper includes tables listing relevant beamlines, a list of all available imaging techniques, and available free and commercial software packages for data visualization and quantification. We highlight potential applications in a review of relevant literature including time-resolved experiments and digital rock physics. The paper concludes with a report on ongoing developments and upgrades at synchrotron facilities to frame the future possibilities for imaging sub-second processes in centimetre-sized samples.
•This paper gives an overview on Synchrotron radiation based microtomography.•Image acquisition, processing and quantification are outlined.•Review of applications in hard rock geology, rock mechanics and porous media research.
The feedback between fluid migration and rock deformation in mid-crustal shear zones is acknowledged as being critical for earthquake nucleation, the initiation of subduction zones and the formation ...of mineral deposits. The importance of this poorly understood feedback is further highlighted by evidence for shear-zone-controlled advective flow of fluids in the ductile lower crust and the recognition that deformation-induced grain-scale porosity is a key to large-scale geodynamics. Fluid migration in the middle crust cannot be explained in terms of classical concepts. The environment is considered too hot for a dynamic fracture-sustained permeability as in the upper crust, and fluid pathways are generally too deformed to be controlled by equilibrium wetting angles that apply to hotter, deeper environments. Here we present evidence that mechanical and chemical potentials control a syndeformational porosity generation in mid-crustal shear zones. High-resolution synchrotron X-ray tomography and scanning electron microscopy observations allow us to formulate a model for fluid migration in shear zones where a permeable porosity is dynamically created by viscous grain-boundary sliding, creep cavitation, dissolution and precipitation. We propose that syndeformational fluid migration in our 'granular fluid pump' model is a self-sustained process controlled by the explicit role of the rate of entropy production of the underlying irreversible mechanical and chemical microprocesses. The model explains fluid transfer through the middle crust, where strain localization in the creep regime is required for plate tectonics, the formation of giant ore deposits, mantle degassing and earthquake nucleation. Our findings provide a key component for the understanding of creep instabilities in the middle crust.
Dispersions of nanoscale precipitates in metallic alloys have been known to play a key role in strengthening, by increasing their strain hardenability and providing resistance to deformation. ...Although these phenomena have been extensively investigated in the last century, the traditional approaches employed in the past have not rendered an authoritative microstructural understanding in such materials. The effect of the precipitates' inherent complex morphology and their 3D spatial distribution on evolution and deformation behavior have often been precluded. This study reports, for the first time, implementation of synchrotron‐based hard X‐ray nanotomography in Al–Cu alloys to measure kinetics of different nanoscale phases in 3D, and reveals insights behind some of the observed novel phase transformation reactions. The experimental results of the present study reconcile with coarsening models from the Lifshitz–Slyozov–Wagner theory to an unprecedented extent, thereby establishing a new paradigm for thermodynamic analysis of precipitate assemblies. Finally, this study sheds light on the possibilities for establishing new theories for dislocation–particle interactions, based on the limitations of using the Orowan equation in estimating precipitation strengthening.
Probing nanoscale, 4D microstructural evolution in aluminum alloys using transmission X‐ray microscopy is reported, and the mechanisms behind the observed novel microstructural transformations are discussed. This approach redefines the understanding of aluminum alloys and overcomes shortcomings of most other characterization techniques by enabling measurement of 3D growth kinetics, validation of existing thermodynamic models, and estimation of 3D crystallographic orientation nondestructively.
Metal matrix composites (MMCs) have a combination of high strength, high stiffness, and low density. The damage behavior of MMCs has been studied extensively by a combination of traditional ...mechanical testing, microstructural characterization, and post-experiment fractographic analysis. X-ray tomography is an excellent technique that eliminates destructive cross-sectioning, and allows for superior resolution and image quality with minimal sample preparation. In this work, we have carried out a detailed investigation of the damage behavior of SiC particle reinforced 2080 Al alloy matrix composites by X-ray synchrotron tomography. This work is unique, relative to the existing work in the literature, because it: (a) focuses on a technologically relevant MMC system (2080/SiC
p), (b) uses a combination of image analysis techniques to enable visualization and damage characterization, and (c) entails a significant amount of quantitative and statistical analyses of particle fracture and void growth in the composite. A statistically significant number of particles and volume of the composite were characterized, enabling a meaningful and realistic interpretation of the results. Based on this, a detailed understanding of the micromechanisms of fracture and the quantitative influence of particle size and aspect ratio were obtained.
► In situ 3D X-ray synchrotron tomography of fatigue crack growth in 7075-T6 aluminum alloy. ► Measurements of crack opening displacement for quantifying fatigue crack closure. ► Quantitative ...analysis of the role of brittle inclusions on fatigue crack propagation.
In situ 3D X-ray synchrotron tomography of fatigue crack growth was conducted in a 7075-T6 aluminum alloy. Local measurements of da/dN were possible with the 3D data sets obtained from tomography. In situ measurements of crack opening displacement (COD) were obtained, illustrating the possibilities for quantifying fatigue crack closure. Quantitative microstructural analysis enabled an assessment of the role of brittle inclusions on fatigue crack propagation. A significant increase in preferential crack growth through the inclusions was observed.
Abstract
Evidence of a relation between the mass accretion rate and the disk mass is established for young, Class II pre-main-sequence stars. This observational result opened an avenue to test ...theoretical models and constrain the initial conditions of disk formation, fundamental in the understanding of the emergence of planetary systems. However, it is becoming clear that planet formation starts even before the Class II stage, in disks around Class 0 and I protostars. We show for the first time evidence for a correlation between the mass accretion rate and the disk mass for a large sample of Class I young stars located in nearby (<500 pc) star-forming regions. We fit our sample, finding that the Class I object relation has a slope flatter than Class II stars, and the former have higher mass accretion rates and disk masses. The results are put in context of disk evolution models.
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