We numerically study the effect of adding quenched disorder in the form of randomly placed pinning sites on jamming transitions in a disk packing that jams at a well defined point J in the clean limit. Quenched disorder decreases the jamming density and introduces a depinning threshold. The onset of a finite threshold coincides with point J at the lowest pinning densities, but for higher pinning densities there is always a finite depinning threshold even well below jamming. We find that proximity to point J strongly affects the transport curves and noise fluctuations, and observe a change from plastic behavior below jamming, where the system is highly heterogeneous, to elastic depinning above jamming. Many of the general features we find are related to other systems containing quenched disorder, including the peak effect observed in vortex systems.
We performed an in-depth exploration of the Al-Mg system for presolar graphite, SiC, and Si 3 N 4 grains found to contain large excesses of 26 ratios that, on average, are ∼1.5-2 times larger than the ratios previously reported for the grains. The majority of presolar graphite and SiC grains are heavily affected by Al contamination, resulting in large negative Mg Mg 26 24 d intercepts of the isochron lines. Al contamination is potentially due to etching of the grains' surfaces and subsequent capture of dissolved Al during the acid dissolution of their meteorite host rocks. From the isochron fits, the magnitude of Al contamination was quantified for each grain. The amount of Al contamination on each grain was found to be random and independent of grain size, following a uniform distribution with an upper bound at 59% contamination. The Al contamination causes conventional whole-grain estimates to underpredict the initial Al Al ratios greatly exceed those predicted in the He/C and He/N zones of SN models.
We report on the results of NanoSIMS isotope imaging of low-density supernova graphite grains from the Orgueil meteorite. 70nm-thick microtomed sections of three supernova graphite grains were deposited on Si wafers and isotopically imaged in the NanoSIMS. These sections contain hotspots of excesses in 18 O and 15 N, which are spatially well-correlated, and are likely carried by internal TiC subgrains. These hotspots are considerably more enriched in 18 O and 15 N than the host graphite grain. Correlations between 18 O and 15 N excesses indicate that the grains incorporated material from the He/C supernova zone. Isotope images of the surfaces of some grains show heterogeneities in their N and O isotope compositions, with extreme excesses in 15 N and 18 O. In the microtome sections we also observe two types of heterogeneities in the grains' C isotopic compositions: smooth, radial gradients in 12 C/ 13 C, with this ratio trending towards solar with increasing radius; and highly anomalous pockets up to 2µm in size with 12 C/ 13 C>>solar that are located near the centers of the grain sections. Partial isotopic equilibration does not likely explain the C isotopic heterogeneities. These grains and their constituent parts probably formed in a stellar environment with changing isotopic composition.
Abstract-Transmission electron microscope (TEM) investigations have revealed Os, Ru, Mo-rich refractory metal nuggets within four different presolar graphites, from both the high-density (HD) Murchison (MUR) and low-density (LD) Orgueil (ORG) fractions. Microstructural and chemical data suggest that these are direct condensates from the gas, rather than forming later by exsolution. The presolar refractory metal nugget (pRMN) compositions are variable (e.g., from 8 < Os atom% < 77), but follow the same chemical fractionation trends as isolated refractory metal nuggets (mRMNs) previously found in meteorites (Berg et al. 2009). From these compositions one can infer a temperature of last equilibration with the gas of 1405-1810 K (e.g., Berg et al. [2009] at approximately 100 dyne cm À2 pressure), which implies that the host graphites form over roughly the same range (in agreement with predictions) and that the pRMNs are chemically isolated from the gas when captured by graphite. Further, the pRMN compositions give evidence that HD graphites form at a higher T than LD ones. Chemical and phase similarities with the isolated mRMNs suggest that the mRMNs also condense directly from a gas, although from the early solar nebula rather than a presolar environment. Although the pRMNs themselves are too small for detection of isotopic anomalies, NanoSIMS isotopic measurements of their host graphites confirm a presolar origin for the assemblages. The two pRMN-containing LD graphites show evidence of a supernova (SN) origin, whereas the stellar origins of the pRMNs in HD graphite are unclear, because only less-diagnostic 12 C enrichments are detectable (as is commonly true for HD graphites).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.