Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCo-Accretion of Chondrules and Dust in the Solar Nebula
235
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present a mechanism for chondrules to stick together by means of compaction of a porous dust rim they sweep up as they move through the dusty nebula gas. It is shown that dust aggregates formed out of micron-sized grains stick to chondrules, forming a porous dust rim. When chondrules collide, this dust can be compacted by means of rolling motions within the porous dust layer. This mechanism dissipates the collisional energy, compacting the rim and allowing chondrules to stick. The structure of the obtained chondrule-dust agglomerates (referred to as compounds) then consists of three phases: chondrules, porous dust, and dust that has been compacted by collisions. Subsequently, these compounds accrete their own dust and collide with other compounds. The evolution of the compound size distribution and the relative importance of the phases is calculated by a Monte Carlo code. Growth ends, and a simulation is terminated when all the dust in the compounds has been compacted. Numerous runs are performed, reflecting the uncertainty in the physical conditions at the chondrule formation time. It is found that compounds can grow by 1-2 orders of magnitudes in radius, upto dm-sizes when turbulence levels are low. However, relative velocities associated with radial drift form a barrier for further growth. Earlier findings that the dust sweep-up by chondrules is proportional to their sizes are confirmed. We contrast two scenarios regarding how this dust evolved further towards the densely packed rims seen in chondrites.
rate research
Read More
Chondrules formed by the melting of dust aggregates in the solar protoplanetary disk and as such provide unique insights into how solid material was transported and mixed within the disk. Here we show that chondrules from enstatite and ordinary chondrites show only small 50Ti variations and scatter closely around the 50Ti composition of their host chondrites. By contrast, chondrules from carbonaceous chondrites have highly variable 50Ti compositions, which, relative to the terrestrial standard, range from the small 50Ti deficits measured for enstatite and ordinary chondrite chondrules to the large 50Ti excesses known from Ca-Al-rich inclusions (CAIs). These 50Ti variations can be attributed to the addition of isotopically heterogeneous CAI-like material to enstatite and ordinary chondrite-like chondrule precursors. The new Ti isotopic data demonstrate that isotopic variations among carbonaceous chondrite chondrules do not require formation over a wide range of orbital distances, but can instead be fully accounted for by the incorporation of isotopically anomalous nuggets into chondrule precursors. As such, these data obviate the need for disk-wide transport of chondrules prior to chondrite parent body accretion and are consistent with formation of chondrules from a given chondrite group in localized regions of the disk. Lastly, the ubiquitous presence of 50Ti-enriched material in carbonaceous chondrites, and the lack of this material in the non-carbonaceous chondrites support the idea that these two meteorite groups derive from areas of the disk that remained isolated from each other through the formation of Jupiter.
Isotopic anomalies in chondrules hold important clues about the dynamics of mixing and transport processes in the solar accretion disk. These anomalies have been interpreted to indicate either disk-wide transport of chondrules or local heterogeneities of chondrule precursors. However, all previous studies relied on isotopic data for a single element (either Cr, Ti, or O), which does not allow distinguishing between source and precursor signatures as the cause of the chondrules isotope anomalies. Here we obtained the first combined O, Ti, and Cr isotope data for individual chondrules from enstatite, ordinary, and carbonaceous chondrites. We find that chondrules from non-carbonaceous (NC) chondrites have relatively homogeneous {Delta}17O, {epsilon}50Ti, and {epsilon}54Cr, which are similar to the compositions of their host chondrites. By contrast, chondrules from carbonaceous chondrites (CC) have more variable compositions. Although the compositions of the analyzed CC and NC chondrules may overlap for either {epsilon}50Ti, {epsilon}54Cr, or {Delta}17O, in multi-isotope space none of the CC chondrules plot in the compositional field of NC chondrites, and no NC chondrule plots within the field of CC chondrites. As such, our data reveal a fundamental isotopic difference between NC and CC chondrules, which is inconsistent with a disk-wide transport of chondrules across and between the NC and CC reservoirs. Instead, the isotopic variations among CC chondrules reflect local precursor heterogeneities, which most likely result from mixing between NC-like dust and a chemically diverse dust component that was isotopically similar to CAIs and AOAs.The same mixing processes, but on a larger, disk-wide scale, were likely responsible for establishing the distinct isotopic compositions of the NC and CC reservoirs, which represent in inner and outer disk, respectively.
Planets form in the disks around young stars. Their formation efficiency and composition are intimately linked to the protoplanetary disk locations of snow lines of abundant volatiles. We present chemical imaging of the CO snow line in the disk around TW Hya, an analog of the solar nebula, using high spatial and spectral resolution Atacama Large Millimeter/Submillimeter Array (ALMA) observations of N2H+, a reactive ion present in large abundance only where CO is frozen out. The N2H+ emission is distributed in a large ring, with an inner radius that matches CO snow line model predictions. The extracted CO snow line radius of ~ 30 AU helps to assess models of the formation dynamics of the Solar System, when combined with measurements of the bulk composition of planets and comets.
Chondrules are often surrounded by fine-grained rims or igneous rims. The properties of these rims reflect their formation histories. While the formation of fine-grained rims is modeled by the accretion of dust grains onto chondrules, the accretion should be followed by the growth of dust grains due to the shorter growth timescale than the accretion. In this paper, we investigate the formation of rims, taking into account the growth of porous dust aggregates. We estimate the rim thickness as a function of the chondrule fraction at a time when dust aggregate accretion onto chondrules is switched to collisions between these chondrules. Our estimations are consistent with the measured thicknesses of fine-grained rims in ordinary chondrites. However, those of igneous rims are thicker than our estimations. The thickness of igneous rims would be enlarged in remelting events.
We have constructed MOCASSIN photoionization plus dust radiative transfer models for the Crab Nebula core-collapse supernova (CCSN) remnant, using either smooth or clumped mass distributions, in order to determine the chemical composition and masses of the nebular gas and dust. We computed models for several different geometries suggested for the nebular matter distribution but found that the observed gas and dust spectra are relatively insensitive to these geometries, being determined mainly by the spectrum of the pulsar wind nebula which ionizes and heats the nebula. Smooth distribution models are ruled out since they require 16-49 Msun of gas to fit the integrated optical nebular line fluxes, whereas our clumped models re quire 7.0 Msun of gas. A global gas-phase C/O ratio of 1.65 by number is derived, along with a He/H number ratio of 1.85, neither of which can be matched by current CCSN yield predictions. A carbonaceous dust composition is favoured by the observed gas-phase C/O ratio: amorphous carbon clumped model fits to the Crabs Herschel and Spitzer infrared spectral energy distribution imply the presence of 0.18-0.27 Msun of dust, corresponding to a gas to dust mass ratio of 26-39. Mixed dust chemistry models can also be accommodated, comprising 0.11-0.13 Msun of amorphous carbon and 0.39-0.47 Msun of silicates. Power-law grain size distributions with mass distributions that are weighted towards the largest grain radii are derived, favouring their longer-term survival when they eventually interact with the interstellar medium. The total mass of gas plus dust in the Crab Nebula is 7.2 +/- 0.5 Msun, consistent with a progenitor star mass of 9 Msun.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
