131 research outputs found
Research Report to the National Aeronautics and Space Administration Cosmochemistry Program
The discovery of presolar grains in meteorites is one of the most exciting recent developments in meteoritics. Six types of presolar grain have been discovered: diamond, Sic, graphite, Si3N4, Al2O3 and MgAl2O4 (NIITLER, 2003). These grains have been identified as presolar because their isotopic compositions are very different from those of Solar System materials. Comparison of their isotopic compositions with astronomical observations and theoretical models indicates that most of the grains formed in the envelopes of highly evolved stars. They are, therefore, a new source of information with which to test astrophysical models of the evolution of these stars. In fact, because several elements can often be measured in the same grain, including elements that are not measurable spectroscopically in stars, the grain data provide some very stringent constraints for these models. Our primary goal is to create large, unbiased, multi-isotope databases of single presolar Sic, Si3N4, oxide and graphite grains in meteorites, as well as any new presolar grain types that are identified in the future. These will be used to: (i) test stellar and nucleosynthetic models, (ii) constrain the galactic chemical evolution (GCE) paths of the isotopes of Si, Ti, O and Mg, (iii) establish how many stellar sources contributed to the Solar System, (iv) constrain relative dust production rates of various stellar types and (v) assess how representative of galactic dust production the record in meteorites is. The primary tool for this project is a highly automated grain analysis system on the Carnegie 6f ion probe. This proposal was part of a long-standing research effort that is still ongoing
STELLAR ORIGINS OF EXTREMELY C-13- AND N-15-ENRICHED PRESOLAR SIC GRAINS: NOVAE OR SUPERNOVAE?
Extreme excesses of 13C (12C/13C < 10) and 15N (14N/15N < 20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron- to micron-sized 13C- and 15N-enriched presolar SiC grains (12C/13C < 16 and 14N/15N < ~100) from Murchison, and their correlated Mg–Al, S, and Ca–Ti isotope data when available. These grains are enriched in 13C and 15N, but with quite diverse Si isotopic signatures. Four grains with 29,30Si excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton- and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with 30Si excesses and 29Si depletions show lower-than-solar 34S/32S ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also 13C enriched, but have a range of higher 14N/15N. We found that 15N-enriched AB grains (~50 < 14N/15N < ~100) have distinctive isotopic signatures compared to putative nova grains, such as higher 14N/15N, lower 26Al/27Al, and lack of 30Si excess, indicating weaker proton-capture nucleosynthetic environments
Stellar origin of 15N-rich presolar SiC grains of type AB: Supernovae with explosive hydrogen burning
© 2017. The American Astronomical Society. All rights reserved. We report C, N, and Si isotopic data for 59 highly 13 C-enriched presolar submicron-to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 15 N-rich ( 14 N/ 15 N ≤ solar) AB grains, and their Mg-Al, S, and Ca-Ti isotope data when available. These 37 grains are enriched in 13 C, 15 N, and 26 Al with the PNGs showing more extreme enhancements. The 15 N-rich AB grains show systematically higher 26 Al and 30 Si excesses than the 14 N-rich AB grains. Thus, we propose to divide the AB grains into groups 1 ( 14 N/ 15 N < solar) and 2 ( 14 N/ 15 N ≥ solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG and found 32 S and/or 50 Ti enhancements. Interestingly, one AB1 grain had the largest 32 S and 50 Ti excesses, strongly suggesting a neutron-capture nucleosynthetic origin of the 32 S excess and thus the initial presence of radiogenic 32 Si (t 1/2 = 153 years). More importantly, we found that the 15 N and 26 Al excesses of AB1 grains form a trend that extends to the region in the N-Al isotope plot occupied by C2 grains, strongly indicating a common stellar origin for both AB1 and C2 grains. Comparison of supernova models with the AB1 and C2 grain data indicates that these grains came from supernovae that experienced H ingestion into the He/C zones of their progenitors
The Compositional Structure of the Asteroid Belt
The past decade has brought major improvements in large-scale asteroid
discovery and characterization with over half a million known asteroids and
over 100,000 with some measurement of physical characterization. This explosion
of data has allowed us to create a new global picture of the Main Asteroid
Belt. Put in context with meteorite measurements and dynamical models, a new
and more complete picture of Solar System evolution has emerged. The question
has changed from "What was the original compositional gradient of the Asteroid
Belt?" to "What was the original compositional gradient of small bodies across
the entire Solar System?" No longer is the leading theory that two belts of
planetesimals are primordial, but instead those belts were formed and sculpted
through evolutionary processes after Solar System formation. This article
reviews the advancements on the fronts of asteroid compositional
characterization, meteorite measurements, and dynamical theories in the context
of the heliocentric distribution of asteroid compositions seen in the Main Belt
today. This chapter also reviews the major outstanding questions relating to
asteroid compositions and distributions and summarizes the progress and current
state of understanding of these questions to form the big picture of the
formation and evolution of asteroids in the Main Belt. Finally, we briefly
review the relevance of asteroids and their compositions in their greater
context within our Solar System and beyond.Comment: Accepted chapter in Asteroids IV in the Space Science Series to be
published Fall 201
Vapor-Melt Exchange -- Constraints on Chondrite Formation Conditions and Processes
The bulk volatile contents of chondritic meteorites provide clues to their
origins. Matrix and chondrules carry differing abundances of moderately
volatile elements, with chondrules carrying a refractory signature. At the high
temperatures of chondrule formation and the low pressures of the solar nebula,
many elements, including Na and Fe, should have been volatile. Yet the evidence
is that even at peak temperatures, at or near the liquidus, Na and Fe (as FeO
and Fe-metal) were present in about their current abundances in molten
chondrules. This seems to require very high solid densities during chondrule
formation to prevent significant evaporation. Evaporation should also be
accompanied by isotopic mass fractionation. Evidence from a wide range of
isotopic systems indicates only slight isotopic mass fractionations of
moderately vola-tile elements, further supporting high solid densities.
However, olivine-rich, FeO-poor chondrules commonly have pyroxene-dominated
outer zones that have been interpreted as the prod-ucts of late condensation of
SiO2 into chondrule melts. Late condensation of more refractory SiO2 is
inconsistent with the apparent abundances of more volatile Na, FeO and Fe-metal
in many chondrules. Despite significant recent experimental work bearing on
this problem, the conditions under which chondrules behaved as open systems
remain enigmatic
Stellar Origins of Extremely - and -enriched Presolar SiC Grains: Novae or Supernovae?
Extreme excesses of (/<10) and
(/<20) in rare presolar SiC grains have been considered
diagnostic of an origin in classical novae, though an origin in core collapse
supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data
for 14 submicron- to micron-sized - and -enriched presolar SiC
grains (/<16 and /<~100) from Murchison, and
their correlated Mg-Al, S, and Ca-Ti isotope data when available. These grains
are enriched in and , but with quite diverse Si isotopic
signatures. Four grains with excesses similar to those of type C
SiC grains likely came from CCSNe, which experienced explosive H burning
occurred during explosions. The independent coexistence of proton- and
neutron-capture isotopic signatures in these grains strongly supports
heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven
putative nova grains with excesses and depletions show
lower-than-solar / ratios that cannot be explained by classical
nova nucleosynthetic models. We discuss these signatures within the CCSN
scenario. For the remaining five putative nova grains, both nova and supernova
origins are viable because explosive H burning in the two stellar sites could
result in quite similar proton-capture isotopic signatures. Three of the grains
are sub-type AB grains that are also enriched, but have a range of
higher /. We found that -enriched AB grains
(~50</<~100) have distinctive isotopic signatures compared to
putative nova grains, such as higher /, lower
/, and lack of excess, indicating weaker
proton-capture nucleosynthetic environments.Comment: fix typo in one of the authors' name
The ancient heritage of water ice in the solar system
Identifying the source of Earth's water is central to understanding the
origins of life-fostering environments and to assessing the prevalence of such
environments in space. Water throughout the solar system exhibits
deuterium-to-hydrogen enrichments, a fossil relic of low-temperature,
ion-derived chemistry within either (i) the parent molecular cloud or (ii) the
solar nebula protoplanetary disk. Utilizing a comprehensive treatment of disk
ionization, we find that ion-driven deuterium pathways are inefficient,
curtailing the disk's deuterated water formation and its viability as the sole
source for the solar system's water. This finding implies that if the solar
system's formation was typical, abundant interstellar ices are available to all
nascent planetary systems.Comment: 33 pages, 7 figures including main text and supplementary materials.
Published in Scienc
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