Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCoordinated Analysis of Two Graphite Grains from the CO3.0 LAP 031117 Meteorite: First Identification of a CO Nova Graphite and a Presolar Iron Sulfide Subgrain
180
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Presolar grains constitute remnants of stars that existed before the formation of the solar system. In addition to providing direct information on the materials from which the solar system formed, these grains provide ground-truth information for models of stellar evolution and nucleosynthesis. Here we report the in-situ identification of two unique presolar graphite grains from the primitive meteorite LaPaz Icefield 031117. Based on these two graphite grains, we estimate a bulk presolar graphite abundance of 5(-3)(+7) ppm in this meteorite. One of the grains (LAP-141) is characterized by an enrichment in 12C and depletions in 33,34S, and contains a small iron sulfide subgrain, representing the first unambiguous identification of presolar iron sulfide. The other grain (LAP-149) is extremely 13C-rich and 15N-poor, with one of the lowest 12C/13C ratios observed among presolar grains. Comparison of its isotopic compositions with new stellar nucleosynthesis and dust condensation models indicates an origin in the ejecta of a low-mass CO nova. Grain LAP-149 is the first putative nova grain that quantitatively best matches nova model predictions, providing the first strong evidence for graphite condensation in nova ejecta. Our discovery confirms that CO nova graphite and presolar iron sulfide contributed to the original building blocks of the solar system.
rate research
Read More
Cluster analysis of presolar silicon carbide grains based on literature data for 12C/13C, 14N/15N, {delta}30Si/28Si, and {delta}29Si/28Si including or not inferred initial 26Al/27Al data, reveals nine clusters agreeing with previously defined grain types but also highlighting new divisions. Mainstream grains reside in three clusters probably representing different parent star metallicities. One of these clusters has a compact core, with a narrow range of composition, pointing to an enhanced production of SiC grains in asymptotic giant branch (AGB) stars with a narrow range of masses and metallicities. The addition of 26Al/27Al data highlights a cluster of mainstream grains, enriched in 15N and 26Al, which cannot be explained by current AGB models. We defined two AB grain clusters, one with 15N and 26Al excesses, and the other with 14N and smaller 26Al excesses, in agreement with recent studies. Their definition does not use the solar N isotopic ratio as a divider, and the contour of the 26Al-rich AB cluster identified in this study is in better agreement with core-collapse supernova models. We also found a cluster with a mixture of putative nova and AB grains, which may have formed in supernova or nova environments. X grains make up two clusters, having either strongly correlated Si isotopic ratios or deviating from the 2/3 slope line in the Si 3-isotope plot. Finally, most Y and Z grains are jointly clustered, suggesting that the previous use of 12C/13C= 100 as a divider for Y grains was arbitrary. Our results show that cluster analysis is a powerful tool to interpret the data in light of stellar evolution and nucleosynthesis modelling and highlight the need of more multi-element isotopic data for better classification.
Measurements of sulphur isotopes in presolar grains can help to identify the astrophysical sites in which these grains were formed. A more precise thermonuclear rate of the 33S(p,gamma)34Cl reaction is required, however, to assess the diagnostic ability of sulphur isotopic ratios. We have studied the 33S(3He,d)34Cl proton-transfer reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Deuteron spectra were measured at ten scattering angles between 10 and 55 degrees. Twenty-four levels in 34Cl over Ex = 4.6 - 5.9 MeV were observed, including three levels for the first time. Proton spectroscopic factors were extracted for the first time for levels above the 33S+p threshold, spanning the energy range required for calculations of the thermonuclear 33S(p,gamma)34Cl rate in classical nova explosions. We have determined a new 33S(p,gamma)34Cl rate using a Monte Carlo method and have performed new hydrodynamic nova simulations to determine the impact on nova nucleosynthesis of remaining nuclear physics uncertainties in the reaction rate. We find that these uncertainties lead to a factor of less than 5 variation in the 33S(p,gamma)34Cl rate over typical nova peak temperatures, and variation in the ejected nova yields of S--Ca isotopes by less than 20%. In particular, the predicted 32S/33S ratio is 110 - 130 for the nova model considered, compared to 110 - 440 with previous rate uncertainties. As recent type II supernova models predict ratios of 130 - 200, the 32S/33S ratio may be used to distinguish between grains of nova and supernova origin.
Among presolar materials recovered in meteorites, abundant SiC and Al$_{2}$O$_{3}$ grains of AGB origins were found. They showed records of C, N, O, $^{26}$Al and s-element isotopic ratios that proved invaluable in constraining the nucleosynthesis models for AGB stars cite{zin,gal}. In particular, when these ratios are measured in SiC grains, they clearly reveal their prevalent origin in cool AGB circumstellar envelopes and provide information on both the local physics and the conditions at the nucleosynthesis site (the H- and He-burning layers deep inside the structure). Among the properties ascertained for the main part of the SiC data (the so-called {it mainstream} ones), we mention a large range of $^{14}$N/$^{15}$N ratios, extending below the solar value cite{mar}, and $^{12}$C/$^{13}$C ratios $gtrsim$ 30. Other classes of grains, instead, display low carbon isotopic ratios ($gtrsim 10$) and a huge dispersion for N isotopes, with cases of large $^{15}$N excess. In the same grains, isotopes currently feeded by slow neutron captures reveal the characteristic pattern expected from this process at an efficiency slightly lower than necessary to explain the solar main s-process component. Complementary constraints can be found in oxide grains, especially Al$_{2}$O$_{3}$ crystals. Here, the oxygen isotopes and the content in $^{26}$Al are of a special importance for clarifying the partial mixing processes that are known to affect evolved low-mass stars. Successes in modeling the data, as well as problems in explaining some of the mentioned isotopic ratios through current nucleosynthesis models are briefly outlined.
We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19) and Z (18), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and Z grain data with data for mainstream grains from AGB stars of close-to-solar metallicity demonstrates that the three types of grains have indistinguishable Mo isotopic compositions. We show that the Mo isotope data can be used to constrain the maximum stellar temperatures (TMAX) during thermal pulses in AGB stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with the grain data for Mo isotopes points to an origin from low-mass (~1.5-3 Msun) rather than intermediate-mass (>3-~9 Msun) AGB stars. Because of the low efficiency of 22Ne({alpha},n)25Mg at the low TMAX values attained in low-mass AGB stars, model calculations cannot explain the large 30Si excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.
We compare element and isotopic fractionations measured in solar wind samples collected by NASAs Genesis mission with those predicted from models incorporating both the ponderomotive force in the chromosphere and conservation of the first adiabatic invariant in the low corona. Generally good agreement is found, suggesting that these factors are consistent with the process of solar wind fractionation. Based on bulk wind measurements, we also consider in more detail the isotopic and elemental abundances of O. We find mild support for an O abundance in the range 8.75 - 8.83, with a value as low as 8.69 disfavored. A stronger conclusion must await solar wind regime specific measurements from the Genesis samples.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
