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

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 Origins of C-13 and N-15-Enriched Presolar SiC Grains

Extreme excesses of 13 C ( C (12 C/ 13 C<10) and 15 N ( N (14 N/ 15 N< 20) in rare presolar SiC 20) in rare presolar SiClar SiC grains have been considered diagnostic of an origin in classical novae [1], though an origin in core-collapse supernovae (CCSNe) has also been proposed [2]. We report multi-element isotopic data for 19 13 C- and 15 N-enriched presolar SiC grains(12 C/13 C<16 and 14 N/ 15 N<~150) from an acid resistant residue of the Murchison meteorite. These grains are enriched in 13 C and15 N, but with quite diverse Si isotopic signatures. Four grains with isotopic signatures. Four grains with isotopic signatures. Four grains with isotopic signatures. Four grains with isotopic signatures

Aluminum-, Calcium- And Titanium-Rich Oxide Stardust In Ordinary Chondrite Meteorites

We report isotopic data for a total of 96 presolar oxide grains found in residues of several unequilibrated ordinary chondrite meteorites. Identified grain types include Al2O3, MgAl2O4, hibonite (CaAl12O19) and Ti oxide. This work greatly increases the presolar hibonite database, and is the first report of presolar Ti oxide. O-isotopic compositions of the grains span previously observed ranges and indicate an origin in red giant and asymptotic giant branch (AGB) stars of low mass (<2.5 MSun) for most grains. Cool bottom processing in the parent AGB stars is required to explain isotopic compositions of many grains. Potassium-41 enrichments in hibonite grains are attributable to in situ decay of now-extinct 41Ca. Inferred initial 41Ca/40Ca ratios are in good agreement with model predictions for low-mass AGB star envelopes, provided that ionization suppresses 41Ca decay. Stable Mg and Ca isotopic ratios of most of the hibonite grains reflect primarily the initial compositions of the parent stars and are generally consistent with expectations for Galactic chemical evolution, but require some local interstellar chemical inhomogeneity. Very high 17O/16O or 25Mg/24Mg ratios suggest an origin for some grains in binary star systems where mass transfer from an evolved companion has altered the parent star compositions. A supernova origin for the hitherto enigmatic 18O-rich Group 4 grains is strongly supported by multi-element isotopic data for two grains. The Group 4 data are consistent with an origin in a single supernova in which variable amounts of material from the deep 16O-rich interior mixed with a unique end-member mixture of the outer layers. The Ti oxide grains primarily formed in low-mass AGB stars. They are smaller and rarer than presolar Al2O3, reflecting the lower abundance of Ti than Al in AGB envelopes.Comment: Accepted for publication in ApJ; 47 pages, 13 figure

Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample

Studies of material returned from Cb asteroid Ryugu have revealed considerable mineralogical and chemical heterogeneity, stemming primarily from brecciation and aqueous alteration. Isotopic anomalies could have also been affected by delivery of exogenous clasts and aqueous mobilization of soluble elements. Here, we show that isotopic anomalies for mildly soluble Cr are highly variable in Ryugu and CI chondrites, whereas those of Ti are relatively uniform. This variation in Cr isotope ratios is most likely due to physicochemical fractionation between 54Cr-rich presolar nanoparticles and Cr-bearing secondary minerals at the millimeter-scale in the bulk samples, likely due to extensive aqueous alteration in their parent bodies that occurred 5:2þ11::84 Ma after Solar System birth. In contrast, Ti isotopes were marginally affected by this process. Our results show that isotopic heterogeneities in asteroids are not all nebular or accretionary in nature but can also reflect element redistribution by water

The chromium isotopic composition of Almahata Sitta

Abstract–Nine ureilitic fragments of the anomalous polymict ureilite Almahata Sitta have been analyzed for their Cr isotopic compositions. All the samples, including both nonmagnetic and magnetic portions, show essentially the same e54Cr deficit ()0.77 ± 0.10), relative to the terrestrial Cr standard. This contrasts with the variable positive 54Cr anomalies observed for carbonaceous chondrites, but agrees with the values measured for eucrites, diogenites, and mesosiderites (Trinquier et al. 2007). This implies that, contrary to previous suggestions based on O isotopes, ureilites were not derived from any known carbonaceous chondrite parent body. Instead, the Almahata Sitta parent body may have accreted in a nebular region ⁄ environment similar to that of the howardite, eucrite, and diogenite (HED) parent body. In addition, the lack of variation in e54Cr combined with variable O isotopic compositions in the meteorite fragments suggests that whatever process(es) caused the O isotopic heterogeneity of the solar system was probably not responsible for heterogeneity in e54Cr. The samples show resolvable variations in e53Cr (0.15–0.41) that are correlated with Mn ⁄Cr ratios, suggesting that live 53Mn was present at the time of formation of Almahata Sitta. The isochron yields an initial 53Mn ⁄ 55Mn value o

Explosive Nucleosynthesis in Core-collapse Type II Supernovae: Insights from New C, N, Si, and Al–Mg Isotopic Compositions of Presolar Grains

We report C, N, Si, and Al–Mg isotope data for 39 presolar X silicon carbide (SiC) and four silicon nitride grains—a group of presolar grains that condensed in the remnants of core-collapse Type II supernovae (CCSNe)—isolated from the Murchison meteorite. Energy dispersive X-ray data were used to determine the Mg and Al contents of the X SiC grains for comparison with the Mg/Al ratios determined by secondary ion mass spectroscopy (SIMS). Previous SIMS studies have used O-rich standards in the absence of alternatives. In this study, the correlated isotopic and elemental data of the X SiC grains enabled accurate determination of the initial ^26 Al/ ^27 Al ratios for the grains. Our new grain data suggest that (i) the literature data for X grains are affected to varying degrees by asteroidal/terrestrial contamination, and (ii) the Al/Mg ratios in SiC are a factor of 2 (with ±6% 1 σ uncertainties) lower than estimated based on the SIMS analyses that used O-rich standards. The lowered Al/Mg ratios result in proportionally higher inferred initial ^26 Al/ ^27 Al ratios for presolar SiC grains. In addition, the suppression of asteroidal/terrestrial contamination in this study leads to the observation of negative trends for ^12 C/ ^13 C– ^30 Si/ ^28 Si and ^26 Al/ ^27 Al– ^30 Si/ ^28 Si among our CCSN grains. We discuss these isotope trends in the light of explosive CCSN nucleosynthesis models, based on which we provide new insights into several nontraditional CCSN nucleosynthesis processes, including explosive H burning, the existence of a C/Si zone in the outer regions of CCSNe, and neutrino–nucleus reactions in deep CCSN regions