Thomas Kevin Croat

Presolar Graphite from AGB Stars: Microstructure and s-Process Enrichment

Correlated transmission electron microscopy and secondary ion mass spectrometry with submicron spatial resolution (NanoSIMS) investigations of the same presolar graphites spherules from the Murchison meteorite were conducted, to link the isotopic anomalies with the mineralogy and chemical composition of the graphite and its internal grains. Refractory carbide grains (especially titanium carbide) are commonly found within the graphite spherules, and most have significant concentrations of Zr, Mo, and Ru in solid solution, elements primarily produced by s-process nucleosynthesis. The effect of chemical fractionation on the Mo/Ti ratio in these carbides is limited, and therefore from this ratio one can infer the degree of s-process enrichment in the gas from which the graphite condensed. The resulting s-process enrichments within carbides are large (~200 times solar on average), showing that most of the carbide-containing graphites formed in the mass outflows of asymptotic giant branch (AGB) stars. NanoSIMS measurements of these graphites also show isotopically light carbon (mostly in the 100 < 12C/13C < 400 range). The enrichment of these presolar graphites in both s-process elements and 12C considerably exceeds that astronomically observed around carbon stars. However, a natural correlation exists between 12C and s-process elements, as both form in the He intershell region of thermally pulsing AGB stars and are dredged up together to the surface. Their observation together suggests that these graphites may have formed in chemically and isotopically inhomogeneous regions around AGB stars, such as high-density knots or jets. As shown in the companion paper, a gas density exceeding that expected for smooth mass outflows is required for graphite of the observed size to condense at all in circumstellar environments, and the spatially inhomogeneous, high-density regions from which they condense may also be incompletely mixed with the surrounding gas. We have greatly expanded the available data set of presolar graphites (N = 847) and characterized them by their morphology (onion type and cauliflower type). This effort has also revealed two new, rare presolar phases (iron carbide and metallic osmium). Due to the peculiar gas composition needed to form these rare presolar grain types, the graphites containing them are more likely to originate in supernova outflows.

CONSTRAINTS ON GRAIN FORMATION AROUND CARBON STARS FROM LABORATORY STUDIES OF PRESOLAR GRAPHITE

We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, with and without previously formed internal crystals of titanium carbide (TiC). A lower mass limit of 1.1 Mfor stars capable of contributing grains to the solar nebula isderived. Thismass limit, in conjunction with a mass-luminosity relation for carbon stars, identifies the region of the H-R diagram relevant to the production of presolar graphite. Detailed dynamical models of AGB outflows, along with constraints provided by kinetics and equilibrium ther- modynamics, indicate that grain formation occurs at radiifrom2.3 to 3.7 AU for AGB carbon stars in the 1.1-5M� range. This analysis also yields time intervals available for graphite growth that are on the order of a few years. By considering the luminosity variations of carbon stars, we show that grains formed during minima in the luminosity are likely to be evaporated subsequently, while those formed at luminosity maxima will survive. We calculate strict upper limits on grain sizes for graphite and TiC in spherically symmetric AGB outflows. Graphite grains can reach diameters in the observed micron size range (1-2 � m) only under ideal growth conditions (perfect sticking efficiency,noevaporation,nodepletionofgasspeciescontributingtograingrowth),andthenonlyinoutflowsfrom carbon stars with masses P2.5 M� . The same is true for TiC grains that are found within presolar graphite, which have mean diameters of 24 � 14 nm. In general, the mass-loss rates that would be required to produce the observed grain sizes in spherically symmetric outflows are at least an order of magnitude larger than the maximum observed AGB carbon star mass-loss rates. These results, as well as pressure constraints derived from equilibrium ther- modynamics,forceustoconcludethatpresolargraphiteandTiCmustforminregionsofenhanceddensity(clumps, jets)inAGBoutflowshavingsmallangularscales.AsshowninthecompanionpaperbyCroatetal.,theenrichment of 12 C in many AGB graphites, and the overabundances of the s-process elements Mo, Zr, and Ru in the carbides found within them, often greatly exceed the values observed astronomically in AGB outflows. These observations notonlylendfurthersupporttotheideathattheoutflowsareclumpy,butalsoimplythattheoutflowingmatterisnot well mixed in the circumstellar envelope out to the radii where grain condensation takes place. Subject headingg dust, extinction — stars: AGB and post-AGB — stars: carbon

UNUSUAL 29,30Si-RICH SiCs OF MASSIVE STAR ORIGIN FOUND WITHIN GRAPHITES FROM THE MURCHISON METEORITE

Correlated transmission electron microscopy and NanoSIMS isotopic studies have revealed two unusual SiCs with large 29,30Si enrichments within micron-sized graphites from the Murchison meteorite. Such anomalies are rare among the overall SiC population (in 0.01% of SiCs yet measured), whereas two of the three SiCs found within graphite show 29,30Si enrichments, in one case as large as 29Si/28Si = (2.28 ± 0.03) × solar and 30Si/28Si = (2.03 ± 0.03)× solar. C-burning and Ne-burning in massive stars (>8 M ☉ initial mass) during their post-main-sequence development are the only processes capable of producing sufficiently large 29,30Si enrichments. This material with heavy Si isotopic enrichments from the O/Ne and O/Si layers is later incorporated into carbonaceous stardust, either in ejecta from Type II supernovae or perhaps in the colliding winds of Wolf-Rayet binaries. Although often too small for Si isotopic measurements, four other SiC-containing graphites show other signatures of a massive star origin. Abundance estimates suggest that such unusual SiCs are present within ~1% of high-density graphites. This abundance can be reconciled with the much lower abundance in the overall SiC population if these unusual SiCs are naturally smaller (~200 nm or less) than SiCs from other isotopic subgroups and if differential destruction of small unusual SiCs occurs in massive star outflows unless these SiCs are encapsulated in graphite.