Ann N. Nguyen

Aluminum-, calcium- and titanium-rich oxide stardust in ordinary chondrite meteorites

We report O-, Al-Mg-, K-, Ca-, and Ti-isotopic data for a total of 96 presolar oxide grains found in residues of several unequilibrated ordinary chondrite meteorites. Identified grain types include Al2O3 ,M gAl2O4, hibonite (CaAl12O19), and Ti oxide. This work greatly increases the presolar hibonite database, and is the first report of presolarTioxide.O-isotopiccompositionsof thegrainsspanpreviouslyobservedrangesandindicateanoriginin red giant and asymptotic giant branch (AGB) stars of low mass (<2.5 M� ) 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 41 Ca. Inferred initial 41 Ca/ 40 Ca ratios are in good agreement with model predictionsfor low-mass AGB star envelopes,provided that ionizationsuppresses 41 Cadecay. Stable Mg and Ca isotopic ratios of most of the hibonite grains reflect primarily the initial compositions of the parent starsandaregenerallyconsistentwithexpectationsforGalacticchemicalevolution,butrequiresomelocalinterstellar chemical inhomogeneity. Very high 17 O/ 16 Oo r 25 Mg/ 24 Mg ratios suggest an origin for some grains in binary star systemswheremasstransferfromanevolvedcompanionhasalteredtheparentstarcompositions.Asupernovaorigin for the hitherto enigmatic 18 O-rich Group 4 grains is strongly supported by multielement 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 16 O-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. Subject headingg dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — supernovae: general

Characterization of Presolar Silicate and Oxide Grains in Primitive Carbonaceous Chondrites

Raster ion imaging of the oxygen isotopes with the NanoSIMS ion microprobe has been used to identify presolar grains in two primitive meteorites. Eleven presolar silicates and eight presolar oxides were identified in the primitive carbonaceous chondrite Acfer 094 for abundances of 325 and 360 parts per million (ppm), respectively. In addition, nine presolar silicates and five presolar oxide grains were identified in the CO3 chondrite ALHA 77307, for abundancesof 320and200ppm,respectively.Theseabundances,whicharematrix-normalizedandcorrectedforinstrumental detectionefficiencies,aremuchhigherthanthoseofotherpresolarphases,withtheexceptionof nanodiamonds,although the latter may not all be presolar. The chemical compositions of six presolar silicate grains from ALHA 77307 were elucidated by Auger spectroscopy. Transmission electron microscopy (TEM) analysis of one presolar silicate grain revealed a nonstoichiometric composition and an amorphous structure as indicated by the diffuse electron diffraction pattern. The oxygen isotopic compositions of the presolar silicates indicate origins in red giant and asymptotic giant branch stars. Analysis of the Si isotopic compositions of 10 presolar silicates provides further constraints on the effects of Galactic chemical evolution. Subject headingg circumstellar matter — dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: evolution

COORDINATED ANALYSES OF PRESOLAR GRAINS IN THE ALLAN HILLS 77307 AND QUEEN ELIZABETH RANGE 99177 METEORITES

We report the identification of presolar silicates ({approx}177 ppm), presolar oxides ({approx}11 ppm), and one presolar SiO{sub 2} grain in the Allan Hills (ALHA) 77307 chondrite. Three grains having Si-isotopic compositions similar to SiC X and Z grains were also identified, though the mineral phases are unconfirmed. Similar abundances of presolar silicates ({approx}152 ppm) and oxides ({approx}8 ppm) were also uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177, along with 13 presolar SiC grains and one presolar silicon nitride. The O-isotopic compositions of the presolar silicates and oxides indicate that most of the grains condensed in low-mass red giant and asymptotic giant branch stars. Interestingly, unlike presolar oxides, few presolar silicate grains have isotopic compositions pointing to low-metallicity, low-mass stars (Group 3). The {sup 18}O-rich (Group 4) silicates, along with the few Group 3 silicates that were identified, likely have origins in supernova outflows. This is supported by their O- and Si-isotopic compositions. Elemental compositions for 74 presolar silicate grains were determined by scanning Auger spectroscopy. Most of the grains have non-stoichiometric elemental compositions inconsistent with pyroxene or olivine, the phases commonly used to fit astronomical spectra, and have comparable Mg and Fe contents. Non-equilibrium condensation and/ormorexa0» secondary alteration could produce the high Fe contents. Transmission electron microscopic analysis of three silicate grains also reveals non-stoichiometric compositions, attributable to non-equilibrium or multistep condensation, and very fine scale elemental heterogeneity, possibly due to subsequent annealing. The mineralogies of presolar silicates identified in meteorites thus far seem to differ from those in interplanetary dust particles.«xa0less

RESOLVING THE STELLAR SOURCES OF ISOTOPICALLY RARE PRESOLAR SILICATE GRAINS THROUGH Mg AND Fe ISOTOPIC ANALYSES

We conducted multi-element isotopic analyses of 11 presolar silicate grains from the Acfer 094 meteorite having unusual O isotopic compositions. Eight grains are 18O-rich, one is 16O-rich, and two are extremely 17O-rich. We constrained the grains stellar sources by measuring their Si and Mg isotopic ratios, and also the 54Fe/56Fe and 57Fe/56Fe ratios for five grains. The Mg and Fe isotopic measurements were conducted after surrounding matrix grains were removed for more accurate ratios. Most of the 18O-rich silicates had anomalous Mg isotopic ratios, and their combined isotopic constraints are consistent with origins in low-mass Type II supernovae (SNe II) rather than high-metallicity stars. The isotopic ratios of the 16O-rich silicate are also consistent with an SN origin. Mixing small amounts of interior stellar material with the stellar envelope replicated all measured isotopic ratios except for 29Si/28Si and 54Fe/56Fe in some grains. The 29Si/28Si ratios of all SN-derived grains are matched by doubling the 29Si yield in the Ne- and Si-burning zones. The 54Fe/56Fe ratios of the grains imply elemental fractionation in the Si/S zone, or introduction of isotopically solar Fe by secondary processing. The two highly 17O-rich silicates exhibited significant 25Mg and/or 26Mg enrichments and their isotopic ratios are best explained by strong dilution of 1.15 M ☉ CO nova matter. We estimate that ~12% and 1% of presolar silicates have SN and nova origins, respectively, similar to presolar SiC and oxides. This implies that asymptotic giant branch stars are the dominant dust producers in the galaxy.

Erratum: “Aluminum-, Calcium- and Titanium-rich Oxide Stardust in Ordinary Chondrite Meteorites” (ApJ, 682, 1450 [2008])

KH2 38 8 42 20 32 8 (4.27 0.07) ; 10 4 KH4 1 7 21 18 16 8 <9.7 ; 10 7 KH7 3 6 7 16 2 7 <9.3 ; 10 7 KH8 36 13 29 29 63 10 <4.2 ; 10 6 KH9 7 7 14 19 4 8 <2.0 ; 10 6 KH10 249 12 118 27 5 9 (1.8 0.3) ; 10 5 KH11 32 7 54 18 44 8 <4.0 ; 10 6 KH12 42 15 74 34 19 11 (1.6 0.3) ; 10 5 KH13 40 6 22 16 115 8 (3.6 0.1) ; 10 5 KH14 22 9 27 21 18 8 (2.09 0.05) ; 10 4 KH15 8.7 7 1 18 12 8 (1.02 0.02) ; 10 4 KH16 45 27 11 46 26 31 (2.07 0.12) ; 10 4 KH17 70 25 2 40 25 31 (5.7 0.5) ; 10 5 KH18 18 20 8 31 5 30 (1.40 0.14) ; 10 5 KH19 8 24 42 40 6 31 <6.0 ; 10 6 KH21 3 20 23 28 20 30 (1.28 0.03) ; 10 4