Stellar Sapphires: The Properties and Origins of Presolar Al₂O₃in Meteorites

Abstract

Thirty-seven isotopically highly anomalous presolar grains and one presolar grain Al 2 O 3 MgAl 2 O 4 from a separate of the Tieschitz H3.6 ordinary chondrite were identiÐed out of 17,000 isotopically normal refractory oxide grains by an automatic 16O/18O low mass resolution ion-imaging mapping technique in the ion microprobe. Eight additional presolar grains were found by high mass resolution Al 2 O 3 ion probe measurements of all three stable O isotopes in individual grains, including several that would have been missed by the ion-imaging search. Forty-Ðve of the grains were analyzed for their 16O/17O and 16O/18O ratios. Twenty-four grains were also analyzed for Al-Mg and 17 of them have large excesses of 26Mg, attributable to the radioactive decay of 26Al. The highly anomalous isotopic composition of the grains is evidence for their presolar, stellar origin.The 46 oxide grains of this study together with 42 previously identiÐed presolar grains were divided into four groups. These groups most likely comprise grains from distinct types of stellar sources. Group 1 grains have 17O excesses and moderate 18O depletions, relative to solar, and many of them exhibit 26Mg excesses as well. Group 2 grains have 17O excesses, large 18O depletions, and high inferred 26Al/27Al ratios. Group 3 grains have solar or higher 16O/17O and 16O/18O ratios. Group 4 grains have 17O and 18O enrichments. One grain of this study, T54, has an 16O/17O ratio of 71, lower than any preAl 2 O 3 viously observed, and 16O/18O much greater than the solar value.The O-isotopic compositions of Group 1 and Group 3 grains are consistent with an origin in O-rich red giant stars, which have undergone the Ðrst dredge-up. The range of O-isotopic ratios of these groups requires multiple stellar sources of di †erent masses and initial isotopic compositions and is well explained by a combination of Galactic chemical evolution and Ðrst dredge-up models. The inferred 26Al/27Al ratios of many of these grains indicate that they formed in thermally pulsing asymptotic branch (TP-AGB) stars that had undergone the third dredge-up. Group 2 grains probably formed in low-mass AGB stars as well, and their substantial 18O depletions are the likely result of "" extra ÏÏ mixing (cool bottom processing). The origin of the 18O enrichments in Group 4 grains is unknown, but it might be due to initial compositional di †erences of the stellar sources or to unusual third dredge-up in lowmass AGB stars. The highly 17O-enriched grain T54 could have formed in an AGB star undergoing hot bottom burning or in a massive star in the Of-WN phase.O-rich circumstellar dust seems to be underrepresented in meteorites, relative to C-rich. Explanations include the possibility that most O-rich stardust grains are silicates and have been destroyed either in the laboratory or in nature and the possibility that presolar has a Ðner grain size distribution Al 2 O 3 than SiC and graphite.

Water ice growth around evolved stars

“…The adopted grain size range is motivated by studies on pre-solar grains (e.g. Nittler et al 1997;Daulton et al 2003;Clayton & Nittler 2004) and detailed radiative modeling of evolved stars such as IRAS 17150-3224 and HD 161796 (Meixner et al 2002). The default model corresponds to model 1 in Table 1.…”

Study of the aluminium content in AGB winds using ALMA

“…The circumstellar Al 2 O 3 -grains are relatively abundant in meteorites (Nittler et al 1997), albeit perhaps not as abundant as one would expect if all aluminium would condense in these grains (Alexander 1997). This result from meteorite studies now gets some support by our analysis of the ALMA data, confirming that a fraction of the aluminium is still present in its gaseous form after its passage through the dust formation region.…”

Dust destruction in the ISM: a re-evaluation of dust lifetimes

“…Studies of meteorite matrix and IDPs have shown that, at a minimum, the following materials are present at some level in interstellar dust: nanodiamonds (Lewis et al 1989;Bernatowicz et al 1990), poorly graphitized carbon (Amari et al 1990), amorphous carbonaceous materials (Zinner 1988 (Huss et al 1994;Nittler et al 1997;Zinner et al 2003;Stroud et al 2004), and crystalline and amorphous silicates (Bradley 1994;Messenger et al 2003;Nguyen & Zinner 2004). Given that a large fraction of the material in the meteorite matrix is of nebular origin (Boynton 1985;Clayton 1985) it is surprising that any presolar grains survived at all.…”