A. Besmehn

A NanoSIMS study of Si- and Ca-Ti-isotopic compositions of presolar silicon carbide grains from supernovae

Abstract We report results from NanoSIMS isotopic measurements on 37 presolar silicon carbide grains of type X which are believed to have formed in the ejecta of supernova explosions. Isotopic data were obtained for Si and Ca-Ti (all grains), C and N (two grains), and Ti (one grain). All X grains exhibit large enrichments in 28Si (up to 5× solar), in agreement with previously studied X grains. On a scale of 200 nm, the Si-isotopic ratios do not vary by more than the analytical uncertainties of several percent in all but one X grain. This implies that most X grains formed from well-mixed regions in supernova ejecta. X grain M9-68-3 is characterized by two regions with distinct Si- and Ti-isotopic signatures which may either represent two distinct grains or overgrowth of matter from two different mixtures in the supernova ejecta. Most of the Ca in the X grains is most likely contamination as indicated by close to normal 42Ca/40Ca ratios. Seven X grains show enhanced 44Ca/40Ca ratios of up to 6× the solar ratio. Spatial distributions of 44Ca excesses and Ti are positively correlated, giving strong support to the view that excesses in 44Ca are due to the decay of radioactive 44Ti. Inferred initial 44Ti/48Ti ratios are between 0.01 and 0.28 and are correlated with Si-isotopic ratios. Radiogenic 44Ca is widely distributed in six X grains. X grain M9-132-4 exhibits a pronounced heterogeneity in the distribution of radiogenic 44Ca and 48Ti as well as in 44Ti/48Ti, pointing to presence of a small Ti-rich subgrain or heterogeneous loss of Ca and Ti after grain formation. This grain has a unique Si-isotopic composition with 30Si/29Si = 2.2× the solar ratio and C- and N-isotopic compositions as typically observed in X grains.

Evidence for extinct vanadium-49 in presolar silicon carbide grains from supernovae

We report Si, Ca, and Ti-V isotopic data for seven presolar silicon carbide grains of type X separated from the Murchison CM2 meteorite. The silicon carbide X grains most likely formed in the ejecta of Type II supernova explosions as indicated by large isotopic anomalies. The X grains of this study have 29Si/28Si and 30Si/28Si ratios of 0.56-0.75 and 0.41-0.62 times the solar value, respectively. Slight enrichments in 44Ca are observed, possibly due to the decay of radioactive 44Ti (t1/2 = 60 yr). While 46Ti/48Ti, 47Ti/48Ti, and 50Ti/48Ti ratios are close to solar, all X grains have enhanced 49Ti/48Ti ratios of up to 1.9 times the solar ratio. The 49Ti/48Ti ratios are positively correlated with 51V/48Ti ratios, suggestive of the incorporation of live 49V (t1/2 = 330 days) during silicon carbide condensation. This implies that the grains formed on a timescale of several months after supernova explosion.

New attempts to understand nanodiamond stardust

We report on a concerted effort aimed at understanding the origin and history of the pre-solar nanodiamonds in meteorites including the astrophysical sources of the observed isotopic abundance signatures. This includes measurement of light elements by secondary ion mass spectrometry (SIMS), analysis of additional heavy trace elements by accelerator mass spectrometry (AMS) and dynamic calculations of r-process nucleosynthesis with updated nuclear properties. Results obtained indicate that: (i) there is no evidence for the former presence of now-extinct 26Al and 44Ti in our diamond samples other than what can be attributed to silicon carbide and other ‘impurities’, and this does not offer support for a supernova (SN) origin but neither does it negate it; (ii) analysis by AMS of platinum in ‘bulk diamond’ yields an overabundance of r-only 198Pt that at face value seems more consistent with the neutron burst than with the separation model for the origin of heavy trace elements in the diamonds, although this conclusion is not firm given analytical uncertainties; (iii) if the Xe–H pattern was established by an unadulterated r-process, it must have been a strong variant of the main r-process, which possibly could also account for the new observations in platinum.