Coordinated EDX and micro‐Raman analysis of presolar silicon carbide: A novel, nondestructive method to identify rare subgroup SiC

Abstract

Abstract-We report the development of a novel method to nondestructively identify presolar silicon carbide (SiC) grains with high initial 26 Al/ 27 Al ratios (>0.01) and extreme 13 C-enrichments ( 12 C/ 13 C ≤ 10) by backscattered electron-energy dispersive X-ray (EDX) and micro-Raman analyses. Our survey of a large number of presolar SiC demonstrates that (1)~80% of core-collapse supernova and putative nova SiC can be identified by quantitative EDX and Raman analyses with >70% confidence; (2)~90% of presolar SiC are predominantly 3C-SiC, as indicated by their Raman transverse optical (TO) peak position and width; (3) presolar 3C-SiC with 12 C/ 13 C ≤ 10 show lower Raman TO phonon frequencies compared to mainstream 3C-SiC. The downward shifted phonon frequencies of the 13 C-enriched SiC with concomitant peak broadening are a natural consequence of isotope substitution. 13C-enriched SiC can therefore be identified by micro-Raman analysis; (4) larger shifts in the Raman TO peak position and width indicate deviations from the ideal 3C structure, including rare polytypes. Coordinated transmission electron microscopy analysis of one X and one mainstream SiC grain found them to be of 6H and 15R polytypes, respectively; (5) our correlated Raman and NanoSIMS study of mainstream SiC shows that high nitrogen content is a dominant factor in causing mainstream SiC Raman peak broadening without significant peak shifts; and (6) we found that the SiC condensation conditions in different stellar sites are astonishingly similar, except for X grains, which often condensed more rapidly and at higher atmospheric densities and temperatures, resulting in a higher fraction of grains with much downward shifted and broadened Raman TO peaks.

Late formation of silicon carbide in type II supernovae

“…Neutron capture taking place in the outer C-rich He/C zone converts 28 Si and 48 Ti to neutronrich Si and Ti isotopes, respectively, whereas a-capture in the inner Si/S zone overproduces a-nuclides, including 28 Si and 48 Ti. Abundant short-lived 49 V is also made in the Si/S zone.…”

“…However, quasi-equilibrium calculations (30) show that, although graphite could be stable in O-dominated SN zones, SiC is not stable under such O-rich conditions. On the other hand, the short-lived (t ½ = 330 day) 49 V, which decays to 49 Ti, is abundantly made in the inner Si/S zone, along with a-nuclides such as 28 Si and 48 Ti ( Fig. 1).…”

“…S1), reflecting variable contributions to their Si budgets from the less 28 Si-rich He/C zone. The 49 Ti budgets of X grains could also be affected by the variable contributions of the He/C zone. Using Ti isotopes to constrain the formation timing of X grains in SNe, therefore, has been hampered by two difficulties: (i) It is difficult to accurately estimate the mixing ratios for different X grains as a result of the large uncertainties in SN nucleosynthetic predictions for the He/C zone (25,31,33), and (ii) potential Ti-V elemental fractionation could occur within the Si/S or He/C zones before zonal mixing, for example, fractionation of S relative to Si by preferential capture of SiS molecules from the Si/S zone into growing SiC grains was previously suggested to explain the large 32 S excesses found in some X grains (34).…”

“…Otherwise, their d 49 Ti values would be significantly affected by the variable amounts of live 49 V relative to 48 Ti incorporated into the grains, and no correlation would exist between d 49 Ti and d 30 Si. Thus, the isotopic systematics of the X grains point to late formations of X grains, relative to the 330-day half-life of 49 V, in their parent SNe and not formation within a few months after the explosions (31).…”

“…Subtraction of 49 Ti from the He/C zone First, we estimate the variable amounts of material from the He/C zone that resulted in the wide range of Ti and Si isotope ratios observed in X grains. Titanium-50 is an ideal index for estimating the contribution to 49 Ti from the He/C zone because 50 Ti is not produced in the Si/S zone but is made along with 49 Ti by a neutron capture process in the He/C zone; therefore, 50 Ti in each X grain is solely from the He/C zone. In addition, Ti could be fractionated from other elements by condensation of small TiC grains before zonal mixing [for example, (37)].…”