Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.235h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 235
Astronomy and Astrophysics
Astrophysics
12
Scientific paper
We have measured the oxygen isotopic composition of 23 graphite grains from Murchison C-alpha density separates KE1 (density = 1.60-2.05 g/cm^3) and KFA1 (density = 2.05-2.10 g/cm^3) (Amari et al., 1992). Previous studies have shown that interstellar graphite has a large range in C-isotopic composition, moderate N- isotopic anomalies, and inferred ^26Al/^27Al ratios up to 0.09 (Amari et al., 1990; Hoppe et al., 1992). While 6 out of 23 grains (O-concentrations are several percent) show large enrichments in ^18O by up to a factor of 13, all ^16O/^17O ratios are normal within experimental uncertainties (Fig. 1a). For two of these grains we measured Mg-Al. They show large excesses in ^26Mg and inferred ^26Al/^27Al ratios are 0.043 and 0.100. The most extreme ^12C/^13C (Fig. 1b) and ^14N/^15N ratios (Fig. 1c) of these 6 grains are shown by the grain with the smallest ^16O/^18O ratio (^12C/^13C = 722, 14N/15N = 134). Round grains show a general correlation between 16O/18O and 14N/15N (Fig. 1c). Excesses in ^18O point to an early phase of He-burning in massive stars. According to model calculations of Prantzos et al. (1986) ^18O is expected to be strongly enriched at the surface of Wolf- Rayet stars (by ^14N(alpha,gamma)^18F(beta,nu)^18O in core He- burning and subsequent convective mixing) during the WN to WC transition. At the same time ^17O is destroyed by ^17O(alpha,n)^20Ne, resulting in a ^16O/^17O ratio well above solar. Dilution of this ^18O-rich and ^17O-depleted stellar component with normal oxygen (either in the solar nebula or in the laboratory) can generate the observed compositions. ^15N excesses of grains with ^18O excesses might be due to the rapid transformation of ^14N since the destruction of ^15N by ^15N(alpha,gamma)^19F is less effective at temperatures typically for He-burning (Caughlan and Fowler, 1988). Model calculations of Prantzos et al. (1986) give ^26Al/^27Al ratios of ~0.1 at the surface of Wolf-Rayet stars during the WN to WC transition, in good aggreement with the inferred ^26Al/^27Al ratios of the two grains analyzed for Mg-Al. The C-isotopic ratios (^12C/^13C = 39- 722) are well above the value (~4) calculated for the surface of a star at the end of the WN phase. They are consistent with the appearence of He-burning products at the surface of massive stars at the beginning of the WC phase, as ^12C is produced by the 3alpha reaction and ^13C is destroyed by ^13C(alpha,n)^16O. REFERENCES: Amari S., Anders E., Virag A. and Zinner E. (1990) Nature 345, 238-240. Amari S., Lewis R. S. and Anders E. (1992) Geochim. Cosmochim. Acta, submitted. Caughlan G. R. and Fowler W. A. (1988) Atomic Data and Nuclear Data Tables 40, 283-344. Hoppe P., Amari S., Zinner E. and Lewis R. S. (1992) Lunar Planet. Sci.(abstract) 23, 553-554. Prantzos N., Doom C., Arnould M. and de Loore C. (1986) Astrophys. J. 304, 695-712.
Amari Sachiko
Hoppe Peter
Lewis Reed S.
Zinner Emst
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