Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.363h&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 363
Astronomy and Astrophysics
Astrophysics
5
Astrophysics, Interstellar Dust, Ion Probe, Isotopic Anomalies, Murchison, Nucleosynthesis, Stellar Evolution
Scientific paper
In search of grain-size dependent characteristics of interstellar SiC, we have measured the C, N, and Si isotopes in 596 single grains from Murchison separate KJE (average size 1.14 micrometers) [1]. So far, extended studies of single SiC grains from the Murchison K-series were made on separates KJG (average size 3.02 micrometers) and KJH (average size 4.57 micrometers) [2] and a limited dataset was also obtained for separate KJF (average size 1.86 micrometers) [2,3]. As it is the case for the larger grains, most grains from KJE have heavy carbon and light nitrogen. ^12C/^13C ratios vary between 2.3 and 270. The distribution of ^12C/^13C ratios is almost identical to those observed for the larger-grain separates, with most grains having ^12C/^13C ~40-100 (Fig. 1a-c). ^14N/^15N ratios vary between 48 and >10,000. The distribution of ^14N/^15N ratios clearly differs from those observed for the larger-grain separates (Fig. 1d-f). Grains from separate KJE, on the average, have much higher ^14N/^15N ratios. The median value is 2800 compared to 1250 for KJG and 550 for KJH [2]. As is evident from these numbers, there is a clear trend with the typical ^14N/^15N ratio increasing with decreasing grain size. In close agreement with the larger grains, most grains from KJE have delta^29Si ~ -50 to 200 per mil and delta^30Si ~ 0 to 160 per mil, in a three-isotope-plot lying along a line with slope ~1.1. However, there are 12 grains clearly off this line with 10 of them being depleted in ^29Si and enriched in ^30Si. Nine grains were identified as grains X [4], being charaterized by light C, heavy N, and light Si. Five of them, in addition, show a very high nitrogen content, ~5x higher than that of a typical grain from KJE. Furthermore, one grain has the Si-isotopic signature of grains X, but heavy C. While the observed C-isotopic ratios for the majority of the grains can be explained by standard stellar evolution models for AGB-stars [5], the ^14N/^15N ratios > ~2000, as it is typical for KJE grains, require special processes, e.g., additional admixture of ^14N to the surface during late pulses in AGB-stars [6]. The clear difference in N isotopes between different size separates implies that grains of different sizes in their majority are formed in different stars or, alternatively, either at different locations in the stellar atmosphere or at different times in the same stars. References: [1] Amari S. et al. (1993) GCA, in press. [2] Hoppe P. et al. (1993) Astrophys. J., submitted. [3] Hoppe P. et al. (1993) GCA, in press. [4] Amari S. et al. (1992) Astrophys. J., 394, L43-L46. [5] Gallino R. et al. (1993) Astrophys. J., submitted. [6] Gallino R., private communication.
Amari Sachiko
Eberhardt Peter
Hoppe Peter
Lewis Reed S.
Strebel Ralph
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