Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.480a&link_type=abstract
Meteoritics, vol. 30, no. 5, page 480
Astronomy and Astrophysics
Astrophysics
3
Anomalies, Isotopic, Dust, Circumstellar, Interstellar, Graphite, Meteorites, Murchison, Nucleosynthesis, Supernova
Scientific paper
We have measured isotopic ratios of C, N, O, Si, K, Ca, and Ti in 13 carbon grains from the low density graphite fraction KE3 (1.65-1.72g/cm3). These grains show the same isotopic signatures as KE3 grains previously measured [1]. Many of them have isotopically heavy C (up to x13 solar) and heavy N (up to x9.7 solar); nine have ^18O excesses (up to x183 solar); 26Al/27Al ratios range up to 0.07. Type II supernovae have previously been proposed as stellar sources of low density graphite grains [1,2,3]. Grain KE3c-551 (14 micrometers in size) has the most extreme C-, N-, and O-isotopic ratios ever measured in a graphite grain (^12C/^13C=7223+/-111, 14N/15N=28+/-2, 16O/18O=2.72+/-0.08). Interestingly, the isotopic ratios of these elements changed during analysis, becoming more anomalous with time. This suggests that isotopically "normal" components had been absorbed onto the grain surface or that partial exchange of the indigenous component had occurred. The above ratios are therefore lower (for C) and upper (for N and O) limits. High ^12C/^13C and ^16O/^17O (4595+/-858) and low ^16O/^18O ratios as well as the Ti isotopic ratios of the grain (delta 46Ti/48Ti=12+/-10 permil, delta ^47Ti/^48Ti=-62+/-9 permil, delta ^49Ti/^48Ti=901+/-16 permil, delta ^50Ti/^48Ti=200+/-12 permil) show the signature of the He/C zone in presupernova stars [4]. Eleven grains were measured for their K, Ca, and Ti isotopic ratios. Four of them, including KE3c-551, have ^41K excesses (218+/-50 permil to 10610+/-326 permil), apparently due to the decay of ^41Ca (T(sub)1/2=1.03 x 10^5a). Inferred ^41Ca/^40Ca ratios range from (1.94+/-0.43) x 10^-3 to (1.65+/-0.38) x 10^-2. In supernovae, ^41Ca is produced by neutron capture in the He/C, the O/C, and the O/Ne zones, as well as by explosive nucleosynthesis in the Si/S zone [4,5]. The expected ^41Ca/^40Ca ratios in the neutron capture zones are 1.6-1.8 x 10^-2 and in the Si/S zone 8 x 10^-4. Two of the 4 grains with ^41K excesses have Ca isotopic anomalies (Fig. 1), with patterns that are consistent with that predicted for the O-rich zones of a 25 (sub)Solar Mass supernova [4] (shown as broad lines in the figure). Thus, ^41Ca in these two grains is likely to have been produced by neutron capture in these zones. One grain (KE3c-242) has a ^44Ca excess due to the decay of ^44Ti (T(sub)1/2=52a), with an inferred ^44Ti/^48Ti ratio of (3.6+/-1.4) x 10^-2. The presence of ^41Ca together with Ca isotopic anomalies in circumstellar graphite grains is evidence for mixing between the C-rich and O-rich zones. Furthermore, evidence for ^44Ti in a few low density graphite grains [6] strongly suggests contributions from the innermost zone to the ejecta from which the grains formed. Observations of SN 1987A [e.g., 7] and hydrodynamic calculations [e.g., 8] indicate the existence of clumps of variable compositions in SN ejecta. The large variety of isotopic compositions in low density graphite grains is evidence for extensive and heterogeneous mixing of SN ejecta, confirming the astronomical observations and the theoretical calculations. References: [1] Amari S. et al. (1994) LPS XXV, 27-28. [2] Zinner E. et al. (1995) LPS XXVI, 1561-1562. [3] Travaglio C. et al. (1995) in preparation. [4] Meyer B. S. et al. (1995) Meteoritics, 30, 319-324. [5] Woosley S. E. and Weaver T. A. (1995) Astrophys. J. Suppl., in press. [6] Amari S. et al. (1995) LPS XXVI, 37-38. [7] Hass M. R. et al. (1990) Astrophys. J., 360, 257-266. [8] Herant M. and Benz W. (1992) Astrophys. J., 387, 294-308.
Amari Sachiko
Lewis Reed S.
Zinner Emst
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