44Ca and 49Ti Excesses in Circumstellar SiC Grains X from the Murchison Carbonaceous Chondrite

Details 44Ca and 49Ti Excesses in Circumstellar SiC Grains X from the Murchison Carbonaceous Chondrite 44Ca and 49Ti Excesses in Circumstellar SiC Grains X from the Murchison Carbonaceous Chondrite

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..557n&link_type=abstract

Meteoritics, vol. 30, no. 5, page 557

Astronomy and Astrophysics

Astrophysics

4

Dust, Circumstellar, Isotopes, Ca, Si, Ti, Meteorites, Murchison, Nucleosynthesis, Sic, Supernovae

Scientific paper

In their original study of five members of the uncommon (~1%) type of circumstellar SiC named grains X, Amari et al. (1992) found one grain, X2, with a large excess of 44Ca, which they attributed to the decay of short-lived (tau(sub)1/2=52 yr) 44Ti, indicating a supernova origin [1]. Four of the grains also had excesses in 49Ti. Recently, 44Ca excesses have been also found in Murchison graphite grains [2]. Ion-imaging searches of the Murchison SiC separate KJG (size 2-4 micrometers) have now identified a total of 77 SiC grains X and three members of the isotopically similar class of presolar grain, Si3N4[3-5]. These circumstellar grains are characterized by 15N and 28Si excesses, relative to solar, and have 12C/13C ratios ranging from 18 to 2200 [1, 3-5 & Table 1]. Most also have extremely high inferred initial 26Al/27Al ratios, ranging up to 0.6. Small grain sizes and low Ca and Ti concentrations precluded isotopic measurements of these elements in most of the grains, but 12 SiC grains X yielded sufficiently high secondary ion signals to allow determination of their Ca and Ti isotopic compositions. Of these, eight grains have 44Ca excesses, with inferred initial 44Ti/48Ti ratios between 0.003 and 0.06 (Table 1). The inferred 44Ti/48Ti ratio in grain KJGM4-205-12 has an unknown uncertainty due to Ca and/or Ti contributions from a nearby hibonite grain on the sample mount. Ti-isotopic measurements of the SiC grains X suffered from large errors, but one grain,KJGM2-66-3, has a large 49Ti excess (delta 49Ti= 978 +/- 333 \per mil),similar to those previously observed in grains X [1]. 44Ti is produced only in the innermost zones of supernovae (SN),and 49Ti is made there primarily as well, in the form of radioactive precursors 49V and 49Cr [6]. The presence of 44Ti and 49Ti excesses in SiC grains X confirms that these grains likely formed in SN ejecta. Moreover, since the grains carry the isotopic signatures of different SN regions (e. g., high 26Al/27Al ratios from the He/N zone, and 44Ti and 28Si enrichments from the Si/S zone), these data give additional evidence for deep heterogeneous mixing in SN explosions [2,4,5,7]. As we have previously noted, however, there are discrepancies between the detailed isotopic compositions of SiC grains X and the compositions predicted by current SN models [1, 4-5]. For example, while all the grains are enriched in 28Si, most have larger 30Si than 29Si depletions, in contrast to predictions by supernova models [6]. In connection to this problem, we note that for the44Ca-enriched SiC grains X, there is a possible correlation between the relative depletions of the heavy Si isotopes and the 44Ti/48Ti ratios. That is, grains with higher 44Ti/48Ti ratios tend to have higher delta 29Si/delta 30Si ratios (Table 1). References: [1] Amari et al. (1992) Astrophys. J. Lett., 394, L43-L46. [2] Amari et al. (1995) LPS XXVI, 37-38. [3] Nittler et al. (1993) Meteoritics, 28, 413. [4] Nittler et al. (1995) LPS XXVI, 1057-1058. [5] Nittler et al. (1995) Astrophys. J. Lett., submitted. [6] Woosley S. E. and Weaver T. A. (1995) Astrophys. J. Suppl., in press. [7] Zinner et al. (1995) LPS XXVI, 1561-1562.

Affiliated with

Also associated with

No associations

Rating

44Ca and 49Ti Excesses in Circumstellar SiC Grains X from the Murchison Carbonaceous Chondrite does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with 44Ca and 49Ti Excesses in Circumstellar SiC Grains X from the Murchison Carbonaceous Chondrite, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and 44Ca and 49Ti Excesses in Circumstellar SiC Grains X from the Murchison Carbonaceous Chondrite will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-829749