Relatore
Peter Hoppe
(Max Planck Institute for Chemistry)
Descrizione
Primitive solar system materials contain small concentrations of presolar grains that formed in the winds of evolved stars and in the ejecta of stellar explosions [1]. Presolar SiC is the best studied presolar mineral. Among them are so-called Type AB grains which have low 12C/13C ratios of <= 10. This population of presolar SiC grains appears to originate from multiple types of stellar sources, namely, supernovae (SNe) for grains with isotopically heavy N (14N/15N < 440) [2], and born-again AGB stars [3] and in particular J-type carbon stars for grains with isotopically light N (14N/15N >= 440) [4].
Here, we report on high resolution (< 100 nm) measurements of C-, N-, Mg-Al-, Si-, and S-isotopic compositions of 10 SiC AB grains from Murchison separate KJD (median size 0.81 micrometer) [5] conducted with the NanoSIMS ion probe at MPI for Chemistry with Cs and Hyperion O ion sources. Except for one grain with the highest 12C/13C ratio we find good correlations between 12C/13C, 14N/15N, and 26Al/27Al. There is an almost perfect 1:1 correlation between Al and N concentrations, suggestive of AlN and low levels of contamination. Magnesium is essentially monoisotopic 26Mg from 26Al decay (half life: 0.72 Myr). Sulfur isotope anomalies are generally small and Si-isotopic compositions plot along the SiC mainstream line. Four of our AB grains have light N with 14N/15N up to 1000. The correlations between C-, N-, and Al-isotopic ratios are well explained by the 25 Msun SN model 25T-H of [6] when matter from the O/nova zone, which experienced explosive H burning, and above (6.847-13.3 Msun) is mixed with matter that experienced only partial H burning, taken from the outer layers in the 12 Msun model of [7], as suggested by [2], and if the 12C/13C ratio in the 25T-H model is decreased by a factor of 3. The comparison of our data with model 25T-H suggests that SNe might have contributed not only AB grains with heavy N but also some of those with light N.
[1] E. Zinner, in Treatise on Geochemistry, ed. A. M. Davis, Vol. 1(2014)181.
[2] N. Liu et al., ApJL 842(2017)L1
[3] S. Amari et al., ApJ 559(2001)463
[4] N. Liu et al., ApJL 844(2017)L12
[5] S. Amari et al., GCA 58(1994)459
[6] M. Pignatari et al., ApJL 808(2015)L43
[7] S. Woosley & A. Heger, PhR 442(2007)269
Autore principale
Peter Hoppe
(Max Planck Institute for Chemistry)
Coautore
Marco Pignatari
(University of Hull)
Sachiko Amari
(Washington University, St. Louis)