Titanium isotopic compositions of rare presolar SiC grain types from the Murchison meteorite. (15th January 2018)

• Record Type: Journal Article
• Authors: Nguyen, Ann N.; Nittler, Larry R.; Alexander, Conel M.O'D.; Hoppe, Peter
• Is Part Of: Geochimica et cosmochimica acta. Volume 221(2018)
• Page Start: 162
• Page End: 181
• Publication Date: 2018-01-15
• Subjects: Circumstellar grains -- Galactic chemical evolution -- Nucleosynthesis -- Stars; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2017.02.026
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: We report the Ti isotopic compositions of 8 mainstream, 22 Y, 9 Z, and 26 AB presolar SiC grains from two SiC-rich residues of the Murchison CM2 meteorite together with Si, C and some Mg-Al isotopic data for the same grains. Mainstream, Y and Z grains are believed to originate in asymptotic giant branch (AGB) stars of varying metallicities, but the stellar sources of AB grains are poorly understood. We find that the 46, 47, 49 Ti/ 48 Ti ratios are correlated with 29 Si/ 28 Si for all of the grain types, indicating that these ratios are mainly dominated by Galactic chemical evolution (GCE). The mainstream, Y and Z grains all show enrichments in 50 Ti from neutron capture nucleosynthesis. However, AGB models predict smaller excesses in 50 Ti (and 49 Ti) than are observed in these grains. For Z grains and especially for Y grains, the enhancement of 50 Ti is greater than the enhancement in 30 Si, indicating that the 13 C neutron source produced a greater total fluence of neutrons than the 22 Ne source in the low metallicity parent AGB stars. The Z grains plot below the mainstream correlation lines at more 48 Ti- and 28 Si-rich compositions in plots of 46, 47, 49 Ti/ 48 Ti vs. 29 Si/ 28 Si. On the other hand, the Y grains plot close to the mainstream correlation line. This could imply that the Ti isotopes evolved non-linearly at metallicities below ∼1/3 solar. The AB grains in this study have Ti isotopic compositions that fall along correlation lines defined by the mainstream grains, suggesting origins in close to solar metallicity stars. However, these grains fall below the mainstream correlation lines in plots of 46, 49, 50 Ti/ 48 Ti vs. 29 Si/ 28 Si and do not show enhancements in 50 Ti, indicating that their parent stars did not undergo significant s -process nucleosynthesis. These data support origins of AB grains in J-type C stars rather than born-again AGB stars that undergo s -process nucleosynthesis. AB grains that do not have 50 Ti excesses may provide the best measure of Si and Ti isotope GCE since their parent stars were less affected by s -process nucleosynthesis than the mainstream grains.