Cluster analysis of presolar silicon carbide grains: evaluation of their
  classification and astrophysical implications
  Cluster analysis of presolar silicon carbide grains based on literature data
for 12C/13C, 14N/15N, {\delta}30Si/28Si, and {\delta}29Si/28Si including or not
inferred initial 26Al/27Al data, reveals nine clusters agreeing with previously
defined grain types but also highlighting new divisions. Mainstream grains
reside in three clusters probably representing different parent star
metallicities. One of these clusters has a compact core, with a narrow range of
composition, pointing to an enhanced production of SiC grains in asymptotic
giant branch (AGB) stars with a narrow range of masses and metallicities. The
addition of 26Al/27Al data highlights a cluster of mainstream grains, enriched
in 15N and 26Al, which cannot be explained by current AGB models. We defined
two AB grain clusters, one with 15N and 26Al excesses, and the other with 14N
and smaller 26Al excesses, in agreement with recent studies. Their definition
does not use the solar N isotopic ratio as a divider, and the contour of the
26Al-rich AB cluster identified in this study is in better agreement with
core-collapse supernova models. We also found a cluster with a mixture of
putative nova and AB grains, which may have formed in supernova or nova
environments. X grains make up two clusters, having either strongly correlated
Si isotopic ratios or deviating from the 2/3 slope line in the Si 3-isotope
plot. Finally, most Y and Z grains are jointly clustered, suggesting that the
previous use of 12C/13C= 100 as a divider for Y grains was arbitrary. Our
results show that cluster analysis is a powerful tool to interpret the data in
light of stellar evolution and nucleosynthesis modelling and highlight the need
of more multi-element isotopic data for better classification.
