Clustering in the Stellar Abundances Space
  We have studied the chemical enrichment history of the interstellar medium
through an analysis of the n-dimensional stellar abundances space. This work is
a non-parametric analysis of the stellar chemical abundance space. The main
goal is to study the stars from their organization within this abundance space.
Within this space, we seek to find clusters (in a statistical sense), that is,
stars likely to share similar chemo-evolutionary history, using two methods:
the hierarchical clustering and the principal component analysis. We analysed
some selected abundance surveys available in the literature. For each sample,
we labelled the group of stars according to its average abundance curve. In all
samples, we identify the existence of a main enrichment pattern of the stars,
which we call chemical enrichment flow. This flow is set by the structured and
well defined mean rate at which the abundances of the interstellar medium
increase, resulting from the mixture of the material ejected from the stars and
stellar mass loss and interstellar medium gas. One of the main results of our
analysis is the identification of subgroups of stars with peculiar chemistry.
These stars are situated in regions outside of the enrichment flow in the
abundance space. These peculiar stars show a mismatch in the enrichment rate of
a few elements, such as Mg, Si, Sc and V, when compared to the mean enrichment
rate of the other elements of the same stars. We believe the existence of these
groups of stars with peculiar chemistry may be related to the accretion of
planetary material onto stellar surfaces or may be due to production of the
same chemical element by different nucleosynthetic sites.
