Starting from recent determination of Fe, O, Na abundances on a restricted
sample (N = 67) of halo and thick disk stars, a natural and well motivated
selection criterion is defined for the classification globular cluster stars.
An application is performed to M13 using a sample (N = 113) for which Fe, O,
Na abundances have been recently inferred from observations. A comparison is
made between the current and earlier M13 star classifications. Both O and Na
empirical differential abundance distributions are determined for each class
and for the whole sample (with the addition of Fe in the last case) and
compared with their theoretical counterparts due to cosmic scatter obeying a
Gaussian distribution whose parameters are inferred from related subsamples.
The occurrence of an agreement between the empirical and theoretical
distributions is interpreted as absence of significant chemical evolution and
vice versa. The procedure is repeated with regard to four additional classes
depending on whether oxygen and sodium abundance is above (stage CE) or below
(stage AF) a selected threshold. Both O and Na empirical differential
abundance distributions, related to the whole sample, exhibit a linear fit
for the AF and CE stage. Within the errors, the oxygen slope for the CE stage
is equal and of opposite sign with respect to the sodium slope for AF stage,
while the contrary holds when dealing with the oxygen slope for the AF stage
with respect to the sodium slope for the CE stage. In the light of simple
models of chemical evolution applied to M13, oxygen depletion appears to be
mainly turned into sodium enrichment for [O/H]? -1.35 and [Na/H]? -1.45,
while one or more largely preferred channels occur for [O/H]< -1.35 and
[Na/H]> -1.45. In addition, the primordial to the current M13 mass ratio can
be inferred from the true sodium yield in units of the sodium solar
abundance. Though the above results are mainly qualitative due to large
(-+1.5 dex) uncertainties in abundance determination, still the exhibited
trend is expected to be real. The proposed classification of globular cluster
stars may be extended in a twofold manner, namely to: (i) elements other than
Na and Fe and (ii) globular clusters other than M13.
High precision differential abundance measurements in globular clusters: chemical inhomogeneities in NGC 6752★