Kavli Affiliate: Alexander P. Ji
| First 5 Authors: Evan N. Kirby, Alexander P. Ji, Mikhail Kovalev, ,
| Summary:
Whereas light element abundance variations are a hallmark of globular
clusters, there is little evidence for variation in neutron-capture elements. A
significant exception is M15, which shows a star-to-star dispersion in
neutron-capture abundances of at least one order of magnitude. The literature
contains evidence both for and against a neutron-capture dispersion in M92. We
conducted an analysis of archival Keck/HIRES spectra of 35 stars in M92, 29 of
which are giants, which we use exclusively for our conclusions. M92 conforms to
the light element abundance variations typical of massive clusters. Like other
globular clusters, its neutron-capture abundances were generated by the
r-process. We confirm a star-to-star dispersion in the r-process. Unlike M15,
the dispersion is limited to "first-generation" (low Na, high Mg) stars, and
the dispersion is smaller for Sr, Y, and Zr than for Ba and the lanthanides.
This is the first detection of a relation between light element and
neutron-capture abundances in a globular cluster. We propose that a source of
the main r-process polluted the cluster shortly before or concurrently with the
first generation of star formation. The heavier r-process abundances were
inhomogeneously distributed while the first-generation stars were forming. The
second-generation stars formed after several crossing times (~0.8 Myr); hence,
the second generation shows no r-process dispersion. This scenario imposes a
minimum temporal separation of 0.8 Myr between the first and second
generations.
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