The Stars in M15 Were Born with the r-process
  High-resolution spectroscopy of stars on the red giant branch (RGB) of the
globular cluster M15 has revealed a large (~1 dex) dispersion in the abundances
of r-process elements, like Ba and Eu. Neutron star mergers (NSMs) have been
proposed as a major source of the r-process. However, most NSM models predict a
delay time longer than the timescale for cluster formation. One possibility is
that a NSM polluted the surfaces of stars in M15 long after the cluster
finished forming. In this case, the abundances of the polluting elements would
decrease in the first dredge-up as stars turn on to the RGB. We present
Keck/DEIMOS abundances of Ba in 66 stars along the entire RGB and the top of
the main sequence. The Ba abundances have no trend with stellar luminosity
(evolutionary phase). Therefore, the stars were born with the Ba they have
today, and Ba did not originate in a source with a delay time longer than the
timescale for cluster formation. In particular, if the source of Ba was a
neutron star merger, it would have had a very short delay time. Alternatively,
if Ba enrichment took place before the formation of the cluster, an
inhomogeneity of a factor of 30 in Ba abundance needs to be able to persist
over the length scale of the gas cloud that formed M15, which is unlikely.
