NGC 1261: an $r$-process enhanced globular cluster from the
  Gaia-Enceladus event
  Our Milky Way (MW) has witnessed a series of major accretion events. One of
the later additions, Gaia-Enceladus, has contributed a considerable mass to the
inner Galaxy, but also generously donated to the outer halo. So far,
associations with present-day MW globular clusters (GCs) have been chiefly
based on their kinematics and ages. Here, we present a chemical abundance study
of the outer halo (R$_{\rm GC}$=18 kpc) GC NGC 1261, which has been suggested
to be an accreted object. We measured 31 species of 29 elements in two stars
from high-resolution Magellan/MIKE spectra and find that the cluster is
moderately metal poor, at [Fe/H]=-1.26. NGC 1261 is moderately
$\alpha$-enhanced to the 0.3-dex level. While from the small sample alone it is
difficult to assert any abundance correlations, the light elements Na,O,Mg, and
Al differ significantly between the two stars in contrast to the majority of
other elements with smaller scatter; this argues in favour of multiple
generations of stars coexisting in this GC. Intriguingly for its metallicity,
NGC 1261 shows heavy element abundances that are consistent with $r$-process
nucleosynthesis and we discuss their origin in various sites. In particular the
Eu overabundance quantitatively suggests that one single $r$-process event,
such as a neutron-star neutron-star merger or a rare kind of supernova, can be
responsible for the stellar enhancement or even the enrichment of the cluster
with the excess $r$-material. Its heavy element pattern makes NGC 1261 resemble
the moderately enhanced r-I stars that are commonly found in the halo and have
been detected in Gaia-Enceladus as well. Therefore, combining all kinematical,
age, and chemical evidence we conclude that NGC 1261 is a chemically intriguing
GC that was born in Gaia-Enceladus and has been subsequently accreted into the
MW halo. [abridged]
