Europium production: neutron star mergers versus core-collapse
  supernovae
  We have explored the Eu production in the Milky Way by means of a very
detailed chemical evolution model. In particular, we have assumed that Eu is
formed in merging neutron star (or neutron star black hole) binaries as well as
in type II supernovae. We have tested the effects of several important
parameters influencing the production of Eu during the merging of two neutron
stars, such as: i) the time scale of coalescence, ii) the Eu yields and iii)
the range of initial masses for the progenitors of the neutron stars. The
yields of Eu from type II supernovae are very uncertain, more than those from
coalescing neutron stars, so we have explored several possibilities. We have
compared our model results with the observed rate of coalescence of neutron
stars, the solar Eu abundance, the [Eu/Fe] versus [Fe/H] relation in the solar
vicinity and the [Eu/H] gradient along the Galactic disc. Our main results can
be summarized as follows: i) neutron star mergers can be entirely responsible
for the production of Eu in the Galaxy if the coalescence time scale is no
longer than 1 Myr for the bulk of binary systems, the Eu yield is around $3
\times 10^{-7}$ M$_\odot$, and the mass range of progenitors of neutron stars
is 9-50 M$_\odot$; ii) both type II supernovae and merging neutron stars can
produce the right amount of Eu if the neutron star mergers produce $2 \times
10^{-7}$ M$_\odot$ per system and type II supernovae, with progenitors in the
range 20-50 M$_\odot$, produce yields of Eu of the order of $10^{-8}-10^{-9}$
M$_\odot$; iii) either models with only neutron stars producing Eu or mixed
ones can reproduce the observed Eu abundance gradient along the Galactic disc.
