On the origins of enigmatic stellar populations in Local Group galactic
  nuclei
  We consider the origins of enigmatic stellar populations in four Local Group
galactic nuclei, specifically the Milky Way, M31, M32 and M33. These are
centrally concentrated blue stars, found in three out of the four nuclear star
clusters (NSCs) considered here. Their origins are unknown, but could include
blue straggler (BS) stars, extended horizontal branch stars and young recently
formed stars. Here, we calculate order-of-magnitude estimates for various
collision rates, as a function of the host NSC environment and distance from
the cluster centre. These rates are sufficiently high that BSs, formed via
collisions between main sequence (MS) stars, could contribute non-negligibly
($\sim$ 1-10% in mass) to every surface brightness profile, with the exception
of the Milky Way. Stellar evolution models show that the envelopes of red giant
branch (RGB) stars must be nearly completely stripped to significantly affect
their photometric appearance, which requires multiple collisions. Hence, the
collision rates for individual RGB stars are only sufficiently high in the
inner $\lesssim$ 0.1 pc of M31 and M32 for RGB destruction to occur. Collisions
between white dwarfs and MS stars, which should ablate the stars, could offer a
steady and significant supply of gas in every NSC in our sample. The gas could
either fragment to form new stars, or accrete onto old MS stars already
present. Thus, collisional processes could contribute significantly to the
observed blue excesses in M31 and M33; future studies should be aimed at better
constraining theoretical predictions to compliment existing and future
observational data.
