Impact of dynamical dark energy on the neutron star equilibrium
  We study the density distribution of the minimally-coupled scalar field dark
energy inside a neutron star. The dark energy is considered in the
hydrodynamical representation as a perfect fluid with three parameters
(background density, equation of state, and effective sound speed). The neutron
star matter is modeled with three unified equations of state, developed by the
Brussels-Montreal group. With the calculated density distribution of the dark
energy inside a neutron star (and its dependence on the dark energy parameters)
we investigate how its presence impacts the macroscopic characteristics and the
value of the mass limit for neutron stars. From this impact we derive the
possible constrains on the effective speed of sound of dark energy with the
help of maximal known masses of observed neutron stars. In this approach, we
have found, that the squared effective speed of sound can not be smaller than
$\sim 10^{-2}$ in units of squared speed of light.
