We study static, spherically symmetric neutron stars in a class of scalar-tensor theories with noncanonical kinetic terms (K-essence) obeying all causality and hyperbolicity conditions. These models have nontrivial dynamics that lead to a type of antiscreening of the scalar. They lead to small corrections in the solar system due to a small coupling, but can lead to large corrections in regimes of high densities, especially neutron stars. We solve the modified Tolman-Oppenheimer-Volkoff equations numerically using realistic equations of state (SLy4, WFF1, MS1, MPA1). For a given central density, we find that two distinct configurations may exist, forming two separate branches of solutions. We find that above a certain critical central density solutions with the correct asymptotic behavior at spatial infinity cannot be obtained. We obtain precise predictions for the mass-radius relation for neutron stars for different values of the parameters in the model and we compare to data.

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• Alternative gravity theories
• Gravitation
• Hypothetical scalars
• Neutron stars & pulsars
• Numerical simulations in gravitation & astrophysics