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Abstract In the present paper we construct novel non-topological, spontaneously scalarized neutron stars in multi-scalar Gauss–Bonnet gravity with maximally symmetric target space, and nontrivial map $$\varphi :spacetime \rightarrow target\ space$$ φ : s p a c e t i m e → t a r g e t s p a c e . The theory is characterized by the fact that for some classes of coupling functions the field equations allow solutions with trivial scalar field, which coincide with the general relativistic ones. For a certain range of parameters those solutions lose stability and new branches of solutions with nontrivial scalar field bifurcate from the trivial branch. For a given set of parameters, those branches are characterized by the number of zeros of the scalar field, and they are energetically more favorable than the general relativistic ones.

Abstract We are investigating universal relations between different normalisations of the moment of inertia and the compactness of neutron stars in slow rotation approximation. We study the relations in particular class of massive scalar-sensor theories with self-interaction, for which significant deviations from General Relativity are allowed for values of the parameters that are in agreement with the observations. Moment of inertia-compactness relations are examined for different normalisation of the moment of inertia. It is shown that for all studied cases the deviations from EOS universality are small for the examined equations of state. On the other hand the scalarization can lead to large deviations from the general relativistic universal relations for values of the parameters that are in agreement with the current observations that can be potentially used to set further test the scalar-tensor theories.

Abstract Binary pulsar observations and gravitational wave detections seriously constrained scalar–tensor theories with massless scalar field allowing only small deviations from general relativity. If we consider a nonzero mass of the scalar field, though, significant deviations from general relativity are allowed for values of the parameters that are in agreement with the observations. In the present paper we extend this idea and we study scalar–tensor theory with massive field with self-interaction term in the potential. The additional term suppresses the scalar field in the neutron star models in addition to the effect of the mass of the scalar field but still, large deviations from pure GR can be observed for values of the parameters that are in agreement with the observations.