Spontaneous scalarization of neutron stars in teleparallel gravity with derivative torsional coupling

TL;DR

This paper explores how neutron stars can develop scalar fields in a teleparallel gravity framework with derivative torsional coupling, affecting their structure and observable properties, and providing potential tests for this modified gravity theory.

Contribution

It introduces a novel teleparallel gravity model with derivative torsional coupling and analyzes neutron star solutions, revealing conditions for scalarization and its dependence on matter and torsion.

Findings

Scalarized neutron stars exist only within specific density ranges.

Torsional coupling can either enhance or suppress scalarization.

Scalarization effects diminish at high densities, approaching general relativity.

Abstract

We study neutron star configurations in a teleparallel gravity model featuring a scalar field coupled to both matter and torsion. In the Einstein frame, the theory includes a derivative coupling between the scalar field and the torsion vector, together with a conformal matter coupling \(A(\phi)=\exp(\beta\phi^{2}/2)\). Static and slowly rotating neutron-star solutions are constructed for realistic equations of state, focusing on the APR and MS1 equations of state. Scalarized solutions appear only within a finite range of central densities and correspond to localized deviations from the general-relativistic mass--radius and mass--central-density relations. The onset and extent of scalarization depend on the equation of state and on the strength of the derivative torsional interaction, which can either enhance or suppress scalarization relative to the general-relativistic scalarized branch. At high central densities, scalarization is quenched and the solutions approach the general-relativistic limit, remaining bounded even for strong torsional couplings. No scalarized solutions are found in the absence of matter coupling (\(\beta=0\)). The normalized scalar charge follows trends consistent with the global mass relations, indicating an intermediate scalarized regime suppressed at high compactness. For slowly rotating stars, the moment of inertia depends systematically on the torsional coupling and the equation of state, with stiffer equations yielding larger values. These results highlight the potential of neutron-star radius and rotational measurements to test teleparallel scalarization scenarios.