[Paper Review] Spontaneous scalarization of neutron stars in teleparallel gravity with derivative torsional coupling

Youcef Kehal, Khireddine Nouicer|arXiv (Cornell University)|Mar 23, 2026

Pulsars and Gravitational Waves Research0 citations

TL;DR

The paper constructs static and slowly rotating neutron-star solutions in a teleparallel gravity model where a scalar field couples to matter and to torsion, revealing a bounded, EOS-dependent spontaneous scalarization and its impact on mass-radius relations and moment of inertia.

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 \(A(ϕ)=\exp(βϕ^{2}/2)\). 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 (\(β=0\)). 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.

Motivation & Objective

• Investigate neutron-star configurations in a teleparallel gravity model with scalar-torsion and matter couplings.
• Determine how derivative torsional coupling and matter coupling influence scalarization in static and slowly rotating NSs.
• Assess the impact of different realistic EOS (APR and MS1) on scalarization and stellar properties.
• Characterize how scalarization alters mass–radius, mass–central density relations, and moment of inertia.
• Identify observational signatures that could test teleparallel scalarization with NS measurements.

Proposed method

• Adopt a scalar-torsion action in the Einstein frame with a derivative coupling between the scalar and torsion and a conformal matter coupling A(phi)=exp(beta phi^2/2).
• Derive modified TOV-like structure equations for static and slowly rotating stars using the Hartle–Thorne formalism in a stationary axisymmetric spacetime.
• Solve the field equations for APR and MS1 equations of state via numerical shooting to obtain mass-radius, central density, and scalar charge relationships.
• Analyze linear perturbations and the effective mass m_eff^2 to understand the onset of spontaneous scalarization via tachyonic instability.
• Explore how the derivative torsional coupling xi and the matter coupling beta influence scalarization and the boundedness of scalarized branches.

Experimental results

Research questions

• RQ1 Does spontaneous scalarization occur for neutron stars in teleparallel gravity with derivative torsional coupling and matter coupling?
• RQ2 How do the derivative torsional coupling xi and the matter coupling beta affect the onset, extent, and quenching of scalarization across EOS APR and MS1?
• RQ3 What are the resulting changes in mass–radius relations, central density thresholds, and scalar charges compared to GR?
• RQ4 How does slow rotation (moment of inertia) respond to the teleparallel coupling and EOS?
• RQ5 Can NS radius and rotational measurements test teleparallel scalarization scenarios?

Key findings

• Scalarized neutron-star solutions exist only within a finite range of central densities for both APR and MS1 EOS.
• Scalarization onset and extent depend on the equation of state and the strength and sign of the derivative torsional coupling xi.
• For high central densities, scalarization is quenched and solutions approach the general-relativistic limit.
• Scalarized branches can lie above or below the GR-like scalarized sequence depending on the sign of xi, indicating enhancement or suppression of support by the teleparallel coupling.
• The normalized scalar charge Q and the mass–radius relationship reflect the intermediate scalarized regime and show deviations that could source scalar dipole radiation.
• For slow rotation, the moment of inertia I systematically increases with stiffer EOS and is influenced by xi, offering potential observational tests.