Spontaneous scalarization with an extremely massive field and heavy neutron stars

TL;DR

This paper explores how a massive scalar field in a scalar-tensor theory influences neutron star structure and mass, revealing potential for neutron stars to surpass traditional mass limits while maintaining stability.

Contribution

It introduces the analysis of spontaneous scalarization with a very short Compton wavelength, showing significant effects on neutron star properties not previously studied.

Findings

Maximum neutron star mass can exceed 2 solar masses.

Scalarization can significantly alter internal structure and gravity.

Some parameter ranges produce super-causal neutron star masses.

Abstract

We investigate the internal structure and the mass-radius relation of neutron stars in a recently proposed scalar-tensor theory dubbed asymmetron in which a massive scalar field undergoes spontaneous scalarization inside neutron stars. We focus on the case where the Compton wavelength is shorter than 10 km, which has not been investigated in the literature. By solving the modified Einstein equations, either purely numerically or by partially using a semianalytic method, we find that not only the weakening of gravity by spontaneous scalarization but also the scalar force affect the internal structure significantly in the massive case. We also find that the maximum mass of neutron stars is larger for certain parameter sets than that in general relativity and reaches 2 solar mass even if the effect of strange hadrons is taken into account. There is even a range of parameters where the maximum mass of neutron stars largely exceeds the threshold that violates the causality bound in general relativity.