Slowly rotating neutron stars in scalar-tensor theories with a massive scalar field

In the scalar-tensor theories with a massive scalar field, the coupling constants, and the coupling functions in general, which are observationally allowed, can differ significantly from those in the massless case. This fact naturally implies that the scalar-tensor neutron stars with a massive scalar field can have rather different structure and properties in comparison with their counterparts in the massless case and in general relativity. In the present paper, we study slowly rotating neutron stars in scalar-tensor theories with a massive gravitational scalar. Two examples of scalar-tensor theories are examined—the first example is the massive Brans-Dicke theory and the second one is a massive scalar-tensor theory indistinguishable from general relativity in the weak-field limit. In the latter case, we study the effect of the scalar field mass on the spontaneous scalarization of neutron stars. Our numerical results show that the inclusion of a mass term for the scalar field indeed changes the picture drastically compared to the massless case. It turns out that mass, radius, and moment of inertia for neutron stars in massive scalar-tensor theories can differ drastically from the pure general relativistic solutions if sufficiently large masses of the scalar field are considered.

Figure 1

The mass as a function of the central energy density (left panel) and as a function of the radius (right panel) for EOS APR. The results for different values of the coupling constant $\beta$ and dimensionless mass of the scalar field $m_{\phi }$ are plotted.

Figure 2

The moment of inertia as a function of the stellar mass. The right panel is a magnification of the left panel.

Figure 3

The mass as a function of the central energy density (left panel) and as a function of the radius (right panel) for neutron stars in the massive Brans-Dicke theory with EOS APR. The results for different values of the coupling constant ${\alpha }_0$ and dimensionless mass of the scalar field $m_{\phi }$ are plotted.

Figure 4

The moment of inertia as a function of the neutron star mass for the massive Brans-Dicke theory.