Mass-Gap Neutron Stars from Vector \texorpdfstring{$f(R)$}{f(R)} Gravity Inflationary Deformations

Abstract

The latest observations from the LIGO-Virgo indicated the existence of mass-gap region astrophysical objects. This is a rather sensational observation and there are two possibilities for the nature of these mass-gap region astrophysical objects, these are either small black holes that result from the mergers of ordinary mass neutron stars, or these are heavy neutron stars. In the line of research implied by the former possibility, in this work we shall examine the implied neutron star phenomenology from vector $f(R)$ gravity inflationary models. These theories are basically scalar-tensor deformations of the Starobinsky inflationary model. We shall present the essential features of cosmologically viable and non-viable deformations of the Starobinsky model, originating from vector $f(R)$ gravity inflationary theories, and we indicate which models and for which equations of state provide a viable neutron star phenomenology. We solve the Tolman-Oppenheimer-Volkov equations using a robust double shooting LSODA python based code, for the following piecewise polytropic equations of state the WFF1, the SLy, the APR, the MS1, the AP3, the AP4, the ENG, the MPA1 and the MS1b. We confront the resulting phenomenology with several well known neutron star constraints and we indicate which equation of state and model fits the phenomenological constraints. A remarkable feature, also known from other inflationary attractor models, is that the MPA1 is the equation of state which is most nicely fitted the constraints, for all the theoretical models used, and actually the maximum mass for this equation of state is well inside the mass-gap region. Another mentionable feature that stroked us with surprise is the fact that even cosmologically non-viable inflationary models produced a viable neutron star phenomenology, which most likely has to be a model-dependent feature.