Inflationary attractors predictions for static neutron stars in the mass-gap region

Abstract

In this work we study static neutron stars in the context of several inflationary models which are popular in cosmology. These inflationary models are nonminimally coupled scalar theories which yield a viable inflationary phenomenology in both Jordan and Einstein frames. By considering the constraints from inflationary theories, which basically determine the values of the potential strength, usually considered as a free parameter in astrophysical neutron star works, we construct and solve the Tolman-Oppenheimer-Volkoff equations using a solid Python-3 LSODA integrator. For our study we consider several popular inflationary models, such as the universal attractors, the ${R}^{p}$ attractors (three distinct model values), the induced inflation, the quadratic inflation, the Higgs inflation and the $a$-attractors (two distinct model values) and for the following popular equations of state the WFF1, the SLy, the APR, the MS1, the AP3, the AP4, the ENG, the MPA1 and the MS1b. We construct the $M\ensuremath{-}R$ diagram and we confront the resulting theory with theoretical and observational constraints. As we demonstrate, remarkably, all the neutron stars produced by all the inflationary models we considered are compatible with all the constraints for the MPA1 equation of state. It is notable that for this particular equation of state, the maximum masses of the neutron stars are in the mass-gap region with $M>2.5{M}_{\ensuremath{\bigodot}}$, but lower than the three solar masses causal limit. Another important feature of our work is that it may be possible to discriminate inflationary attractors which at the cosmological level are indistinguishable using the $M\ensuremath{-}R$ graphs of static neutron stars, however we point out the limitations in discriminating the inflationary attractors. Also we show that the WFF1, MS1 and MS1b seem to be entirely ruled out, regarding a viable description of static neutron stars. We also make the observation that as the NICER constraints are pushed towards larger radii, as for example in the case of the black widow pulsar PSR J0952-0607, it seems that equations of state that produce neutron stars with maximum masses in the mass gap region, with $M>2.5{M}_{\ensuremath{\bigodot}}$, but lower than the three solar masses causal limit, are favored and are compatible with the modified NICER constraints. Finally we question the ability of the MPA1 equation of state to pass all the theoretical and observational constraints and we impose the question whether this equation of state plays any fundamental role in static neutron star physics.