Compactness, masses and radii of compact stars within the Eddington-inspired Born-Infeld theory

Abstract: We investigate the compactness, masses and radii of realistic neutron stars (NSs) and quark stars (QSs) within the Eddington-inspired Born-Infeld (EiBI) theory of gravity, and the energy conditions of their apparent equation of states (EOSs). We show that the maximum compactness and maximum masses constraints extracted from the recent pulsars masses and radii observations can provide the upper and lower limits of $\kappa$ value of EiBI theory. By using BSP parameter set of relativistic mean field (RMF) model to describe NS core EOS including hyperons, it can be estimated that $ (2.7 \le \kappa_g \le 7.9) \times$ $\rm 10^{-2} m^5 kg^{-1} s^{-2}$. If we use confined-isospin-density-dependent-mass (CIDDM) model with additional scalar Coulomb term with QSK046 parameter set to describe QS EOS, we can obtain lesser value i.e., $ (1.6 \le \kappa_g\le 2.2) \times$ $\rm 10^{-2} m^5 kg^{-1} s^{-2}$. We have also observed that for large $\kappa$ values, mass-radius relations of NSs and QSs do not exceed causality restrictions because the compactness of NS and QS are saturated after passing certain large critical value. This observation is in agreement with the results obtained in Ref.\cite{Delsate12} for the case of pressure-less stars. We have also found that if $\kappa$ larger than certain non-zero value, the CIDDM with vector Coulomb model prediction of QS can reach the maximum mass $\gtrsim$ 2 $M_\odot$. We have found also that if the NS or QS EOS becomes stiffer, the upper limit of $\kappa$ from compactness constraint becomes smaller and lower limit of $\kappa$ from maximum masses constraint becomes larger. We also observe that the uncertainty of the systematic in canonical mass compact stars radii measurements data can affect the $\kappa$ range. It is also shown that the non-physical apparent EOS of NSs and QSs can satisfy the energy conditions.