Critical compactness bound of a class of compact stars

Tolman VII solution (Tolman in Phys Rev 55(4):364, 1939) is an exact analytic solution to the Einstein field equations describing the space-time of a static spherically symmetric distribution of matter. The solution has been shown to be capable of describing the interior of compact objects like neutron stars. Generalized (Raghoonundun and Hobill in Phys Rev D 92(12):124005, 2015) and modified (Jiang and Yagi in Phys Rev D 99(12):124029, 2019) versions of the solution are also available in the literature, which have been subsequently developed to accommodate a wide range of neutron star EOS. The stability of the modified Tolman VII solution has recently been analyzed (Posada et al in Phys Rev D 103(10):104067, 2021), which provides a critical value of the adiabatic index above which the stellar configuration becomes unstable against radial oscillations. In this paper, making use of the generalized version of the Tolman VII solution, we prescribe an upper bound on the compactness (M/R) beyond which the star becomes unstable against radial oscillations (Chandrasekhar in Phys Rev Lett 12(4):114, 1964). Our study brings to attention the role of model parameters in the generalized Tolman VII solution. The analysis also provides new insight into the role of inhomogeneity of the matter distribution vis-a-vis equation of state (EOS) on the compactness of a relativistic star.