Causal limit of neutron star maximum mass in f(R) gravity in view of GW190814

• Published in: Physics letters. B Vol. 816; p. 136222
• Main Authors: Astashenok, A.V., Capozziello, S., Odintsov, S.D., Oikonomou, V.K.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 10-05-2021; Elsevier
• ISSN: 0370-2693; 1873-2445
• DOI: 10.1016/j.physletb.2021.136222

We investigate the causal limit of maximum mass for stars in the framework of f(R) gravity. We choose a causal equation of state, with variable speed of sound, and with the transition density and pressure corresponding to the SLy equation of state. The transition density is chosen to be equal to twice the saturation density ρt∼2ρ0, and also the analysis is performed for the transition density, chosen to be equal to the saturation density ρt∼ρ0. We examine numerically the combined effect of the stiff causal equation of state and of the sound speed on the maximum mass of static neutron stars, in the context of Jordan frame of f(R) gravity. This yields the most extreme upper bound for neutron star masses in the context of extended gravity. As we will evince for the case of R2 model, the upper causal mass limit lies within, but not deeply in, the mass-gap region, and is marginally the same with the general relativistic causal maximum mass, indicating that the ∼3M⊙ general relativistic limit is respected. In view of the modified gravity perspective for the secondary component of the GW190814 event, we also discuss the strange star possibility. Using several well established facts for neutron star physics and the Occam's razor approach, although the strange star is exciting, for the moment, it remains a possibility for describing the secondary component of GW190814. We underpin the fact that the secondary component of the compact binary GW190814 is probably a neutron star, a black hole or even a rapidly rotating neutron star, but not a strange star. We also discuss, in general, the potential role of the extended gravity description for the binary merging.