Compact Stars in F(R)Gravity Theory

F(R)gravity is a modified gravity theory,and its applications for the compact star have attracted attention in the last decades.We review the basics of the F(R)gravity theory and the modified Tolman-Oppenheimer-Volkoff(TOV)equation.Recent studies show that the model dependence of equation of state(EOS)and modification of gravity degenerate to each other,which suggests the mass-radius(M-R)relation of the compact star alone cannot completely determine the EOS of the inner matter.Moreover,the effects of a new scalar field predicted in F(R)gravity on both the internal and external structure of the compact star are illustrated in the benchmark R^(2)model.Finally,We discuss the future directions for testing gravitational theories by observational measurements of the compact stars.

Dark matter with chiral symmetry admixed with hadronic matterin compact stars

Using the two-fluid Tolman-Oppenheimer-Volkoff equation,the properties of dark matter(DM)admixed neutron stars(DANSs)have been investigated.In contrast to previous studies,we find that an increase in the maximum mass and a decrease in the radius of 1.4 M_(⊙)NSs can occur simultaneously in DANSs.This stems from the ability of the equation of state(EOS)for DM to be very soft at low density but very stiff at high density.It is well known that the IU-FSU and XS models are unable to produce a neutron star(NS)with a maximum mass greater than 2.0 M_(⊙).However,by considering the IU-FSU and XS models for DANSs,there are interactions with DM that can produce a maximum mass greater than 2.0 M_(⊙)and a radius of 1.4 M_(⊙)NSs below 13.7 km.When considering a DANS,the difference between DM with chiral symmetry(DMC)and DM with meson exchange(DMM)becomes obvious when the central energy density of DM is greater than that of nuclear matter(NM).In this case,the DMC model with a DM mass of 1000 MeV can still produce a maximum mass greater than 2.0 M_(⊙)and a radius of a 1.4 M_(⊙)NS below 13.7 km.Additionally,although the maximum mass of the DANS using the DMM model is greater than 2.0 M_(⊙),the radius of a 1.4 M_(⊙)NS can surpass 13.7 km.In the two-fluid system,the maximum mass of a DANS can be larger than 3.0 M_(⊙).Consequently,the dimensionless tidal deformabilityΛCP of a DANS with 1.4 M_(⊙),which increases with increasing maximum mass,may be larger than 800 when the radius of the 1.4 M_(⊙)DANS is approximately 13.0 km.