Structure, maximum mass, and stability of compact stars in f (Q, T) gravity

Abstract We investigate the properties of compact objects in the f (Q, T) theory, where Q Q is the non-metricity scalar and {T} T is the trace of the energy–momentum tensor. We derive an interior analytical solution for anisotropic perfect-fluid spheres in hydrostatic equilibrium using the linear form of f (Q, {T}) = Q+ {T} f (Q, T) = Q + ψ T, where ψ represents a dimensional parameter. Based on the observational constraints related to the mass and radius of the pulsar SAX J1748. 9-2021, ψ is set to a maximum negative value of ₁= / ²=-0. 04 ψ 1 = ψ / κ 2 = - 0. 04, where ² κ 2 is the gravitational coupling constant. The solution results in a stable compact object, which does not violate the speed of sound condition cₛ² c²3 c s 2 ≤ c 2 3. The effective equation of state is similar to the quark matter equation of state, and involves the presence of an effective bag constant. When ψ is negative, the star has a slightly larger size as compared to GR stars with the same mass. The difference in the predicted star size between the theory with a negative ψ and GR for the same mass is attributed to an additional force appearing in the hydrodynamic equilibrium equation. The maximum compactness allowed by the strong energy condition for f (Q, {T}) f (Q, T) theory and for GR is C = 0. 514 C = 0. 514 and 0. 419, respectively, with the f (Q, {T}) f (Q, T) prediction about 10\% 10 % higher than the GR one. Assuming a surface density at saturation nuclear density of ₍ₔ₂ = 4 10^14~ g/ cm³ ρ nuc = 4 × 10 14 g / cm 3, the maximum mass of the star is 4. 66 M_ 4. 66 M ⊙, with a radius of 14. 9 km.