Effects of Axion Interactions on Quark Stars in 4D Einstein-Gauss-Bonnet Gravity
We explore the properties of quark stars by combining the microscopic axion-extended Polyakov--NambuJona-Lasinio model with the macroscopic framework of four-dimensional Einstein--Gauss--Bonnet (4D EGB) gravity. Specifically, axion effects are incorporated through the background phase $θ=a/f_a$ in the thermodynamics of quark matter. Our results show that the inclusion of axion-induced interactions stiffens the quark-matter equation of state, thereby increasing the sound speed and the maximum mass of quark stars. The 4D EGB correction effectively weakens gravitational compression and further modifies the stellar structure, allowing for larger radii and higher maximum masses while reducing the compactness and the surface gravitational redshift. The combined effects can support quark stars above $2\,M_\odot$, with mass--radius sequences overlapping the observational regions in the indicative phenomenological comparison. These findings reveal that the interplay between axion-induced modifications of dense quark matter and 4D EGB gravity may provide a viable phenomenological framework for describing massive quark stars.