Teleparallel torsion and white dwarf structure in \(f(T)=T+\Xi T^2\) gravity
We investigate how quadratic torsion modifies the equilibrium structure of white dwarfs in covariant $f(T)=T+\Xi T^2$ gravity. Static spherical configurations are calculated with a fixed equation of state for cold carbon matter, including relativistic electron degeneracy and Coulomb lattice corrections. The stellar interior is matched to a vacuum exterior, and the mass is determined from the asymptotic geometry. At fixed central density, negative couplings produce more massive and more compact configurations than general relativity, whereas positive couplings give smaller masses and larger radii. The deviations increase with central density and are more pronounced in mass than in radius. The first limiting feature of each sequence depends on the coupling. For $\Xi=-10^{18}\,\mathrm{cm}^{2}$, the sequence remains monotonic up to the electron capture density and reaches $1.508\,M_\odot$. General relativity and $\Xi=+10^{18}\,\mathrm{cm}^{2}$ instead reach mass turning points at $1.385\,M_\odot$ and $1.366\,M_\odot$, respectively. The positive extreme can be continued as an equilibrium solution to the capture density, where its mass decreases to $1.24\,M_\odot$, but this configuration lies beyond the turning point and is not the maximum mass along that sequence. These results identify a density dependent structural response to torsion that changes the stellar mass scale without modifying the matter equation of state.