In this work we investigate the equilibrium configurations of white dwarfs in a modified gravity theory, namely, f (R, T ) gravity, for which R and T stand for the Ricci scalar and trace of the energy-momentum tensor, respectively. Considering the functional form f (R, T ) = R +2λT , with λ being a constant, we obtain the hydrostatic equilibrium equation for the theory. Some physical properties of white dwarfs, such as: mass, radius, pressure and energy density, as well as their dependence on the parameter λ are derived. More massive and larger white dwarfs are found for negative values of λ when it decreases. The equilibrium configurations predict a maximum mass limit for white dwarfs slightly above the Chandrasekhar limit, with larger radii and lower central densities when compared to standard gravity outcomes. The most important effect of f (R, T ) theory for massive white dwarfs is the increase of the radius in comparison with GR and also f (R) results. By comparing our results with some observational data of massive white dwarfs we also find a lower limit for λ, namely, λ > −3 × 10 −4 .

In this work, we study the properties of strongly magnetized white dwarfs (WDs), taking into account the electron capture and pycnonuclear fusion reactions instabilities. The structure of WDs is obtained by solving the Einstein–Maxwell equations with a poloidal magnetic field in a fully general relativistic treatment. The stellar fluid is assumed to be composed of a regular crystal lattice made of carbon ions immersed in a degenerate relativistic electron gas. The onset of electron capture reactions and pycnonuclear reactions are determined with and without magnetic fields. We find that magnetized WDs significantly exceed the standard Chandrasekhar mass limit, even when electron capture and pycnonuclear fusion reactions are present in the stellar interior. We obtain a maximum white dwarf mass of around 2.14 M⊙ for a central magnetic field of ∼3.85 × 1014 G, which indicates that magnetized WDs may play a crucial role for the interpretation of superluminous type Ia supernovae. Furthermore, we show that the critical density for pycnonuclear fusion reactions limits the central white dwarf density to 9.35 × 109 g cm−3. As a consequence, equatorial radii of WDs cannot be smaller than ∼1100 km. Another interesting feature concerns the relationship between the central stellar density and the strength of the magnetic field at the core of a magnetized white dwarf. For high magnetic fields, we find that the central density increases (stellar radius decrease) with magnetic field strength, which makes highly magnetized WDs more compact. The situation is reversed if the central magnetic field is less than ∼1013 G.

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In this work, we discuss white dwarf pulsars found recently making also reference of the possibility of some SGRs/AXPs being part of this class of pulsars. We also study the properties of very massive compact ultra magnetized white dwarfs that could be the progenitors candidates of super luminous type Ia supernovae, and also a previous stage of these white dwarf pulsars before the magnetic field decay. The structure of this ultra magnetized white dwarfs is obtained by solving the Einstein-Maxwell equations with a poloidal magnetic field in a fully general relativistic approach. The stellar interior is composed of a regular crystal lattice made of carbon ions immersed in a degenerate relativistic electron gas. We find that magnetized white dwarfs violate the standard Chandrasekhar mass limit significantly. We obtain a maximum white dwarf mass of around 2.12 M with an equatorial radius R ∼ 1596 Km, a central magnetic field of Bc = 1.74 × 10 14 G and Bs = 3.6 × 10 13 G at the stellar surface.

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