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Due to the high-density nuclear matter equation of state (EOS) being as yet unknown, neutron stars (NSs) do not have a confirmed limiting “Chandrasekhar” type maximum mass. However, observations of NSs (PSR J1614-2230, PSR J0348+0432, PSR J0740+6620, PSR J0952–0607) indicate that the NS’s limiting mass, if there is any, could be well over 2M⊙. On the other hand, there seems to be an observational mass gap (of around 2.5−5M⊙ ) between the lightest black hole and the heaviest NS. Therefore, the “massive NSs” are prime candidates to fill that gap. Several NS EOSs have been proposed using both microscopic and phenomenological approaches. In this project, we look at a class of phenomenological nuclear matter EOSs—relativistic mean field models—and see what kind of NS is formed from them. We compute the max- imum mass supported by each model EOS to observe if the mass of the NS is indeed in the “massive” NS (>2M⊙) regime. We also observe the effects of including exotic particles (hyperons, Δs) in the NS EOS and how that affects the NS mass. However, only by introducing the magnetic field, i.e. for magnetized anisotro- pic NSs, we find the mass exceeding 2.5M⊙. Using tidal deformability constraints from gravitational wave observations, we place a further check on how physical the EOS and NSs are. 

The equilibrium configuration of white dwarfs composed of anisotropic fluid distribution in the presence of a strong magnetic field is investigated in this work. By considering a functional form of the anisotropic stress and magnetic field profile, some physical properties of magnetized white dwarfs, such as mass, radius, density, radial and tangential pressures, are derived; their dependency on the anisotropy and central magnetic field is also explored. We show that the orientations of the magnetic field along the radial direction or orthogonal to the radial direction influence the stellar structure and physical properties of white dwarfs significantly. Importantly, we show that ignoring anisotropy governed by the fluid due to its high density in the presence of a strong magnetic field would destabilize the star. Through this work, we can explain the highly massive progenitor for peculiar over-luminous type Ia supernovae, and low massive progenitor for under-luminous type Ia supernovae, which poses a question of considering 1.4 solar mass white dwarf to be related to the standard candle. 

Abstract We investigate the properties of anisotropic, spherically symmetric compact stars, especially neutron stars (NSs) and strange quark stars (SQSs), made of strongly magnetized matter. The NSs are described by the SLy equation of state (EOS) and the SQSs by an EOS based on the MIT Bag model. The stellar models are based on an a priori assumed density dependence of the magnetic field and thus anisotropy. Our study shows that not only the presence of a strong magnetic field and anisotropy, but also the orientation of the magnetic field itself, have an important influence on the physical properties of stars. Two possible magnetic field orientations are considered: a radial orientation where the local magnetic fields point in the radial direction, and a transverse orientation, where the local magnetic fields are perpendicular to the radial direction. Interestingly, we find that for a transverse orientation of the magnetic field, the stars become more massive with increasing anisotropy and magnetic-field strength and increase in size since the repulsive, effective anisotropic force increases in this case. In the case of a radially oriented magnetic field, however, the masses and radii of the stars decrease with increasing magnetic-field strength because of the decreasing effective anisotropic force. Importantly, we also show that in order to achieve hydrostatic equilibrium configurations of magnetized matter, it is essential to account for both the local anisotropy effects as well as the anisotropy effects caused by a strong magnetic field. Otherwise, hydrostatic equilibrium is not achieved for magnetized stellar models. 

Abstract Ever since the observation of peculiar overluminous Type Ia supernovae (SNeIa), exploring possible violations of the canonical Chandrasekhar mass limit (CML) has become a pressing research area of modern astrophysics. Since its first detection in 2003, more than a dozen of peculiar overluminous SNeIa has been detected, but the true nature of the underlying progenitors is still under dispute. Furthermore there are also underluminous SNeIa whose progenitor masses appear to be well below the CML (sub-Chandrasekhar progenitors). These observations call into question how sacrosanct the CML is. We have shown recently in Paper I that the presence of a strong magnetic field, the anisotropy of dense matter, as well as the orientation of the magnetic field itself significantly influence the properties of neutron and quark stars. Here, we study these effects for white dwarfs (WDs), showing that their properties are also severely impacted. Most importantly, we arrive at a variety of mass–radius relations of WDs that accommodate sub- to super-Chandrasekhar mass limits. This urges caution when using WDs associated with SNeIa as standard candles.