super-Chandrasekhar White Dwarfs Research Articles

Massive white dwarfs in Rastall-Rainbow gravity

We explore the properties of white dwarfs within the framework of Rastall-Rainbow gravity. Employing the Chandrasekhar equation of state in conjunction with the modified Tolman-Oppenheimer-Volkoff equation, we calculate the mass-radius relations of white dwarfs. We also obtain the confidence levels for the Rastall-Rainbow parameters based on some observed white dwarfs. Numerical results show that Rastall-Rainbow gravity can not only align well with these observations, but also can account for the existence of the super-Chandrasekhar white dwarfs. In addition, the gravitational redshift, compactness and dynamical stability of white dwarfs are discussed in this modified gravitational theory.

Cooling Process of White Dwarf Stars in Palatini f(R) Gravity

A simple cooling model of white dwarf stars is re-analyzed in Palatini f(R) gravity. Modified gravity affects the white dwarf structures and consequently their ages. We find that the resulting super-Chandrasekhar white dwarfs need more time to cool down than sub-Chandrasekhar ones, or when compared to the Newtonian models.

Instability of Super-Chandrasekhar White Dwarfs in Modified Gravity

Instability of Super-Chandrasekhar White Dwarfs in Modified Gravity

Massive Neutron Stars and White Dwarfs as Noncommutative Fuzzy Spheres

Over the last couple of decades, there have been direct and indirect evidences for massive compact objects than their conventional counterparts. A couple of such examples are super-Chandrasekhar white dwarfs and massive neutron stars. The observations of more than a dozen peculiar over-luminous type Ia supernovae predict their origins from super-Chandrasekhar white dwarf progenitors. On the other hand, recent gravitational wave detection and some pulsar observations provide arguments for massive neutron stars, lying in the famous mass-gap between lowest astrophysical black hole and conventional highest neutron star masses. We show that the idea of a squashed fuzzy sphere, which brings in noncommutative geometry, can self-consistently explain either of the massive objects as if they are actually fuzzy or squashed fuzzy spheres. Noncommutative geometry is a branch of quantum gravity. If the above proposal is correct, it will provide observational evidences for noncommutativity.

Massive white dwarfs in f(mathtt {R,L_m}) gravity

In this work, we investigate the equilibrium configurations of massive white dwarfs (MWD) in the context of modified gravity, namely f(mathtt {R,L_m}) gravity, where mathtt {R} stands for the Ricci scalar and mathtt {L_m} is the Lagrangian matter density. We focused on the specific case f(mathtt {R,L_m})= mathtt {R}/2 + mathtt {L_m}+ sigma mathtt {R}mathtt {L_m}, i.e., we have considered a non-minimal coupling between the gravity field and the matter field, with sigma being the coupling constant. For the first time, the theory is applied to white dwarfs, in particular to study massive white dwarfs, which is a topic of great interest in the last years. The equilibrium configurations predict maximum masses which are above the Chandrasekhar mass limit. The most important effect of the theory is to increase significantly the mass for stars with radius < 2000 km. We found that the theory can accommodate the super-Chandrasekhar white dwarfs for different star compositions. Apart from this, the theory recovers the General Relativity results for stars with radii larger than 3000 km, independent of the value of sigma .

Evolution of Highly Magnetic White Dwarfs by Field Decay and Cooling: Theory and Simulations

Abstract We investigate the luminosity suppression and its effect on the mass–radius relation and cooling evolution of highly magnetized white dwarfs. Based on the effect of magnetic field relative to gravitational energy, we suitably modify our treatment of the radiative opacity, magnetostatic equilibrium, and degenerate core equation of state to obtain the structural properties of these stars. Although the Chandrasekhar mass limit is retained in the absence of magnetic field and irrespective of the luminosity, strong central fields of about 1014 G can yield super-Chandrasekhar white dwarfs with masses ∼2.0 M ⊙. Smaller white dwarfs tend to remain super-Chandrasekhar for sufficiently strong central fields even when their luminosity is significantly suppressed to 10−16 L ⊙. Nevertheless, owing to the cooling evolution and simultaneous field decay over 10 Gyr, the limiting masses of small magnetized white dwarfs can fall to 1.5 M ⊙ over time. However, the majority of these systems still remain practically hidden throughout their cooling evolution because of their high fields and correspondingly low luminosities. Utilizing the stellar evolution code stars, we obtain close agreement with the analytical mass limit estimates, which suggests that our analytical formalism is physically motivated. Our results argue that super-Chandrasekhar white dwarfs born as a result of strong-field effects may not remain so forever. This explains their apparent scarcity, in addition to making them hard to detect because of their suppressed luminosities.

Instability Due to Electron Captures of Nuclei 56Ni, 56Co, 56Fe, 56Mn, 56Cr, and 56V in Strongly Magnetized Super-Chandrasekhar White Dwarfs

Based on the theory of the relativistic mean-field effective interactions, we explore the effect of superstrong magnetic fields (SMFs) on nuclear single-particle level structure, nuclear blinding energy, and the electron Fermi energy in strongly magnetized super-Chandrasekhar white dwarfs (SMSWDs). We also discuss the electron capture (EC) instability from the threshold mass density (TMD) and threshold pressure (TP) for some typical iron group nuclei in SMSWD due to SMFs. At relatively low densities (e.g., ρ 7 = 57, 97, 197), the EC rates increase and they then decrease by more than four orders of magnitude. However, at relatively higher density (e.g., ρ 7 = 4197), the rates increase by about two orders of magnitude when 2 × 103 < B 12 < 5 × 104, and then the SMFs have a minor effect on EC rates. The TMD (TP) can increase by about 23% ∼ 29%, due to an increase in the electron–ion interaction and nuclear binding energy in SMFs. As SMFs further increase, the TMD (TP) increase almost linearly for one Landau levels system. For a two and three Landau level system, the SMFs have a slight influence on the TMD (TP) (e.g., at B 12 = 103.5), and then TMD (TP) increases by about 13% ∼ 14%, and finally decreases by about 27.85% (27.34%). The shift of the onset of EC can make the maximum mass of the SMSWD increase by more than 2.28% for a one Landau level system. This influence from TMD (TP) can be larger than that of general relativity for SMSWD, which contains some iron group nuclei.

Suppression of luminosity and mass–radius relation of highly magnetized white dwarfs

ABSTRACT We explore the luminosity L of magnetized white dwarfs and its effect on the mass–radius relation. We self-consistently obtain the interface between the electron degenerate-gas dominated inner core and the outer ideal gas surface layer or envelope by incorporating both the components of gas throughout the model white dwarf. This is obtained by solving the set of magnetostatic equilibrium, photon diffusion, and mass conservation equations in the Newtonian framework, for different sets of luminosity and magnetic field. We appropriately use magnetic opacity, instead of Kramer’s opacity, wherever required. We show that the Chandrasekhar limit is retained, even at high luminosity up to about $10^{-2}\, L_\odot$ but without magnetic field, if the temperature is set constant inside the interface. However, there is an increased mass for large-radius white dwarfs, an effect of photon diffusion. Nevertheless, in the presence of strong magnetic fields, with central strength of about 1014 G, super-Chandrasekhar white dwarfs, with masses of about $1.9\, {\rm M}_{\odot }$, are obtained even when the temperature inside the interface is kept constant. Most interestingly, small-radius magnetic white dwarfs remain super-Chandrasekhar even if their luminosity decreases to as low as about $10^{-20}\, L_{\odot }$. However, their large-radius counterparts in the same mass–radius relation merge with Chandrasekhar’s result at low L. Hence, we argue for the possibility of highly magnetized, low luminous super-Chandrasekhar mass white dwarfs that, owing to their faintness, can be practically hidden.

Timescales for Detection of Super-Chandrasekhar White Dwarfs by Gravitational-wave Astronomy

Abstract Over the past two decades, the inference of the violation of the Chandrasekhar mass limit of white dwarfs (WDs) from indirect observation has been a revolutionary discovery in astronomy. Various researchers have already proposed different theories to explain this interesting phenomenon. However, such massive WDs usually possess very little luminosity, hence they so far cannot be detected directly by any observations. We have already proposed that the continuous gravitational wave may be one of the probes to detect them directly, and in the future various space-based detectors, such as LISA, DECIGO, and BBO, should be able to detect many of those WDs (provided they behave like pulsars). In this paper, we address various timescales related to the emission of gravitational as well as dipole radiations. This exploration sets a timescale for the detectors to observe the massive WDs.

Gravitational waves from r-mode instability of massive young sub- and super-Chandrasekhar white dwarfs

In the present work we investigate the r-mode instability windows, spindown and spindown rates of sub- and super-Chandrasekhar magnetized white dwarfs in presence of Landau quantization of the electron gas and magnetic braking. The gravitational wave strain amplitudes due to r-mode instability is also calculated. The dominant damping mechanism is taken to be the shear viscosity arising due to scattering of the degenerate electrons from the ion liquid. We find that the critical frequencies of Landau quantized magnetized white dwarfs are the lowest, those of non-Landau quantized ones are higher and those of non-magnetized ones are the highest at the same temperature. This implies that magnetic braking and Landau quantization both enhance r-mode instability. We have also seen that there is rapid spindown of magnetized white dwarfs due to additional magnetic braking term but there is no considerable effect of Landau quantization on the spindown and spindown rates for magnetic field strengths relevant for white dwarf interiors. We find that the r-mode gravitational wave strain amplitude for a rapidly rotating super-Chandrasekhar white dwarf at 1 kpc is $\sim 10^{-27}$, making isolated massive rapidly rotating hot magnetized white dwarfs prime candidates for search of gravitational waves in the future.

Erratum: ‘Continuous gravitational wave from magnetized white dwarfs and neutron stars: possible missions for LISA, DECIGO, BBO, ET detectors’

Recent detection of gravitational wave from nine black hole merger events and one neutron star merger event by LIGO and VIRGO shed a new light in the field of astrophysics. On the other hand, in the past decade, a few super-Chandrasekhar white dwarf candidates have been inferred through the peak luminosity of the light-curves of a few peculiar type Ia supernovae, though there is no direct detection of these objects so far. Similarly, a number of neutron stars with mass $>2M_\odot$ have also been observed. Continuous gravitational wave can be one of the alternate ways to detect these compact objects directly. It was already argued that magnetic field is one of the prominent physics to form super-Chandrasekhar white dwarfs and massive neutron stars. If such compact objects are rotating with certain angular frequency, then they can efficiently emit gravitational radiation, provided their magnetic field and rotation axes are not aligned, and these gravitational waves can be detected by some of the upcoming detectors, e.g. LISA, BBO, DECIGO, Einstein Telescope etc. This will certainly be a direct detection of rotating magnetized white dwarfs as well as massive neutron stars.

Continuous gravitational wave from magnetized white dwarfs and neutron stars: possible missions for LISA, DECIGO, BBO, ET detectors

ABSTRACT Recent detection of gravitational wave from nine black hole merger events and one neutron star merger event by LIGO and VIRGO shed a new light in the field of astrophysics. On the other hand, in the past decade, a few super-Chandrasekhar white dwarf candidates have been inferred through the peak luminosity of the light curves of a few peculiar Type Ia supernovae, though there is no direct detection of these objects so far. Similarly, a number of neutron stars with mass $&gt;\! 2\, \mathrm{M}_\odot$ have also been observed. Continuous gravitational wave can be one of the alternate ways to detect these compact objects directly. It was already argued that magnetic field is one of the prominent physics to form super-Chandrasekhar white dwarfs and massive neutron stars. If such compact objects are rotating with certain angular frequency, then they can efficiently emit gravitational radiation, provided their magnetic field and rotation axes are not aligned, and these gravitational waves can be detected by some of the upcoming detectors, e.g. LISA, BBO, DECIGO, Einstein Telescope, etc. This will certainly be a direct detection of rotating magnetized white dwarfs as well as massive neutron stars.

Study on charged strange stars in f(R, T) gravity

We investigate the effects of the modified f(R, T) gravity on the charged strange quark stars with the standard choice of f(R, T)=R+2χ T. Those types of stars are supposed to be made of strange quark matter (SQM) whose distribution is governed by the phenomenological MIT bag EOS as p=1/3(ρ−4B), where B is the bag constant, while the form of charge distribution is chosen to be q(r)=Q(r/ℛ)3=α r3 with α as a constant. We derive the values of the unknown parameters by matching the interior spacetime to the exterior Reissner-Nordstr{öm metric followed by the appropriate choice of the values of the parameters χ and α. Our study reveals that besides SQM, a new kind of matter distribution originates due to the interaction between the matter and the extra geometric term, while the modification of the Tolman-Oppenheimer-Volkoff (TOV) equation invokes the presence of a new force Fc. The accumulation of the electric charge distribution reaches its maximum at the surface, and the predicted values of the corresponding electric charge and electric field are of the order of 1019−20C and 1021−22 V/cm, respectively. To examine the physical validity of our solutions, we perform tests of the energy conditions, stability against the equilibrium of the forces, the adiabatic index, etc., and find that the proposed f(R, T) model survives all these critical tests. Therefore, our model can describe the non-singular charged strange stars and justify the supermassive compact stellar objects having their masses beyond the maximum mass limit for the compact stars in the standard scenario. Our model also supports the existence of several exotic astrophysical objects like super-Chandrasekhar white dwarfs, massive pulsars, and even magnetars, which remain unexplained in the framework of General Relativity (GR).

Continuous gravitational waves from magnetized white dwarfs

AbstractRecent evidence of super-Chandrasekhar white dwarfs (WDs), from the observations of over-luminous type Ia supernovae (SNeIa), has been a great astrophysical discovery. However, no such massive WDs have so far been observed directly as their luminosities are generally quite low. Hence it immediately raises the question of whether there is any possibility of detecting them directly. The search for super-Chandrasekhar WDs is very important as SNeIa are used as standard candles in cosmology. In this article, we show that continuous gravitational wave can allow us to detect such super-Chandrasekhar WDs directly.

Relativistic Thomas-Fermi equation of state for magnetized white dwarfs

The relativistic Feynman-Metropolis-Teller treatment of compressed atoms is extended to treat magnetized matter. Each atomic configuration is confined by a Wigner-Seitz cell and is characterized by a positive electron Fermi energy which varies insignificantly with the magnetic field. In the relativistic treatment the limiting configuration is reached when the Wigner-Seitz cell radius equals the radius of the nucleus with a maximum value of the electron Fermi energy which can not be attained in presence of magnetic field due to the effect of Landau quantization of electrons within the Wigner-Seitz cell. This treatment is implemented to develop the Equation of State for magnetized White Dwarf stars in presence of Coulomb screening. The mass-radius relations for magnetized White Dwarfs are obtained by solving the general relativistic hydrostatic equilibrium equations using Schwarzschild metric description suitable for non rotating and slowly rotating stars. The explicit effects of the magnetic energy density and pressure contributed by a density-dependent magnetic field are included to find the stable configurations of magnetized Super-Chandrasekhar White Dwarfs.

White dwarfs in de Rham-Gabadadze-Tolley like massive gravity

The existence of possible massive white dwarfs more than the Chandrasekhar limit ($1.45\text{ }{M}_{\ensuremath{\bigodot}}$, in which ${M}_{\ensuremath{\bigodot}}$ is mass of the sun) is a challenging topic. In this regard, and motivated by the important effect of massive graviton on the structure of white dwarfs, we study the white dwarfs in Vegh's massive gravity which is known as one of theories of de Rham, Gabadadze, and Tolley (dRGT) like massive gravity. First, we consider the modified Tolman-Oppenheimer-Volkoff equation in this theory of massive gravity and solve it numerically by using the Chandrasekhar's equation of state. Our results show that the maximum mass of white dwarfs in massive gravity can be more than the Chandrasekhar limit ($M&gt;1.45\text{ }{M}_{\ensuremath{\bigodot}}$), and this result imposes some constraints on parameters of massive gravity. Then, we investigate the effects of various parameters on other properties of the white dwarfs such as mass-radius relation, mass-central density relation, Schwarzschild radius, average density, and Kretschmann scalar. Next, we study dynamical stability condition for super-Chandrasekhar white dwarfs and show that these massive compact objects enjoy dynamical stability. Finally, in order to have a better insight, we compare the super-Chandrasekhar white dwarfs with the obtained massive neutron stars in dRGT like massive theory of gravity.

Optical and Radio Transients after the Collapse of Super-Chandrasekhar White Dwarf Merger Remnants

Abstract Super-Chandrasekhar remnants of double white dwarf mergers could sometimes collapse into a rapidly rotating neutron star (NS), accompanying with a mass ejection of a few times 0.01 M ⊙. Bright optical transient emission can be produced by the ejecta due to heating by radioactivities and particularly by energy injection from the NS. Since the merger remnants before collapse resemble a star evolving from the asymptotic giant branch phase to the planetary nebula phase, an intense dusty wind is considered to be driven about several thousand years ago before the collapse and surround the remnant at large radii. Therefore, the optical transient emission can be somewhat absorbed and scattered by the dusty wind, which can suppress the peak emission and cause a scattering plateau in optical light curves. Several years later, as the ejecta finally catches up with the wind material, the shock interaction between them can further give rise to a detectable radio transient emission on a timescale of several tens of days. Discovery of and observations to such dust-affected optical transients and shock-driven radio transients can help to explore the nature of super-Chandrasekhar merger remnants and as well as the density and type ratios of double white dwarf systems, which is beneficial in assessing their gravitational wave contributions.

The Spin Evolution of Fast-rotating, Magnetized Super-Chandrasekhar White Dwarfs in the Aftermath of White Dwarf Mergers

Abstract The evolution of the remnant of the merger of two white dwarfs is still an open problem. Furthermore, few studies have addressed the case in which the remnant is a magnetic white dwarf with a mass larger than the Chandrasekhar limiting mass. Angular momentum losses might bring the remnant of the merger to the physical conditions suitable for developing a thermonuclear explosion. Alternatively, the remnant may be prone to gravitational or rotational instabilities, depending on the initial conditions reached after the coalescence. Dipole magnetic braking is one of the mechanisms that can drive such losses of angular momentum. However, the timescale on which these losses occur depends on several parameters, like the strength of the magnetic field. In addition, the coalescence leaves a surrounding Keplerian disk that can be accreted by the newly formed white dwarf. Here we compute the post-merger evolution of a super-Chandrasekhar magnetized white dwarf taking into account all the relevant physical processes. These include magnetic torques acting on the star, accretion from the Keplerian disk, the threading of the magnetic field lines through the disk, and the thermal evolution of the white dwarf core. We find that the central remnant can reach the conditions suitable to develop a thermonuclear explosion before other instabilities (such as the inverse beta-decay instability or the secular axisymmetric instability) are reached, which would instead lead to gravitational collapse of the magnetized remnant.

Structure formation through self-gravitational instability in degenerate and non-degenerate anisotropic magnetized plasma

The self-gravitational instability is examined for non-degenerate and degenerate magnetized plasma. In the case of non-degenerate collisionless magnetized plasma the pressure is considered as anisotropic while in the case of degenerate situations it is taken as isotropic. The effect of finite Larmor radius correction of non-degenerate ions and viscous dissipation is taken into account in both the cases. Firstly in non-degenerate anisotropic plasma the conventional magnetohydrodynamic model is used to construct basic set of equations within the framework of modified Chew–Goldberger and Low theory. Secondly, in the case of degenerate isotropic plasma, which is considered to be composed of degenerate electrons and non-degenerate ions, the model equations are constructed using quantum magneto hydrodynamic model. The dynamics of degenerate particles are governed by Bohm and exchange potentials. The general dispersion relations are derived for both degenerate and non-degenerate situations separately using linearized perturbation equations. The results are discussed analytically and numerically for various modes of propagation. In case of non degenerate strongly magnetized plasma the effects of stress tensor anisotropy dominate over the influence of FLR effects while the FLR effects prevail in the weak magnetic field region. In case of isotropic degenerate plasma the implications of exchange parameter on the Jeans mass have been estimated and it is found that the increase in exchange parameter increases the limit of Jeans mass. The Jeans length and Jeans mass have been estimated for the white dwarf stars as $L_{J} \approx 2.1 \times 10^{11}~\mbox{m}$ and $M_{J} \approx 5 \times 10^{39}~\mbox{kg}$ respectively assist the existence of super Chandrasekhar white dwarfs.

A perturbation study of axisymmetric strongly magnetic degenerate stars: the case of super-Chandrasekhar white dwarfs

In the presence of a strong magnetic field a stellar equilibrium configuration, aided by the Lorentz force, can support a larger mass than a non-magnetic one. This has been considered a possible explanation of the super-Chandrasekhar mass white dwarfs giving rise to over-luminous Type-Ia supernovae. We present here linear and non-linear perturbation studies of such strongly magetised configurations and show that axisymmetric configurations with poloidal or toroidal fields are unstable. The numerical evolution of the perturbations shows instability after about an Alfv\'en crossing time. This time scale is very short for the magnetically supported super-Chandrasekhar mass white dwarfs. Uniform rotation about the symmetry axis can reduce the growth rate but can not stabilize the super-massive configurations. It is concluded that long-lived super-Chandrasekhar mass white dwarfs supported by magnetic field are unlikely to occur in Nature.