Sinha, Monika

The massive stars end their lives by supernova explosions leaving central compact objects that may evolve into neutron stars. Initially, after birth, the star remains hot and gradually cools down. We explore the matter and star properties during this initial stage of the compact stars considering the possibility of the appearance of deconfined quark matter in the core of the star. At the initial stage after the supernova explosion, the occurrence of non-radial oscillation in the newly born compact object is highly possible. Non-radial oscillations are an important source of gra vitational wa ves (GWs). There is a high chance for GWs from these oscillations, especially the nodeless fundamental (f) mode to be detected by next-generation GW detectors. We study the evolution in frequencies of non-radial oscillation after birth considering phase transition and predicting the possible signature for different possibilities of theoretical compact star models.

In this work, we study the effect of (anti)kaon condensation on the properties of compact stars that develop hypernuclear cores with and without an admixture of Δ-resonances. We work within the covariant density functional theory with the parameters adjusted to K-atomic and kaon-nucleon scattering data in the kaonic sector. The density-dependent parameters in the hyperonic sector are adjusted to the data on Λ and Ξ- hypernuclei data. The Δ-resonance couplings are tuned to the data obtained from their scattering off nuclei and heavy-ion collision experiments. We find that (anti)kaon condensate leads to a softening of the equation of state and lower maximum masses of compact stars than in the absence of the condensate. Both the K- and K̄0 condensations occur through a second-order phase transition, which implies no mixed-phase formation. For large values of (anti)kaon and Δ-resonance potentials in symmetric nuclear matter, we observe that condensation leads to an extinction of Ξ-,0 hyperons. We also investigate the influence of inclusion of additional hidden-strangeness σ∗ meson in the functional and find that it leads to a substantial softening of the equation of state and delay in the onset of (anti)kaons.

Compact stars (CS) are stellar remnants of massive stars. Inside CSs the density is so high that matter is in subatomic form composed of nucleons. With an increase of density of matter toward the center of the objects, other degrees of freedom like hyperons, heavier nonstrange baryons, meson condensates may appear. Not only that, at higher densities the nucleons may get decomposed into quarks and form deconfined strange quark matter (SQM). If it is so then CSs may contain SQM in the core surrounded by nucleonic matter forming hybrid stars (HSs). However, the nature and composition of matter inside CSs can only be inferred from the astrophysical observations of these CSs. Recent astrophysical observations in terms of CS mass-radius (M-R) relation and gravitational wave (GW) observation indicate that the matter should be soft in the intermediate density range and stiff enough at higher density range to attain the maximum possible mass above 2M⊙ which is not compatible with pure hadronic equations of states (EOSs). Consequently, we study the HS properties with different models of SQM and find that within vector bag model considering density dependent bag parameter, the model goes well with the astrophysical observations so far.

Compact stars with strong magnetic fields (magnetars) have been observationally determined to have surface magnetic fields of order of 1014-1015 G, the implied internal field strength being several orders larger. We study the equation of state and composition of dense hypernuclear matter in strong magnetic fields in a range expected in the interiors of magnetars. Within the non-linear Boguta-Bodmer-Walecka model we find that the magnetic field has sizable influence on the properties of matter for central magnetic field B ≥ 1017 G, in particular the matter properties become anisotropic. Moreover, for the central fields B ≥ 1018 G, the magnetized hypernuclear matter shows instability, which is signalled by the negative sign of the derivative of the pressure parallel to the field with respect to the density, and leads to vanishing parallel pressure at the critical value Bcr ≃ 1019 G. This limits the range of admissible homogeneously distributed fields in magnetars to fields below the critical value Bcr. © 2012 Elsevier B.V..

We construct a new equation of state for the baryonic matter under an intense magnetic field within the framework of covariant density functional theory. The composition of matter includes hyperons as well as (Formula presented.) -resonances. The extension of the nucleonic functional to the hypernuclear sector is constrained by the experimental data on (Formula presented.) and (Formula presented.) -hypernuclei. We find that the equation of state stiffens with the inclusion of the magnetic field, which increases the maximum mass of neutron star compared to the non-magnetic case. In addition, the strangeness fraction in the matter is enhanced. Several observables, like the Dirac effective mass, particle abundances, etc. show typical oscillatory behavior as a function of the magnetic field and/or density which is traced back to the occupation pattern of Landau levels.

We construct equilibrium models of compact stars using a realistic equation of state and obtain the density range occupied by the proton superconductor in strong B-fields. We do so by combining the density profiles of our models with microscopic calculations of proton pairing gaps and the critical unpairing field Hc2 above which the proton type-II superconductivity is destroyed. We find that magnetars with interior homogeneous field within the range 0.1 ≤ B16 ≤ 2, where B16 = B/10¹⁶ G, are partially superconducting, whereas those with B16 > 2 are void of superconductivity. We briefly discuss the neutrino emissivity and superfluid dynamics of magnetars in the light of their (non)-superconductivity.

We describe the microphysics, phenomenology, and astrophysical implication of a B-field induced unpairing effect that may occur in magnetars, if the local B field in the core of a magnetar exceeds a critical value Hc2. Using the Ginzburg-Landau theory of superconductivity, we derive the Hc2 field for proton condensate taking into the correction (≤30%) which arises from its coupling to the background neutron condensate. The density dependence of pairing of proton condensate implies that Hc2 is maximal at the crust-core interface and decreases towards the center of the star. As a consequence, magnetar cores with homogenous constant fields will be partially superconducting for "medium-field" magnetars (1015≤B≤5×1016G) whereas "strong-field" magnetars (B>5×1016G) will be void of superconductivity. The neutrino emissivity of a magnetar's core changes in a twofold manner: (i) the B-field assisted direct Urca process is enhanced by orders of magnitude, because of the unpairing effect in regions where B≥Hc2; (ii) the Cooper-pair breaking processes on protons vanish in these regions and the overall emissivity by the pair-breaking processes is reduced by a factor of only a few.

The detection of gravitational waves (GWs) from the merger of binary neutron star (NS) events (GW170817 and GW190425) and subsequent estimations of tidal deformability play a key role in constraining the behaviour of dense matter. In addition, massive NS candidates (∼2 M) along with NICER mass-radius measurements also set sturdy constraints on the dense matter equation of state. Strict bounds from GWs and massive NS observations constrain the theoretical models of nuclear matter comportment at large density regimes. On the other hand, model parameters providing the highly dense matter response are bounded by nuclear saturation properties. This work analyses coupling parametrizations from two classes based on covariant density functional models: non-linear and density-dependent schemes. Considering these constraints together, we study possible models and parametrization schemes with the feasibility of exotic degrees of freedom in dense matter which go well with the astrophysical observations as well as the terrestrial laboratory experiments. We show that most parametrizations with non-linear schemes do not support the observations and experiments while density-dependent scheme goes well with both. Astrophysical observations are well explained if the inclusion of heavier non-strange baryons is considered as one fraction of the dense matter particle spectrum.

During very early age of neutron stars, the core cools down faster compared to the crust creating a large thermal gradient in the interior of the star. During 10−100 years, a cooling wave propagates from the core to the crust causing the interior of the star to thermalize. During this duration thermal properties of the core material is of great importance to understand the dynamics of the interior of the star. The heat capacity and thermal conductivity of the core depends on the behaviour of matter inside the core. We investigate these two properties in case of magnetars. Due to presence of large magnetic field, the proton superconductivity is quenched partially inside the magnetars depending upon the comparative values of upper critical field and the strength of the magnetic field present. This produces non-uniformity in the behaviour of matter throughout the star. Moreover, such non-uniformity arises from the variation of nature of the pairing and values of the pairing gap energy. We find that the heat capacity is substantially reduced due to the presence of superfluidity. On the other hand, the thermal conductivity of neutron is enhanced due to proton superconductivity and gets reduced due to neutron superfluidity. Hence, the variation of the thermal properties due to superfluidity in presence of magnetic field is different at different radius inside the star. However, in all the cases the maximum variation is of the order one. This affects the thermal relaxation time of the star and eventually its the thermal evolution.

In this work, we study the effects of the nuclear symmetry energy slope on the neutron star dense matter equation of state and its impact on neutron star observables (mass-radius, tidal response). We construct the equation of state within the framework of covariant density functional theory implementing coupling schemes of nonlinear and density-dependent models with viability of heavier non-nucleonic degrees of freedom. The slope of the symmetry energy parameter (Lsym) is adjusted following the density dependence of isovector meson coupling to baryons. We find that smaller values of Lsym at saturation favor early appearance of Δ resonances in comparison to hyperons, leading to latter's threshold at higher matter densities. We also investigate the dependence of Lsym on tidal deformability and the compactness parameter of a 1.4M⊙ neutron star for different equations of state and observe similar converging behavior for larger Lsym values.