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 $\mathrm{\Delta }$-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 $\mathrm{\Lambda }$ and ${\mathrm{\Xi }}^{-}$ hypernuclei data. The $\mathrm{\Delta }$-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 ${\overline{K}}^0$ condensations occur through a second-order phase transition, which implies no mixed-phase formation. For large values of (anti)kaon and $\mathrm{\Delta }$-resonance potentials in symmetric nuclear matter, we observe that condensation leads to an extinction of ${\mathrm{\Xi }}^{-,0}$ hyperons. We also investigate the influence of inclusion of additional hidden-strangeness ${\sigma }^{*}$ 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.

Recent measurements of neutron star mass from several candidates (PSR $\mathrm{J}1614-2230$, PSR $\mathrm{J}0348+0432$, MSP $\mathrm{J}0740+6620$) set the lower bound on the maximum possible mass for this class of compact objects $\sim 2{\mathrm{M}}_{\odot }$. Existence of stars with high mass brings the possibility of existence of exotic matter (hyperons, meson condensates) at the core region of the objects. In this work, we investigate the (anti)kaon ($K^{-}$, ${\overline{K}}^0$) condensation in $\beta$-equilibrated nuclear matter within the framework of covariant density functional theory. The functionals in the kaonic sector are constrained by the experimental studies on $K^{-}$ atomic, kaon-nucleon scattering data fits. We find that the equation of state softens with the inclusion of (anti)kaon condensates, which lowers the maximum mass of neutron star. In one of the density-independent coupling cases, the $K^{-}$ condensation is through a first order phase transition type, which produces a $2{\mathrm{M}}_{\odot }$ neutron star. The first order phase transition results in mixed phase region in the inner core of the stars. While ${\overline{K}}^0$ condensation appears via second-order phase transition for all the models we consider here.

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 $\mathsf{\Delta }$-resonances. The extension of the nucleonic functional to the hypernuclear sector is constrained by the experimental data on $\mathsf{\Lambda }$ and $\mathsf{\Xi }$-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.

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 $2M_{\odot }$ 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.

We investigate the properties of stars participating in double compact star merger events considering interacting model of stable strange quark matter. We model the matter making it compatible with the recent astrophysical observations of compact star mass-radius and gravitational wave events. In this context we consider modified MIT bag model and vector bag model with and without self interaction. We find new upper bound on tidal deformability of 1.4 M_⊙ strange star corresponding to the upper bound of effective tidal deformability inferred from gravitational wave event. Range of compactness of 1.4 M_⊙ strange star is obtained as 0.175 ≤ C_1.4 ≤ 0.199. Radius range of 1.5 M_⊙ primary star is deduced to be 10.57 ≤ R_1.5 ≤ 12.04 km, following stringent GW170817 constraints. GW190425 constraints provide with upper limit on radius of 1.7 solar mass strange star that it should be less than 13.41 km.

The nature of the highly dense matter inside the supernova remnant compact star is not constrained by terrestrial experiments and hence modeled phenomenologically to accommodate the astrophysical observations from compact stars. The observable properties of the compact stars are highly sensitive to the microscopic model of highly dense matter. However, some universal relations exist between some macroscopic properties of compact stars independent of the matter model. We study the universal relation including the stars containing exotic degrees of freedom such as heavier strange and non-strange baryons, strange quark matter in normal and superconducting phases, etc. We examine the universal relations for quantities moment of inertia - tidal love number - quadrupole moment. We also study the correlation of non-radial f-mode and p-mode frequencies with stellar properties. We find the f-mode frequency observes the universal relation with dimensionless tidal deformability but the p-mode frequency does not show a good correlation with stellar properties. The p-mode frequency is sensitive to the composition of the matter. We find that universal relation is also applicable for stars with exotic matter in the core of the star with several models of exotic matter.