In this work, we investigate the tidal deformability of a neutron star admixed with dark matter, modeled as a massive, self-interacting, complex scalar field. We derive the equations to compute the tidal deformability of the full Einstein-Hilbert-Klein-Gordon system self-consistently, and we probe the influence of the scalar field mass and self-interaction strength on the total mass and tidal properties of the combined system. We find that dark matter corelike configurations lead to more compact objects with smaller tidal deformability, and dark matter cloudlike configurations lead to larger tidal deformability. Electromagnetic observations of certain cloudlike configurations would appear to violate the Buchdahl limit. The self-interaction strength is found to have a significant effect on both mass and tidal deformability. We discuss observational constraints and the connection to anomalous detections. We also investigate how this model compares to those with an effective bosonic equation of state and find the interaction strength where they converge sufficiently.

Dark matter could accumulate around neutron stars in sufficient amounts to affect their global properties. In this work, we study the effect of a specific model for dark matter—a massive and self-interacting vector (spin-1) field—on neutron stars. We describe the combined systems of neutron stars and vector dark matter using Einstein–Proca theory coupled to a nuclear matter term and find scaling relations between the field and metric components in the equations of motion. We construct equilibrium solutions of the combined systems, compute their masses and radii, and also analyze their stability and higher modes. The combined systems admit dark matter (DM) core and cloud solutions. Core solutions compactify the neutron star component and tend to decrease the total mass of the combined system. Cloud solutions have the inverse effect. Electromagnetic observations of certain cloud-like configurations would appear to violate the Buchdahl limit. This could make Buchdahl-limit-violating objects smoking gun signals for dark matter in neutron stars. The self-interaction strength is found to significantly affect both mass and radius. We also compare fermion Proca stars to objects where the dark matter is modeled using a complex scalar field. We find that fermion Proca stars tend to be more massive and geometrically larger than their scalar field counterparts for equal boson masses and self-interaction strengths. Both systems can produce degenerate masses and radii for different amounts of DM and DM particle masses.

We perform new general relativistic hydrodynamics simulations for collapses of rotating supermassive star cores with an approximate nuclear burning up to carbon and a detailed equation of state. For all the models we investigate, the energy generation by nuclear burning plays only a minor role, leading to the formation of a black hole without a nuclear-powered explosion. For rotating models, however, the stellar explosion associated with shock heating is driven from a torus, which forms after the black hole formation. The explosion energy is up to 10⁻⁴ of the mass energy of the supermassive star cores (∼10⁵⁵–10⁵⁶ erg). We find that, even if we increase the rotational angular momentum of the progenitor, the ejecta mass saturates at ∼1% of the total mass of the initial stellar core. The average ejecta velocity also saturates at ≈20% of the speed of light. As a result, the ejecta kinetic energy is approximately proportional to the initial mass of the supermassive star core for the rapidly rotating case. We also perform viscous hydrodynamics simulations to explore the evolution of the remnant torus. Although the viscous heating drives an outflow from the torus, we find that its effect is subdominant in terms of the kinetic energy because of the small velocity (≈0.07c) of the ejecta component.

Einstein-Cartan gravity is a close historical sibling of general relativity that allows for spacetime torsion. As a result, angular momentum couples to spacetime geometry in a similar way to energy. While consequences of this are well studied on cosmological scales, their role in neutron star physics is largely underexplored. We study the effects that torsion, sourced by either microphysical spin or macroscopic angular momentum, has on neutron stars. For this, we use a simplified polytropic model to quantify the microphysical coupling to torsion. We also derive expressions to model rotation-induced torsion effects and estimate the consequences for rotating neutron stars with different rotation rates. We find that the presence of torsion in general leads to neutron stars with smaller radii and masses but higher central densities. Realistic models for microphysical spin lead to torsion effects that have no relevant influence on the neutron star structure. Rotation-induced torsion effects, however, can decrease the radius by up to 900 m, which is comparable to the increase due to centrifugal forces. Depending on which effect dominates, this leads to a torsion-induced spin-up or spin-down of the neutron star. We conclude that torsion effects due to rotation can not be neglected and are large enough to be tested using current or near-future technology.

The dependence of the final fate of supermassive star (SMS) cores on their mass and angular momentum is studied with simple modeling. SMS cores in the hydrogen-burning phase encounter the general-relativistic instability during stellar evolution if the mass is larger than ∼3 × 10⁴ M _⊙. Spherical SMS cores in the helium-burning phase encounter the general-relativistic instability prior to the onset of the electron–positron pair instability if the mass is larger than ∼1 × 10⁴ M _⊙. For rapidly rotating SMS cores, these values for the threshold mass are enhanced by up to a factor of ∼5, and thus, for SMSs with mass smaller than ∼10⁴ M _⊙, the collapse is triggered by the pair instability, irrespective of the rotation. After the onset of the general-relativistic instability, SMS cores in the hydrogen-burning phase with reasonable metallicity are likely to collapse to a black hole irrespective of the degree of rotation, whereas SMS cores in the helium-burning phase could explode via nuclear burning with no black hole formation, as previous works demonstrate.

Recently discovered supermassive black holes with masses of $\sim 10^8\, \mathrm{ M}_\odot$ at redshifts $z\sim 9$–11 in active galactic nuclei (AGNs) pose severe challenges to our understanding of supermassive black hole formation. One proposed channel are rapidly accreting supermassive PopIII stars (SMSs) that form in large primordial gas haloes and grow up to $< 10^6\, {\rm {M}}_\odot$. They eventually collapse due to the general relativistic instability and could lead to supernova-like explosions. This releases massive and energetic ejecta that then interact with the halo medium via an optically thick shock. We develop a semi-analytic model to compute the shock properties, bolometric luminosity, emission spectrum, and photometry over time. The initial data are informed by stellar evolution and general relativistic SMS collapse simulations. We find that SMS explosion light curves reach a brightness $\sim 10^{45\mathrm{-}47}\, \mathrm{erg\,s^{-1}}$ and last 10–200 yr in the source frame – up to 250–3000 yr with cosmic time dilation. This makes them quasi-persistent sources which vary indistinguishably to little red dots and AGN within 0.5–$9\, (1+z)$ yr. Bright SMS explosions are observable in long-wavelength JWST (James Webb Space Telescope) filters up to $z\le 20$ (24–26 mag) and pulsating SMSs up to $z\le 15$. EUCLID and the Roman Space Telescope (RST) can detect SMS explosions at $z< 11$–12. Their deep fields could constrain the SMS rate down to $10^{-11}$Mpc$^{-3}$yr$^{-1}$, which is much deeper than JWST bounds. Based on cosmological simulations and observed star formation rates, we expect to image up to several hundred SMS explosions with EUCLID and dozens with RST deep fields.