Dynamics and Merger Rate of Primordial Black Holes in a Cluster

The PBH clusters can be sources of gravitational waves, and the merger rate depends on the spatial distribution of PBHs in the cluster which changes over time. It is well known that gravitational collisional systems experience the core collapse that leads to significant increase of the central density and shrinking of the core. After core collapse, the cluster expands almost self-similarly (i.e., density profile extends in size without changing its shape). These dynamic processes affect the merger rate of PBHs. In this paper, the dynamics of the PBH cluster is considered using the Fokker–Planck equation. We calculate the merger rate of PBHs on cosmic time scales and show that its time dependence has a unique signature. Namely, it grows by about an order of magnitude at the moment of core collapse which depends on the characteristics of the cluster, and then decreases according to the dependence R∝t−1.48. It was obtained for monochromatic and power-law PBH mass distributions with some fixed parameters. Obtained results can be used to test the model of the PBH clusters via observation of gravitational waves at high redshift.

Stars on the Verge: Analyses of the Complex Light Variations of the Hyper-luminous Red Supergiant VY Canis Majoris: On the Nature of the Star’s “Great Dimming” Episodes

VY Canis Majoris (VY CMa) is one of the largest, most luminous red supergiant (RSG) stars in the Galaxy. This huge oxygen-rich RSG may be on the verge of becoming a bright core-collapse Type-II supernova. Over the 20th century, VY CMa has been undergoing episodic large quasi-periodic light variations. It has experienced two “great dimming” episodes during ∼1920–1950 and ∼1985–1995. During these, the star underwent large (1–2.5 mag) quasi-periodic light variations with dominant periods ranging between ∼1415 and 1565 days. In addition to experiencing large quasi-periodic light variations, VY CMa became fainter by 1.0–1.5 mag during the 1920–1950 episode. There is evidence that the star’s brightness decreased from ∼6.5–7.0 to ∼8.0–8.5 mag from 1850 to after 1900. These long-term dimming events may arise from dust formation from previously expelled gas. The light variations and the “dimmings” of VY CMa far surpass the recent “great dimming” of Betelgeuse.

Imprints of the jittering jets explosion mechanism in the morphology of the supernova remnant SNR~0540-69.3

I identify a point-symmetric structure in recently published VLT/MUSE velocity maps of different elements in a plane along the line of sight at the center of the supernova remnant SNR~0540-69.3, and argue that jittering jets that exploded this core collapse supernova shaped this point-symmetric structure. The four pairs of two opposite clumps that compose this point symmetric structure suggest that two to four pairs of jittering jets shaped the inner ejecta in this plane. In addition, intensity images of several spectral lines reveal a faint strip (the main jet-axis) that is part of this plane of jittering jets and its similarity to morphological features in a few other SNRs and in some planetary nebulae further suggests shaping by jets. My interpretation implies that in addition to instabilities, jets also mix elements in the ejecta of core collapse supernovae. Based on the point-symmetric structure and under the assumption that jittering jets exploded this supernova, I estimate the component of the neutron star natal kick velocity on the plane of the sky to be $\simeq 235 \km\s^{-1}$, and at an angle of $\simeq 47^\circ$ to the direction of the main jet-axis. I analyse this natal kick direction together with other 12 SNRs in the frame of the jittering jets explosion mechanism.

Radiative diffusion in a time-dependent outflow: a model for fast blue optical transients

Fast Blue Optical Transients (FBOTs) are luminous transients with fast evolving (typically trise < 12 days) light curve and blue color (usually−0.2 > g−r > −0.3)that cannot be explained by a supernova-like explosion. We propose a radiative diffusion in a time-dependent outflow model to interpret such special transients. In this model, we assume a stellar-mass black hole is formed from stellar core-collapse. As a central engine, the black hole accretes the infalling stellar envelope material via an accretion disk. Due to the extremely super- Eddington accretion rate, the disk ejects continuous outflow during a few days. We consider the ejection of the outflow to be time-dependent. The outflow is optically thick initially and photons are frozen in it. As the outflow expands over time, photons gradually escape, and our work is to model such an evolution. Numerical and analytical calculations are considered separately, and the results are consistent. We apply the model to three typical FBOTs: PS1-10bjp, ZTF18abukavn, and ATLAS19dqr. The modeling finds the total mass of the outflow (∼ 1M⊙), and the total time of the ejection (∼ a few days) for them, leading us to speculate that they may be the result of the collapse of massive stars.

Core-collapse Supernova Simulations and the Formation of Neutron Stars, Hybrid Stars, and Black Holes

We investigate observable signatures of a first-order quantum chromodynamics (QCD) phase transition in the context of core-collapse supernovae. To this end, we conduct axially symmetric numerical relativity simulations with multi-energy neutrino transport, using a hadron–quark hybrid equation of state (EOS). We consider four nonrotating progenitor models, whose masses range from 9.6 to 70 M ⊙. We find that the two less-massive progenitor stars (9.6 and 11.2 M ⊙) show a successful explosion, which is driven by the neutrino heating. They do not undergo the QCD phase transition and leave behind a neutron star. As for the more massive progenitor stars (50 and 70 M ⊙), the proto-neutron star (PNS) core enters the phase transition region and experiences the second collapse. Because of a sudden stiffening of the EOS entering to the pure quark matter regime, a strong shock wave is formed and blows off the PNS envelope in the 50 M ⊙ model. Consequently the remnant becomes a quark core surrounded by hadronic matter, leading to the formation of the hybrid star. However, for the 70 M ⊙ model, the shock wave cannot overcome the continuous mass accretion and it readily becomes a black hole. We find that the neutrino and gravitational wave (GW) signals from supernova explosions driven by the hadron–quark phase transition are detectable for the present generation of neutrino and GW detectors. Furthermore, the analysis of the GW detector response reveals unique kHz signatures, which will allow us to distinguish this class of supernova explosions from failed and neutrino-driven explosions.

Probing neutrino decay scenarios by using the Earth matter effects on supernova neutrinos

The observation of Earth matter effects in the spectrum of neutrinos coming from a next galactic core-collapse supernova (CCSN) could, in principle, reveal if neutrino mass ordering is normal or inverted. One of the possible ways to identify the mass ordering is through the observation of the modulations that appear in the spectrum when neutrinos travel through the Earth before they arrive at the detector. These features in the neutrino spectrum depend on two factors, the average neutrino energies, and the difference between the primary neutrino fluxes of electron and other flavors produced inside the supernova. However, recent studies indicate that the Earth matter effect for CCSN neutrinos is expected to be rather small and difficult to be observed by currently operating or planned neutrino detectors mainly because of the similarity of average energies and fluxes between electron and other flavors of neutrinos, unless the distance to CCSN is significantly smaller than the typically expected one, ∼ 10 kpc. Here, we are looking towards the possibility if the non-standard neutrino properties such as decay of neutrinos can enhance the Earth matter effect. In this work we show that invisible neutrino decay can potentially enhance significantly the Earth matter effect for both νe and ν̅e channels at the same time for both mass orderings, even if the neutrino spectra between electron and other flavors of neutrinos are very similar, which is a different feature not expected for CCSN neutrinos with standard oscillation without the decay effect.

Impact of Hypernova νp-process Nucleosynthesis on the Galactic Chemical Evolution of Mo and Ru

We calculate the Galactic Chemical Evolution of Mo and Ru by taking into account the contribution from ν p-process nucleosynthesis. We estimate yields of p-nuclei such as 92,94Mo and 96,98Ru through the ν p-process in various supernova progenitors based upon recent models. In particular, the ν p-process in energetic hypernovae produces a large amount of p-nuclei compared to the yield in ordinary core-collapse SNe. Because of this, the abundances of 92,94Mo and 96,98Ru in the Galaxy are significantly enhanced at [Fe/H] = 0 by the ν p-process. We find that the ν p-process in hypernovae is the main contributor to the elemental abundance of 92Mo at low metallicity [Fe/H] < −2. Our theoretical prediction of the elemental abundances in metal-poor stars becomes more consistent with observational data when the ν p-process in hypernovae is taken into account.

The Three-dimensional Collapse of a Rapidly Rotating 16 M ⊙ Star

I report on the three-dimensional (3D) hydrodynamic evolution of a rapidly rotating 16 M ⊙ star to iron core collapse. For the first time, I follow the 3D evolution of the angular momentum (AM) distribution in the iron core and convective shell burning regions for the final 10 minutes up to and including gravitational instability and core collapse. In 3D, convective regions show efficient AM transport that leads to an AM profile that differs in shape and magnitude from MESA within a few shell convective turnover timescales. For different progenitor models, such as those with tightly coupled Si/O convective shells, efficient AM transport in 3D simulations could lead to a significantly different AM distribution in the stellar interior affecting estimates of the natal neutron star or black hole spin. The results suggest that 3D AM transport in convective and rotating shell burning regions are critical components in models of massive stars and could qualitatively alter the explosion outcome and inferred compact remnant properties.

Neutrino beam driven instability of magnetosonic waves in the presence of obliquemagnetic field and ion-neutral collisional effect in plasmas

The neutrino beam driven instability of fast and slow magnetosonic waves with oblique applied magnetic field in multi-component ion, electron, and neutrino beam plasma is studied. The dissipation effects of ion-neutral collisions are also included in the model. The neutrino and electron interactions through electro-weak force are included. It is found that the dissipation of ion collisions has significant effect on the phase velocity of the wave propagation and growth rate of the neutrino beam driven instability. The analytical expression of the growth rate of the fast and slow magnetosonic waves instability is found under the weak neutrino beam approximation and in the absence of ions and neutrals (atoms) collision effect. The numerical illustration of growth rates of the fast and slow magnetosonic waves are also presented with variations of magnetic field angle, neutrino beam energy, neutrino beam density, magnetic field intensity. It is found that the growth rate of the fast magnetosonic wave is maximum in case of the perpendicular directed magnetic field to the direction of wave propagation, while growth rate of slow magnetosonic wave is minimum in that case. It is also noticed that growth rate of fast magnetosonic wave comes out to be larger (of the order tens) than the slow magnetosonic wave case, which is quite different from earlier published results of Type II core-collapse supernova.