Collapse Of Stars Research Articles

Black hole formation in gravitational collapse and their astrophysical implications

In this study, we have explored the process of black hole (BH) formation occurring in the collapse of a self-gravitating configuration using an innovative approach. The exact solution of the Einstein field equations is obtained in a model-independent way by considering a parametrization of the expansion scalar (Θ) in the background of spherically symmetric space-time geometry governed by the FLRW metric. Smooth matching of the interior solution with the Schwarzschild exterior metric across the boundary hypersurface of the star, together with the condition that the mass function m(t, r) is equal to Schwarzschild mass M, is used to obtain all the physical and geometrical parameters in terms of the stellar mass. The four known massive stars namely R136a3, Melnick, R136c, and R136b with their known astrophysical data (mass, radius, and present age) are used to study the physics of the model both numerically and graphically. We demonstrate that the formation of the apparent horizon occurs earlier than the singular state that is, the collapse of massive stars in our model results in the eventual formation of black holes as their final state. We have conducted an analysis indicating that the lifespans of massive stars are closely related to their respective masses. Our findings demonstrate that more massive stars exhibit considerably shorter lifespans in comparison to their lighter counterparts. Thus, the presented model corresponds to the evolutionary stages of astrophysical stellar objects and theoretically predicts their possible lifespan. We have also shown that our model satisfies the energy conditions and stability requirements via Herrera’s cracking method.

Supernovalike explosions of massive rotating stars from disks surrounding a black hole

We perform a new general-relativistic viscous-radiation hydrodynamics simulation for supernovalike explosions associated with stellar core collapse of rotating massive stars to a system of a black hole and a massive torus, paying particular attention to large-mass progenitor stars with the zero-age main-sequence mass of MZAMS=20, 35, and 45M⊙ of Aguilera-Dena [ ]. Assuming that a black hole is formed in a short timescale after the onset of the stellar collapse, the new simulations are started from initial data of a spinning black hole and infalling matter that self-consistently satisfy the constraint equations of general relativity. It is found that, with a reasonable size of the viscous parameter, the supernovalike explosion is driven by the viscous heating effect in the torus around the black hole, irrespective of the progenitor mass. The typical explosion energy and ejecta mass for the large-mass cases (MZAMS=35 and 45M⊙) are ∼1052 erg and ∼5M⊙, respectively, with Ni56 mass larger than 0.15M⊙. These are consistent with the observational data of stripped-envelope and high-energy supernovae such as broad-lined type Ic supernovae. This indicates that rotating stellar collapses of massive stars to a black hole surrounded by a massive torus can be a central engine for high-energy supernovae. By artificially varying the angular velocity of the initial data, we explore the dependence of the explosion energy and ejecta mass on the initial angular momentum and find that the large explosion energy ∼1052 erg and large Ni56 mass ≥0.15M⊙ are possible only when a large-mass compact torus with mass ≳1M⊙ is formed. Published by the American Physical Society 2024

Dark black holes in the mass gap

In the standard picture of stellar evolution, pair-instability — the energy loss in stellar cores due to electron-positron pair production — is predicted to prevent the collapse of massive stars into black holes with mass in the range between approximately 50 and 130 solar masses — a range known as the “black hole mass gap”. LIGO and Virgo detection of black hole binary mergers containing one or both black holes with masses in this mass gap thus challenges the standard picture, possibly pointing to an unexpected merger history, unanticipated or poorly understood astrophysical mechanisms, or new physics. Here, we entertain the possibility that a “dark sector” exists, consisting of dark electrons, dark protons, and electromagnetic-like interactions, but no nuclear forces. Dark stars would inevitably form given such dark sector constituents, possibly collapsing into black holes with masses within the mass gap. We study in detail the cooling processes necessary for successful stellar collapse in the dark sector and show that for suitable choices of the particle masses, we indeed predict populating the mass gap with dark sector black holes. In particular, we numerically find that the heavier of the two dark sector massive particles cannot be lighter than, approximately, the visible sector proton for the resulting dark sector black holes to have masses within the mass gap. We discuss constraints on this scenario and how to test it with future, larger black hole merger statistics.

Nonlinear Evolution of Fast Collective Neutrino Oscillations in Core-Collapse Supernovae

According to one of the most promising supernova theories, the neutrino-heating mechanism, neutrinos are responsible for transferring the energy released during the gravitational collapse of massive stars to their surroundings. If neutrino flavors are converted fast in the cores, the efficiency of neutrino heating is enhanced and can change the dynamics of the shock wave in supernovae. In this article, we investigate the dynamics of fast neutrino flavor conversions with collisions under energy-dependent treatment in detail. For the first time, we use a realistic initial condition, which is taken from the results of the self-consistent, realistic Boltzmann simulations in two spatial dimensions under axisymmetry. We report that the neutrino flavor conversion will be significantly enhanced if the energy-dependent collision term is considered in the neutrino transport equation which has been ignored in previous studies. We present the preliminary results of our investigation here, which are consistent with other studies which were conducted under simple treatments. It is believed that such findings may have an impact on the explosion mechanism of core-collapse supernovae, nucleosynthesis and neutrino astronomy.

Nonlinear Evolution of Fast Collective Neutrino Oscillations in Core-collapse Supernovae

According to one of the most promising supernova theories, the neutrino-heating mechanism, neutrinos are responsible for transferring the energy released during the gravitational collapse of massive stars to their surroundings. If neutrino flavors are converted fast in the cores, the efficiency of neutrino heating is enhanced and can change the dynamics of the shock wave in supernovae. In this article, we investigate the dynamics of fast neutrino flavor conversions with collisions under energy-dependent treatment in detail. For the first time, we use a realistic initial condition, which is taken from the results of the self-consistent, realistic Boltzmann simulations in two spatial dimensions under axisymmetry. We report that the neutrino flavor conversion will be significantly enhanced if the energy-dependent collision term is considered in the neutrino transport equation which has been ignored in previous studies. We present the preliminary results of our investigation here, which are consistent with other studies which were conducted under simple treatments. It is believed that such findings may have an impact on the explosion mechanism of core-collapse supernovae, \mbox{nucleosynthesis and neutrino astronomy.

Identifying the Physical Origin of Gamma-Ray Bursts with Supervised Machine Learning

The empirical classification of gamma-ray bursts (GRBs) into long and short GRBs based on their durations is already firmly established. This empirical classification is generally linked to the physical classification of GRBs originating from compact binary mergers and GRBs originating from massive star collapses, or Type I and II GRBs, with the majority of short GRBs belonging to Type I and the majority of long GRBs belonging to Type II. However, there is a significant overlap in the duration distributions of long and short GRBs. Furthermore, some intermingled GRBs, i.e., short-duration Type II and long-duration Type I GRBs, have been reported. A multiparameter classification scheme of GRBs is evidently needed. In this paper, we seek to build such a classification scheme with supervised machine-learning methods, chiefly XGBoost. We utilize the GRB Big Table and Greiner’s GRB catalog and divide the input features into three subgroups: prompt emission, afterglow, and host galaxy. We find that the prompt emission subgroup performs the best in distinguishing between Type I and II GRBs. We also find the most important distinguishing features in prompt emission to be T 90, the hardness ratio, and fluence. After building the machine-learning model, we apply it to the currently unclassified GRBs to predict their probabilities of being either GRB class, and we assign the most probable class of each GRB to be its possible physical class.

The formation and evolution of black holes

The concept of Black Holes was first introduced as a prediction by Eisteins Theory of General Relativity and later solved by the Schwarzschild Solution in 1915. A point of mass to great that the bend in space time as a result doesnt allow even light to escape its gravitational pull within the Event Horizon, a point of infinite density and a point at which the laws of time break down. Formation of Black Holes are of interest in many fields of not only Astrophysics but the Physics as a whole. For example it can tell us about the structure and rotation of galaxies which in turn reveals properties of Dark Matter while features such as Hawking Radiation help to further link the world of quantum mechanics to Eisteins Theory of General Relativity. Stellar Black Holes are the remains of enormous stars which has collapsed after running out of fuel for nuclear fusion to continue. These objects can be found throughout the galaxy but most importantly at the center of each galaxy lies a Supermassive Black Hole such as Sagittarius A* at the centre of the Milky Way which has a massive of 4.6 million solar masses, the process by which these Supermassive Black Holes form remain unknown till this day. Black Holes have been notoriously hard to detect due to the fact that they cannot be seen and only through the analysis of orbits of objects around it can its properties like mass be determined. Their mystical nature is exactly why Black Holes draw so much interest within the Astrophysics community today. The formation of such large celestial objects has been debated among Physicists for decades and this paper aims to review current understandings and existing research on the formation of Black Holes to provide an overview on the collapse of stars and the processes with which the core may evolve into a black hole.

Theory and prospects of the pulsar study

After the pulsar was discovered by Hewish, the relevant research has become one of the hottest research fields. Astronomers have noticed that pulsars are of great academic significance in the field of basic scientific research. Due to the large mass and small radius of pulsars, their surface gravitational field is very strong. The existence of general relativity effects cannot be ignored, making pulsars a natural laboratory for the study of strong gravitational fields. the strong magnetic field provides an identical place for the study of radio radiation process, magnetospheric particle acceleration mechanism and high energy radiation. Pulsar as the product of supernova burst after the collapse of massive stars, it is very important for the study of supernova burst theory and understand the formation mechanism of pulsars. In terms of application research, due to the high stability of its rotation period, it has significant applicating prospect in measuring time standard and X-ray pulsar navigation system. The Pulsar has made great remarkable achievements in the past 50 years since its discovery. In short, the discovery and research significance of pulsars are highly anticipated.

Assessment of a new sub-grid model for magnetohydrodynamical turbulence – II. Kelvin–Helmholtz instability

ABSTRACT The modelling of astrophysical systems such as binary neutron star mergers or the formation of magnetars from the collapse of massive stars involves the numerical evolution of magnetized fluids at extremely large Reynolds numbers. This is a major challenge for (unresolved) direct numerical simulations which may struggle to resolve highly dynamical features as, e.g. turbulence, magnetic field amplification, or the transport of angular momentum. Sub-grid models offer a means to overcome those difficulties. In a recent paper we presented MInIT, an MHD-instability-induced-turbulence mean-field, sub-grid model based on the modelling of the turbulent (Maxwell, Reynolds, and Faraday) stress tensors. While in our previous work MInIT was assessed within the framework of the magnetorotational instability, in this paper we further evaluate the model in the context of the Kelvin–Helmholtz instability (KHI). The main difference with other sub-grid models (as e.g. the alpha-viscosity model or the gradient model) is that in MInIT, we track independently the turbulent energy density at sub-grid scales, which is used, via a simple closure relation, to compute the different turbulent stresses relevant for the dynamics. The free coefficients of the model are calibrated using well-resolved box simulations of magnetic turbulence generated by the KHI. We test the model against these simulations and show that it yields order-of-magnitude accurate predictions for the evolution of the turbulent Reynolds and Maxwell stresses.

Radio Emission From a z = 10.1 Black Hole in UHZ1

The recent discovery of a 4 × 107 M ⊙ black hole (BH) in UHZ1 at z = 10.3, just 450 Myr after the Big Bang, suggests that the seeds of the first quasars may have been direct-collapse BHs from the collapse of supermassive primordial stars at z ∼ 20. This object was identified in James Webb Space Telescope NIRcam and Chandra X-ray data, but recent studies suggest that radio emission from such a BH should also be visible to the Square Kilometer Array (SKA) and the next-generation Very Large Array (ngVLA). Here, we present estimates of radio flux densities for UHZ1 from 0.1 to 10 GHz, and find that SKA and ngVLA could detect it with integration times of 10–100 hr and just 1–10 hr, respectively. It may be possible to see this object with VLA now with longer integration times. The detection of radio emission from UHZ1 would be a first test of exciting new synergies between near-infrared and radio observatories that could open the era of z ∼ 5–15 quasar astronomy in the coming decade.

Magnetorotational instability in dense electron–positron–ion plasmas

We in this manuscript analyzed the magnetorotational instability (MRI) by using a multi-component quantum fluid model with the effect of spin magnetization in a differentially rotating degenerate electron–positron–ion (e–p–i) quantum plasma. The electrons and positron having the same mass but opposite charge are taken to be degenerate whereas ions are considered as classical owing to their large inertia. The general dispersion relation is derived and a local dispersion relation for MRI is obtained by applying MHD approximations. To obtained MRI and to analyze the results numerically, reduced dispersion relation is derived using the local approximations. The obtained results are applied to the astrophysical situations exist there in the interiors of White Dwarfs and neutron stars. Contribution from spin magnetization and the number densities of electrons and positrons plays a vital role in the dynamics and can alter the instability. The increase in the electron number density, hence spin magnetization enhances the growth rate of the mode and leads the system to instability which results in the core collapse of certain massive stars.

The r-process and νp-process in CCSNe, collapsars, hypernovae and mergers, and their effect on galactic chemical evolution

In spite of many years of effort, some aspects of the origin and evolution of heavy elements in nature are yet to be understood. Here, we overview the current status of models for the formation of both r-process and νp-process elements. We summarize recent state-of the art developments of supernova and binary neutron star evolution in both r-process and νp-process nucleosynthesis. In particular, we highlight two recent recent works detailing the emerging evidence for the important role of hypernovae (energetic supernovae) and collapsars (jets from the collapse of massive stars to a black hole). These studies illuminate how such events may play a key role in the origin and early explosive nucleosynthesis and evolution of some heavy-elements.

Modeling the Central Supermassive Black Hole Mass of Quasars via the LSTM Approach

One of the fundamental questions about quasars is related to their central supermassive black holes. The reason for the existence of these black holes with such a huge mass is still unclear, and various models have been proposed to explain them. However, there is still no comprehensive explanation that is accepted by the community. The only thing we are sure of is that these black holes were not created by the collapse of giant stars or the accretion of matter around them. Moreover, another important question is related to the mass distribution of these black holes over time. Observations have shown that if we go back through redshift, we see black holes with more mass, and after passing the peak of star formation redshift, this procedure decreases. Nevertheless, the exact redshift of this peak is still controversial. In this paper, with the help of deep learning and the LSTM algorithm, we try to find a suitable model for the mass of the central black holes of quasars over time by considering both the QUOTAS and QuasarNET data sets. Our model was built with these data reported from redshift 3 to 7 and for two redshift intervals, 0–3 and 7–10, and it predicted the mass of the quasars’ central supermassive black holes. We have also tested our model for the specified intervals with observed data from central black holes and discussed the results.

Radio emission from the first quasars at z = 6–15

ABSTRACT Nearly 300 quasars have now been found at z > 6, including nine at z > 7. They are thought to form from the collapse of supermassive primordial stars to 104–105 M⊙ black holes at z ∼ 20–25, which then rapidly grow in the low-shear environments of rare, massive haloes fed by strong accretion flows. Sensitive new radio telescopes such as the Next-Generation Very Large Array (ngVLA) and the Square Kilometre Array (SKA) could probe the evolution of these objects at much earlier times. Here, we estimate radio flux from the first quasars at z ∼ 6–15 at 0.5–12.5 GHz. We find that SKA and ngVLA could detect a quasar like ULAS J1120+0641, a 1.35 × 109 M⊙ black hole at z = 7.1, at much earlier stages of evolution, z ∼ 14–16, with 100 h integration times in targeted searches. The advent of these new observatories, together with the JWST, Euclid, and the Roman Space Telescope, will inaugurate the era of z ≲ 15 quasar astronomy in the coming decade.

3D Radiation-hydrodynamic Simulations Resolving Interior of Rapidly Accreting Primordial Protostar

Direct collapse of supermassive stars is a possible pathway to form supermassive black hole seeds at high redshifts. Whereas previous three-dimensional (3D) simulations demonstrate that supermassive stars form via rapid mass accretion, those resolving the stellar interior have been limited. Here, we report 3D radiation-hydrodynamic (RHD) simulations following the evolution of rapidly accreting protostars resolving the stellar interior. We use an adaptive mesh refinement code with our newly developed RHD solver employing an explicit M1 closure method. We follow the early evolution until the stellar mass reaches ∼10 M ⊙ from two different initial configurations of spherical and turbulent clouds. We demonstrate that, in both cases, a swollen protostar whose radius is 100–1000 R ⊙ appears, as predicted by the stellar evolution calculations. Its effective temperature remains a few thousand Kelvin, and the radiative feedback by ionizing photons is too weak to disturb the accretion flow up to the epoch examined in this work. In the turbulent case, the protostar rotates rapidly at more than 0.4 times the Keplerian velocity owing to the angular momentum provided by the initial turbulence. The protostar approximates an oblate spheroid, and its equatorial radius is more than twice the polar radius. Our results suggest that we need to consider the rapid stellar rotation to elucidate the realistic 3D protostellar evolution in the supermassive star formation.

Evidence for Two Distinct Populations of Kilonova-associated Gamma-Ray Bursts

Identification of gamma-ray burst (GRB) progenitors based on the duration of their prompt emission (T 90) has faced several roadblocks recently. Long-duration GRBs (with T 90 > 2 s) have traditionally been thought to be originating from the collapse of massive stars and the short-duration ones (with T 90 < 2 s) from compact binary mergers. However, recent observations of a long GRB associated with a kilonova (KN) and a short GRB with supernova association demand a more detailed classification of the GRB population. In this Letter, we focus on GRBs associated with KNe, believed to be originating from mergers of binaries involving neutron stars (NSs). We make use of the GRB prompt-emission light curves of the Swift/BAT 2022 GRB catalog and employ machine-learning algorithms to study the classification of GRB progenitors. Our analysis reveals that there are five distinct clusters of GRBs, of which the KN-associated GRBs are located in two separate clusters, indicating they may have been produced by different progenitors. We argue that these clusters may be due to subclasses of binary neutron star and/or NS–black hole mergers. We also discuss the implications of these findings for future gravitational-wave observations and how those observations may help in understanding these clusters better.

Exceptionally bright optical emission from a rare and distant gamma-ray burst

Long $\rm \gamma$-ray bursts (GRBs) are produced by the dissipation of ultra-relativistic jets launched by newly-born black holes after the collapse of massive stars. Right after the luminous and highly variable $\gamma$-ray emission, the multi-wavelength afterglow is released by the external dissipation of the jet in circumburst medium. We report the discovery of a very bright ($\rm \sim 10$ mag) optical emission $\rm \sim 28$ s after the explosion of the extremely luminous and energetic GRB 210619B located at redshift 1.937. Early multi-filter observations allowed us to witness the end of the shock wave propagation into the GRB ejecta. We observed the spectral transition from a bright reverse to the forward shock emission, demonstrating that the early and late GRB multi-wavelength emission is originated from a very narrow jet propagating into an unusually rarefied interstellar medium. We also find evidence of an additional component of radiation, coming from the jet wings which is able explain the uncorrelated optical/X-ray emission.

Probing Diversity of Type II Supernovae with the Chinese Space Station Telescope

Type II supernovae (SNe II), which show abundant hydrogen in their spectra, belong to a class of SNe with diverse observed properties. It is commonly accepted that SNe II are produced by core collapse and explosion of massive stars. However, the large photometric and spectroscopic diversity of SNe II and the mechanisms responsible for this diversity are not thoroughly understood. In this review, we first briefly introduce the optical characteristics and possible progenitors of each subtype of SNe II. We then highlight the role of the Chinese Space Station Telescope in future SN studies. With a deep limiting magnitude, the main survey project could detect SN IIP-like objects as distant as z∼1.2 and obtain UV-optical follow-up for peculiar transients, especially those long-lived events. With a high resolution and a large field of view, the main survey camera is powerful in linking a nearby SN with its progenitor, while the integral field spectrograph is powerful in revealing the SN environment. All this information has the potential to help enrich our understanding of supernova physics.

Production of PBHs from inflaton structures

At times prior to Big Bang Nucleosynthesis, the universe could show a primordial structure formation period if dominated by a fast oscillating inflaton field during reheating. In this context, we have postulated a new mechanism of primordial black hole formation [L. E. Padilla, J. C. Hidalgo, and K. A. Malik, Phys. Rev. D, vol. 106, p. 023519, Jul 2022], that draws the analogy between an extended reheating era and the scalar field dark matter model, stipulating the gravitational collapse of inflaton halos and inflaton stars. In this paper we look at the requirements for the realization of this new mechanism. We show that a generic primordial power spectrum with a peak at small scales is most suitable for the production of a considerable number of PBHs. When such requirement is met, and if reheating lasts long enough, large populations of PBHs with $M_{\rm PBH}\sim 1~\mathrm{gram}$ may be produced. We find in particular, that the mass fraction of PBHs is orders of magnitude larger than that obtained when PBHs form via direct collapse in a universe dominated by radiation or pressure-less dust. Looking at observable implications of our findings, we explore the possibility that the PBHs component may dominate the energy density of the universe at some point after the end of reheating.

Probing the global dust properties and cluster formation potential of the giant molecular cloud G148.24+00.41

ABSTRACT Clouds more massive than about 105 M⊙ are potential sites of massive cluster formation. Studying the properties of such clouds in the early stages of their evolution offers an opportunity to test various cluster formation processes. We make use of CO, Herschel, and UKIDSS observations to study one such cloud, G148.24+00.41. Our results show the cloud to be of high mass ($\sim \, 1.1\times 10^5$ M⊙), low dust temperature (∼ 14.5 K), nearly circular (projected radius ∼ 26 pc), and gravitationally bound with a dense gas fraction of ∼18 per cent and a density profile with a power-law index of ∼−1.5. Comparing its properties with those of nearby molecular clouds, we find that G148.24+00.41 is comparable to the Orion-A molecular cloud in terms of mass, size, and dense gas fraction. From our analyses, we find that the central area of the cloud is actively forming protostars and is moderately fractal with a Q-value of ∼ 0.66. We also find evidence of global mass-segregation with a degree of mass-segregation (ΛMSR) ≈ 3.2. We discuss these results along with the structure and compactness of the cloud, the spatial and temporal distribution of embedded stellar population and their correlation with the cold dust distribution, in the context of high-mass cluster formation. We compare our results with models of star cluster formation, and conclude that the cloud has the potential to form a cluster in the mass range ∼ 2000–3000 M⊙ through dynamical hierarchical collapse and assembly of both gas and stars.