Collisions Of Stars Research Articles

Black hole mergers in compact star clusters and massive black hole formation beyond the mass gap

ABSTRACT We present direct N-body simulations, carried out with nbody6+ + gpu, of young and compact low-metallicity (Z = 0.0002) star clusters with 1.1 × 105 stars, a velocity dispersion of ∼15 $\mathrm{km\, s^{-1}}$, a half-mass radius Rh = 0.6 pc, and a binary fraction of $10{{\ \rm per\,cent}}$ including updated evolution models for stellar winds and (pulsation) pair-instability supernovae (PSNe). Within the first tens of megayears, each cluster hosts several black hole (BH) merger events which nearly cover the complete mass range of primary and secondary BH masses for current LIGO-Virgo-KAGRA gravitational wave detections. The importance of gravitational recoil is estimated statistically during post-processing analysis. We present possible formation paths of massive BHs above the assumed lower PSN mass-gap limit ($45\, {\rm M}_\odot$) into the intermediate-mass black hole (IMBH) regime ($\gt 100\, {\rm M}_\odot$) which include collisions of stars, BHs, and the direct collapse of stellar merger remnants with low core masses. The stellar evolution updates result in the early formation of heavier stellar BHs compared to the previous model. The resulting higher collision rates with massive stars support the rapid formation of massive BHs. For models assuming a high accretion efficiency for star–BH mergers, we present a first-generation formation scenario for GW190521-like events: a merger of two BHs which reached the PSN mass-gap merging with massive stars. This event is independent of gravitational recoil and therefore conceivable in dense stellar systems with low escape velocities. One simulated cluster even forms an IMBH binary (153, 173 M⊙) which is expected to merge within a Hubble time.

Measurements of electrons from heavy-flavor hadron decays in 27, 54.4, and 200 GeV Au+Au collisions in STAR

In these proceedings, we present the measurements of nuclear modification factors for electrons from open charm (c→e) and bottom hadron decays (b→e) at mid-rapidity region in Au+Au collisions at √SNN = 200 GeV. The significance of the difference in nuclear modification factors between b→e and c→e is ≥ 3σ. We also present the measurements of the elliptic flow (v2) of inclusive heavy-flavor hadron decayed electrons (HFE) in Au+Au collisions at √SNN = 27 and 54.4 GeV. The results show non-zero HFE v2 in 54.4 GeV Au+Au collisions and a hint of smaller HFE v2 in 27 GeV Au+Au collisions than those at 54.4 and 200 GeV.

Diluted axion star collisions with neutron stars

Diluted axion star, a self-gravitating object with the quantum pressure balancing gravity, has been predicted in many models with a QCD axion or axion-like particle. It can be formed in the early universe and composes a sizable fraction of dark matter. One could detect the transient radio signals when it passes by a magnetar with the axion particle converted into photon in the magnetic field. Using both numerical and semi-analytic approaches, we simulate the axion star's dynamic evolution and estimate the fraction of axion particles that can have a resonance conversion during such a collision event. We have found that both self-gravity and quantum pressure are not important after the diluted axion star enters the Roche radius. A free-fall approximate can capture individual particle trajectories very well. With some optimistic cosmological and astrophysical assumptions, the QCD axion parameter space can be probed from detecting such a collision event by radio telescopes.

Laser frequency noise in next generation gravitational-wave detectors

Future ground-based gravitational-wave detectors are slated to detect black hole and neutron star collisions from the entire stellar history of the universe. To achieve the designed detector sensitivities, frequency noise from the laser source must be reduced below the level achieved in current Advanced LIGO detectors. This paper reviews the laser frequency noise suppression scheme in Advanced LIGO, and quantifies the noise coupling to the gravitational-wave readout. The laser frequency noise incident on the current Advanced LIGO detectors is 8 × 10 − 5 H z / H z at 1~kHz. Future detectors will require even lower incident frequency noise levels to ensure this technical noise source does not limit sensitivity. The frequency noise requirement for a gravitational wave detector with arm lengths of 40~km is estimated to be 7 × 10 − 7 H z / H z . To reach this goal a new frequency noise suppression scheme is proposed, utilizing two input mode cleaner cavities, and the limits of this scheme are explored. Using this scheme the frequency noise requirement is met, even in pessimistic noise coupling scenarios.

Search for nuclearites with the KM3NeT detector

Strange quark matter (SQM) is a hypothetical type of matter composed of almost equal quantities of up, down and strange quarks. Edward Witten presented the SQM as a denser and more stable matter that could represent the ground state of Quantum Chromodynamics (QCD). Massive SQM particles are commonly called nuclearites. These particles could have been produced in violent astrophysical processes, such as neutron star collisions and could be present in the cosmic radiation. Nuclearites with masses greater than 1013 GeV/c2 and velocities of about 250 km/s (typical galactic velocities) could reach the Earth and interact with atoms and molecules of sea water within the sensitive volume of the deep-sea neutrino telescopes. The SQM particles can be detected with the KM3NeT telescope (whose first lines are already installed and taking data in the Mediterranean Sea) through the visible blackbody radiation generated along their path inside or near the instrumented area. In this work the results of a study using Monte Carlo simulation of down-going nuclearites are discussed.

Spectral-temporal features of repeating and one-off fast radio bursts and axion star

The fast radio bursts ( FRBs ) are energetic radio bursts with millisecond duration only observed at radio frequencies. The generation mechanism is still mysterious. We have proposed a generation mechanism of both repeating and one-off FRBs. They arise from the axion star collision with neutron star or magnetized accretion disk of galactic black hole. Once we accept the existence of the axions, we find that the mechanism well explain previously observed spectral-temporal features. In this paper we show that it also explains recently observed phenomena such as downward drifting in the repeating FRBs, etc.. Analysis of the downward drifting based on Doppler effects has been presented in recent papers, in which a superradiance system of molecular or atom has been proposed as a source of FRBs. We apply the analysis to our mechanism and find that it well explains the relation between the downward drifting rate and the duration of the repeating FRBs. The Doppler effects lead to the fact that the duration of radio burst with higher center frequency is shorter than that of radio burst with lower center frequency in the repeating FRBs. Our generation mechanism naturally explain polarization angle swing observed in the repeating FRB180301 and one-off FRBs. We also discuss the association between the FRB200428 and magnetar SGR J1935+2154. The X ray burst observed just after the observation of the FRB could be triggered by the axion star collision with the magnetar. We also explain the consistency of our generation mechanism with observed spectral-temporal differences in the repeating and one-off FRBs, e.g. longer duration ( smaller flux density ) of repeating FRBs than duration ( flux density ) of one-off FRBs.

From cosmic matter to the laboratory

AbstractThe recent discovery of binary neutron star mergers has opened a new and exciting venue of research into hot and dense strongly interacting matter. For the first time, this elusive state of matter, described by the theory of quantum chromo dynamics, can be studied in two very different environments. On the macroscopic scale, in the collisions of neutron stars; and on the microscopic scale, in collisions of heavy ions at particle collider facilities. We will discuss the conditions that are created in these mergers and the corresponding high energy nuclear collisions. This includes the properties of quantum chromo dynamics matter, that is, the expected equation of state as well as expected chemical and thermodynamic properties of this exotic matter. To explore this matter in the laboratory, a new research prospect is available at the Facility for Antiproton and Ion Research, FAIR. The new facility is being constructed adjacent to the existing accelerator complex of the GSI Helmholtz Centre for Heavy Ion Research at Darmstadt/Germany, expanding the research goals and technical possibilities substantially. The worldwide unique accelerator and experimental facilities of FAIR will open the way for a broad spectrum of unprecedented research supplying a variety of experiments in hadron, nuclear, atomic, and plasma physics as well as biomedical and material science, which will be briefly described.

Neutron star collisions and gravitational waves

AbstractThe long‐awaited detection of a gravitational wave from the merger of a binary neutron star in August 2017 (GW170817) marked the beginning of the new field of multi‐messenger gravitational wave astronomy. By exploiting the extracted tidal deformations of the two neutron stars from the late inspiral phase of GW170817, it was possible to constrain several global properties of the equation of state of neutron star matter. By means of fully general‐relativistic hydrodynamic simulations, it is possible to get an insight into the hydrodynamic evolution of matter and into the structure of the space–time deformation caused by the remnant of binary neutron star merger. Neutron star mergers represent an optimal astrophysical laboratory to investigate the phase transition from confined hadronic matter to deconfined quark matter. With future gravitational wave detectors, it will most likely be possible in the near future to investigate the hadron‐quark phase transition by analyzing the spectrum of the post‐merger gravitational wave of the differentially rotating hypermassive hybrid star. In contrast to hypermassive neutron stars, these highly differentially rotating objects contain deconfined strange quark matter in their slowly rotating inner region.

White dwarf–main sequence star collisions from wide triples in the field

ABSTRACT Multiple star systems interact strongly with galactic field stars when the outer semi-major axis of a triple or multiple star is >103 AU. Stable triples composed of two white-dwarfs (WD) and a low-mass main sequence (MS) star in a wide outer orbit can thus be destabilized by gravitational interactions with random field stars. Such interactions excite the eccentricity of the distant third star sufficiently so that it begins to interact significantly with the inner binary. When this occurs, the triple undergoes multiple binary-single resonant encounters. These encounters may result either in a collision between the non-degenerate component and a WD, or the breakup of the triple into a compact binary and a third object which is ejected. The compact binary can be either a MS–WD pair which survives, or collides or a double WD (DWD), which may inspiral through gravitational wave emission. We calculate the collision rate between a MS and WD star, and the merger rate of DWDs. Additionally, we describe the prospects of detectability of such a collision, which may resemble a sub-luminous supernovae event.

Holographic quark matter with color superconductivity and a stiff equation of state for compact stars

We present a holographic model of QCD with a first order chiral restoration phase transition with chemical potential, mu. The first order behaviour follows from allowing a discontinuity in the dual description as the quarks are integrated out below their constituent mass. The model predicts a deconfined yet massive quark phase at intermediate densities (350 MeV< mu <500 MeV), above the nuclear density phase, which has a very stiff equation of state and a speed of sound close to one. We also include a holographic description of a colour superconducting condensate in the chirally restored vacuum and study the resulting equation of state. They provides a well behaved first order transition from the deconfined massive quark phase at very high density (mu>500 MeV). We solve the Tolman-Oppenheimer-Volkoff equations with the resulting equations of state and find stable hybrid stars with quark cores. We compute the tidal deformability for these hybrid stars and show they are consistent with LIGO/Virgo data on a neutron star collision. Our holographic model shows that quark matter could be present at the core of such compact stars.

Tuning Advanced LIGO to kilohertz signals from neutron-star collisions

Gravitational waves produced at kilohertz frequencies in the aftermath of a neutron star collision can shed light on the behavior of matter at extreme temperatures and densities that are inaccessible to laboratory experiments. Gravitational-wave interferometers are limited by quantum noise at these frequencies but can be tuned via their optical configuration to maximize the probability of post-merger signal detection. We compare two such tuning strategies to turn Advanced LIGO into a post-merger-focused instrument: first, a wideband tuning that enhances the instrument's signal-to-noise ratio 40--80\% broadly above \SI{1}{\kHz} relative to the baseline, with a modest sensitivity penalty at lower frequencies; second, a "detuned" configuration that provides even more enhancement than the wideband tuning, but over only a narrow frequency band and at the expense of substantially worse quantum noise performance elsewhere. With an optimistic accounting for instrument loss and uncertainty in post-merger parameters, the detuned instrument has a ${\lesssim}40\%$ sensitivity improvement compared to the wideband instrument.

Enhanced Fusion Mechanisms Towards Synthesizing Superheavy Elements

In nature all of the heavy elements are produced by nuclear fusion reactions, mostly in supernova explosions and neutron star collisions, so, this is to date the only known and proven mechanism to produce heavy elements in usable quantities. In this work we approach a difficult challenge, namely, the possibility of fusion of heavy elements, taking as a test case the heaviest observationally stable element - ²³⁸U, showing that it is feasible, at least in principle with the help of existing technologies. The main idea behind is to show that fusion of lighter - than z=184 - nuclei is conceptually viable examining the tunnel effect assisted by an auxiliary field that will produce a Sauter like effect, and this is the pathway to explore the synthesis of elements higher than z=118. The production of theoretical untested elements like Unoctquadium-184 or close Z species could open a new chapter in the physics of super-heavy elements, and leads to a deeper understanding of nuclear decay channels and stability conditions. Nuclear fusion of heavy elements will open the breach to produce neutron rich elements, so we may obtain a deep insight into the physics of the island of stability. This work will review basic aspects of fusion physics related to the assisted fusion mechanism. An enhanced fusion perspective is found generalizing the work of [1] to space dependent fields and the cases of ²H, ¹⁰⁶Pd and ²³⁸U are presented for several test fields. A final section reviewing laser confinement fusion actual experiments capable of achieving the required energies is also reported.

Enhanced Fusion Mechanisms Towards Synthesizing Superheavy Elements

In nature all of the heavy elements are produced by nuclear fusion reactions, mostly in supernova explosions and neutron star collisions, so, this is to date the only known and proven mechanism to produce heavy elements in usable quantities. In this work we approach a difficult challenge, namely, the possibility of fusion of heavy elements, taking as a test case the heaviest observationally stable element - ²³?U, showing that it is feasible, at least in principle with the help of existing technologies. The main idea behind is to show that fusion of lighter - than z=184 - nuclei is conceptually viable examining the tunnel effect assisted by an auxiliary field that will produce a Sauter like effect, and this is the pathway to explore the synthesis of elements higher than z=118. The production of theoretical untested elements like Unoctquadium-184 or close Z species could open a new chapter in the physics of super-heavy elements, and leads to a deeper understanding of nuclear decay channels and stability conditions. Nuclear fusion of heavy elements will open the breach to produce neutron rich elements, so we may obtain a deep insight into the physics of the island of stability. This work will review basic aspects of fusion physics related to the assisted fusion mechanism. An enhanced fusion perspective is found generalizing the work of [1] to space dependent fields and the cases of ²H, ¹??Pd and ²³?U are presented for several test fields. A final section reviewing laser confinement fusion actual experiments capable of achieving the required energies is also reported.

On the critical endpoint and the first‐order phase transition in the extended Polyakov Nambu Jona‐Lasinio Lagrangian

AbstractThe (Polyakov) Nambu‐ Jona‐Lasinio (PNJL) approach is an effective theory for low energies which shares with Quantumchromodynamics (QCD) the global symmetries. This approach is easy to handle and allows for analytical results for many of the observables which are of interest for the studies of ultrarelativistic heavy‐ion collisions but also for the merger and collisions of neutron stars. It allows in particular to study the density and temperature dependence of quark masses and of the masses of mesons and baryons as well as the chiral phase transition. In recent times, this transition came in the focus of interest because it is expected that the upcoming experiments at the Fair (Germany) and NICA (Russia) facilities will observe a Quark‐Gluon‐Plasma (QGP) at finite baryon chemical potential. In this contribution, we show that the extended PNJL model is able to describe the properties of symmetric matter at vanishing chemical potential, as determined by lQCD calculations.

The Astrophysical Processes of Cosmological Hydrogen that Generate the Chemical Elements that Make up the Universe

The objective of the present article is to explain how all the chemical elements were formed from the big bang generated element: hydrogen. The methodology used was to analyze the main cosmological and astrophysical processes in order to explain the origin of all the known chemical elements. The main results are: Hydrogen cannot be formed in any part of the actual universe; it must come from the Big Bang. Helium and a little bit of lithium can have a cosmological origin associated to the Big Bang nucleosynthesis and the recombination process. The elements with an atomic number between 3 and 26 were, and continue to be, synthetized by nuclear fusion reactions inside the core of massive stars and liberated by explosion when the stars go supernovae, at the end of their lives. In the process of going supernova, elements with a medium atomic number, between 27 and 40, are created. All the elements with an atomic number larger than 40 were generated by neutron star collisions. When Mendeleev and Lothar Meyer designed an ordered arrangement of chemical elements, their tables included the 63 chemical elements known in 1869. A century and a half later, the known elements are 118. By studying different topics related to the elements, it was possible to uncover fundamental particles, such as quarks and leptons, and the strong and weak nuclear forces that form the baryonic part of the universe. The Sun was formed 6000 million years ago and its planets, including earth, were formed 4600 million years ago when and where there were debris of different stars that went supernova, in particular 1A type, and also debris, of at least one of a binary neutron star collision, so to attain, all the elements that have been identified in the solar system, and especially in earth. In addition, the current “periodic table” includes 26 synthetic elements that were produced in neutron star collisions but, because of their short lifetimes, they are not found, on earth. The vast quantities of the elements, produced during the aforementioned astrophysical processes, clustered into planets, stars and galaxies; and at least in one planet, our earth, some chemical elements organized themselves into living creatures.

Search for CME in U+U and Au+Au collisions in STAR with different approaches of handling backgrounds

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field between left- and right-handed quarks, caused by interactions with topological gluon fields from QCD vacuum fluctuations. We present two approaches to handle the dominant elliptic flow (v2) background in the three-particle correlator (Δγ112), sensitive to CME.In the first approach, we present the Δγ112 and Δγ123 measurements in U+U and Au+Au collisions. While hydrodynamic simulations including resonance decays and local charge conservation predict that Δγ112 scaled by Npart/ν2 will be similar in U+U and Au+Au collisions, the projected B-field exhibits a distinct difference between the two systems and with varying Npart. Therefore, U+U and Au+Au collisions provide configurations with different expectations for both CME signal and background. Moreover, the three-particle observable Δγ123 scaled by Npart/ν3 provide baseline measurement for only the background.In the second approach, we handle the ν2 background by measuring Δγ112 with respect to the planes of spectators measured by Zero Degree Calorimeters and participants measured by Time Projection Chamber. These measurements contain different amounts of contributions from CME signal (along B-field, due to spectators) and ν2 background (determined by the participant geometry). With the two Δγ112 measurements, the possible CME signal and the background contribution can be determined. We report such a measurement in Au+Au collisions at sNN=27GeV with the newly installed event plane detector, and report the new findings in U+U system where the spectator-participant plane correlations are expected to differ from those in Au+Au collisions.

Populating the Upper Black Hole Mass Gap through Stellar Collisions in Young Star Clusters

Abstract Theoretical modeling of massive stars predicts a gap in the black hole (BH) mass function above ∼40–50 M ⊙ for BHs formed through single star evolution, arising from (pulsational) pair-instability supernovae (PISNe). However, in dense star clusters, dynamical channels may exist that allow construction of BHs with masses in excess of those allowed from single star evolution. The detection of BHs in this so-called “upper-mass gap” would provide strong evidence for the dynamical processing of BHs prior to their eventual merger. Here, we explore in detail the formation of BHs with masses within or above the pair-instability gap through collisions of young massive stars in dense star clusters. We run a suite of 68 independent cluster simulations, exploring a variety of physical assumptions pertaining to growth through stellar collisions, including primordial cluster mass segregation and the efficiency of envelope stripping during collisions. We find that as many as ∼20% of all BH progenitors undergo one or more collisions prior to stellar collapse and up to ∼1% of all BHs reside within or above the pair-instability gap through the effects of these collisions. We show that these BHs readily go on to merge with other BHs in the cluster, creating a population of massive BH mergers at a rate that may compete with the “multiple-generation” merger channel described in other analyses. This has clear relevance for the formation of very massive BH binaries as recently detected by the Laser Interferometer Gravitational-Wave Observatory/Virgo in GW190521. Finally, we describe how stellar collisions in clusters may provide a unique pathway to PISNe and briefly discuss the expected rate of these events and other electromagnetic transients.

Understanding the origin of white dwarf atmospheric pollution by dynamical simulations based on detected three-planet systems

ABSTRACT Between 25 and 50 ${{\ \rm per\ cent}}$ of white dwarfs (WD) present atmospheric pollution by metals, mainly by rocky material, which has been detected as gas/dust discs, or in the form of photometric transits in some WDs. Planets might be responsible for scattering minor bodies that can reach stargazing orbits, where the tidal forces of the WD can disrupt them and enhance the chances of debris to fall on to the WD surface. The planet–planet scattering process can be triggered by the stellar mass-loss during the post main-sequence (MS) evolution of planetary systems. In this work, we continue the exploration of the dynamical instabilities that can lead to WD pollution. In a previous work, we explored two-planet systems found around MS stars and here we extend the study to three-planet system architectures. We evolved 135 detected three-planet systems orbiting MS stars to the WD phase by scaling their orbital architectures in a way that their dynamical properties are preserved using the N-body integrator package mercury. We find that 100 simulations (8.6 ${{\ \rm per\ cent}}$) are dynamically active (having planet losses, orbit crossing, and scattering) on the WD phase, where low-mass planets (1–100 M⊕) tend to have instabilities in Gyr time-scales, while high-mass planets (&amp;gt;100 M⊕) decrease the dynamical events more rapidly as the WD ages. Besides, 19 simulations (1.6 ${{\ \rm per\ cent}}$) were found to have planets crossing the Roche radius of the WD, where 9 of them had planet–star collisions. Our three-planet simulations have a slight increase in percentage of simulations that may contribute to the WD pollution than the previous study involving two-planet systems and have shown that planet–planet scattering is responsible of sending planets close to the WD, where they may collide directly to the WD, become tidally disrupted or circularize their orbits, hence producing pollution on the WD atmosphere.

Phase diagram of the Polyakov–Nambu–Jona-Lasinio approach for finite chemical potentials

We extend the SU(3) (Polyakov) Nambu Jona-Lasinio in two ways: We introduce the next to leading order contribution (in $N_c$) in the partition function. This contribution contains explicit mesonic terms. We introduce a coupling between the gluon field and the quark degrees of freedom which goes beyond a simple rescaling of the critical temperature. With both these improvements we can reproduce, for vanishing chemical potentials, the lattice results for the thermal properties of a strongly interacting system like pressure, energy density, entropy density, interaction measure and the speed of sound. Also the expansion parameter towards small but finite chemical potentials agrees with the lattice results. Extending the calculations to finite chemical potentials (what does not require any new parameter) we find a first order phase transition up to a critical end point of $T_{CEP}= 110\ MeV$ and $\mu_q = 320\ MeV$. For very large chemical potentials, we find agreement with pQCD calculations. We calculate the mass of mesons and baryons as a function of temperature and chemical potential and the transition between the hadronic and the chirally restored phase. These calculations provide an equation of state in the whole $T,\mu$ plane an essential ingredient for dynamical calculations of ultra-relativistic heavy ion collisions but also for the physics of neutron stars and neutron star collisions.

5D memberships and fundamental properties of the old open cluster NGC 6791 based on $\mathit{Gaia}$-DR2

This paper investigates reliable memberships and fundamental astrophysical properties of the old (∼8 Gyr) open cluster NGC 6791 using the astrometric and photometric data of the Gaia data release 2 ($\mathit{Gaia}$-DR2). We identify 2536 likely cluster members (≥0.6) in a five-dimensional (5D) parameter space using a Gaussian mixture model (GMM) clustering method. Based on these reliable 5D cluster members, we determine the core and tidal radii of the cluster to be $R_{c}=3.69^{\prime } \pm 0.23^{\prime }$ and $R_{t}= 16.96^{\prime } \pm 2.29^{\prime }$, respectively. We find evidence for the presence of mass segregation in the cluster. Among the likely cluster members, we identify six blue subdwarfs, 43 blue stragglers, and 27 red clump giants. We find that 34 blue stragglers (∼79%) lie within the core radius (${<}3.69^{\prime }$) of the cluster, indicating that these stars may be formed by star collisions or by the evolution of primordial binaries within the dense core. The 27 red clump giants are used to accurately estimate the distance of the cluster. We determine a distance of 4118 ± 23 pc for the cluster using a Monte-Carlo simulation method. We confirm the presence of a significant zero-point offset (${\sim}\, {-}0.055 \pm 0.004$ mas) in the $\mathit{Gaia}$ parallaxes of the cluster members. Using the same simulation method, the mean proper motion and radial velocity of the cluster are found to be ($\langle \mu _{\alpha }\cos \delta \rangle , \langle \mu _{\delta }\rangle )= (-0.434\pm 0.008,-2.266\pm 0.010)$ mas/yr and $\langle V_{r} \rangle =-46.7\pm 0.6$ km/s, respectively.