Search

The discovery of a binary comprising a black hole (BH) and a millisecond pulsar (MSP) would yield insights into stellar evolution and facilitate exquisitely sensitive tests of general relativity. Globular clusters (GCs) are known to harbour large MSP populations and recent studies suggest that GCs may also retain a substantial population of stellar mass BHs. We modelled the formation of BH+MSP binaries in GCs through exchange interactions between binary and single stars. We found that in dense, massive clusters most of the dynamically formed BH+MSP binaries will have orbital periods of 2–10 d, regardless of the mass of the BH, the number of BHs retained by the cluster, and the nature of the GC's binary population. The size of the BH+MSP population is sensitive to several uncertain parameters, including the BH mass function, the BH retention fraction, and the binary fraction in GCs. Based on our models, we estimate that there are 0.6 ± 0.2 dynamically formed BH+MSP binaries in the Milky Way GC system, and place an upper limit on the size of this population of ∼10. Interestingly, we find that BH+MSP binaries will be rare even if GCs retain large BH populations. [ABSTRACT FROM AUTHOR]

Material from the surface of a planet can be ejected into space by a large impact and could carry primitive life-forms with it. We performed n-body simulations of such ejecta to determine where in the Solar System rock from Earth and Mars may end up. We found that, in addition to frequent transfer of material among the terrestrial planets, transfer of material from Earth and Mars to the moons of Jupiter and Saturn is also possible, but rare. We expect that such transfers were most likely to occur during the Late Heavy Bombardment or during the ensuing 1-2 billion years. At this time, the icy moons were warmer and likely had little or no ice shell to prevent meteorites from reaching their liquid interiors. We also note significant rates of re-impact in the first million years after ejection. This could re-seed life on a planet after partial or complete sterilization by a large impact, which would aid the survival of early life during the Late Heavy Bombardment. Key Words: Panspermia-Impact-Meteorites-Titan-Europa. Astrobiology 13, 1155-1165. [ABSTRACT FROM AUTHOR]

We model the formation of black hole–neutron star (BH–NS) binaries via dynamical interactions in globular clusters. We find that in dense, massive clusters, 16–61 per cent of the BH–NS binaries formed by interactions with existing BH binaries will undergo mergers driven by the emission of gravitational radiation. If the BHs are retained by the cluster after merging with an NS, the BHs acquire subsequent NS companions and undergo several mergers. Thus, the merger rate depends critically upon whether or not the BH is retained by the cluster after the merger. Results from numerical relativity suggest that kick imparted to a ∼7 M⊙ BH after it merges with an NS will greatly exceed the cluster's escape velocity. In this case, the models suggest that the majority of BH–NS mergers in globular clusters occur within 4 Gyr of the cluster's formation and would be unobservable by Advanced LIGO. For more massive BHs, on the other hand, the post-merger kick is suppressed and the BH is retained. Models with 35 M⊙ BHs predict Advanced LIGO detection rates in the range 0.04–0.7 yr−1. On the pessimistic end of this range, BH–NS mergers resulting from binary–single star interactions in globular clusters could account for an interesting fraction of all BH–NS mergers. On the optimistic end, this channel may dominate the rate of detectable BH– NS mergers. [ABSTRACT FROM AUTHOR]

ABSTRACT We model the emission lines generated in the photoionized debris of a tidally disrupted horizontal branch (HB) star. We find that at late times, the brightest optical emission lines are [N ii] λλ6548, 6583 and [O iii] λλ4959, 5007. Models of a red clump HB star undergoing mild disruption by a massive (50-100 M⊙) black hole (BH) yield an emission-line spectrum that is in good agreement with that observed in the NGC 1399 globular cluster hosting the ultraluminous X-ray source CXOJ033831.8−352604. We make predictions for the ultraviolet emission-line spectrum that can verify the tidal disruption scenario and constrain the mass of the BH. [ABSTRACT FROM AUTHOR]

The addition of metals to any gas can significantly alter its evolution by increasing the rate of radiative cooling. In star-forming environments, enhanced cooling can potentially lead to fragmentation and the formation of low-mass stars, where metal-free gas-clouds have been shown not to fragment. Adding metal cooling to numerical simulations has traditionally required a choice between speed and accuracy. We introduce a method that uses the sophisticated chemical network of the photoionization software,cloudy, to include radiative cooling from a complete set of metals up to atomic number 30 (Zn) that can be used with large-scale three-dimensional hydrodynamic simulations. Our method is valid over an extremely large temperature range , up to hydrogen number densities of . At this density, a sphere of has a radius of roughly 40 au. We implement our method in the adaptive mesh refinement hydrodynamic/ N-body code,enzo. Using cooling rates generated with this method, we study the physical conditions that led to the transition from Population III to Population II star formation. While C, O, Fe and Si have been previously shown to make the strongest contribution to the cooling in low-metallicity gas, we find that up to 40 per cent of the metal cooling comes from fine-structure emission by S, when solar abundance patterns are present. At metallicities, , regions of density and temperature exist where gas is both thermally unstable and has a cooling time less than its dynamical time. We identify these doubly unstable regions as the most inducive to fragmentation. At high redshifts, the cosmic microwave background inhibits efficient cooling at low temperatures and, thus, reduces the size of the doubly unstable regions, making fragmentation more difficult. [ABSTRACT FROM AUTHOR]

The pulsar B1620-26 has two companions, one of stellar mass and one of planetary mass. We detected the stellar companion with the use of Hubble Space Telescope observations. The color and magnitude of the stellar companion indicate that it is an undermassive white dwarf (0.34 ± 0.04 solar mass) of age 480 X 10[sup6] ± 140 X 10[sup6] years. This places a constraint on the recent history of this triple system and supports a scenario in which the current configuration arose through a dynamical exchange interaction in the cluster core. This implies that planets may be relatively common in low-metallicity globular clusters and that planet formation is more widespread and has happened earlier than previously believed. [ABSTRACT FROM AUTHOR]

Recent conjectures that there are mesoscopically "large" extra dimensions, through which gravity propagates have interesting implications for much of physics. The scenario implies gross departures from Newton's law of gravity at small length scales. Testing departures from Coulomb's law on sub-millimetre scales is hard. It is now possible to routinely create Bose-Einstein condensates with de Broglie wavelengths of order a µm and total size of order 10 µm. BEC condensates move coherently under gravitational acceleration, and I propose that the transverse fringe shift due to the acceleration of a pair of interfering BECs passing a dense linear mass may be measurable, and provide direct evidence for anomalous gravitational acceleration. Ideally such experiments are best carried out in free fall to maximise the time spent by a BEC in the non-Newtonian regime. [ABSTRACT FROM AUTHOR]

Recent conjectures suggest the universe may have large extra dimensions, through which gravity propagates. This implies gross departures from Newton's law of gravity at small length scales. Here I consider some implications for particle dynamics on scales comparable to the compactification radius, R[sub c] ... 1 mm. During planet formation, coalescence of micron sized dust grains to planetesimals is a rate critical step. Blum et al. (2000) found dust grain aggregates form low fractal dimension structures in microgravity, consistent with high angular momentum coalescence. I consider the effects of non-Newtonian gravity on dust aggregation on scales less than R[sub c] and show they naturally coalesce into low-dimensional structures with high specific angular momentum. We infer R[sub c] ≈ 80 microns. [ABSTRACT FROM AUTHOR]

Proposes the alternative physical processes producing detectable Sunyaev-Zeldovich decrements in quasars or quasar pairs. Argument on the thermal and kinematic decrements producing baryons; Effects of quasar magnitude outflows depending on the luminosity of the QSO; Enumeration on the effects of quasar outflow; Production of cosmic microwave background from quasar outflow effect.

The article reports on the detection of the collision of two superdense neutron stars that spiraled into each other on August 17, 2017. The event lasted for 100 seconds at frequencies climbed to thousands of cycles per second. When neutron stars collide, they spew debris glowing in all wavelengths. The event, connecting gravitational waves to both gamma-ray bursts and a shortlived electromagnetic transient, represented a watershed moment in astrophysics.

Explores the dynamical and observational consequences of the presence of primordial black-hole population in a globular cluster of stars. Segregation; Number of retained black-holes; Disruption of main-sequence and giant stellar binaries; Tidal interactions.

The article discusses the astrophysics concept of gamma ray bursts (GRB). According to studies, GRB is defined as a brief but extremely intense flashes of high-energy radiation which is divided into two distinct types. Several space missions reveal that GRB is emitted from relativistic collimated fireballs.

We explore the growth of black holes that reside at the centres of spiral and field dwarf galaxies in a Local Group type of environment. We use merger trees from a cosmological N-body simulation known as Via Lactea 2 (VL-2) as a framework to test two merger-driven semi-analytic recipes for black hole growth that include dynamical friction, tidal stripping and gravitational wave recoil in over 20 000 merger tree realizations. First, we apply a Fundamental Plane limited (FPL) model to the growth of Sgr A*, which drives the central black hole to a maximum mass limited by the black hole Fundamental Plane after every merger. Next, we present a new model that allows for low-level prolonged gas accretion (PGA) during the merger. We find that both models can generate an Sgr A* mass black hole. We predict a population of massive black holes in local field dwarf galaxies - if the VL-2 simulation is representative of the growth of the Local Group, we predict up to 35 massive black holes () in Local Group field dwarfs. We also predict that hundreds of black holes fail to merge, and instead populate the Milky Way halo, with the most massive of them at roughly the virial radius. In addition, we find that there may be hundreds of massive black holes ejected from their hosts into the nearby intergalactic medium due to gravitational wave recoil. We discuss how the black hole population in the Local Group field dwarfs may help to constrain the growth mechanism for Sgr A*. [ABSTRACT FROM AUTHOR]

Abstract: Detection of electromagnetic (EM) counterparts of pre-coalescence binaries has very important implications for our understanding of the evolution of these systems as well as the associated accretion physics. In addition, a combination of EM and gravitational wave signatures observed from coalescing supermassive black hole binaries (SBHBs) would provide independent measurements of redshift and luminosity distance, thus allowing for high precision cosmological measurements. However, a statistically significant sample of these objects is yet to be attained and finding them observationally has proven to be a difficult task. Here we discuss existing observational evidence and how further advancements in the theoretical understanding of observational signatures of SBHBs before and after the coalescence can help in future searches. [Copyright &y& Elsevier]

Understanding how seed black holes (BHs) grow into intermediate-mass and supermassive black holes (IMBHs and SMBHs, respectively) has important implications for the duty cycle of active galactic nuclei (AGN), galaxy evolution and gravitational wave astronomy. Most studies of the cosmological growth and merger history of BHs have used semianalytic models and have concentrated on SMBH growth in luminous galaxies. We have developed a ‘hybrid method’ that combines high-resolution cosmological N-body simulations for the haloes' merger history, with semi-analytical recipes for BH pair dynamics and BH gas accretion. We track the assembly of BHs over a large range of final masses – from seed BHs to SMBHs – over widely varying dynamical histories. We used the dynamics of dark matter haloes to track the evolution of seed BHs in three different gas accretion scenarios. We have found that growth of a Sagittarius A*– size of SMBH reaches its maximum mass at through early gaseous accretion episodes, after which it stays at near constant mass. At the same redshift, the duty cycle of the host AGN ends, hence redshift marks the transition from an AGN to a starburst galaxy which eventually becomes the Milky Way. By tracking BH growth as a function of time and mass, we estimate that the IMBH merger rate reaches a maximum of at . From IMBH merger rates we calculate per Milky Way type galaxy per redshift in redshift range . [ABSTRACT FROM AUTHOR]

Primordial stars are likely to be very massive (≥ 30 M⊙), form in isolation, and will likely leave black holes as remnants in the centres of their host dark matter haloes. We expect primordial stars to form in haloes in the mass range 106–1010 M⊙. Some of these early black holes, formed at redshifts z≳ 10, could be the seed black holes for a significant fraction of the supermassive black holes (SMBHs) found in galaxies in the local Universe. If the black hole descendants of the primordial stars exist, their merger with nearby SMBHs may be a prime candidate for long wavelength gravitational wave detectors. We simulate formation and evolution of dark matter haloes in a Λ cold dark matter universe. We seed high-redshift dark matter haloes with early black holes, and explore the merger history of the host haloes and the implications of black hole kick velocities arising from their coalescence. The central concentration of low-mass early black holes in present-day galaxies is reduced if they experience even moderate kicks of tens of km s−1. Even such modest kicks allow the black holes to leave their parent halo, which consequently leads to dynamical friction being less effective on the low-mass black holes that were ejected, compared to those still embedded in their parent haloes. Therefore, merger rates with central SMBHs in the largest halo may be reduced by more than an order of magnitude. Using analytical and illustrative cosmological N-body simulations, we quantify the role of kicks on the merger rates of black holes formed from massive metal-free stars with SMBHs in present-day galaxies. [ABSTRACT FROM AUTHOR]

Close-in giant planets (e.g., ‘hot Jupiters’) are thought to form far from their host stars and migrate inward, through the terrestrial planet zone, via torques with a massive gaseous disk. Here we simulate terrestrial planet growth during and after giant planet migration. Several-Earth-mass planets also form interior to the migrating jovian planet, analogous to recently discovered ‘hot Earths.’ Very-water-rich, Earth-mass planets form from surviving material outside the giant planet's orbit, often in the habitable zone and with low orbital eccentricities. More than a third of the known systems of giant planets may harbor Earth-like planets. [ABSTRACT FROM AUTHOR]

In several previous studies, quasars exhibiting broad emission lines with ≳1000 km s−1 velocity offsets with respect to the host galaxy rest frame have been discovered. One leading hypothesis for the origin of these velocity-offset broad lines is the dynamics of a binary supermassive black hole (SMBH). We present high-resolution radio imaging of 34 quasars showing these velocity-offset broad lines with the Very Long Baseline Array (VLBA), aiming to find evidence for the putative binary SMBHs (such as dual radio cores), and testing the competing physical models. We detect exactly half of the target sample from our VLBA imaging, after implementing a 5σ detection limit. While we do not resolve double radio sources in any of the targets, we obtain limits on the instantaneous projected separations of a radio-emitting binary for all of the detected sources under the assumption that a binary still exists within our VLBA angular resolution limits. We also assess the likelihood that a radio-emitting companion SMBH exists outside of our angular resolution limits, but its radio luminosity is too weak to produce a detectable signal in the VLBA data. Additionally, we compare the precise sky positions afforded by these data to optical positions from both the SDSS and Gaia DR2 source catalogs. We find projected radio/optical separations on the order of ∼10 pc for three quasars. Finally, we explore how future multi-wavelength campaigns with optical, radio, and X-ray observatories can help discriminate further between the competing physical models. [ABSTRACT FROM AUTHOR]

We present a method for comparing the Hβ emission-line profiles of observed supermassive black hole (SBHB) candidates and models of subparsec SBHBs in circumbinary disks. Using the approach based on principal component analysis, we infer the values of the binary parameters for the spectroscopic SBHB candidates and evaluate the parameter degeneracies, representative of the uncertainties intrinsic to such measurements. We find that as a population, the SBHB candidates favor the average value of the semimajor axis corresponding to log(a/M) ≈ 4.20 ± 0.42 and comparable mass ratios, q > 0.5. If the SBHB candidates considered are true binaries, this result would suggest that there is a physical process that allows initially unequal-mass systems to evolve toward comparable mass ratios (e.g., accretion that occurs preferentially onto the smaller of the black holes) or point to some, yet unspecified, selection bias. Our method also indicates that the SBHB candidates equally favor configurations in which the mini-disks are coplanar or misaligned with the binary orbital plane. If confirmed for true SBHBs, this finding would indicate the presence of a physical mechanism that maintains misalignment of the mini-disks down to subparsec binary separations (e.g., precession driven by gravitational torques). The probability distributions of the SBHB parameters inferred for the observed SBHB candidates and our control group of AGNs are statistically indistinguishable, implying that this method can in principle be used to interpret the observed emission-line profiles once a sample of confirmed SBHBs is available but cannot be used as a conclusive test of binarity. [ABSTRACT FROM AUTHOR]