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Several stars show deep transits consistent with discs of roughly |$1\mathrm{\, R}_\odot$| seen at moderate inclinations, likely surrounding planets on eccentric orbits. We show that this configuration arises naturally as a result of planet–planet scattering when the planets possess satellite systems. Planet–planet scattering explains the orbital eccentricities of the discs' host bodies, while the close encounters during scattering lead to the exchange of satellites between planets and/or their destabilization. This leads to collisions between satellites and their tidal disruption close to the planet. Both of these events lead to large quantities of debris being produced, which in time will settle into a disc such as those observed. The mass of debris required is comparable to a Ceres-sized satellite. Through N -body simulations of planets with clones of the Galilean satellite system undergoing scattering, we show that 90 per cent of planets undergoing scattering will possess debris from satellite destruction. Extrapolating to smaller numbers of satellites suggests that tens of per cent of such planets should still possess circumplanetary debris discs. The debris trails arising from these events are often tilted at tens of degrees to the planetary orbit, consistent with the inclinations of the observed discs. Disruption of satellite systems during scattering thus simultaneously explains the existence of debris, the tilt of the discs, and the eccentricity of the planets they orbit. [ABSTRACT FROM AUTHOR]

Observations suggested that the occurrence rate of hot Jupiters (HJs) in open clusters is largely consistent with the field (⁠|$\sim 1{{\ \rm per\ cent}}$|⁠) but in the binary-rich cluster M67, the rate is |$\sim 5{{\ \rm per\ cent}}$|⁠. How does the cluster environment boost HJ formation via the high-eccentricity tidal migration initiated by the extreme-amplitude von Zeipel–Lidov–Kozai (XZKL) mechanism forced by a companion star? Our analytical treatment shows that the cluster's collective gravitational potential alters the companion's orbit slowly, which may render the star–planet–companion configuration XZKL-favourable. We have also performed direct Gyr N -body simulations of the star cluster evolution and XZKL of planets' orbit around member stars. We find that an initially single star may acquire a companion star via stellar scattering and the companion may enable XZKL in the planets' orbit. Planets around an initially binary star may also be XZKL-activated by the companion. In both scenarios, the companion's orbit has likely been significantly changed by stellar scattering and the cluster potential before XZKL occurs. Across different cluster models, 0.8–3 per cent of the planets orbiting initially single stars have experienced XZKL while the fraction is 2–26 per cent for initially binary stars. Around a star that is binary at 1 Gyr, 13–32 per cent of its planets have undergone XZKL, and combined with single stars, the overall XZKL fraction is 3–21 per cent, most affected by the cluster binarity. If 10 per cent of the stars in M67 host a giant planet, our model predicts an HJ occurrence rate of |$\sim 1{{\ \rm per\ cent}}$|⁠. We suggest that HJ surveys target old, high-binarity, not-too-dense open clusters and prioritize wide binaries to maximize HJ yield. [ABSTRACT FROM AUTHOR]

It has been suggested that the occurrence rate of hot Jupiters (HJs) in open clusters might reach several per cent, significantly higher than that of the field (∼a per cent). In a stellar cluster, when a planetary system scatters with a stellar binary, it may acquire a companion star, which may excite large-amplitude von Zeipel–Lidov–Kozai oscillations in the planet's orbital eccentricity, triggering high-eccentricity migration, and the formation of an HJ. We quantify the efficiency of this mechanism by modelling the evolution of a gas giant around a solar mass star under the influence of successive scatterings with binary and single stars. We show that the chance that a planet ∈ (1, 10) au becomes an HJ in a Gyr in a cluster of stellar density n * = 50 pc−3, and binary fraction f bin = 0.5 is about 2 per cent and an additional 4 per cent are forced by the companion star into collision with or tidal disruption by the central host. An empirical fit shows that the total percentage of those outcomes asymptotically reaches an upper limit determined solely by f bin (e.g. 10 per cent at f bin = 0.3 and 18 per cent at f bin = 1) on a time-scale inversely proportional to n * (∼Gyr for n * ∼ 100 pc−3). The ratio of collisions to tidal disruptions is roughly a few, and depends on the tidal model. Therefore, if the giant planet occurrence rate is 10 per cent, our mechanism implies an HJ occurrence rate of a few times 0.1 per cent in a Gyr and can thus explain a substantial fraction of the observed rate. [ABSTRACT FROM AUTHOR]

A large fraction of stars are formed in dense clusters. In the cluster, close encounters between stars at distances less than 100 au are common. It has been shown that during close encounters planets can transfer between stars. Such captured planets will be on different orbits compared to planets formed in the system, often on very wide, eccentric, and inclined orbits. We examine how these captured planets affect Kuiper belt-like planetesimal belts in their new systems by examining the effects on habitable planets in systems containing an outer gas giant. We show that these captured planets can destabilize the belt, and we show that the fraction of the planetesimals that make it past the giant planets into the system to impact the habitable planet is independent of the captured planet's orbital plane, whereas the fraction of the planetesimals that are removed and the rate at which they are removed depend strongly on the captured planet's pericentre and inclination. We then examine a wide range of outcomes of planet capture and find that when a Jupiter-mass planet is captured it will in 40 per cent of cases destabilize the giant planets in the system and in 40 per cent of cases deplete the belt in a few Myr, i.e. not posing much risk to life on terrestrial planets that would be expected to develop later. In the final 20 per cent of cases, the result will be a flux of impactors 10–20 times greater than that on Earth that can persist for several Gyr, detrimental to the development of life on the planet. [ABSTRACT FROM AUTHOR]

In many galactic nuclei, a nuclear stellar cluster (NSC) co-exists with a supermassive black hole (SMBH). In this second one in a series of papers, we further explore the idea that the NSC forms before the SMBH through the merger of several stellar clusters that may contain intermediate-mass black holes (IMBHs). These IMBHs can subsequently grow by mergers and accretion to form an SMBH. To check the observable consequences of this proposed SMBH seeding mechanism, we created an observationally motivated mock population of galaxies, in which NSCs are constructed by aggregating stellar clusters that may or may not contain IMBHs. Based on several assumptions, we model the growth of IMBHs in the NSCs through gravitational wave (GW) mergers with other IMBHs and gas accretion. In the case of GW mergers, the merged BH can either be retained or ejected depending on the GW recoil kick it receives. The likelihood of retaining the merged BH increases if we consider the growth of IMBHs in the NSC through gas accretion. We find that nucleated lower mass galaxies (⁠|${\it M}_{\star } \lesssim 10^{9}\, {\rm M_{\odot }}$|⁠ ; e.g. M33) have an SMBH seed occupation fraction of about 0.3–0.5. This occupation fraction increases with galaxy stellar mass and for more massive galaxies (⁠|$\rm 10^{9} \ \lesssim {\it M}_{\star } \lesssim 10^{11}\, {\rm M_{\odot }}$|⁠), it is between 0.5 and 0.8, depending on how BH growth is modelled. These occupation fractions are consistent with observational constraints. Furthermore, allowing for BH growth also allows us to reproduce the observed diversity in the mass range of SMBHs in the |${\it M}_{\rm NSC}\!-\!{\it M}_{\rm BH}$| plane. [ABSTRACT FROM AUTHOR]

We model the effects of collisions and close encounters on the stellar populations observed in the Milky Way nuclear stellar cluster (NSC). Our analysis is based on N -body simulations in which the NSC forms by accretion of massive stellar clusters around a supermassive black hole. We attach stellar populations to our N -body particles and follow the evolution of their stars, and the rate of collisions and close encounters. The most common encounters are collisions between pairs of main-sequence (MS) stars, which lead to mergers: destructive collisions between MS stars and compact objects are rare. We find that the effects of collisions on the stellar populations are small for three reasons. First, our models possess a core that limits the maximum stellar density. Secondly, the velocity dispersion in the NSC is similar to the surface escape velocities of the stars, which minimizes the collision rate. Finally, whilst collisions between MS stars destroy bright giants by accelerating their evolution, they also create them by accelerating the evolution of lower mass stars. These two effects approximately cancel out. We also investigate whether the G2 cloud could be a fuzzball: a compact stellar core that has accreted a tenuous envelope in a close encounter with a red giant. We conclude that fuzzballs with cores below |$2\, {\rm M_{\odot }}$| have thermal times-scales too short to reproduce G2. A fuzzball with a black hole core could reproduce the surface properties of G2 but the production rate of such objects in our model is low. [ABSTRACT FROM AUTHOR]

Supermassive black holes (SMBHs) are found in most galactic nuclei. A significant fraction of these nuclei also contains a nuclear stellar cluster (NSC) surrounding the SMBH. In this paper, we consider the idea that the NSC forms first, from the merger of several stellar clusters that may contain intermediate-mass black holes (IMBHs). These IMBHs can subsequently grow in the NSC and form an SMBH. We carry out N -body simulations of the simultaneous merger of three stellar clusters to form an NSC, and investigate the outcome of simulated runs containing zero, one, two, and three IMBHs. We find that IMBHs can efficiently sink to the centre of the merged cluster. If multiple merging clusters contain an IMBH, we find that an IMBH binary is likely to form and subsequently merge by gravitational wave emission. We show that these mergers are catalyzed by dynamical interactions with surrounding stars, which systematically harden the binary and increase its orbital eccentricity. The seed SMBH will be ejected from the NSC by the recoil kick produced when two IMBHs merge, if their mass ratio q ≳ 0.15. If the seed is ejected then no SMBH will form in the NSC. This is a natural pathway to explain those galactic nuclei that contain an NSC but apparently lack an SMBH, such as M33. However, if an IMBH is retained then it can seed the growth of an SMBH through gas accretion and tidal disruption of stars. [ABSTRACT FROM AUTHOR]

The origin of the population of very massive stars observed within ∼0.4 pc of the supermassive black hole in the Galactic Centre is a mystery. Tidal forces from the black hole would likely inhibit in situ star formation whilst the youth of the massive stars would seem to exclude formation elsewhere followed by transportation (somehow) into the Galactic Centre. Here, we consider a third way to produce these massive stars from the lower mass stars contained in the nuclear stellar cluster which surrounds the supermassive black hole. A passing gas cloud can be tidally shredded by the supermassive black hole forming an accretion disc around the black hole. Stars embedded within this accretion disc will accrete gas from the disc via Bondi–Hoyle accretion, where the accretion rate on to a star, |$\dot{M}_\star \propto M_\star ^2$|⁠. This super-exponential growth of accretion can lead to a steep increase in stellar masses, reaching the required 40–50 M⊙ in some cases. The mass growth rate depends sensitively on the stellar orbital eccentricities and their inclinations. The evolution of the orbital inclinations and/or their eccentricities as stars are trapped by the disc, and their orbits are circularized, will increase the number of massive stars produced. Thus accretion on to low-mass stars can lead to a top heavy stellar mass function in the Galactic Centre and other galactic nuclei. The massive stars produced will pollute the environment via supernova explosions and potentially produce compact binaries whose mergers may be detectable by the LIGO–VIRGO gravitational waves observatories. [ABSTRACT FROM AUTHOR]

The forthcoming Laser Interferometer Space Antenna (LISA) will probe the population of coalescing massive black hole (MBH) binaries up to the onset of structure formation. Here, we simulate the galactic-scale pairing of ∼106 M⊙ MBHs in a typical, non-clumpy main-sequence galaxy embedded in a cosmological environment at z  = 7–6. In order to increase our statistical sample, we adopt a strategy that allows us to follow the evolution of six secondary MBHs concomitantly. We find that the magnitude of the dynamical-friction-induced torques is significantly smaller than that of the large-scale, stochastic gravitational torques arising from the perturbed and morphologically evolving galactic disc, suggesting that the standard dynamical friction treatment is inadequate for realistic galaxies at high redshift. The dynamical evolution of MBHs is very stochastic, and a variation in the initial orbital phase can lead to a drastically different time-scale for the inspiral. Most remarkably, the development of a galactic bar in the host system either significantly accelerates the inspiral by dragging a secondary MBH into the centre, or ultimately hinders the orbital decay by scattering the MBH in the galaxy outskirts. The latter occurs more rarely, suggesting that galactic bars overall promote MBH inspiral and binary coalescence. The orbital decay time can be an order of magnitude shorter than what would be predicted relying on dynamical friction alone. The stochasticity and the important role of global torques have crucial implications for the rates of MBH coalescences in the early Universe: both have to be accounted for when making predictions for the upcoming LISA observatory. [ABSTRACT FROM AUTHOR]

K2-19 hosts a planetary system composed of two outer planets, b and c, with size of 7.0 ± 0.2 R⊕ and 4.1 ± 0.2 R⊕, and an inner planet, d, with a radius of 1.11 ± 0.05 R⊕. A recent analysis of Transit-Timing Variations (TTVs) suggested b and c are close to but not in 3:2 mean motion resonance (MMR) because the classical resonant angles circulate. Such an architecture challenges our understanding of planet formation. Indeed, planet migration through the protoplanetary disc should lead to a capture into the MMR. Here, we show that the planets are in fact, locked into the 3:2 resonance despite circulation of the conventional resonant angles and aligned periapses. However, we show that such an orbital configuration cannot be maintained for more than a few hundred million years due to the tidal dissipation experienced by planet d. The tidal dissipation remains efficient because of a secular forcing of the innermost planet eccentricity by planets b and c. While the observations strongly rule out an orbital solution where the three planets are on close to circular orbits, it remains possible that a fourth planet is affecting the TTVs such that the four planet system is consistent with the tidal constraints. [ABSTRACT FROM AUTHOR]

Planetary systems formed in clusters may be subject to stellar encounter flybys. Here, we create a diverse range of representative planetary systems with different orbital scales and planets' masses and examine encounters between them in a typical open cluster. We first explore the close-in multisuper Earth systems ≲0.1 au. They are resistant to flybys in that only ones inside a few au can destabilize a planet or break the resonance between such planets. But these systems may capture giant planets on to wide orbits from the intruding star during distant flybys. If so, the original close-in small planets' orbits may be tilted together through Kozai–Lidov mechanism, forming a 'cold' system that is significantly inclined against the equator of the central host. Moving to the intermediately placed planets around solar-like stars, we find that the planets' mass gradient governs the systems' long-term evolution post-encounter: more massive planets have better chances to survive. Also, a system's angular momentum deficit, a quantity describing how eccentric/inclined the orbits are, measured immediately after the encounter, closely relates to the longevity of the systems – whether or not and when the systems turn unstable in the ensuing evolution millions of years post-encounter. We compare the orbits of the surviving planets in the unstable systems through (1) the immediate consequence of the stellar fly or (2) internal interplanetary scattering long post-encounter and find that those for the former are systematically colder. Finally, we show that massive wide-orbit multiplanet systems like that of HR 8799 can be easily disrupted and encounters at a few hundreds of au suffice. [ABSTRACT FROM AUTHOR]

Primordial black holes formed through the collapse of cosmological density fluctuations have been hypothesized as contributors to the dark matter content of the Universe. At the same time, their mergers could contribute to the recently observed population of gravitational-wave sources. We investigate the scenario in which primordial black holes form binaries at late times in the Universe. Specifically, we re-examine the mergers of primordial black holes in small clusters of ∼30 objects in the absence of initial binaries. Binaries form dynamically through Newtonian gravitational interactions. These binaries act as heat sources for the cluster, increasing the cluster's velocity dispersion, which inhibits direct mergers through gravitational-wave two-body captures. Meanwhile, three-body encounters of tight binaries are too rare to tighten binaries sufficiently to allow them to merge through gravitational-wave emission. We conclude that in the absence of initial binaries, merger rates of primordial black holes in the considered scenario are at least an order of magnitude lower than previously suggested, which makes gravitational-wave detections of such sources improbable. [ABSTRACT FROM AUTHOR]

We investigate the possibility of finding Earth-like planets in the habitable zone of 34 nearby FGK-dwarfs, each known to host one giant planet exterior to their habitable zone detected by RV. First we simulate the dynamics of the planetary systems in their present day configurations and determine the fraction of stable planetary orbits within their habitable zones. Then, we postulate that the eccentricity of the giant planet is a result of an instability in their past during which one or more other planets were ejected from the system. We simulate these scenarios and investigate whether planets orbiting in the habitable zone survive the instability. Explicitly we determine the fraction of test particles, originally found in the habitable zone, which remain in the habitable zone today. We label this fraction the resilient habitability of a system. We find that for most systems the probability of planets existing [or surviving] on stable orbits in the habitable zone becomes significantly smaller when we include a phase of instability in their history. We present a list of candidate systems with high resilient habitability for future observations. These are: HD 95872, HD 154345, HD 102843, HD 25015, GJ 328, HD 6718, and HD 150706. The known planets in the last two systems have large observational uncertainties on their eccentricities, which propagate into large uncertainties on their resilient habitability. Further observational constraints of these two eccentricities will allow us to better constrain the survivability of Earth-like planets in these systems. [ABSTRACT FROM AUTHOR]

Giant molecular clouds (GMCs) are believed to affect the biospheres of planets as their host star passes through them. We simulate the trajectories of stars and GMCs in the Galaxy and determine how often stars pass through GMCs. We find a strong decreasing dependence with Galactocentric radius, and with the velocity perpendicular to the Galactic plane, V |$\mathrm{ z}$| . The XY -component of the kinematic heating of stars was shown to not affect the GMC hit rate, unlike the Z -dependence (V |$\mathrm{ z}$| ) implies that stars hit fewer GMCs as they age. GMCs are locations of star formation, therefore we also determine how often stars pass near supernovae. For the supernovae the decrease with V |$\mathrm{ z}$| is steeper as how fast the star passes through the GMC determines the probability of a supernova encounter. We then integrate a set of Sun-like trajectories to see the implications for the Sun. We find that the Sun hits 1.6 ± 1.3 GMCs per Gyr which results in 1.5 ± 1.1 or (with correction for clustering) 0.8 ± 0.6 supernova closer than 10 pc per Gyr. The different the supernova frequencies are from whether one considers multiple supernovae per GMC crossing (few Myr) as separate events. We then discuss the effect of the GMC hits on the Oort cloud, and the Earth's climate due to accretion, we also discuss the records of distant supernova. Finally, we determine Galactic Habitable Zone using our model. For the thin disc, we find it to lie between 5.8 and 8.7 kpc and for the thick disc to lie between 4.5 and 7.7 kpc. [ABSTRACT FROM AUTHOR]

Stars formed in clusters can encounter other stars at close distances. In typical open clusters in the Solar neighbourhood containing hundreds or thousands of member stars, 10–20 per cent of Solar-mass member stars are expected to encounter another star at distances closer than 100 au. These close encounters strongly perturb the planetary systems, directly causing ejection of planets or their capture by the intruding star, as well as exciting the orbits. Using extensive N -body simulations, we study such fly-by encounters between two Solar system analogues, each with four giant planets from Jupiter to Neptune. We quantify the rates of loss and capture immediately after the encounter, e.g. the Neptune analogue is lost in one in four encounters within 100 au, and captured by the flying-by star in 1 in 12 encounters. We then perform long-term (up to 1 Gyr) simulations investigating the ensuing post-encounter evolution. We show that large numbers of planets are removed from systems due to planet–planet interactions and that captured planets further enhance the system instability. While encounters can initially leave a planetary system containing more planets by inserting additional ones, the long-term instability causes a net reduction in planet number. A captured planet ends up on a retrograde orbit in half of the runs in which it survives for 1Gyr; also, a planet bound to its original host star but flipped during the encounter may survive. Thus, encounters between planetary systems are a channel to create counter-rotating planets, This would happen in around 1 per cent of systems, and such planets are potentially detectable through astrometry or direct imaging. [ABSTRACT FROM AUTHOR]

We analyse N -body simulations of star-forming regions to investigate the effects of external far- and extreme-ultraviolet photoevaporation from massive stars on protoplanetary discs. By varying the initial conditions of simulated star-forming regions, such as the spatial distribution, net bulk motion (virial ratio), and density, we investigate which parameters most affect the rate at which discs are dispersed due to external photoevaporation. We find that disc dispersal due to external photoevaporation is faster in highly substructured star-forming regions than in smooth and centrally concentrated regions. Subvirial star-forming regions undergoing collapse also show higher rates of disc dispersal than regions that are in virial equilibrium or are expanding. In moderately dense (∼100 M⊙ pc−3) regions, half of all protoplanetary discs with radii ≥100 au are photoevaporated within 1 Myr, three times faster than is currently suggested by observational studies. Discs in lower density star-forming regions (∼10 M⊙ pc−3) survive for longer, but half are still dispersed on short time-scales (∼2 Myr). This demonstrates that the initial conditions of the star-forming regions will greatly impact the evolution and lifetime of protoplanetary discs. These results also imply that either gas giant planet formation is extremely rapid and occurs before the gas component of discs is  evaporated, or gas giants only form in low-density star-forming regions where no massive stars are present to photoevaporate gas from protoplanetary discs. [ABSTRACT FROM AUTHOR]

The globular clusters of large spiral galaxies can be divided into two populations: one that formed in situ and one that comprises clusters tidally stripped away from other galaxies. In this paper, we investigate the contribution to the outer globular cluster population in the M31 galaxy through donation of clusters from dwarf galaxies. We test this numerically by comparing the contribution of globular clusters from simulated encounters to the observed M31 globular cluster population. To constrain our simulations, we specifically investigate the outermost globular cluster in the M31 system, MGC1. The remote location of MGC1 favours the idea of it being captured; however, the cluster is devoid of features associated with tidal interactions. Hence, we separate simulations where tidal features are present and where they are hidden. We find that our simulated encounters can place clusters on MGC1-like orbits. In addition, we find that tidal stripping of clusters from dwarf galaxies leaves them on orbits having a range of separations, broadly matching those observed in M31. We find that the specific energies of globular clusters captured by M31 closely match those of the incoming host dwarf galaxies. Furthermore, in our simulations we find an equal number of accreted clusters on co-rotating and counter-rotating orbits within M31 and use this to infer the fraction of clusters that has been accreted. We find that even close in roughly |$50{{\ \rm per\ cent}}$| of the clusters are accreted, while this figure increases to over |$80{{\ \rm per\ cent}}$| further out. [ABSTRACT FROM AUTHOR]

Planet–planet collisions are a common outcome of instability in systems of transiting planets close to the star, as well as occurring during in-situ formation of such planets from embryos. Previous N -body studies of instability amongst transiting planets have assumed that collisions result in perfect merging. Here, we explore the effects of implementing a more realistic collision prescription on the outcomes of instability and in-situ formation at orbital radii of a few tenths of an au. There is a strong effect on the outcome of the growth of planetary embryos, so long as the debris thrown off in collisions is rapidly removed from the system (which happens by collisional processing to dust, and then removal by radiation forces) and embryos are small (<0.1 M⊕). If this is the case, then systems form fewer detectable (≥1 M⊕) planets than systems evolved under the assumption of perfect merging in collisions. This provides some contribution to the ‘ Kepler dichotomy’: the observed overabundance of single-planet systems. The effects of changing the collision prescription on unstable mature systems of super-Earths are less pronounced. Perfect mergers only account for a minority of collision outcomes in such systems, but most collisions resulting in mass loss are grazing impacts in which only a few per cent of mass is lost. As a result, there is little impact on the final masses and multiplicities of the systems after instability when compared to systems evolved under the assumption that collisions always result in perfect merging. [ABSTRACT FROM AUTHOR]

The distribution of exoplanet masses is not primordial. After the initial stage of planet formation, gravitational interactions between planets can lead to the physical collision of two planets, or the ejection of one or more planets from the system. When this occurs, the remaining planets are typically left in more eccentric orbits. In this report we demonstrate how the present-day eccentricities of the observed exoplanet population can be used to reconstruct the initial mass function of exoplanets before the onset of dynamical instability. We developed a Bayesian framework that combines data from N-body simulations with present-day observations to compute a probability distribution for the mass of the planets that were ejected or collided in the past. Integrating across the exoplanet population, one can estimate the initial mass function of exoplanets. We find that the ejected planets are primarily sub-Saturn-type planets. While the present-day distribution appears to be bimodal, with peaks around ∼1MJ and ∼20M⊕, this bimodality does not seem to be primordial. Instead, planets around ∼60M⊕ appear to be preferentially removed by dynamical instabilities. Attempts to reproduce exoplanet populations using population synthesis codes should be mindful of the fact that the present population may have been depleted of sub-Saturn-mass planets. Future observations may reveal that young giant planets have a more continuous size distribution with lower eccentricities and more sub-Saturn-type planets. Lastly, there is a need for additional data and for more research on how the system architecture and multiplicity might alter our results. [ABSTRACT FROM AUTHOR]

We study how close-in systems such as those detected by Kepler are affected by the dynamics of bodies in the outer system. We consider two scenarios: outer systems of giant planets potentially unstable to planet-planet scattering and wide binaries that may be capable of driving Kozai or other secular variations of outer planets' eccentricities. Dynamical excitation of planets in the outer system reduces the multiplicity of Kepler-detectable planets in the inner system in ~20-25 per cent of our systems. Accounting for the occurrence rates of wide-orbit planets and binary stars, ≈18 per cent of close-in systems could be destabilized by their outer companions in this way. This provides some contribution to the apparent excess of systems with a single transiting planet compared to multiple; however, it only contributes at most 25 per cent of the excess. The effects of the outer dynamics can generate systems similar to Kepler-56 (two coplanar planets significantly misaligned with the host star) and Kepler-108 (two significantly non-coplanar planets in a binary). We also identify three pathways to the formation of eccentric warm Jupiters resulting from the interaction between outer and inner systems: direct inelastic collision between an eccentric outer and an inner planet; secular eccentricity oscillations that may 'freeze out' when scattering resolves in the outer system; and scattering in the inner system followed by 'uplift', where inner planets are removed by interaction with the outer planets. In these scenarios, the formation of eccentric warm Jupiters is a signature of a past history of violent dynamics among massive planets beyond ~1 au. [ABSTRACT FROM AUTHOR]

We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs (WDs) with masses less than a critical mass of MWD, crit = 0.2 M⊙ undergo stable mass transfer and evolve into ultracompact X-ray binaries. Systems with higher mass WDs experience unstable mass transfer, which leads to tidal disruption of the WD. Our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-Eddington mass-transfer rates. We find that the eccentricities expected for WD-NS binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. Our results are broadly consistent with the observed numbers of both ultracompact X-ray binaries and radio pulsars with WD companions. The observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass WDs. [ABSTRACT FROM AUTHOR]

Many observed giant planets lie on eccentric orbits. Such orbits could be the result of strong scatterings with other giant planets. The same dynamical instability that produces these scatterings may also cause habitable planets in interior orbits to become ejected, destroyed, or be transported out of the habitable zone. We say that a habitable planet has resilient habitability if it is able to avoid ejections and collisions and its orbit remains inside the habitable zone. Here we model the orbital evolution of rocky planets in planetary systems where giant planets become dynamically unstable. We measure the resilience of habitable planets as a function of the observed, present-day masses and orbits of the giant planets.We find that the survival rate of habitable planets depends strongly on the giant planet architecture. Equal-mass planetary systems are far more destructive than systems with giant planets of unequal masses. We also establish a link with observation; we find that giant planets with present-day eccentricities higher than 0.4 almost never have a habitable interior planet. For a giant planet with a presentday eccentricity of 0.2 and semimajor axis of 5 au orbiting a Sun-like star, 50 per cent of the orbits in the habitable zone are resilient to the instability. As semimajor axis increases and eccentricity decreases, a higher fraction of habitable planets survive and remain habitable. However, if the habitable planet has rocky siblings, there is a significant risk of rocky planet collisions that would sterilize the planet. [ABSTRACT FROM AUTHOR]

Instabilities in planetary systems can result in the ejection of planets from their host system, resulting in free-floating planets (FFPs). If this occurs in a star cluster, the FFP may remain bound to the star cluster for some time and interact with the other cluster members until it is ejected. Here, we use N-body simulations to characterize close star-planet and planet-planet encounters and the dynamical fate of the FFP population in star clusters containing 500-2000 single or binary star members. We find that FFPs ejected from their planetary system at low velocities typically leave the star cluster 40 per cent earlier than their host stars, and experience tens of close (<1000 au) encounters with other stars and planets before they escape. The fraction of FFPs that experiences a close encounter depends on both the stellar density and the initial velocity distribution of the FFPs. Approximately half of the close encounters occur within the first 30 Myr, and only 10 per cent occur after 100 Myr. The periastron velocity distribution for all encounters is well described by a modified Maxwell-Bolzmann distribution, and the periastron distance distribution is linear over almost the entire range of distances considered, and flattens off for very close encounters due to strong gravitational focusing. Close encounters with FFPs can perturb existing planetary systems and their debris structures, and they can result in re-capture of FFPs. In addition, these FFP populations may be observed in young star clusters in imaging surveys; a comparison between observations and dynamical predictions may provide clues to the early phases of stellar and planetary dynamics in star clusters. [ABSTRACT FROM AUTHOR]

ABSTRACT We consider a popular model for long-duration gamma-ray bursts, in which the progenitor star, a stripped helium core, is spun up by tidal interactions with a black hole companion in a compact binary. We perform population synthesis calculations to produce a representative sample of such binaries, and model the effect that the companion has on material that falls back on to the newly formed black hole. Taking the results of hydrodynamic models of black hole formation by fallback as our starting point, we show that the companion has two principal effects on the fallback process. First, a break forms in the accretion curve at around 104 s. Secondly, subsequent to the break, we expect to see a flare of total energy around 1050 erg. We show that the break and flare times are set largely by the semimajor axis of the binary at the time of explosion, and that this correlates negatively with the flare energy. Although comparison with observations is non-trivial, we show that our predicted break times are comparable to those found in the X-ray light curves of canonical long-duration gamma-ray bursts. Similarly, the flare properties that we predict are consistent with the late-time flares observed in a subsample of bursts. [ABSTRACT FROM AUTHOR]

We present the observed offsets of short-duration gamma-ray bursts (SGRBs) from their putative host galaxies and compare them with the expected distributions of merging compact object binaries, given the observed properties of the hosts. We find that for all but one burst in our sample the offsets are consistent with this model. For the case of bursts with massive elliptical host galaxies, the circular velocities of the hosts' haloes exceed the natal velocities of almost all our compact object binaries. Hence, the extents of the predicted offset distributions for elliptical galaxies are determined largely by their spatial extents. In contrast, for spiral hosts, the galactic rotation velocities are smaller than typical binary natal velocities and the predicted burst offset distributions are more extended than the galaxies. One SGRB, 060502B, apparently has a large radial offset that is inconsistent with an origin in a merging galactic compact binary. Although it is plausible that the host of GRB 060502B is misidentified, our results show that the large offset is compatible with a scenario where at least a few per cent of SGRBs are created by the merger of compact binaries that form dynamically in globular clusters. [ABSTRACT FROM AUTHOR]

To date, there have been several detections of high-mass black hole binaries in both the Milky Way and other galaxies. For some of these, the spin parameter of the black hole has been estimated. As many of these systems are quite tight, a suggested origin of the spin is angular momentum imparted by the synchronous rotation of the black hole progenitor with its binary companion. Using Cygnus X-1, the best studied high-mass black hole binary, we investigate this possibility. We find that such an origin of the spin is not likely, and our results point rather to the spin being the result of processes during the collapse. [ABSTRACT FROM AUTHOR]

Most of the observed extrasolar planets are found on tight and often eccentric orbits. The high eccentricities are not easily explained by planet-formation models, which predict that planets should be on rather circular orbits. Here we explore whether fly-bys involving planetary systems with properties similar to those of the gas giants in the Solar system can produce planets with properties similar to the observed planets. Using numerical simulations, we show that fly-bys can cause the immediate ejection of planets, and sometimes also lead to the capture of one or more planets by the intruder. More common, however, is that fly-bys only perturb the orbits of planets, sometimes leaving the system in an unstable state. Over time-scales of a few million to several hundred million years after the fly-by, this perturbation can trigger planet-planet scatterings, leading to the ejection of one or more planets. For example, in the case of the four gas giants of the Solar system, the fraction of systems from which at least one planet is ejected more than doubles in yr after the fly-by. The remaining planets are often left on more eccentric orbits, similar to the eccentricities of the observed extrasolar planets. We combine our results of how fly-bys affect Solar-system-like planetary systems, with the rate at which encounters in young stellar clusters occur. For example, we measure the effects of fly-bys on the four gas giants in the Solar system. We find, that for such systems, between 5 and 15 per cent suffer ejections of planets in yr after fly-bys in typical open clusters. Thus, encounters in young stellar clusters can significantly alter the properties of any planets orbiting stars in clusters. As a large fraction of stars which populate the solar neighbourhood form in stellar clusters, encounters can significantly affect the properties of the observed extrasolar planets. [ABSTRACT FROM AUTHOR]

To measure the onset of mass transfer in eccentric binaries, we have developed a two-phase smoothed particle hydrodynamics (SPH) technique. Mass transfer is important in the evolution of close binaries, and a key issue is to determine the separation at which mass transfer begins. The circular case is well understood and can be treated through the use of the Roche formalism. To treat the eccentric case, we use a newly developed two-phase system. The body of the donor star is made up from high-mass water particles, whilst the atmosphere is modelled with low-mass oil particles. Both sets of particles take part fully in SPH interactions. To test the technique, we model circular mass-transfer binaries containing a donor star and a white dwarf; such binaries are thought to form cataclysmic variable (CV) systems. We find that we can reproduce a reasonable CV mass-transfer rate, and that our extended atmosphere gives a separation that is too large by approximately 16 per cent, although its pressure scale height is considerably exaggerated. We use the technique to measure the semimajor axis required for the onset of mass transfer in binaries with a mass ratio of and a range of eccentricities. Comparing to the value obtained by considering the instantaneous Roche lobe at pericentre, we find that the radius of the star required for mass transfer to begin decreases systematically with increasing eccentricity. [ABSTRACT FROM AUTHOR]

Most observed extrasolar planets have masses similar to, but orbits very different from, the gas giants of our Solar system. Many are much closer to their parent stars than would have been expected and their orbits are often rather eccentric. We show that some of these planets might have formed in systems much like our Solar system, i.e. in systems where the gas giants were originally on orbits with a semimajor axis of several au, but where the masses of the gas giants were all rather similar. If such a system is perturbed by another star, strong planet–planet interactions follow, causing the ejection of several planets while leaving those remaining on much tighter and more eccentric orbits. The eccentricity distribution of these perturbed systems is very similar to that of the observed extrasolar planets with semimajor axis between 1 and 6 au. [ABSTRACT FROM AUTHOR]

We show that collisions with stellar-mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al. We show that the missing objects are low-mass giants and asymptotic giant branch stars in the range . Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of smoothed particle hydrodynamic calculations, restricted three-body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main-sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the band. However, depleting the brighter objects out to the required radius would require a large population of black hole impactors which would in turn deplete the giants in a region much larger than is observed. We conclude that collisions with stellar-mass black holes cannot account for the depletion of the very brightest giants, and we use our results to place limits on the population of stellar-mass black holes in the Galactic Centre. [ABSTRACT FROM AUTHOR]

The stars that populate the solar neighbourhood were formed in stellar clusters. Through N-body simulations of these clusters, we measure the rate of close encounters between stars. By monitoring the interaction histories of each star, we investigate the singleton fraction in the solar neighbourhood. A singleton is a star which formed as a single star, has never experienced any close encounters with other stars or binaries, or undergone an exchange encounter with a binary. We find that, of the stars which formed as single stars, a significant fraction is not singletons once the clusters have dispersed. If some of these stars had planetary systems, with properties similar to those of the Solar System, the planets' orbits may have been perturbed by the effects of close encounters with other stars or the effects of a companion star within a binary. Such perturbations can lead to strong planet–planet interactions which eject several planets, leaving the remaining planets on eccentric orbits. Some of the single stars exchange into binaries. Most of these binaries are broken up via subsequent interactions within the cluster, but some remain intact beyond the lifetime of the cluster. The properties of these binaries are similar to those of the observed binary systems containing extrasolar planets. Thus, dynamical processes in young stellar clusters will alter significantly any population of Solar System-like planetary systems. In addition, beginning with a population of planetary systems exactly resembling the Solar System around single stars, dynamical encounters in young stellar clusters may produce at least some of the extrasolar planetary systems observed in the solar neighbourhood. [ABSTRACT FROM AUTHOR]

Recent comparative observations of long-duration gamma-ray bursts (LGRBs) and core collapse supernova (cc SN) host galaxies demonstrate that these two, highly energetic transient events are distributed very differently upon their hosts. LGRBs are much more concentrated on their host galaxy light than cc SN. Here we explore the suggestion that this differing distribution reflects different progenitor masses for LGRBs and cc SN. Using a simple model we show that, assuming cc SN arise from stars with main-sequence masses >8 , GRBs are likely to arise from stars with initial masses . This difference can naturally be explained by the requirement that stars which create a LGRB must also create a black hole. [ABSTRACT FROM AUTHOR]

Cosmological long-duration gamma-ray bursts (LGRBs) are thought to originate from the core collapse to black holes (BHs) of stripped massive stars. Those with sufficient rotation form a centrifugally supported torus whose collapse powers the GRB. We investigate the role of tidal locking within a tight binary as a source of the necessary angular momentum. We find that the binary orbit must be no wider than a few solar radii for a torus to form upon core collapse. Comparing this criterion to the observed population of binaries containing two compact objects suggests that rotation may have been important in the formation of up to 50 per cent of the observed systems. As these systems created a neutron star and not a BH, they presumably did not produce highly luminous GRBs. We suggest instead that they make the subset of GRBs in the relatively local universe which have much lower luminosity. [ABSTRACT FROM AUTHOR]

We have modelled the formation of the eccentric double-degenerate binaries J1141−6545 and B2303+46 using the Henyey-type full stellar evolution code STARS and the population synthesis code BSE. We find that the outcome depends strongly on the common envelope (CE) evolution efficiency parameter and show that both systems can be modelled with a single value of . The final orbit of the system depends critically on the order of Roche lobe filling events. The phase space of progenitors and the different evolutionary pathways followed by binary stars that form eccentric double-degenerate binaries in the two codes are compared. We show that the pathways are similar between the codes and that the distribution of progenitors in mass and separation phase space is qualitatively the same, thus validating the use of BSE-like population synthesis for simulations of this type. The phase space of initial parameters is very different to that obtained using ad hoc arguments about the evolution, which shows that such arguments are insufficient to model evolutionary pathways of this complexity. There are some differences associated with the prescription adopted for CE evolution but these are not qualitatively significant. We investigate the dependence of the formation mechanism on wind mass loss and the CE efficiency parameter , showing that it depends strongly on the latter but rather less on the former. [ABSTRACT FROM AUTHOR]

We model collisions between pre-main-sequence stars using a smoothed particle hydrodynamics method. Assuming that all collisions lead to simple mergers, we use derived merger cross-sections to calculate the time-scale to make a star by collisions within the core of a stellar cluster as a function of stellar number density. We show that a star may be produced in this manner within 106 yr beginning with a cluster core of 200 stars within a radius of 0.0025 pc. Encounters between one high-mass star and one low-mass star tend to result in the tidal shredding of the latter, producing a massive disc around the former. This disc spreads viscously and provided a much larger target than any star for subsequent collisions. If a star strikes the disc, it is likely to be captured, and so forms a binary with the other star. Subsequent encounters between the binary and single stars lead either to exchanges or to the formation of merged objects. The inclusion of this effect leads to a significant reduction in the time taken to produce a star. We also consider the role played by primordial binaries. We show that the time-scale required to produce a star decreases with increasing binary fraction. We find that the number of primordial binaries is reduced by encounters. The core of a cluster must therefore contain a very high binary fraction initially if a large fraction of the massive stars are to be contained within binaries when the star is produced. [ABSTRACT FROM AUTHOR]

We study close encounters involving massive main-sequence stars and the evolution of the exotic products of these encounters as common-envelope systems or possible hypernova progenitors. We show that parabolic encounters between low- and high-mass stars and between two high-mass stars with small periastrons result in mergers on time-scales of a few tens of stellar free-fall times (a few tens of hours). We show that such mergers of unevolved low-mass stars with evolved high-mass stars result in little mass-loss and can deliver sufficient fresh hydrogen to the core of the collision product to allow the collision product to burn for several million years. We find that grazing encounters enter a common-envelope phase which may expel the envelope of the merger product. The deposition of energy in the envelopes of our merger products causes them to swell by factors of ∼100. If these remnants exist in very densely populated environments , they will suffer further collisions which may drive off their envelopes, leaving behind hard binaries. We show that the products of collisions have cores rotating sufficiently rapidly to make them candidate hypernova/gamma-ray burst progenitors and that ∼0.1 per cent of massive stars may suffer collisions, sufficient for such events to contribute significantly to the observed rates of hypernovae and gamma-ray bursts. [ABSTRACT FROM AUTHOR]

We investigate the structure and evolution of blue straggler stars, which were formed from direct stellar collisions between main-sequence stars in globular clusters. In particular, we look at the rotational evolution of the products of off-axis collisions. As found in previous work, such blue stragglers initially have too high an angular momentum to contract down to the main sequence. We consider angular momentum loss through either disc locking or locking to an outflowing wind, and we show that both methods allow the merged object to shed sufficient angular momentum to contract down to the main sequence. [ABSTRACT FROM AUTHOR]

We present a simple, semi-analytical description for the final stages of mergers of black hole–neutron star (BH–NS) systems. Such systems are of much interest as gravitational wave sources and gamma-ray burst progenitors. Numerical studies show that in general the neutron star is not disrupted at the first phase of mass transfer. Instead, what remains of the neutron star is left on a wider, eccentric, orbit. We consider the evolution of such systems as they lose angular momentum via gravitational radiation and come into contact for further phases of mass transfer. During each mass transfer event the neutron star mass is reduced until a critical value where mass loss leads to a rapid increase in the stellar radius. At this point Roche lobe overflow shreds what remains of the neutron star, most of the mass forming a disc around the black hole. Such a disc may be massive enough to power a gamma-ray burst. The mass of the neutron star at the time of disruption (and therefore the disc mass) is largely independent of the initial masses of the black hole and neutron star, indicating that BH–NS star mergers may be standard candles. [ABSTRACT FROM AUTHOR]

Recent Hubble Space Telescope observations of a large sample of globular clusters reveal that every cluster contains between 40 and 400 blue stragglers. The population does not correlate with either stellar collision rate (as would be expected if all blue stragglers were formed via collisions) or total mass (as would be expected if all blue stragglers were formed via the unhindered evolution of a subset of the stellar population). In this paper, we support the idea that blue stragglers are made through both channels. The number produced via collisions tends to increase with cluster mass. In this paper we show how the current population produced from primordial binaries decreases with increasing cluster mass; exchange encounters with third, single stars in the most massive clusters tend to reduce the fraction of binaries containing a primary close to the current turn-off mass. Rather, their primaries tend to be somewhat more massive ( ) and have evolved off the main sequence, filling their Roche lobes in the past, often converting their secondaries into blue stragglers (but more than 1 Gyr or so ago and thus they are no longer visible as blue stragglers). We show that this decline in the primordial blue straggler population is likely to be offset by the increase in the number of blue stragglers produced via collisions. The predicted total blue straggler population is therefore relatively independent of cluster mass, thus matching the observed population. This result does not depend on any particular assumed blue straggler lifetime. [ABSTRACT FROM AUTHOR]

We investigate a means of explaining the apparent paucity of red giant stars within post-core-collapse globular clusters. We propose that collisions between the red giants and binary systems can lead to the destruction of some proportion of the red giant population, by either knocking out the core of the red giant or by forming a common envelope system which will lead to the dissipation of the red giant envelope. Treating the red giant as two point masses, one for the core and another for the envelope (with an appropriate force law to take account of the distribution of mass), and the components of the binary system also treated as point masses, we utilize a four-body code to calculate the time-scales on which the collisions will occur. We then perform a series of smooth particle hydrodynamics runs to examine the details of mass transfer within the system. In addition, we show that collisions between single stars and red giants lead to the formation of a common envelope system which will destroy the red giant star. We find that low-velocity collision between binary systems and red giants can lead to the destruction of up to 13 per cent of the red giant population. This could help to explain the colour gradients observed in PCC globular clusters. We also find that there is the possibility that binary systems formed through both sorts of collision could eventually come into contact perhaps producing a population of cataclysmic variables. [ABSTRACT FROM AUTHOR]

We investigate the observed depletion of red giants in the cores of post-core-collapse globular clusters. In particular, the evolutionary scenario we consider is a binary consisting of two low-mass stars which undergoes two common-envelope phases. The first common-envelope phase occurs when the primary is a red giant resulting in a helium white dwarf and main-sequence star in a detached binary. The second common-envelope phase occurs shortly after the secondary becomes a red giant. During the second common-envelope phase, the degenerate helium cores merge resulting in a core mass greater than the helium burning limit and the formation of a horizontal branch star. We show that stellar encounters enhance this evolutionary route in post-core-collapse clusters. These encounters increase the population of binary secondaries which would have evolved on to the red giant branch in the recent past. [ABSTRACT FROM AUTHOR]

Recent three-dimensional, high-resolution simulations of neutron star coalescences are analysed to assess whether short gamma-ray bursts (GRBs) could originate from such encounters. The two most popular modes of energy extraction – namely the annihilation of and magnetohydrodynamic processes – are explored in order to investigate their viability in launching GRBs. We find that annihilation can provide the necessary stresses to drive a highly relativistic expansion. However, unless the outflow is beamed into less than 1 per cent of the solid angle, this mechanism may fail to explain the apparent isotropized energies implied for short GRBs at cosmological distances. We argue that the energetic, neutrino-driven wind that accompanies the merger event will have enough pressure to provide adequate collimation to the -annihilation-driven jet, thereby comfortably satisfying constraints on event rate and apparent luminosity. We also assess magnetic mechanisms to transform the available energy into a GRB. If the central object does not collapse immediately into a black hole, it will be convective and it is expected to act as an effective large scale dynamo, amplifying the seed magnetic fields to a few times 1017 G within a small fraction of a second. The associated spindown time-scale is 0.2 s, coinciding with the typical duration of a short GRB. The efficiencies of the various assessed magnetic processes are high enough to produce isotropized luminosities in excess of 1052 erg s−1 even without beaming. [ABSTRACT FROM AUTHOR]

We present the results of 3D high-resolution calculations of the last inspiral stages and the final coalescence of neutron star binary systems. The equations of hydrodynamics are solved using the smoothed particle hydrodynamics method with up to 10[sup 6] particles. Using Newtonian gravity, but adding the forces emerging from the emission of gravitational waves, we focus on the impact of microphysics on the dynamical evolution of the merger; namely, we use a new equation of state based on the relativistic mean field approach of Shen et al. Neutrino emission of all flavours, the resulting cooling and the change in the electron fraction are accounted for with a detailed leakage scheme. The new equation of state is substantially stiffer than the Lattimer–Swesty equation of state that has been used in previous investigations. This leads the system to become dynamically unstable at a separation as large as 3.3 stellar radii, where the secular orbital decay undergoes a transition towards a dynamical ‘plunge’ of the binary components towards each other. As soon as the stars come into contact a Kelvin–Helmholtz instability forms at the interface of both stars. The peak temperatures are found in the vortex rolls that form during this process. We generally find slightly lower temperatures than previously found using the Lattimer–Swesty equation of state. The central object is surrounded by a very thick disc that shows cool equatorial flows: inflows in the inner parts of the disc and outflow further out. The cool inflows become shock heated in the innermost parts of the disc and lead to an outflow of hot material in the vertical direction. The temperatures in the disc have typical values of 3–4 MeV , lower than the temperatures found in previous investigations using the Lattimer–Swesty equation of state. These conditions allow for the existence of heavy nuclei even in the inner parts of the disc, we find typical mass fractions of ∼0.1, which is enough for scattering off heavy nuclei to be the dominant source of neutrino opacity. The central object formed during the coalescence shows a rapid, differential rotation with periods of ∼2 ms. Although a final conclusion on this point is not possible from our basically Newtonian approach, we argue that the central object will remain stable without collapsing to a black hole, at least on the simulation time-scale of 20 ms, but possibly for as long as ∼100 s, mainly stabilized by differential rotation. The massive, differentially rotating central object is expected to wind up initial magnetic fields to enormous field strengths of ∼10[sup 17] G and may therefore have important implications for this event as a central engine of gamma-ray bursts. [ABSTRACT FROM AUTHOR]

We present the results of multiple simulations of open clusters, modelling the dynamics of a population of brown dwarf members. We consider the effects of a large range of primordial binary populations, including the possibilities of having brown dwarf members contained within a binary system. We also examine the effects of various cluster diameters and masses. Our examination of a population of wide binary systems containing brown dwarfs, reveals evidence for exchange reactions whereby the brown dwarf is ejected from the system and replaced by a heavier main-sequence star. We find that there exists the possibility of hiding a large fraction of the brown dwarfs contained within the primordial binary population. We conclude that it is probable that the majority of brown dwarfs are contained within primordial binary systems which then hides a large proportion of them from detection. [ABSTRACT FROM AUTHOR]

We performed high-resolution simulations of two stellar collisions relevant for stars in globular clusters. We considered one head-on collision and one off-axis collision between two 0.6-M[sub ⊙] main-sequence stars. We show that a resolution of about 100 000 particles is sufficient for most studies of the structure and evolution of blue stragglers. We demonstrate conclusively that collision products between main-sequence stars in globular clusters do not have surface convection zones larger than 0.004 M[sub ⊙] after the collision, nor do they develop convection zones during the ‘pre-main-sequence’ thermal relaxation phase of their post-collision evolution. Therefore, any mechanism which requires a surface convection zone (i.e. chemical mixing or angular momentum loss via a magnetic wind) cannot operate in these stars. We show that no disc of material surrounding the collision product is produced in off-axis collisions. The lack of both a convection zone and a disc proves a continuing problem for the angular momentum evolution of blue stragglers in globular clusters. [ABSTRACT FROM AUTHOR]

We consider gamma-ray bursts (GRBs) produced by the merger of a massive white dwarf with a neutron star. We show that these are likely to produce long-duration GRBs, in some cases definitely without an accompanying supernova, as observed recently. This class of burst would have a strong correlation with star formation, and occur close to the host galaxy. However, rare members of the class need not be near star-forming regions, and could have any type of host galaxy. Thus a long-duration burst far from any star-forming region would also be a signature of this class. Estimates based on the existence of a known progenitor suggest that our proposed class may be an important contributor to the observed GRB rate. [ABSTRACT FROM AUTHOR]

We have computed the Galactic trajectories of 12 hypervelocity stars (HVSs) under the assumption that they originated in the Galactic Centre. We show that eight of these 12 stars are bound to the Galaxy. We consider the subsequent trajectories of the bound stars to compute their characteristic orbital period, which is 2 Gyr. All eight bound stars are moving away from the centre of the Galaxy, which implies that the stars' lifetimes are less than 2 Gyr. We thus infer that the observed HVSs are massive main-sequence stars, rather than blue horizontal branch stars. The observations suggest that blue HVSs are ejected from the Galactic Centre roughly every 15 Myr. This is consistent with the observed population of blue stars in extremely tight orbits round the central supermassive black hole (SMBH), the so-called S-stars, if we assume that the HVSs are produced by the breakup of binaries. One of the stars in such a binary is ejected at high velocities to form a HVS; the other remains bound to the SMBH as an S-star. We further show that the one high-velocity system observed to be moving towards the Galactic Centre, SDSS J172226.55+594155.9, could not have originated in the Galactic Centre; rather, we identify it as a halo object. [ABSTRACT FROM AUTHOR]