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In hierarchical triple systems, the inner binary is slowly perturbed by a distant companion, giving rise to large-scale oscillations in eccentricity and inclination, known as von-Zeipel–Lidov–Kozai oscillations. Stable systems with a mild hierarchy, where the period ratio is not too small, require an additional corrective term, known as the Brown Hamiltonian, to adequately account for their long-term evolution. Although the Brown Hamiltonian has been used to accurately describe the highly eccentric systems on circulating orbits where the periapse completes a complete revolution, the analysis near its elliptical fixed points had been overlooked. We derive analytically the modified fixed points including the Brown Hamiltonian and analyse its librating orbits (where the periapse motion is limited in range). We compare our result to the direct three-body integrations of millions of orbits and discuss the regimes of validity. We numerically discover the regions of orbital instability, allowed and forbidden librating zones with a complex, fractal, structure. The retrograde orbits, where the mutual inclination is |$\iota \ \gt\ 90\ \rm deg$|⁠ , are more stable and allowed to librate for larger areas of the parameter space. We find numerical fits for the librating-circulating boundary. Finally, we discuss the astrophysical implications for systems of satellites, stars, and compact objects. In a companion paper (Paper II), we apply our formalism to the orbits of irregular satellites around giant planets. [ABSTRACT FROM AUTHOR]

Irregular satellites (ISs) are believed to have been captured during the Solar system's dynamical history and provide clues for the Solar system's formation and evolution. ISs occupy a large fraction of the Hill sphere of their host planet and their orbits are highly perturbed by the Sun. We use a novel formalism developed in Paper I to characterize their orbits in terms of an effective secular Hamiltonian (the Brown Hamiltonian) that accounts for their large orbital separations. We find that prograde satellites generally follow the Brown Hamiltonian, while retrograde satellites (which extend further) deviate more significantly. Nevertheless, the phase portrait is much better described by the Brown Hamiltonian for all satellites. We construct a semi-analytical criterion that predicts the librating orbit based on the effective energy due to the Brown Hamiltonian. We also check our results with highly accurate N -body integrations of satellite orbits, where initial conditions are loaded directly from the updated ephemeris from the NASA Horizons data base. Although the retrograde librating orbits occupy more area in the parameter space, the vast majority of librating ISs are prograde. Using our method, we find 13 librating satellites, 8 of them previously known to librate, and the rest shown to librate for the first time. Further observations of existing and new satellites could shed more light on the dynamical history of the Solar system and satellite formation and test our results. [ABSTRACT FROM AUTHOR]

Accretion discs in active galactic nuclei (AGNs) foster black hole (BH) formation, growth, and mergers. Stellar mass BHs migrate inwards under the influence of hydrodynamical torques unless they encounter a region where the torque flips sign. At these migration traps, BHs accumulate and merge via dynamical or gas-assisted interactions, producing high-frequency LIGO/Virgo/KAGRA (LVK) gravitational wave (GW) sources and potentially cutting off the supply of extreme mass ratio inspirals that would otherwise make low-frequency, LISA -band GWs. In this paper, we study the interplay between different types of migration torques, focusing especially on the 'thermal torques' generated by the thermal response of the AGN to embedded stellar-mass BHs that accrete through their own mini-discs. In contrast to previous work, we find that Type I torques cannot produce migration traps on their own, but thermal torques often do, particularly in low-mass AGN. The migration traps produced by thermal torques exist at much larger distances (∼103−5 gravitational radii) than do previously identified Type I traps, carrying implications for GW populations at multiple frequencies. Finally, we identify a bifurcation of AGN discs into two regimes: migration traps exist below a critical AGN luminosity, and do not at higher luminosities. This critical luminosity is fit as |$\log _{10} L_{\rm AGN}^c = 45 {\!-\!} 0.32 \log _{10}{(\alpha /0.01)}$| where α is the Shakura–Sunyaev viscosity parameter, a range compatible with recent claims that LVK GWs are not preferentially associated with high-luminosity AGN. [ABSTRACT FROM AUTHOR]

Following its flyby and first imaging of the Pluto-Charon binary, the New Horizons spacecraft visited the Kuiper belt object (KBO) 2014 MU.sub.69 (also known as (486958) Arrokoth). The imaging showed MU.sub.69 to be a contact binary that rotates at a low spin period (15.92 hours), is made of two individual lobes connected by a narrow neck and has a high obliquity (about 98 degrees).sup.1, properties that are similar to those of other KBO contact binaries inferred through photometric observations.sup.2. However, all scenarios suggested so far for the origins of such configurations.sup.3-5 have failed to reproduce these properties and their probable frequent occurrence in the Kuiper belt. Here we show that semi-secular perturbations.sup.6,7 operating on only ultrawide KBO binaries close to their stability limit can robustly lead to gentle, slow binary mergers at arbitrarily high obliquities but low rotational velocities, reproducing the characteristics of MU.sub.69 and other similar oblique contact binaries. Using N-body simulations, we find that approximately 15 per cent of all ultrawide binaries with a cosine-uniform inclination distribution.sup.5,9 are likely to merge through this process. Moreover, we find that such mergers are sufficiently gentle to deform the shape of the KBO only slightly. The semi-secular contact binary formation channel not only explains the observed properties of MU.sub.69, but may also apply to other Kuiper belt or asteroid belt binaries and in the Solar System and extra-solar moon systems. The high obliquity and low rotation period of the Kuiper belt object (2014) MU.sub.69 and other similar contact binaries is successfully reproduced from the collision and post-collision characteristics of initially wide binaries., Author(s): Evgeni Grishin [sup.1] , Uri Malamud [sup.1] , Hagai B. Perets [sup.1] , Oliver Wandel [sup.2] , Christoph M. Schäfer [sup.2] Author Affiliations: (1) Physics Department, Technion-Israel Institute of [...]

Many stars live in hierarchical triple systems, but the physics of such systems are still poorly understood. One understudied physical aspect of these systems is tertiary tides, wherein the tidal deformation of a tertiary in a hierarchical triple drains energy from the inner binary, causing the inner binary's orbital separation to decrease. This tidal process is difficult to observe directly, since such an observation requires a very compact hierarchical triple, the tertiary of which must be almost large enough to fill its Roche lobe at the epoch of observation. Concurrently, the recently discovered stellar system TIC242132789 is the fourth most compact observed hierarchical triple, and the most compact in which the tertiary is a giant. In this paper, we demonstrate that TIC242132789 provides a rare opportunity to place constraints on the model parameters for tertiary tides, and can even be a rare opportunity to directly observe tertiary-tides-induced orbital shrinkage of the inner binary. We calculate our expectations of how fast the inner orbit will shrink, and demonstrate that our estimates of this rate of shrinkage should be observable using current techniques. We conclude with a call for relevant observations of this system to commence. [ABSTRACT FROM AUTHOR]

TIC 470710327, a massive compact hierarchical triple-star system, was recently identified by NASA's Transiting Exoplanet Survey Satellite. TIC 470710327 is comprised of a compact (1.10 d) circular eclipsing binary, with total mass |$\approx 10.9\!-\!13.2\, \rm {M_{\odot }}$|⁠ , and a more massive |$\approx 14\!-\!17\, \rm {M_{\odot }}$| eccentric non-eclipsing tertiary in a 52.04 d orbit. Here, we present a progenitor scenario for TIC 470710327 in which '2 + 2' quadruple dynamics result in Zeipel–Lidov–Kozai oscillations that lead to a contact phase of the more massive binary. In this scenario, the two binary systems should form in a very similar manner, and dynamics trigger the merger of the more massive binary either during late phases of star formation or several Myr after the zero-age main sequence, when the stars begin to expand. Any evidence that the tertiary is a highly magnetized (∼1–10 kG), slowly rotating blue main-sequence star would hint towards a quadruple origin. Finally, our scenario suggests that the population of inclined compact multiple-stellar systems is reduced into coplanar systems, via mergers, late during star formation or early in the main sequence. The elucidation of the origin of TIC 470710327 is crucial in our understanding of multiple massive star formation and evolution. [ABSTRACT FROM AUTHOR]

Recent surveys show that wide (>104 AU) binaries and triples are abundant in the field. We study the long-term evolution of wide hierarchical triple systems and the role played by the Galactic tidal (GT) field. We find that when the time-scales of the secular von–Ziepel–Lidov–Kozai and the GT oscillations are comparable, triple evolution becomes chaotic which leads to extreme eccentricities. Consequently, the close pericentre approaches of the inner-binary components lead to strong interactions, mergers, and collisions. We use a novel secular evolution code to quantify the key parameters and carry out a population-synthesis study of low and intermediate-mass wide-orbit triples. We find that in |$\sim 9{{\ \rm per\ cent}}$| of low-mass wide-triples the inner main-sequence binaries collide or tidally-inspiral within |$10\ \rm Gyr$|⁠ , with direct collisions are 6 times more likely to occur. For the intermediate-mass sample, |$\sim 7.6{{\ \rm per\ cent}}$| of the systems merge or inspiral with roughly equal probabilities. We discuss the relative fractions of different stellar merger/inspiral outcomes as a function of their evolutionary stage (Main Sequence, MS; Red Giant, RG; or White Dwarf, WD), their transient electromagnetic signatures and the final products of the merger/inspiral. In particular, the rate of WD–WD direct-collisions that lead to type-Ia Supernovae is comparable to other dynamical channels and accounts for at most |$0.1{{\ \rm per\ cent}}$| of the observed rate. RG inspirals provide a novel channel for the formation of eccentric common-envelope-evolution binaries. The catalysis of mergers/collisions in triples due to GT could explain a significant fraction, or even the vast majority, of blue-stragglers in the field, produce progenitors for cataclysmic-variables, and give-rise to mergers/collisions of double-RG binaries. [ABSTRACT FROM AUTHOR]

Active galactic nuclei (AGNs) are prominent environments for stellar capture, growth, and formation. These environments may catalyse stellar mergers and explosive transients, such as thermonuclear and core-collapse supernovae (SNe). SN explosions in AGN discs generate strong shocks, leading to unique observable signatures. We develop an analytical model that follows the evolution of the shock propagating in the disc until it eventually breaks out. We derive the peak luminosity, bolometric light curve, and breakout time. The peak luminosities may exceed 1045 erg s−1 and last from hours to days. The brightest explosions occur in regions of reduced density: either off-plane, or in discs around low-mass central black holes (⁠|${\sim} 10^6\ \rm {M}_\odot$|⁠), or in starved subluminous AGNs. Explosions in the latter two sites are easier to observe due to a reduced AGN background luminosity. We perform suites of 1D Lagrangian radiative hydrodynamics snec code simulations to validate our results and obtain the luminosity in different bands, and 2D axisymmetric Eulerian hydrodynamics code hormone simulations to study the morphology of the ejecta and its deviation from spherical symmetry. The observed signature is expected to be a bright blue, UV or X-ray flare on top of the AGN luminosity from the initial shock breakout, while the subsequent red part of the light curve will largely be unobservable. We estimate the upper limit for the total event rate to be |$\mathcal {R}\lesssim 100\ \rm yr^{-1}\ Gpc^{-3}$| for optimal conditions and discuss the large uncertainties in this estimate. Future high-cadence transient searches may reveal these events. Some existing tidal disruption event candidates may originate from AGN SNe. [ABSTRACT FROM AUTHOR]

The Pluto–Charon binary system is the best studied representative of the binary Kuiper-belt population. Its origins are vital to understanding the formation of other Kuiper-belt objects (KBO) and binaries and the evolution of the outer Solar system. The Pluto–Charon system is believed to form following a giant impact between two massive KBOs at relatively low velocities. However, the likelihood of a random direct collision between two of the most massive KBOs is low and is further constrained by the requirement of a low-velocity collision, making this a potentially fine-tuned scenario. Here, we expand our previous studies and suggest that the proto-Pluto–Charon system was formed as a highly inclined wide-binary, which was then driven through secular/quasi-secular evolution into a direct impact. Since wide-binaries are ubiquitous in the Kuiper belt with many expected to be highly inclined, our scenario is expected to be robust. We use analytic tools and few-body simulations of the triple Sun–(proto-)Pluto–Charon system to show that a large parameter space of initial conditions leads to such collisions. The velocity of such an impact is the escape velocity of a bound system, which naturally explains the low-velocity impact. The dynamical evolution and the origins of the Pluto–Charon system could therefore be traced to similar secular origins as those of other binaries and contact-binaries (e.g. Arrokoth) and suggest that they play a key role in the evolution of KBOs. [ABSTRACT FROM AUTHOR]

Scalar, tensor waves induce oscillatory perturbations in Keplerian systems that can be probed with measurements of pulsar timing residuals. In this paper, we consider the imprint of coherent oscillations produced by ultralight axion dark matter on the Roemer time delay. We use the angle-action formalism to calculate the time evolution of the observed signal and its dependence on the orbital parameters and the axion phase. We derive exact analytical expressions for arbitrary binary pulsar mass ratio and eccentricity, alleviating the need for long numerical integrations. We emphasize the similarity of the expected signal-to-noise ratio with the response of a harmonic oscillator to an external oscillatory driving. We validate our theoretical predictions with numerical simulations. Our results furnish a useful benchmark for numerical codes and analysis procedures and, hopefully, will motivate the search for such imprints in real data. [ABSTRACT FROM AUTHOR]

Detection of gravitational-wave (GW) sources enables the characterization of binary compact objects (COs) and of their in-spiral. However, other dissipative processes can affect the in-spiral. Here, we show that the in-spiral of COs through a gaseous common envelope (CE) arising from an evolved stellar companion produces a novel type of GW sources, whose evolution is dominated by the dissipative gas dynamical friction effects from the CE, rather than the GW emission itself. The evolution and properties of the GW signals differ from those of isolated gas-poor mergers significantly. We find characteristic strains of ∼10−23–10−21 (⁠|$10\, {\rm kpc}/{D}$|⁠) for such sources – observable by next-generation space-based GW detectors (at rates of once per a few centuries for LISA, and about once a year for BBO). The evolution of the GW signal can serve as a probe of the interior regions of the evolved star, and the final stages of CE evolution, otherwise inaccessible through other observational means. Moreover, such CE mergers are frequently followed by observable explosive electromagnetic counterparts and/or the formation of exotic stars. [ABSTRACT FROM AUTHOR]

Tertiary tides (TTs), or the continuous tidal distortion of the tertiary in a hierarchical triple system, can extract energy from the inner binary, inducing within it a proclivity to merge. Despite previous work on the subject, which established that it is significant for certain close triple systems, it is still not a well-understood process. A portion of our ignorance in this regard stems from our inability to integrate a simulation of this phenomenon into conventional stellar evolution codes, since full calculations of these tidal interactions are computationally expensive on stellar evolution time-scales. Thus, to attain a better understanding of how these TTs act on longer time-scales, an empirical expression of its effects as a function of parameters of the triple system involved is required. In this work, we evaluate the rate at which TTs extract energy from the inner binary within a series of constructed hierarchical triple systems under varying parameters, and study the rate at which the inner binary orbital separation shrinks as a function of those parameters. We find that this rate varies little with the absolute values of the masses of the three component objects, but is very sensitive to the mass ratio of the inner binary q , the tertiary radius R 3, the inner binary orbital separation a 1, the outer orbital separation a 2, and the viscoelastic relaxation time of the tertiary τ. More specifically, we find that the percentage by which a 1 shrinks per unit time can be reasonably approximated by (1/ a 1)(d a 1/d t) = (2.22 × 10−8 yr−1)4 q (1 +  q)−2(R 3/100 R⊙)5.2(a 1/0.2 au)4.8(a 2/2 au)−10.2 (τ/0.534 yr)−1.0. We also provide tests of how precise this fitting function is. [ABSTRACT FROM AUTHOR]

The discovery of the intact minor planet embedded in the debris disc orbiting SDSS J1228+1040 raises questions about the dynamical history of the system. Further, the recent passage of the potentially interstellar object 1I/'Oumuamua within the Solar system has re-ignited interest in minor body flux through exoplanetary systems. Here, we utilize the new analytical formalism from Grishin et al. (2019) to estimate the rate at which the gaseous components of typical white dwarf discs trap an exo-planetesimal. We compare the types of captured orbits which arise from planetesimals originating from the interstellar medium, exo-Kuiper belts, and exo-Oort clouds. We find that the rate of interstellar medium injection is negligible, whereas capture of both exo-Kuiper and exo-Oort cloud planetesimals is viable, but strongly size-dependent. For a gaseous disc which extends much beyond its Roche limit, capture is more probable than disruption at the Roche limit. We find that the capture probability linearly increases with the radial extent of the disc. Even in systems without minor planets, capture of smaller bodies will change the disc size distribution and potentially its temporal variability. Our formalism is general enough to be applied to future discoveries of embedded planetesimals in white dwarf debris discs. [ABSTRACT FROM AUTHOR]

Nuclear star clusters surrounding supermassive black holes (SMBHs) in galactic nuclei contain large numbers of stars, black holes (BHs), and neutron stars (NSs), a fraction of which are likely to form binaries. These binaries were suggested to form a triple system with the SMBH, which acts as a perturber and may enhance BH and NS mergers via the Lidov–Kozai mechanism. We follow-up previous studies, but for the first time perform an extensive statistical study of BH–BH, NS–NS, and BH–NS binary mergers by means of direct high-precision regularized N -body simulations, including post-Newtonian (PN) terms up to order PN2.5. We consider different SMBH masses, slopes for the BH mass function, binary semimajor axis and eccentricity distributions, and different spatial distributions for the binaries. We find that the merger rates are a decreasing function of the SMBH mass and are in the ranges ∼0.17–0.52, ∼0.06–0.10, and ∼0.04–0.16 Gpc−3 yr−1 for BH–BH, BH–NS, and NS–NS binaries, respectively. However, the rate estimate from this channel remains highly uncertain and depends on the specific assumptions regarding the star formation history in galactic nuclei and the supply rate of compact objects (COs). We find that |${\sim } 10\!-\!20{{\ \rm per\ cent}}$| of the mergers enter the LIGO band with eccentricities ≳0.1. We also compare our results to the secular approximation, and show that N -body simulations generally predict a larger number of mergers. Finally, these events can also be observable via their electromagnetic counterparts, thus making these CO mergers especially valuable for cosmological and astrophysical purposes. [ABSTRACT FROM AUTHOR]

Planet formation begins with collisional growth of small planetesimals accumulating into larger ones. Such growth occurs while planetesimals are embedded in a gaseous protoplanetary disc. However, small planetesimals experience collisions and gas drag that lead to their destruction on short time-scales, not allowing, or requiring fine-tuned conditions for the efficient growth of ∼metre-sized objects. Here we show that ∼104 interstellar objects such as the recently detected 1I/2017-U1 ('Oumuamua) could have been captured, and become part of the young Solar system, together with up to hundreds of ∼km-sized ones. The capture rates are robust even for conservative assumptions on the protoplanetary disc structure, local stellar environment, and planetesimal interstellar medium density. 'Seeding' of such planetesimals then catalyses further planetary growth into planetary embryos, and potentially alleviates the main challenges with the metre-sized growth barrier. The capture model is in synergy with the current leading planet formation theories, providing the missing link to the first planetesimals. Moreover, planetesimal capture provides a far more efficient route for lithopanspermia than previously thought. [ABSTRACT FROM AUTHOR]

In hierarchical triple systems, the inner binary is perturbed by a distant companion. For large mutual inclinations, the Lidov–Kozai mechanism secularly excites large eccentricity and inclination oscillations of the inner binary. The maximal eccentricity attained, e max, is simply derived and widely used. However, for mildly hierarchical systems (i.e. the companion is relatively close and massive), non-secular perturbations affect the evolution. Here we account for fast non-secular variations and find a new analytic formula for e max, in terms of the system's hierarchy level, correcting previous work and reproducing the orbital flip criteria. We find that e maxis generally enhanced, allowing closer encounters between the inner binary components, thus significantly changing their interaction and its final outcome. We then extend our approach to include additional relativistic and tidal forces. Using our results, we show that the merger time of gravitational-wave (GW) sources orbiting massive black holes in galactic nuclei is enhanced compared with previous analysis accounting only for the secular regime. Consequently, this affects the distribution and rates of such GW sources in the relevant mild-hierarchy regime. We test and confirm our predictions with direct N -body and 2.5-level post-Newtonian codes. Finally, we calculate the formation and disruption rates of hot Jupiters (HJs) in planetary systems using a statistical approach, which incorporates our novel results for e max. We find that more HJs migrate from further out, but they are also tidally disrupted more frequently. Remarkably, the overall formation rate of HJs remains similar to that found in previous studies. Nevertheless, the different rates could manifest in different underlying distribution of observed warm Jupiters. [ABSTRACT FROM AUTHOR]

We study the chaotic and secular evolution of hierarchical quadruple systems in the 3 + 1 configuration, focusing on the evolution of mutual inclination of the inner binaries as the system undergoes coupled Lidov-Kozai (LK) oscillations. We include short-range forces (SRF; such as those due to tidal and rotational distortions) that control the eccentricity excitation of the inner binary. The evolution of mutual inclination is described, a priori, by two dimensionless parameters, R0, the ratio between the inner and outer LK time-scales and ϵSRF, the ratio between the SRF precession and the inner LK precession rates.We find that the chaotic zones for the mutual inclination depend mainly on R0, while ϵSRF controls mainly the range of eccentricity excitation. The mutual inclination evolves chaotically for 1 ≲ R0 ≲ 10, leading to large misalignments. For 0.4 ≲ R0 ≲ 0.8, the system could be weakly excited and produce bimodal distribution of mutual inclination angles. Our results can be applied to exomoons- planets in stellar binaries andWarm/Hot Jupiters in stellar triples. Such systems could develop large mutual inclination angles if the inner binary is tight enough, and also high eccentricities, depending on the strength of the SRFs. Future detections of tilted Warm/Hot Jupiters and exomoons could put our mechanism under observational tests. [ABSTRACT FROM AUTHOR]