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Register a new userGiant planet swaps during close stellar encounters
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The discovery of planetary systems outside of the solar system has challenged some of the tenets of planetary formation. Among the difficult-to-explain observations, are systems with a giant planet orbiting a very-low mass star, such as the recently discovered GJ~3512b planetary system, where a Jupiter-like planet orbits an $M$-star in a tight and eccentric orbit. Systems such as this one are not predicted by the core accretion theory of planet formation. Here we suggest a novel mechanism, in which the giant planet is born around a more typical Sun-like star ($M_{*,1}$), but is subsequently exchanged during a dynamical interaction with a flyby low-mass star ($M_{*,2}$). We perform state-of-the-art $N$-body simulations with $M_{*,1}=1M_odot$ and $M_{*,2}=0.1M_odot$ to study the statistical outcomes of this interaction, and show that exchanges result in high eccentricities for the new orbit around the low-mass star, while about half of the outcomes result in tighter orbits than the planet had around its birth star. We numerically compute the cross section for planet exchange, and show that an upper limit for the probability per planetary system to have undergone such an event is $Gammasim 4.4(M_{rm c}/100M_odot)^{-2}(a_{rm p}/{rm AU}) (sigma/1,{rm km},{rm s}^{-1})^{5}$Gyr$^{-1}$, where $a_{rm p}$ is the planet semi-major axis around the birth star, $sigma$ the velocity dispersion of the star cluster, and $M_{rm c}$ the total mass of the star cluster. Hence these planet exchanges could be relatively common for stars born in open clusters and groups, should already be observed in the exoplanet database, and provide new avenues to create unexpected planetary architectures.
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We study mass transfers between debris discs during stellar encounters. We carried out numerical simulations of close flybys of two stars, one of which has a disc of planetesimals represented by test particles. We explored the parameter space of the encounters, varying the mass ratio of the two stars, their pericentre and eccentricity of the encounter, and its geometry. We find that particles are transferred to the other star from a restricted radial range in the disc and the limiting radii of this transfer region depend on the parameters of the encounter. We derive an approximate analytic description of the inner radius of the region. The efficiency of the mass transfer generally decreases with increasing encounter pericentre and increasing mass of the star initially possessing the disc. Depending on the parameters of the encounter, the transfer particles have a specific distributions in the space of orbital elements (semimajor axis, eccentricity, inclination, and argument of pericentre) around their new host star. The population of the transferred particles can be used to constrain the encounter through which it was delivered. We expect that many stars experienced transfer among their debris discs and planetary systems in their birth environment. This mechanism presents a formation channel for objects on wide orbits of arbitrary inclinations, typically having high eccentricity but possibly also close-to-circular (eccentricities of about 0.1). Depending on the geometry, such orbital elements can be distinct from those of the objects formed around the star.
The majority of stars form in a clustered environment. This has an impact on the evolution of surrounding protoplanetary discs (PPDs) due to either photoevaporation or tidal truncation. Consequently, the development of planets depends on formation environment. Here we present the first thorough investigation of tidally induced angular momentum loss in PPDs in the distant regime, partly motivated by claims in the literature for the importance of distant encounters in disc evolution. We employ both theoretical predictions and dynamical/hydrodynamical simulations in 2D and 3D. Our theoretical analysis is based on that of Ostriker (1994) and leads us to conclude that in the limit that the closest approach distance $x_{min} gg r$, the radius of a particle ring, the fractional change in angular momentum scales as $(x_{min}/r)^{-5}$. This asymptotic limit ensures that the cumulative effect of distant encounters is minor in terms of its influence on disc evolution. The angular momentum transfer is dominated by the $m=2$ Lindblad resonance for closer encounters and by the $m=1$, $omega = 0$ Lindblad resonance at large $x_{min}/r$. We contextualise these results by comparing expected angular momentum loss for the outer edge of a PPD due to distant and close encounters. Contrary to the suggestions of previous works we do not find that distant encounters contribute significantly to angular momentum loss in PPDs. We define an upper limit for closest approach distance where interactions are significant as a function of arbitrary host to perturber mass ratio $M_2/M_1$.
Aims: We aim to investigate the consequences of a fast massive stellar remnant - a black hole (BH) or a neutron star (NS) - encountering a planetary system. Methods: We modelled a close encounter between the actual Solar System (SS) and a $2,M_odot$ NS and a $10,M_odot$ BH, using a few-body symplectic integrator. We used a range of impact parameters, orbital phases at the start of the simulation derived from the current SS orbital parameters, encounter velocities, and incidence angles relative to the plane of the SS. Results: We give the distribution of possible outcomes, such as when the SS remains bound, when it suffers a partial or complete disruption, and in which cases the intruder is able to capture one or more planets, yielding planetary systems around a BH or a NS. We also show examples of the long-term stability of the captured planetary systems.
(abbreviated) We extend the theory of close encounters of a planet on a parabolic orbit with a star to include the effects of tides induced on the central rotating star. Orbits with arbitrary inclination to the stellar rotation axis are considered. We obtain results both from an analytic treatment and numerical one that are in satisfactory agreement. These results are applied to the initial phase of the tidal circularisation problem. We find that both tides induced in the star and planet can lead to a significant decrease of the orbital semi-major axis for orbits having periastron distances smaller than 5-6 stellar radii (corresponding to periods $sim 4-5$ days after the circularisation has been completed) with tides in the star being much stronger for retrograde orbits compared to prograde orbits. We use the simple Skumanich law for the stellar rotation with its rotational period equal to one month at the age of 5Gyr. The strength of tidal interactions is characterised by circularisation time scale, $t_{ev}$ defined as a time scale of evolution of the planets semi-major axis due to tides considered as a function of orbital period $P_{obs}$ after the process of tidal circularisation has been completed. We find that the ratio of the initial circularisation time scales corresponding to prograde and retrograde orbits is of order 1.5-2 for a planet of one Jupiter mass and $P_{obs}sim $ four days. It grows with the mass of the planet, being of order five for a five Jupiter mass planet with the same $P_{orb}$. Thus, the effect of stellar rotation may provide a bias in the formation of planetary systems having planets on close orbits around their host stars, as a consequence of planet-planet scattering, favouring systems with retrograde orbits. The results may also be applied to the problem of tidal capture of stars in young stellar clusters.
We analyze KMT-2019-BLG-1339, a microlensing event with an obvious but incompletely resolved brief anomaly feature around the peak of the light curve. Although the origin of the anomaly is identified to be a companion to the lens with a low mass ratio $q$, the interpretation is subject to two different degeneracy types. The first type is the ambiguity in $rho$, representing the angular source radius scaled to the angular radius of the Einstein ring, $theta_{rm E}$, and the other is the $sleftrightarrow s^{-1}$ degeneracy. The former type, `finite-source degeneracy, causes ambiguities in both $s$ and $q$, while the latter induces an ambiguity only in $s$. Here $s$ denotes the separation (in units of $theta_{rm E}$) in projection between the lens components. We estimate that the lens components have masses $(M_1, M_2)sim (0.27^{+0.36}_{-0.15}~M_odot, 11^{+16}_{-7}~M_{rm J})$ and $sim (0.48^{+0.40}_{-0.28}~M_odot, 1.3^{+1.1}_{-0.7}~M_{rm J})$ according to the two solutions subject to the finite-source degeneracy, indicating that the lens comprises an M dwarf and a companion with a mass around the planet/brown dwarf boundary or a Jovian-mass planet. It is possible to lift the finite-source degeneracy by conducting future observations utilizing a high resolution instrument because the relative lens-source proper motion predicted by the solutions are widely different.
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