Do Hot Jupiters Migrate due to Stellar Encounters?

Exoplanets: Migration of giants

Author:

Triaud

Abstract:

The origin of hot Jupiters, large gaseous planets in close orbits around stars, is unknown. Observations suggest that such planets are abundant in stellar clusters, and can result from encounters with other celestial bodies.

On stability of planetary motion during stellar approaches

On stability of planetary motion during stellar approaches

Authors:

Gasanov et al

Abstract:

We consider motion of a passive-gravitating body when a test star (perturbing body) approaches the central body. An integral invariant relation – a quasi-integral (Lukyanov, 2010) – is found by using exact expression of the force function, and regions of possible motion of a passive-gravitating body are determined. The surfaces of minimal energy (a generalization of zero velocity surfaces) are plotted, singular points of these surfaces are determined, their type and Lyapunov stability are established. Hill's stability of planetary motion is investigated for the case of a test star approaching the Solar system. Criteria for capture of a passive-gravitating body by a test star being possible and impossible are derived. Based on Hill stability criteria, we find critical parameters of the test star's orbit that leave planets bound to the Solar system.

Mass transfer between debris disks during close stellar encounters

Mass transfer between debris discs during close stellar encounters

Authors:

Jilkova et al

Abstract:

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.

Misaligned hot Jupiters can be Produced by Stellar Encounters

Generation of highly inclined protoplanetary discs through single stellar flybys

Authors:

Xiang-Gruess et al

Abstract:

We study the three-dimensional evolution of a viscous protoplanetary disc which is perturbed by a passing star on a parabolic orbit. The aim is to test whether a single stellar flyby is capable to excite significant disc inclinations which would favour the formation of so-called misaligned planets. We use smoothed particle hydrodynamics to study inclination, disc mass and angular momentum changes of the disc for passing stars with different masses. We explore different orbital configurations for the perturber's orbit to find the parameter spaces which allow significant disc inclination generation. Prograde inclined parabolic orbits are most destructive leading to significant disc mass and angular momentum loss. In the remaining disc, the final disc inclination is only below 20∘. This is due to the removal of disc particles which have experienced the strongest perturbing effects. Retrograde inclined parabolic orbits are less destructive and can generate disc inclinations up to 60∘. The final disc orientation is determined by the precession of the disc angular momentum vector about the perturber's orbital angular momentum vector and by disc orbital inclination changes.

We propose a sequence of stellar flybys for the generation of misalignment angles above 60∘. The results taken together show that stellar flybys are promising and realistic for the explanation of misaligned Hot Jupiters with misalignment angles up to 60\degr.

Sedna: The Exoplanet in our own Solar System

How Sedna and family were captured in a close encounter with a solar sibling

Authors:

Jilkova et al

Abstract:

The discovery of 2012VP113 initiated the debate on the origin of the Sedna family of planetesimals in orbit around the Sun. Sednitos roam the outer regions of the Solar System between the Egeworth--Kuiper belt and the Oort cloud, in extraordinary wide (a greater than 150au) orbits with a large perihelion distance of q greater than 30au compared to the Earth's (a=1au and eccentricity e=(1-q/a) ~ 0.0167 or q=1au). This population is composed of a dozen objects, which we consider a family because they have similar perihelion distance and inclination with respect to the ecliptic i=10--30deg. They also have similar argument of perihelion omega=340+/-55deg. There is no ready explanation for their origin. Here we show that these orbital parameters are typical for a captured population from the planetesimal disk of another star.Assuming the orbital elements of Sednitos have not changed since they acquired their orbits, we reconstruct the encounter that led to their capture. We conclude that they might have been captured in a near miss with a 1.8MSun star that impacted the Sun at ~340au at an inclination with respect to the ecliptic of 17--34deg with a relative velocity at infinity of ~4.3km/s. We predict that the Sednitos-region is populated by 930 planetesimals and the inner Oort cloud acquired ~440 planetesimals through the same encounter.

How Much Will a Stellar Cluster Disrupt Exoplanetary Formation?

The fragility of planetary systems

Authors:

Portegies et al

Abstract:

We specify the range to which perturbations penetrate a planetesimal system. Such perturbations can originate from massive planets or from encounters with other stars. The latter can have an origin in the star cluster in which the planetary system was born, or from random encounters once the planetary system has escaped its parental cluster. The probability of a random encounter, either in a star cluster or in the Galactic field depends on the local stellar density, the velocity dispersion and the time spend in that environment. By adopting order of magnitude estimates we argue that the majority of planetary systems born in open clusters will have a {\em Parking zone}, in which planetesimals are affected by encounters in their parental star cluster but remain unperturbed after the star has left the cluster. Objects found in this range of semi-major axis and eccentricity preserve the memory of the encounter that last affected their orbits, and they can therefore be used to reconstruct this encounter. Planetary systems born in a denser environment, such as in a globular cluster are unlikely to have a Parking zone. We further argue that some planetary systems may have a {\em Frozen zone}, in which orbits are not affected either by the more inner massive planets or by external influences. Objects discovered in this zone will have preserved information about their formation in their orbital parameters.

Free Floating Exoplanets in Stellar Clusters

Close encounters involving free-floating planets in star clusters

Authors:

Wang et al

Abstract:

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 characterise 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% earlier than their host stars, and experience tens of close (less than 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% 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.

Binary Stellar System RW Aurigae's Disk Explained by Tidal Encounter

A tidal encounter caught in the act: modelling a star-disc fly-by in the young RW Aurigae system

Authors:

Dai et al

Abstract:

RW Aurigae (RW Aur) is a binary star system with a long molecular arm trailing the primary star. Cabrit et al. (2006) noted the resemblance between this extended structure and the tidal arm stripped from the primary star in the simulations of star-disc encounters by Clarke & Pringle (1993). In this paper we use new hydrodynamical models and synthetic observations to fit many of the parameters of RW Aur. Using hydrodynamic models we find that the morphological appearance of RW Aur can be indeed explained by a tidal encounter with the secondary star. We reproduce all the major morphological and kinematic features of the system. Using radiative transfer calculations, we find that synthetic CO and dust continuum observations of our hydrodynamic models agree well with observations. We reproduce all the main features of the line profiles, from emission fluxes to the optical depth of the different components of the system. The agreement between observations and simulations thus lends strong support to the hypothesis of a tidal encounter scenario. Finally, we propose a possible solution for the origin of the dimming of the primary star observed in 2010/2011 by Rodriguez et al. (2013).

Dancing With the Stars, the Menage au Trois at Fomalhaut

Dancing with the stars: formation of the Fomalhaut triple system and its effect on the debris discs

Authors:

Shannon et al

Abstract:

Fomalhaut is a triple system, with all components widely separated (∼105 au). Such widely separated binaries are thought to form during cluster dissolution, but that process is unlikely to form such a triple system. We explore an alternative scenario, where A and C form as a tighter binary from a single molecular cloud core (with semimajor axis ∼104 au), and B is captured during cluster dispersal. We use N-body simulations augmented with the Galactic tidal forces to show that such a system naturally evolves into a Fomalhaut-like system in about half of cases, on a time-scale compatible with the age of Fomalhaut. From initial non-interacting orbits, Galactic tides drive cycles in B's eccentricity that lead to a close encounter with C. After several close encounters, typically lasting tens of millions of years, one of the stars is ejected. The Fomalhaut-like case with both components at large separations is almost invariably a precursor to the ejection of one component, most commonly Fomalhaut C. By including circumstellar debris in a subset of the simulations, we also show that such an evolution usually does not disrupt the coherently eccentric debris disc around Fomalhaut A, and in some cases can even produce such a disc. We also find that the final eccentricity of the disc around A and the disc around C are correlated, which may indicate that the dynamics of the three stars stirred C's disc, explaining its unusual brightness.

What Happens to Protoplanetary Disks After Encountering Another Star

Sizes of protoplanetary discs after star-disc encounters

Authors:

Breslau et al

Abstract:

Most stars do not form in isolation, but as part of a star cluster or association. These young stars are initially surrounded by protoplanetary discs. In these cluster environments tidal interactions with other cluster members can alter the disc properties. Besides the disc frequency, its mass, angular momentum, and energy, in particular the disc's size is prone to being changed by a passing star. So far the change in disc size was only investigated for a small number of very specific encounters. Several studies investigated the effect of the cluster environment on the sizes of planetary systems, like our own solar system, based on a generalisation of information from this limited sample. We performed numerical simulations covering the wide parameter space typical for young star clusters, to test the validity of this approach. Here the sizes of discs after encounters are presented, based on a size definition which is comparable to that one used in observational studies. We find that, except for encounters between equal-mass stars, the usually applied estimates are insufficient. They tend to severely overestimate the remaining disc size. We show that the disc size after an encounter can be described by a relatively simple dependence on the periastron distance and the mass ratio of the encounter partners. This knowledge allows, for example, to pin down the types of encounter possibly responsible for the structure of today's solar system.

Class O Stars can be Disruptive to Planet Formation in Nebulae

The Orion Nebula is home to hundreds of young stars and even younger protostars known as proplyds. Many of these nascent systems will go on to develop planets, while others will have their planet-forming dust and gas blasted away by the fierce ultraviolet radiation emitted by massive O-type stars that lurk nearby.

A team of astronomers from Canada and the United States has used the Atacama Large Millimeter/submillimeter Array (ALMA) to study the often deadly relationship between highly luminous O-type stars and nearby protostars in the Orion Nebula. Their data reveal that protostars within 0.1 light-years (about 600 billion miles) of an O-type star are doomed to have their cocoons of dust and gas stripped away in just a few millions years, much faster than planets are able to form.

"O-type stars, which are really monsters compared to our Sun, emit tremendous amounts of ultraviolet radiation and this can play havoc during the development of young planetary systems," remarked Rita Mann, an astronomer with the National Research Council of Canada in Victoria, and lead author on a paper in the Astrophysical Journal. "Using ALMA, we looked at dozens of embryonic stars with planet-forming potential and, for the first time, found clear indications where protoplanetary disks simply vanished under the intense glow of a neighboring massive star."

Many, if not all, Sun-like stars are born in crowded stellar nurseries similar to the Orion Nebula. Over the course of just a few million years, grains of dust and reservoirs of gas combine into larger, denser bodies. Left relatively undisturbed, these systems will eventually evolve into fully fledged star systems, with planets -- large and small -- and ultimately drift away to become part of the galactic stellar population.

Astronomers believe that massive yet short-lived stars in and around large interstellar clouds are essential for this ongoing process of star formation. At the end of their lives, massive stars explode as supernovas, seeding the surrounding area with dust and heavy elements that will get taken up in the next generation of stars. These explosions also provide the kick necessary to initiate a new round of star and planet formation. But while they still shine bright, these larger stars can be downright deadly to planets if an embryonic solar systems strays too close.

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Simulating the Effects of Stars Passing Close to Exoplanet Systems

Effects of stellar flybys on planetary systems: 3D modeling of the circumstellar disks damping effects

Authors:

Picogna et al

Abstract:

Stellar flybys in star clusters are suspected to affect the orbital architecture of planetary systems causing eccentricity excitation and orbital misalignment between the planet orbit and the equatorial plane of the star. We explore whether the impulsive changes in the orbital elements of planets, caused by an hyperbolic stellar flyby, can be fully damped by the circumstellar disk surrounding the star. The time required to disperse stellar clusters is in fact comparable to circumstellar disk's lifetime. We have modelled in 3D a system made of a solar type star surrounded by a low density disk with a giant planet embedded in it approached on a hyperbolic encounter trajectory by a second star, of similar mass and with its own disk. We focus on extreme configurations where a very deep stellar flyby perturbs a Jovian planet on an external orbit. This allows to test in full the ability of the disk to erase the effects of the stellar encounter. We find that the amount of mass lost by the disk during the stellar flyby is less than in 2D models where a single disk was considered due to the mass exchange between the two disks at the encounter. The damping in eccentricity is slightly faster than in 2D models and it occurs on timescales of the order of a few kyr. The only trace of the flyby left in the planet system, after about 10^4 yr, is a small misalignment, lower than 9 degrees, between the star equatorial plane and the planet orbit. In a realistic model based on 3D simulations of star--planet--disk interactions, we find that stellar flybys cannot excite significant eccentricities and inclinations of planets in stellar clusters. The circumstellar disks hosting the planets damp on a short timescale all the step changes in the two orbital parameters produced during any stellar encounter. All records of past encounters are erased.