Hot Jupiters Driven by High-eccentricity Migration in Globular Clusters

Hot Jupiters Driven by High-eccentricity Migration in Globular Clusters

Authors:

Hammers et al

Abstract:

Hot Jupiters (HJs) are short-period giant planets that are observed around $\sim 1 \% $ of solar-type field stars. One possible formation scenario for HJs is high-eccentricity (high-e) migration, in which the planet forms at much larger radii, is excited to high eccentricity by some mechanism, and migrates to its current orbit due to tidal dissipation occurring near periapsis. We consider high-e migration in dense stellar systems such as the cores of globular clusters (GCs), in which encounters with passing stars can excite planets to the high eccentricities needed to initiate migration. We study this process via Monte Carlo simulations of encounters with a star+planet system including the effects of tidal dissipation, using an efficient regularized restricted three-body code. HJs are produced in our simulations over a significant range of the stellar number density ${n}_{\star }$. Assuming the planet is initially on a low-eccentricity orbit with semimajor axis 1 au, for ${n}_{\star }\lesssim {10}^{3}\,{\mathrm{pc}}^{-3}$ the encounter rate is too low to induce orbital migration, whereas for ${n}_{\star }\gtrsim {10}^{6}\,{\mathrm{pc}}^{-3}$ HJ formation is suppressed because the planet is more likely ejected from its host star, tidally disrupted, or transferred to a perturbing star. The fraction of planets that are converted to HJs peaks at $\approx 2 \% $ for intermediate number densities of $\approx 4\times {10}^{4}\,{\mathrm{pc}}^{-3}$. Warm Jupiters, giant planets with periods between 10 and 100 days, are produced in our simulations with an efficiency of up to $\approx 0.5 \% $. Our results suggest that HJs can form through high-e migration induced by stellar encounters in the centers of of dense GCs, but not in their outskirts where the densities are lower.

The discovery of WASP-151b, WASP-153b, WASP-156b: Insights on giant planet migration and the upper boundary of the Neptunian desert

The discovery of WASP-151b, WASP-153b, WASP-156b: Insights on giant planet migration and the upper boundary of the Neptunian desert
Authors:

Demangeon et al 

Abstract:
To investigate the origin of the features discovered in the exoplanet population, the knowledge of exoplanets' mass and radius with a good precision is essential. In this paper, we report the discovery of three transiting exoplanets by the SuperWASP survey and the SOPHIE spectrograph with mass and radius determined with a precision better than 15 %. WASP-151b and WASP-153b are two hot Saturns with masses, radii, densities and equilibrium temperatures of 0.31^{+0.04}_{-0.03} MJ, 1.13^{+0.03}_{-0.03} RJ, 0.22^{-0.03}_{-0.02} rhoJ and 1, 290^{+20}_{-10} K, and 0.39^{+0.02}_{-0.02} MJ, 1.55^{+0.10}_{-0.08} RJ, 0.11^{+0.02}_{-0.02} rhoJ and 1, 700^{+40}_{-40} K, respectively. Their host stars are early G type stars (with magV ~ 13) and their orbital periods are 4.53 and 3.33 days, respectively. WASP-156b is a Super-Neptune orbiting a K type star (magV = 11.6) . It has a mass of 0.128^{+0.010}_{-0.009} MJ, a radius of 0.51^{+0.02}_{-0.02} RJ, a density of 1.0^{+0.1}_{-0.1} rhoJ, an equilibrium temperature of 970^{+30}_{-20} K and an orbital period of 3.83 days. WASP-151b is slightly inflated, while WASP-153b presents a significant radius anomaly. WASP-156b, being one of the few well characterised Super-Neptunes, will help to constrain the formation of Neptune size planets and the transition between gas and ice giants. The estimates of the age of these three stars confirms the tendency for some stars to have gyrochronological ages significantly lower than their isochronal ages. We propose that high eccentricity migration could partially explain this behaviour for stars hosting a short period planet. Finally, these three planets also lie close to (WASP-151b and WASP-153b) or below (WASP-156b) the upper boundary of the Neptunian desert. Their characteristics support that the ultra-violet irradiation plays an important role in this depletion of planets observed in the exoplanet population.

The fate of close-in planets: tidal or magnetic migration?

The fate of close-in planets: tidal or magnetic migration?

Authors: 

Strugarek et al 

Abstract: 

Planets in close-in orbits interact magnetically and tidally with their host stars. These interactions lead to a net torque that makes close-in planets migrate inward or outward depending on their orbital distance. We compare systematically the strength of magnetic and tidal torques for typical observed star-planet systems (T-Tauri & hot Jupiter, M dwarf & Earth-like planet, K star & hot Jupiter) based on state-of-the-art scaling-laws. We find that depending on the characteristics of the system, tidal or magnetic effects can dominate. For very close-in planets, we find that both torques can make a planet migrate on a timescale as small as 10 to 100 thousands of years. Both effects thus have to be taken into account when predicting the evolution of compact systems.

Diffusive Tidal Evolution for Migrating hot Jupiters

Diffusive Tidal Evolution for Migrating hot Jupiters

Author:

Wu

Abstract:

I consider a Jovian planet on a highly eccentric orbit around its host star, a situation possibly produced by secular interactions with its planetary or stellar companions. At every periastron passage, tidal interactions lead to an energy exchange between the orbit and the planet's internal oscillations (predominantly an l=2 f-mode). Starting from zero energy, this f-mode can be diffusively excited if the one-kick energy gain is greater than (ωPorb)−1 of the orbital energy. This occurs at a pericentre distance of 4 tidal radii (or 1.6 Roche radius). Furthermore, when the f-mode has a non-negligible initial energy, this diffusive evolution can set in at a much reduced threshold. The first finding is important for stalling the secular migration. The f-mode can absorb orbital energy and decouple the planet from its secular perturbers, parking all migrating jupiters safely outside the zone of tidal disruption. The second finding is important for circularizing the planet's orbit. It allows an excited f-mode to continuously absorb orbital energy even when the one-kick energy is weakening along the path of circularization (due to increasing pericentre distance). So without any explicit dissipation, other than the fact that the f-mode will damp nonlinearly when its amplitude reaches unity, the planet can be transported from a few AU to 0.2 AU in 10^4 yrs. Such a rapid circularization corresponds to an equivalent tidal dissipation factor Q ~ 1, and it explains the observed deficit of super-eccentric Jovian planets. Lastly, the repeated f-mode breaking deposits energy and angular momentum in the outer shells of the planet. This likely alters the planet's thermal structure, but should fall short of ablating it. Overall, this work boosts the case for forming hot Jupiters through high-eccentricity secular migration.

The scattering outcomes of Kepler circumbinary planets: planet mass ratio

The scattering outcomes of Kepler circumbinary planets: planet mass ratio

Authors:

Gong et al

Abstract:
Recent studies reveal that the free eccentricities of Kepler-34b and Kepler-413b are much larger than their forced eccentricities, implying that the scattering events may take place in their formation. The observed orbital configuration of Kepler-34b cannot be well reproduced in disk-driven migration models, whereas a two-planet scattering scenario can play a significant role of shaping the planetary configuration. These studies indicate that circumbinary planets discovered by Kepler may have experienced scattering process. In this work, we extensively investigate the scattering outcomes of circumbinary planets focusing on the effects of planet mass ratio. We find that the planetary mass ratio and the the initial relative locations of planets act as two important parameters which affect the eccentricity distribution of the surviving planets. As an application of our model, we discuss the observed orbital configurations of Kepler-34b and Kepler-413b. We first adopt the results from the disk-driven models as the initial conditions, then simulate the scattering process occurred in the late evolution stage of circumbinary planets. We show that the present orbital configurations of Kepler-34b and Kepler-413b can be well reproduced when considering two unequal-mass planet ejection model. Our work further suggests that some of the currently discovered circumbinary single-planet systems may be the survivals of original multiple-planet systems. The disk-driven migration and the scattering events occurring in the late stage both play an irreplaceable role in sculpting the final systems.

Forming Different Planetary Architectures. I. The Formation Efficiency of Hot Jupiters from High-eccentricity Mechanisms

Forming Different Planetary Architectures. I. The Formation Efficiency of Hot Jupiters from High-eccentricity Mechanisms 

Authors: 

Wang et al 

Abstract: 

Exoplanets discovered over the past decades have provided a new sample of giant exoplanets: hot Jupiters. For lack of enough materials in the current locations of hot Jupiters, they are perceived to form outside the snowline. Then, they migrate to the locations observed through interactions with gas disks or high-eccentricity mechanisms. We examined the efficiencies of different high-eccentricity mechanisms for forming hot Jupiters in near-coplanar multi-planet systems. These mechanisms include planet–planet scattering, the Kozai–Lidov mechanism, coplanar high-eccentricity migration, and secular chaos, as well as other two new mechanisms that we present in this work, which can produce hot Jupiters with high inclinations even in retrograde. We find that the Kozai–Lidov mechanism plays the most important role in producing hot Jupiters among these mechanisms. Secular chaos is not the usual channel for the formation of hot Jupiters due to the lack of an angular momentum deficit within ${10}^{7}{T}_{\mathrm{in}}$ (periods of the inner orbit). According to comparisons between the observations and simulations, we speculate that there are at least two populations of hot Jupiters. One population migrates into the boundary of tidal effects due to interactions with the gas disk, such as ups And b, WASP-47 b, and HIP 14810 b. These systems usually have at least two planets with lower eccentricities, and remain dynamically stable in compact orbital configurations. Another population forms through high-eccentricity mechanisms after the excitation of eccentricity due to dynamical instability. These kinds of hot Jupiters usually have Jupiter-like companions in distant orbits with moderate or high eccentricities.

Hot Jupiters driven by high-eccentricity migration in globular clusters

Hot Jupiters driven by high-eccentricity migration in globular clusters

Authors:

Hamers et al

Abstract:
Hot Jupiters (HJs) are short-period giant planets that are observed around ∼1% of solar-type field stars. One possible formation scenario for HJs is high-eccentricity (high-e) migration, in which the planet forms at much larger radii, is excited to high eccentricity by some mechanism, and migrates to its current orbit due to tidal dissipation occurring near periapsis. We consider high-e migration in dense stellar systems such as the cores of globular clusters (GCs), in which encounters with passing stars can excite planets to the high eccentricities needed to initiate migration. We study this process via Monte-Carlo simulations of encounters with a star+planet system including the effects of tidal dissipation, using an efficient regularized restricted three-body code. HJs are produced in our simulations over a significant range of the stellar number density n⋆. Assuming the planet is initially on a low-eccentricity orbit with semimajor axis 1 AU, for n⋆≲103pc−3 the encounter rate is too low to induce orbital migration, whereas for n⋆≳106pc−3 HJ formation is suppressed because the planet is more likely ejected from its host star, tidally disrupted, or transferred to a perturbing star. The fraction of planets that are converted to HJs peaks at ≈2% for intermediate number densities of ≈4×104pc−3. Warm Jupiters, giant planets with periods between 10 and 100 days, are produced in our simulations with an efficiency of up to ≈0.5%. Our results suggest that HJs can form through high-e migration induced by stellar encounters in the centers of of dense GCs, but not in their outskirts where the densities are lower.

Analytical model of multi-planetary resonant chains and constraints on migration scenarios

Analytical model of multi-planetary resonant chains and constraints on migration scenarios


Author:

Delisle

Abstract:

Resonant chains are groups of planets for which each pair is in resonance, with an orbital period ratio locked at a rational value (2/1, 3/2, etc.). Such chains naturally form as a result of convergent migration of the planets in the proto-planetary disk. In this article, I present an analytical model of resonant chains of any number of planets. Using this model, I show that a system captured in a resonant chain can librate around several possible equilibrium configurations. The probability of capture around each equilibrium depends on how the chain formed, and especially on the order in which the planets have been captured in the chain. Therefore, for an observed resonant chain, knowing around which equilibrium the chain is librating allows for constraints to be put on the formation and migration scenario of the system. I apply this reasoning to the four planets orbiting Kepler-223 in a 3:4:6:8 resonant chain. I show that the system is observed around one of the six equilibria predicted by the analytical model. Using N-body integrations, I show that the most favorable scenario to reproduce the observed configuration is to first capture the two intermediate planets, then the outermost, and finally the innermost.

Resonant structure, formation and stability of the planetary system HD 155358

Resonant structure, formation and stability of the planetary system HD155358

Authors:

Silburt et al

Abstract:

Two Jovian-sized planets are orbiting the star HD155358 near exact mean motion resonance (MMR) commensurability. In this work we re-analyze the radial velocity (RV) data previously collected by Robertson et al. (2012). Using a Bayesian framework we construct two models - one that includes and one that excludes gravitational planet-planet interactions (PPI). We find that the orbital parameters from our PPI and noPPI models differ by up to 2σ, with our noPPI model being statistically consistent with previous results. In addition, our new PPI model strongly favours the planets being in MMR while our noPPI model strongly disfavours MMR. We conduct a stability analysis by drawing samples from our PPI model's posterior distribution and simulating them for 109 years, finding that our best-fit values land firmly in a stable region of parameter space.

We explore a series of formation models that migrate the planets into their observed MMR. We then use these models to directly fit to the observed RV data, where each model is uniquely parameterized by only three constants describing its migration history. Using a Bayesian framework we find that a number of migration models fit the RV data surprisingly well, with some migration parameters being ruled out.

Our analysis shows that planet-planet interactions are important to take into account when modelling observations of multi-planetary systems. The additional information that one can gain from interacting models can help constrain planet migration parameters.

The Migration Origin Story for Warm Jupiters Questioned

DYNAMICAL CONSTRAINTS ON THE ORIGIN OF HOT AND WARM JUPITERS WITH CLOSE FRIENDS

Authors:

Antonini et al

Abstract:

Gas giants orbiting their host star within the ice line are thought to have migrated to their current locations from farther out. Here we consider the origin and dynamical evolution of observed Jupiters, focusing on hot and warm Jupiters with outer friends. We show that the majority of the observed Jupiter pairs (20 out of 24) are dynamically unstable if the inner planet is placed at gsim1 au distance from the stellar host. This finding is at odds with formation theories that invoke the migration of such planets from semimajor axes gsim1 au due to secular dynamical processes (e.g., secular chaos, Lidov–Kozai [LK] oscillations) coupled with tidal dissipation. In fact, the results of N-body integrations show that the evolution of dynamically unstable systems does not lead to tidal migration but rather to planet ejections and collisions with the host star. This and other arguments lead us to suggest that most of the observed planets with a companion could not have been transported from farther out through secular migration processes. More generally, by using a combination of numerical and analytic techniques, we show that the high-e LK migration scenario can only account for less than 10% of all gas giants observed between 0.1 and 1 au. Simulations of multiplanet systems support this result. Our study indicates that rather than starting on highly eccentric orbits with orbital periods above 1 yr, these "warm" Jupiters are more likely to have reached the region where they are observed today without having experienced significant tidal dissipation.

Do Gas Giant Planets Form at Migration Convergence Zones?

THE FORMATION OF CORES OF GIANT PLANETS AT CONVERGENCE ZONES OF PLANETARY MIGRATION

Authors:

Sirono et al

Abstract:

The formation of solid cores in giant planets of mass $\sim 10\,{M}_{\oplus }$ is numerically simulated following the scenario of Sándor et al. In this scenario, there are two convergence zones, corresponding to the outer and inner edges of the dead zone, where the torque exerted on planetary embryos by the gas nebula is zero. At the outer edge of the dead zone, anticyclonic vortices accumulate infalling dust aggregates, and planetary embryos are continuously formed in this scenario. We performed N-body simulations and show that massive objects of $\simeq 10\,{M}_{\oplus }$ are formed in ~2.5 Myr, starting from the embryos. The largest object is formed at the inner convergence zone, although planetary embryos are placed at the outer convergence zone. This is due to the scattering of embryos from the outer to the inner convergence zone, and the shorter damping timescale of eccentricity at the inner convergence zone compared to the outer one. We varied the migration timescale due to the torque from gas by changing the gas surface density around the convergence zones. We found that there is a critical migration timescale below which $10\,{M}_{\oplus }$-sized objects are formed. Furthermore, we conducted simulations in which the gas surface density evolves according to viscous accretion. The largest object is also formed at the inner convergence zone irrespective of the strength of turbulence. Throughout the simulations, the location of the largest mass is the inner convergence zone. We confirmed that the formation timescale of a core of a Jovian planet can be explained in this scenario.

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.

Migration of accreting planets in radiative discs from dynamical torques

Migration of accreting planets in radiative discs from dynamical torques

Authors:

Pierens et al

Abstract:

We present the results of hydrodynamical simulations of the orbital evolution of planets undergoing runaway gas accretion in radiative discs. We consider accreting disc models with constant mass flux through the disc, and where radiative cooling balances the effect of viscous heating and stellar irradiation. We assume that 20-30 M⊕ giant planet cores are formed in the region where viscous heating dominates and migrate outward under the action of a strong corotation torque. In the case where gas accretion is neglected, we find evidence for strong dynamical torques in accreting discs with accretion rates M˙≳7×10−8M⊙/yr. Their main effect is to increase outward migration rates by a factor of ∼2 typically. In the presence of gas accretion, however, runaway outward migration is observed with the planet passing through the zero-torque radius and the transition between the viscous heating and stellar heating dominated regimes. The ability for an accreting planet to enter a fast migration regime is found to depend strongly on the planet growth rate, but can occur for values of the mass flux through the disc of M˙≳5×10−8M⊙/yr. We find that an episode of runaway outward migration can cause an accreting planet formed in the 5-10 AU region to temporarily orbit at star-planet separations as large as ∼60-70 AU. However, increase in the amplitude of the Lindblad torque associated with planet growth plus change in the streamline topology near the planet systematically cause the direction of migration to be reversed. Our results indicate that a planet can reach large orbital distances under the combined effect of dynamical torques and gas accretion, but an alternative mechanism is required to explain the presence of massive planets on wide orbits.

The Origin of Hot Jupiter CI Tau b's Orbital Eccentricity

The origin of the eccentricity of the hot Jupiter in CI Tau

Authors:

Rosotti et al

Abstract:

Following the recent discovery of the first radial velocity planet in a star still possessing a protoplanetary disc (CI Tau), we examine the origin of the planet's eccentricity (e ∼0.3). We show through long timescale (105 orbits) simulations that the planetary eccentricity can be pumped by the disc, even when its local surface density is well below the threshold previously derived from short timescale integrations. We show that the disc may be able to excite the planet's orbital eccentricity in < a Myr for the system parameters of CI Tau. We also perform two planet scattering experiments and show that alternatively the observed planet may plausibly have acquired its eccentricity through dynamical scattering of a migrating lower mass planet, which has either been ejected from the system or swallowed by the central star. In the latter case the present location and eccentricity of the observed planet can be recovered if it was previously stalled within the disc's magnetospheric cavity.

Are SuperEarths & Mini Neptunes Failed Gas Giants?

Super-Earths as Failed Cores in Orbital Migration Traps

Authors:

Hasegawa et al

Abstract:

We explore whether close-in super-Earths were formed as rocky bodies that failed to grow fast enough to become the cores of gas giants before the natal protostellar disk dispersed. We model the failed cores' inward orbital migration in the low-mass or type I regime, to stopping points at distances where the tidal interaction with the protostellar disk applies zero net torque. The three kinds of migration traps considered are those due to the dead zone's outer edge, the ice line, and the transition from accretion to starlight as the disk's main heat source. As the disk disperses, the traps move toward final positions near or just outside 1~au. Planets at this location exceeding about 3~M⊕ open a gap, decouple from their host trap, and migrate inward in the high-mass or type II regime to reach the vicinity of the star. We synthesize the population of planets formed in this scenario, finding that some fraction of the observed super-Earths can be failed cores. Most super-Earths formed this way have more than 4~M⊕, so their orbits when the disk disperses are governed by type II migration. These planets have solid cores surrounded by gaseous envelopes. Their subsequent photoevaporative mass loss is most effective for masses originally below about 6 M⊕. The failed core scenario suggests a division of the observed super-Earth mass-radius diagram into five zones according to the inferred formation history.

Migration of accreting planets in radiative discs from dynamical torques

Migration of accreting planets in radiative discs from dynamical torques

Authors:

Pierens et al

Abstract:

We present the results of hydrodynamical simulations of the orbital evolution of planets undergoing runaway gas accretion in radiative discs. We consider accreting disc models with constant mass flux through the disc, and where radiative cooling balances the effect of viscous heating and stellar irradiation. We assume that 20-30 M⊕ giant planet cores are formed in the region where viscous heating dominates and migrate outward under the action of a strong entropy-related corotation torque. In the case where gas accretion is neglected and for an α viscous stress parameter α = 2 × 10−3, we find evidence for strong dynamical torques in accreting discs with accretion rates M˙≳7×10−8M⊙/yr. Their main effect is to increase outward migration rates by a factor of ∼2 typically. In the presence of gas accretion, however, runaway outward migration is observed with the planet passing through the zero-torque radius and the transition between the viscous heating and stellar heating dominated regimes. The ability for an accreting planet to enter a fast migration regime is found to depend strongly on the planet growth rate, but can occur for values of the mass flux through the disc of M˙≳5×10−8M⊙/yr. We find that an episode of runaway outward migration can cause an accreting planet formed in the 5-10 AU region to temporarily orbit at star-planet separations as large as ∼60-70 AU. However, increase in the amplitude of the Lindblad torque associated with planet growth plus change in the streamline topology near the planet systematically cause the direction of migration to be reversed. Subsequent evolution corresponds to the planet migrating inward rapidly until it becomes massive enough to open a gap in the disc and migrate in the Type II regime. Our results indicate that a planet can reach large orbital distances under the combined effect of dynamical torques and gas accretion, but an alternative mechanism is required to explain the presence of massive planets on wide orbits.

Magnitude and timing of the giant planet instability: A reassessment from the perspective of the asteroid belt

Magnitude and timing of the giant planet instability: A reassessment from the perspective of the asteroid belt

Authors:

Toliou et al

Abstract:

It is generally accepted today that our solar system has undergone a phase during which the orbits of the giant planets became very unstable. In recent years, many studies have identified traces of this event and have provided reasonable justification for this occurrence. The magnitude (in terms of orbital variation) and the timing of the instability though (early or late with respect to the dispersal of the gas disk) still remains an open debate. The terrestrial planets seem to set a strict constraint: either the giant planet instability happened early, while the terrestrial planets were still forming, or the orbits of Jupiter and Saturn had to separate from each other impulsively, with a large enough `jump' in semimajor axis (Brasser et al. 2009; Kaib and Chambers 2016) for the terrestrial planets to remain stable. Because a large orbital jump is a low probability event, the early instability hypothesis seems to be favored. However, the asteroid belt would also evolve in a different way, assuming different instability amplitudes. These two constraints need to match each other in order to favor one scenario over the other. Considering an initially dynamically cold disk of asteroids, Morbidelli et al. (2010) concluded that a comparably large jump is needed to reconstruct the current asteroid belt. Here we confirm the same conclusion, but considering an asteroid population already strongly excited in eccentricity, such as that produced in the Grand Tack scenario (Walsh et al. 2011). Because the asteroids existed since the time of removal of the gas disk, unlike the terrestrial planets, this constraint on the width of the giant planet jump is valid regardless of whether the instability happened early or late. Hence, at this stage, assuming an early instability does not appear to provide any advantage in terms of the probabilistic reconstruction of the solar system structure.

LONG-TERM AND LARGE-SCALE HYDRODYNAMICAL SIMULATIONS OF MIGRATING PLANETS

LONG-TERM AND LARGE-SCALE HYDRODYNAMICAL SIMULATIONS OF MIGRATING PLANETS

Authors:

Benítez-Llambay et al

Abstract:

We present a new method that allows for long-term and large-scale hydrodynamical simulations of migrating planets over a grid-based Eulerian code. This technique, which consists of a remapping of the disk by tracking the planetary migration, enables runs of migrating planets over a time comparable to the age of protoplanetary disks. This method also has the potential to address efficiency problems related to the migration of multi-planet systems in gaseous disks and to improve the current results of the migration of massive planets by including global viscous evolution as well as detailed studies of the co-orbital region during migration. We perform different tests using the public code FARGO3D to validate this method and compare its results with those obtained using a classical fixed grid.

implications for the formation of hot and warm Jupiters via high-eccentricity migration

Secular dynamics of multiplanet systems: implications for the formation of hot and warm Jupiters via high-eccentricity migration

Authors:

Hamers et al

Abstract:

Hot Jupiters (HJs) are Jupiter-like planets that reside very closely to their host star, within ∼0.1AU. Their formation is not well understood. It is generally believed that they cannot have formed in situ, implying that some form of migration must have occurred after their initial formation. We study the production of HJs through secular evolution in multiplanet systems with three to five planets. In this variant of high-e migration, the eccentricity of the orbit of the innermost planet is excited on secular time-scales, triggering orbital migration due to tidal dissipation. We use a secular dynamics code and carry out a population synthesis study. We find that HJs are only produced if the viscous time-scale is short (≈0.014 yr). In contrast, in up to ≈0.3 of systems, the innermost planet is tidally disrupted. The orbital period distribution is peaked around 5 d, consistent with observations. The median HJ mass is 1MJ with a maximum of ≈2MJ, similar to observed HJs. Approximately 0.1 of the HJs have retrograde orbits with respect to the stellar spin. We do not find any warm Jupiters in our simulations, i.e. planets with semimajor axes between 0.1 and 1 AU.

Migration and Growth of Protoplanetary Embryos III: Mass and Metallicity Dependence for FGKM main-sequence stars

Migration and Growth of Protoplanetary Embryos III: Mass and Metallicity Dependence for FGKM main-sequence stars

Authors:

Liu et al

Abstract:

Radial velocity and transit surveys have found that the fraction of FGKM stars with close-in super-Earth(s) (η⊕) is around 30%−50%, independent of the stellar mass M∗ and metallicity Z∗. In contrast, the fraction of solar-type stars harboring one or more gas giants (ηJ) with masses Mp>100 M⊕ is nearly 10%−15%, and it appears to increase with both M∗ and Z∗. Regardless of the properties of their host stars, the total mass of some multiple super-Earth systems exceeds the core mass of Jupiter and Saturn. We suggest that both super-Earths and supercritical cores of gas giants were assembled from a population of embryos that underwent convergent type I migration from their birthplaces to a transition location between viscously heated and irradiation heated disk regions. We attribute the cause for the η⊕-ηJ dichotomy to conditions required for embryos to merge and to acquire supercritical core mass (Mc∼10 M⊕) for the onset of efficient gaseous envelope accretion. We translate this condition into a critical disk accretion rate, and our analysis and simulation results show that it weakly depends on M∗ and decreases with metallicity of disk gas Zd. We find that embryos are more likely to merge into supercritical cores around relatively massive and metal-rich stars. This dependence accounts for the observed ηJ-M∗. We also consider the Zd-Z∗ dispersed relationship and reproduce the observed ηJ-Z∗ correlation.