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Register a new userDisc-planet, planet-planet, and star-planet interactions during planet formation
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In this article we present results from three on-going projects related to the formation of protoplanets in protostellar discs. We present the results of simulations that model the interaction between embedded protoplanets and disc models undergoing MHD turbulence. We review the similarities and differences that arise when the disc is turbulent as opposed to laminar (but viscous), and present the first results of simulations that examine the tidal interaction between low mass protoplanets and turbulent discs. We describe the results of simulations of Jovian mass protoplanets forming in circumbinary discs, and discuss the range of possible outcomes that arise in hydrodynamic simulations. Finally, we report on some preliminary simulations of three protoplanets of Jovian mass that form approximately coevally within a protostellar disc. We describe the conditions under which such a system can form a stable three planet resonance.
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The discovery of close orbiting extrasolar giant planets led to extensive studies of disk planet interactions and the forms of migration that can result as a means of accounting for their location. Early work established the type I and type II migration regimes for low mass embedded planets and high mass gap forming planets respectively. While providing an attractive means of accounting for close orbiting planets intially formed at several AU, inward migration times for objects in the earth mass range were found to be disturbingly short, making the survival of giant planet cores an issue. Recent progress in this area has come from the application of modern numerical techniques which make use of up to date supercomputer resources. These have enabled higher resolution studies of the regions close to the planet and the initiation of studies of planets interacting with disks undergoing MHD turbulence. This work has led to indications of how the inward migration of low to intermediate mass planets could be slowed down or reversed. In addition, the possibility of a new very fast type III migration regime, that can be directed inwards or outwards, that is relevant to partial gap forming planets in massive disks has been investigated.
Much effort has been invested in recent years, both observationally and theoretically, to understand the interacting processes taking place in planetary systems consisting of a hot Jupiter orbiting its star within 10 stellar radii. Several independent studies have converged on the same scenario: that a short-period planet can induce activity on the photosphere and upper atmosphere of its host star. The growing body of evidence for such magnetic star-planet interactions includes a diverse array of photometric, spectroscopic and spectropolarimetric studies. The nature of which is modeled to be strongly affected by both the stellar and planetary magnetic fields, possibly influencing the magnetic activity of both bodies, as well as affecting irradiation and non-thermal and dynamical processes. Tidal interactions are responsible for the circularization of the planet orbit, for the synchronization of the planet rotation with the orbital period, and may also synchronize the outer convective envelope of the star with the planet. Studying such star-planet interactions (SPI) aids our understanding of the formation, migration and evolution of hot Jupiters.
We consider several processes operating during the late stages of planet formation that can affect observed orbital elements. Disk-planet interactions, tidal interactions with the central star, long term orbital instability and the Kozai mechanism are discussed.
The formation of planets within a disc must operate within the time frame of disc dispersal, it is thus crucial to establish what is the dominant process that disperses the gaseous component of discs around young stars. Planet formation itself as well as photoevaporation by energetic radiation from the central young stellar object have been proposed as plausible dispersal mechanisms. [abridged]. In this paper we use the different metallicity dependance of X-ray photoevaporation and planet formation to discriminate between these two processes. We study the effects of metallicity, Z, on the dispersal timescale, t_phot, in the context of a photoevaporation model, by means of detailed thermal calculations of a disc in hydrostatic equilibrium irradiated by EUV and X-ray radiation from the central source. Our models show t_phot propto Z^0.52 for a pure photoevaporation model. By means of analytical estimates we derive instead a much stronger negative power dependance on metallicity of the disc lifetime for a dispersal model based on planet formation. A census of disc fractions in lower metallicity regions should therefore be able to distinguish between the two models. A recent study by Yasui et al. in low metallicity clusters of the extreme outer Galaxy ([O/H] ~- 0.7dex and dust to gas ratio of ~0.001) provides preliminary observational evidence for shorter disc lifetimes at lower metallicities, in agreement with the predictions of a pure photoevaporation model. [abridged] We finally develop an analytical framework to study the effects of metallicity dependent photoevaporation on the formation of gas giants in the core accretion scenario. We show that accounting for this effect strengthens the conclusion that planet formation is favoured at higher metallicity. [abridged]
During the process of planet formation, the planet-discs interactions might excite (or damp) the orbital eccentricity of the planet. In this paper, we present two long ($tsim 3times 10^5$ orbits) numerical simulations: (a) one (with a relatively light disc, $M_{rm d}/M_{rm p}=0.2$) where the eccentricity initially stalls before growing at later times and (b) one (with a more massive disc, $M_{rm d}/M_{rm p}=0.65$) with fast growth and a late decrease of the eccentricity. We recover the well-known result that a more massive disc promotes a faster initial growth of the planet eccentricity. However, at late times the planet eccentricity decreases in the massive disc case, but increases in the light disc case. Both simulations show periodic eccentricity oscillations superimposed on a growing/decreasing trend and a rapid transition between fast and slow pericentre precession. The peculiar and contrasting evolution of the eccentricity of both planet and disc in the two simulations can be understood by invoking a simple toy model where the disc is treated as a second point-like gravitating body, subject to secular planet-planet interaction and eccentricity pumping/damping provided by the disc. We show how the counterintuitive result that the more massive simulation produces a lower planet eccentricity at late times can be understood in terms of the different ratios of the disc-to-planet angular momentum in the two simulations. In our interpretation, at late times the planet eccentricity can increase more in low-mass discs rather than in high-mass discs, contrary to previous claims in the literature.
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