Star-planet interactions

Stars interact with their planets through gravitation, radiation, and magnetic fields. I shall focus on the interactions between late-type stars with an outer convection zone and close-in planets, i.e., with an orbital semimajor axis smaller than approximately 0.15 AU. I shall review the roles of tides and magnetic fields considering some key observations and discussing theoretical scenarios for their interpretation with an emphasis on open questions.Comment: 20 pages, 5 figures, invited talk at the 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, Proceedings of Lowell Observatory, edited by G. van Belle & H. Harri

Star-Planet Interactions

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.Comment: 8 pages, proceedings of Cool Stars 15, St. Andrews, July 2008, to be published in the Conference Proceedings Series of the American Institute of Physics - "Star-planet interactions" splinter session summar

Signatures of Star-planet interactions

Planets interact with their host stars through gravity, radiation and magnetic fields, and for those giant planets that orbit their stars within $\sim$10 stellar radii ($\sim$0.1 AU for a sun-like star), star-planet interactions (SPI) are observable with a wide variety of photometric, spectroscopic and spectropolarimetric studies. At such close distances, the planet orbits within the sub-alfv\'enic radius of the star in which the transfer of energy and angular momentum between the two bodies is particularly efficient. The magnetic interactions appear as enhanced stellar activity modulated by the planet as it orbits the star rather than only by stellar rotation. These SPI effects are informative for the study of the internal dynamics and atmospheric evolution of exoplanets. The nature of magnetic SPI is modeled to be strongly affected by both the stellar and planetary magnetic fields, possibly influencing the magnetic activity of both, as well as affecting the irradiation and even the migration of the planet and rotational evolution of the star. As phase-resolved observational techniques are applied to a large statistical sample of hot Jupiter systems, extensions to other tightly orbiting stellar systems, such as smaller planets close to M dwarfs become possible. In these systems, star-planet separations of tens of stellar radii begin to coincide with the radiative habitable zone where planetary magnetic fields are likely a necessary condition for surface habitability.Comment: Accepted for publication in the handbook of exoplanet

Models of Star-Planet Magnetic Interaction

Magnetic interactions between a planet and its environment are known to lead to phenomena such as aurorae and shocks in the solar system. The large number of close-in exoplanets that were discovered triggered a renewed interest in magnetic interactions in star-planet systems. Multiple other magnetic effects were then unveiled, such as planet inflation or heating, planet migration, planetary material escape, and even modification of the host star properties. We review here the recent efforts in modelling and understanding magnetic interactions between stars and planets in the context of compact systems. We first provide simple estimates of the effects of magnetic interactions and then detail analytical and numerical models for different representative scenarii. We finally lay out a series of future developments that are needed today to better understand and constrain these fascinating interactions.Comment: 23 pages, 10 figures, accepted as a chapter in the Handbook of Exoplanet

Star-planet interactions

Context. Tidal interactions and planetary evaporation processes impact the evolution of close-in star–planet systems. Aims. We study the impact of stellar rotation on these processes. Methods. We compute the time evolution of star–planet systems consisting of a planet with an initial mass between 0.02 and 2.5 MJup (6 and 800 MEarth) in a quasi-circular orbit with an initial orbital distance between 0.01 and 0.10 au, around a solar-type star evolving from the pre-main-sequence (PMS) phase until the end of the main-sequence phase. We account for the evolution of: the stellar structure, the stellar angular momentum due to tides and magnetic braking, the tidal interactions (equilibrium and dynamical tides in stellar convective zones), the mass evaporation of the planet, and the secular evolution of the planetary orbit. We consider that at the beginning of the evolution, the proto-planetary disk has fully dissipated and planet formation is complete. Results. We find that both a rapid initial stellar rotation and a more efficient angular momentum transport inside the star, in general, contribute to the enlargement of the domain that is devoid of planets after the PMS phase, in the plane of planet mass versus orbital distance. Comparisons with the observed distribution of exoplanets orbiting solar mass stars, in the plane of planet mass versus orbital distance (addressing the “Neptunian desert” feature), show an encouraging agreement with the present simulations, especially since no attempts have been made to fine-tune the initial parameters of the models to fit the observations. We also obtain an upper limit for the orbital period of bare-core planets that agrees with observations of the “radius valley” feature in the plane of planetary radius versus the orbital period. Conclusions. The two effects, namely, tides and planetary evaporation, should be accounted for simultaneously and in a consistent way, with a detailed model for the evolution of the star

Star-planet interactions: I. Stellar rotation and planetary orbits

Context. As a star evolves, the planet orbits change with time due to tidal interactions, stellar mass losses, friction and gravitational drag forces, mass accretion and evaporation on/by the planet. Stellar rotation modifies the structure of the star and therefore the way these different processes occur. Changes of the orbits, at their turn, have an impact on the rotation of the star. Aims. Models accounting in a consistent way for these interactions between the orbital evolution of the planet and the evolution of the rotation of the star are still missing. The present work is a first attempt to fill this gap. Methods. We compute the evolution of stellar models including a comprehensive treatment of rotational effects together with the evolution of planetary orbits, so that the exchanges of angular momentum between the star and the planetary orbit are treated in a self-consistent way. The evolution of the rotation of the star accounts for the angular momentum exchange with the planet and also follows the effects of the internal transport of angular momentum and chemicals. Results. We show that rotating stellar models without tidal interactions can well reproduce the surface rotations of the bulk of the red giants. However, models without any interactions cannot account for fast rotating red giants in the upper part of the red giant branch, where, such models, whatever the initial rotation considered on the ZAMS, always predict very low velocities. For those stars some interaction with a companion is highly probable and the present rotating stellar models with planets confirm that tidal interaction can reproduce their high surface velocities. We show also that the minimum distance between the planet and the star on the ZAMS that will allow the planet to avoid engulfment and survive is decreased around faster rotating stars. [abridged]Comment: 14 pages, abstract abridged for arXiv submission, accepted for publication in Astronomy & Astrophysic