MagIcS
magnetic interaction betwen host stars and close-in giant planets

	© David Aguilar & Lockheed Martin

Star-planet magnetic interaction

Planetary systems characterized by a giant planet at a few stellar radii from their parent stars (hot Jupiters) currently make up 20% of all known extrasolar planets. Spectroscopic observations of such systems reveal that chromospheric activity indices (such as core emission in Ca H&K lines) are sometimes modulated with the orbital period of the giant planet (rather than with the rotation period of the star), suggesting that such giant planets may significantly boost the activity level of the host star (eg Shkolnik et al 2003, ApJ 597, 1092) through some sort of star-planet magnetospheric interaction. By densely monitoring the activity level of a sample of stars hosting giant planets on both rotation and orbital cycles, we can separate the genuine stellar activity from that induced by planets; we can then work out the physics of the magnetospheric interaction by examining how the planet-induced activity depends on the stellar and planet characteristics.

Magnetic fields of close-in giant-planet hosting stars

Monitoring Zeeman signatures of giant-planet hosting stars, eg with ESPaDOnS, can give access to the surface magnetic topology of the host star (eg Catala et al, 2007, MNRAS 374, L42; Moutou et al 2007, A&A 473, 651), and thus to its complete magnetospheric structure thanks to magnetic field extrapolation techniques (eg Jardine et al 2002, MNRAS 333, 339). This is by far the best method available to study both the statistical difference in the magnetic topologies of active stars with and without close-in giant planets, and to work out the details of the magnetospheric interaction (eg tidal effects vs reconnection) triggered by close-in giant planets.

Stellar light reflected off the planet surface/atmosphere

Spectropolarimetric observations can also be used to detect the planet in a different way, ie not through the velocity wobble it produces on its host star, but rather through the light from the host star reflected off the planet atmosphere/surface towards the observer (eg Cameron et al 2002, MNRAS 330, 187). Since the reflected light is likely to be polarised, spectropolarimetry brings additional reliability on the detection itself and further constraints on the albedo of the planet surface/atmosphere, with respect to what spectroscopy alone can do. No definite detection has been secured yet for any known close-in giant planets, although such a detection is at reach of exisiting instruments (provided enough observing time is allocated).

Planet atmospheres and evaporation

Through this research program, we can also study the atmosphere and atmospheric evaporation of close-in giant planets through the absorption signatures that can potentially show up in the spectrum of eclipsing systems when the planet transits in front of the stellar disc. Composition of exoplanet atmospheres and rates of atmospheric evaporation can (in principle) be studied this way.

Related threads and associated Large Program

This research shares obvious links with that on dynamos of low-mass stars, from which we will be able to study statistical differences between active stars with and without close-in giant planets. It also has links with the research program focussing on magnetised protostellar/protoplanetary discs, which should provide with information on how, where and when close-in giant planets form and migrate. The Large Program with ESPaDOnS at CFHT adressing these various issues was not awarded time in 2008a and will be resubmitted later on.

Core team and collaborators

Thread coordinators: C Moutou, E Shkolnik, M Deleuil, D Bohlender

Agency/Country	Core team	Collaborators
F	Deleuil, Donati, Fares, Griessmeier, Moutou, P Petit	Bouchy, Guillot, Vidal-Madjar
O	Cameron, Cuntz, Jardine	Udry
C	Bohlender, Walker	Croll, Matthews
H	Shkolnik
US	Saar, Valenti