Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2K052

Kozai migration with tidal friction (KCTF) as a source for Gliese 436b\'s eccentricity

Beust, Hervé (IPAG, Grenoble, France)
Bonfils, Xavier (IPAG, Grenoble, France)
Delfosse, Xavier (IPAG, Grenoble, France)
Forveille, Thierry (IPAG, Grenoble, France)

The planet GJ 436b presents a puzzling orbital eccentricity considering its short orbital period. Given the age of the star, it should have been already tidally circularized. Many attempts to explain this were proposed, either involving abnormally weak tides, or perturbations by an outer companion. We address here the latter issue based on Kozai migration with tidal friction. We propose that GJ 436b wasformerly located further away from the star and that it underwent a migration induced by an inclined perturber via Kozai mechanism: The companion triggers high amplitude eccentricity variations to GJ 436b that cause tides to act at periastron. Then the orbit shrinks to reach its present day location. We have explored numerically this system including tides and General Relativity (Beust et al., 2012, A&A 545, A88). We first show that starting from the present-day location of GJ 436b inevitably leads to damp the Kozai oscillations and to a rapid circularization. Conversely, starting from 5-10 times further away allows the onset of Kozai cycles, with tides acting in peak eccentricity phases only. The net result is a two fold evolution : a first phase with Kozai cycles and a slowly shrinking semi-major axis, and a second one once the planet gets out of the Kozai resonance characterized by a more rapid decrease. The timescale of this process is often much longer than the standard circularization time by a factor of 50 or above. This model can provide a solution to the eccentricity paradox of GJ 436b. Depending on the initial configuration, it can take several Gyrs to the planet to achieve a full orbital decrease and circularization. According to this scenario, we could be witnessing today the second phase with an already reduced semi-major axis and a still significant eccentricity. We explore then the parameter space and derive in which conditions this model can be realistic given the age of the star. This yields constraints on the characteristics of the putative companion.

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