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WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star

L. Delrez, A. Santerne, J.-M. Almenara, D. R. Anderson, A. Collier Cameron, R. F. Díaz, M. Gillon, C. Hellier, E. Jehin, M. Lendl, P. F. L. Maxted, M. Neveu-VanMalle, F. Pepe, D. Pollacco, D. Queloz, D. Ségransan, B. Smalley, A. M. S. Smith, A. H. M. J. Triaud, S. Udry, V. Van Grootel, R. G. West

Abstract
We present the discovery by the WASP-South survey of WASP-121 b, a new remarkable short-period transiting hot Jupiter. The planet has a mass of 1.183-0.062+0.064 MJup, a radius of 1.865 ± 0.044 RJup, and transits every 1.274 9255-0.000 0025+0.000 0020 days an active F6-type main-sequence star (V = 10.4, 1.353-0.079+0.080 M, 1.458 ± 0.030 R, Teff = 6460 ± 140 K). A notable property of WASP-121 b is that its orbital semimajor axis is only ̃1.15 times larger than its Roche limit, which suggests that the planet is close to tidal disruption. Furthermore, its large size and extreme irradiation (~7.1 109 erg s-1 cm-2) make it an excellent target for atmospheric studies via secondary eclipse observations. Using the TRAnsiting Planets and PlanetesImals Small Telescope, we indeed detect its emission in the z'-band at better than ~4σ, the measured occultation depth being 603 ± 130 ppm. Finally, from a measurement of the Rossiter-McLaughlin effect with the CORALIE spectrograph, we infer a sky-projected spin-orbit angle of 257.8°-5.5°+5.3°. This result may suggest a significant misalignment between the spin axis of the host star and the orbital plane of the planet. If confirmed, this high misalignment would favour a migration of the planet involving strong dynamical events with a third body.

Keywords
techniques: photometric, techniques: radial velocities, techniques: spectroscopic, stars: individual: WASP-121, planetary systems

Monthly Notices of the Royal Astronomical Society
Volume 458, Issue 4, Page 4025
2016 June

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia