* Astronomy

Title: Transit confirmation and improved stellar and planet parameters for the super-Earth HD 97658 b and its host star
Author: V. Van Grootel, M. Gillon, D. Valencia, N. Madhusudhan, D. Dragomir, A.R. Howe, A.S. Burrows, B.-O. Demory, D. Deming, D. Ehrenreich, C. Lovis, M. Mayor, F. Pepe, D. Queloz, R. Scuflaire, S. Seager, D. Segransan, S. Udry

Super-Earths transiting nearby bright stars are key objects that simultaneously allow for accurate measurements of both their mass and radius, providing essential constraints on their internal composition. We present here the confirmation, based on Spitzer transit observations, that the super-Earth HD 97658 b transits its host star. HD 97658 is a low-mass (M*=0.77±0.05 solar masses) K1 dwarf, as determined from the Hipparcos parallax and stellar evolution modeling. To constrain the planet parameters, we carry out Bayesian global analyses of Keck-HIRES radial velocities, and MOST and Spitzer photometry. HD 97658 b is a massive (MP=7.55+0.83-0.79 earth masses) and large (RP=2.247+0.098-0.095 earth radii at 4.5 m) super-Earth. We investigate the possible internal compositions for HD 97658 b. Our results indicate a large rocky component, by at least 60% by mass, and very little H-He components, at most 2% by mass. We also discuss how future asteroseismic observations can improve the knowledge of the HD 97658 system, in particular by constraining its age. Orbiting a bright host star, HD 97658 b will be a key target for coming space missions TESS, CHEOPS, PLATO, and also JWST, to characterise thoroughly its structure and atmosphere.

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