Hot Saturn Kelt-6b has a Sister Exoplanet Gas Giant

The GAPS Programme with HARPS-N@TNG X. The multi-planet system KELT-6: detection of the planet KELT-6 c and measurement of the Rossiter-McLaughlin effect for KELT-6 b

Authors:

Damasso et al

Abstract:

Aims.

For more than 1.5 years we monitored spectroscopically the star KELT-6 (BD+312447), known to host the transiting hot Saturn KELT-6b, because a previously observed long-term trend in radial velocity time series suggested the existence of an outer companion.

Methods.

We collected a total of 93 new spectra with the HARPS-N and TRES spectrographs. A spectroscopic transit of KELT-6b was observed with HARPS-N, and simultaneous photometry was obtained with the IAC-80 telescope.

Results.

We proved the existence of an outer planet with a mininum mass Mpsini=3.71±0.21 MJup and a moderately eccentric orbit (e=0.21+0.039−0.036) of period P∼3.5 years. We improved the orbital solution of KELT-6b and obtained the first measurement of the Rossiter-McLaughlin effect, showing that the planet has a likely circular, prograde, and slightly misaligned orbit, with a projected spin-orbit angle λ=−36±11 degrees. We improved the KELT-6b transit ephemeris from photometry, and we provided new measurements of the stellar parameters. KELT-6 appears as an interesting case to study the formation and evolution of multi-planet systems.