The Transiting Multi-planet System HD3167: A 5.7M(circle plus) Super-Earth and an 8.3M(circle plus) Mini-Neptune

HD3167 is a bright (V = 8.9 mag) K0V star observed by the NASA's K2 space mission during its Campaign 8. It has been recently found to host two small transiting planets, namely, HD 3167b, an ultra short period (0.96 days) super-Earth, and HD 3167c, a mini-Neptune on a relatively long-period orbit (29.85 d). Here we present an intensive radial velocity follow-up of HD 3167 performed with the FIES@NOT, HARPS@ESO-3.6m, and HARPS-N@TNG spectrographs. We revise the system parameters and determine radii, masses, and densities of the two transiting planets by combining the K2 photometry with our spectroscopic data. With a mass of 5.69 +/- -0.44 M-circle plus, radius of 1.574 +/- -0.054 R-circle plus, and mean density of 8.00(-0.98)(+1.0) g/cm(-3), HD 3167b joins the small group of ultra-short period planets known to have a rocky terrestrial composition. HD 3167c has a mass of 8.33(-1.85)(+1.79) M-circle plus and a radius of 2.740(-0.100)(+0.106) R-circle plus, yielding a mean density of 2.21(-0.53)(+0.56) g/cm(-3), indicative of a planet with a composition comprising a solid core surrounded by a thick atmospheric envelope. The rather large pressure scale height (similar to 350 km) and the brightness of the host star make HD 3167c an ideal target for atmospheric characterization via transmission spectroscopy across a broad range of wavelengths. We found evidence of additional signals in the radial velocity measurements but the currently available data set does not allow us to draw any firm conclusion on the origin of the observed variation