Super-Earth of 8M_⊕in a 2.2-day orbit around the K5V star K2-216

Abstract

Context. Although thousands of exoplanets have been discovered to date, far fewer have been fully characterised, in particular super-Earths. The KESPRINT consortium identified K2-216 as a planetary candidate host star in the K2 space mission Campaign 8 field with a transiting super-Earth. The planet has recently been validated as well. Aims. Our aim was to confirm the detection and derive the main physical characteristics of K2-216b, including the mass. Methods. We performed a series of follow-up observations: high resolution imaging with the FastCam camera at the TCS and the Infrared Camera and Spectrograph at Subaru, and high resolution spectroscopy with HARPS (La Silla), HARPS-N (TNG) and FIES (NOT). The stellar spectra were analyzed with the SpecMatch-Emp and SME codes to derive the fundamental stellar properties. We analyzed the K2 light curve with the pyaneti software. The radial velocity measurements were modelled with both a Gaussian process (GP) regression and the so-called floating chunk offset (FCO) technique to simultaneously model the planetary signal and correlated noise associated with stellar activity. Results. Imaging confirms that K2-216 is a single star. Our analysis discloses that the star is a moderately active K5V star of mass 0.70 ± 0.03 M and radius 0.72 ± 0.03 R . Planet b is found to have a radius of 1.75 +0.17 −0.10 R ⊕ and a 2.17-day orbit in agreement with previous results. We find consistent results for the planet mass from both models: M p ≈ 7.4 ± 2.2 M ⊕ from the GP regression and M p ≈ 8.0 ± 1.6 M ⊕ from the FCO technique, which implies that this planet is a super-Earth. The incident stellar flux is 248 +220 −48 F ⊕ . Conclusions. The planet parameters put planet b in the middle of, or just below, the gap of the radius distribution of small planets. The density is consistent with a rocky composition of primarily iron and magnesium silicate. In agreement with theoretical predictions, we find that the planet is a remnant core, stripped of its atmosphere, and is one of the largest planets found that has lost its atmosphere.

Radial velocity confirmation of K2-100b: a young, highly irradiated, and low-density transiting hot Neptune

“…We determined the spectroscopic parameters of K2-100 from the coadded HARPS-N spectrum using the software Spectroscopy Made Easy (SME, version 5.22;Piskunov & Valenti 2017;Valenti & Piskunov 1996) along with ATLAS12 model atmospheres (Kurucz 2013) and atomic/molecular parameters from the VALD database (Ryabchikova et al 2015). The effective temperature T eff , surface gravity log g , iron abundance [Fe/H], and projected rotational velocity v sin i were measured following the same techniques described in, e.g., Fridlund et al (2017), Gandolfi et al (2017), and Persson et al (2018). The micro-(v mic ) and macro-turbulent (v mac ) velocities were fixed through the empirical calibration equations of Bruntt et al (2010) and Doyle et al (2014).…”

The Multiplanet System TOI-421: A Warm Neptune and a Super Puffy Mini-Neptune Transiting a G9 V Star in a Visual Binary*

“…We used the ATLAS12 model spectra (Kurucz 2013) and the line lists from the Vienna Atomic Line Database 4 (Ryabchikova et al 2015) to model the co-added HARPS spectrum. Following the modelling procedure detailed in Fridlund et al (2017) and Persson et al (2018), we measured T eff from the wings of H-α line, and log g from the line wings of the Ca and Mg triplets around 6100 Å and 5100 Å, respectively. v sin i and [Fe/H] were derived from narrow unblended iron lines between 6000 and 6600 Å.…”

Three planets transiting the evolved star EPIC 249893012: a hot 8.8-M⊕ super-Earth and two warm 14.7 and 10.2-M⊕ sub-Neptunes

“…SME calculates the equation of state, the line and continuum opacities, and the radiative transfer over the stellar surface with the help of a library of stellar models. A chi-square minimization procedure is then used to extract spectroscopy parameters, that is, the effective temperature T eff , the surface gravity log g , the metal content, the micro V mic and macro V mac turbulent velocities, and the projected-rotational velocity V sin i , as described in Fridlund et al (2017) and Persson et al (2018Persson et al ( , 2019. When any of the parameters listed above an be determined with another method and it can be held fixed during the iterative procedure, this improves the determination of the remaining parameters.…”