A Low-Mass Planet In The Temperate Zone
A Low-Mass Planet In The Temperate Zone
Of The Nearby K2-18
Of The Nearby K2-18
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Abstract
Abstract
K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting planet, which was first discovered by the K2 mission and later confirmed with Spitzer Space Telescope observations. With a radius of ~ 2 Rearth and an orbital period of ~ 33 days, the planet lies in the temperate zone of its host star and receives stellar irradiation similar to Earth. Here we perform radial velocity follow-up observations with the visual channel of CARMENES with the goal of determining the mass and density of the planet. We measure a planetary semi-amplitude of Kb ~ 3.5 m s -1 and a mass of Mb ~ 9 Mearth , yielding a bulk density around 4 gcm -3. This indicates a low-mass planet with a composition consistent with a solid core and a volatile-rich envelope. A signal at 9 days was recently reported using radial velocity measurements taken with the HARPS spectrograph. This was interpreted as being due to a second planet. We see a weaker, time- and wavelength-dependent signal in the CARMENES data set and thus favor stellar activity for its origin. K2-18 b joins the growing group of low-mass planets detected in the temperate zone of M dwarfs. The brightness of the host star in the near-infrared makes the system a good target for detailed atmospheric studies with the James Webb Space Telescope.
We performed joint modeling of the photometric light curves obtained with STELLA and the CARMENES RV measurements. We modelled the photometric data of both bands with a sine wave function and modelled the RV measurements with a Keplerian model assuming a circular orbit. We accounted for stellar activity in the RV data by assuming that it has a sinusoidal function whose frequency is constrained from the photometric light curves. The best fit model gave a planetary semi-amplitude of Kb = 3.60 +/- 0.52 m/s, corresponding to a planetary mass of Mb = 9.07 +/- 1.54 Mearth. With a radius of Rb = 2.37 +/- 0.22 Rearth, this corresponds to a density of 4.18 +/- 1.44 g/cm^3.
We performed joint modeling of the photometric light curves obtained with STELLA and the CARMENES RV measurements. We modelled the photometric data of both bands with a sine wave function and modelled the RV measurements with a Keplerian model assuming a circular orbit. We accounted for stellar activity in the RV data by assuming that it has a sinusoidal function whose frequency is constrained from the photometric light curves. The best fit model gave a planetary semi-amplitude of Kb = 3.60 +/- 0.52 m/s, corresponding to a planetary mass of Mb = 9.07 +/- 1.54 Mearth. With a radius of Rb = 2.37 +/- 0.22 Rearth, this corresponds to a density of 4.18 +/- 1.44 g/cm^3.
Phase-folded activity-corrected RVs along with the best-fit planetary model.
We show that although we found evidence of the second planet signal (Pc) announced by Cloutier et al. (2017), the peak is not significant in the CARMENES RVs. We demonstrate that the signal is only present in the second half of the blue RV data set, which occurs at the same time when all the CaIRT and Halpha lines show a prominent peak at the stellar rotation period (Prot). The signal is therefore time- and wavelength-dependent and we favor stellar activity for its origin.
We show that although we found evidence of the second planet signal (Pc) announced by Cloutier et al. (2017), the peak is not significant in the CARMENES RVs. We demonstrate that the signal is only present in the second half of the blue RV data set, which occurs at the same time when all the CaIRT and Halpha lines show a prominent peak at the stellar rotation period (Prot). The signal is therefore time- and wavelength-dependent and we favor stellar activity for its origin.
Periodograms of the first (top panels) and second (bottom panels) halves of the CaIRT and Halpha lines (left panels) and full wavelength, blue and red RVs (right panels). Prot indicates the stellar rotation period and Pc the 9-day planetary signal.
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