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Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES

Abstract : In an earlier campaign to characterize the mass of the transiting temperate super-Earth K2-18b with HARPS, a second, non-transiting planet was posited to exist in the system at ∼9 days. Further radial velocity follow-up with the CARMENES spectrograph visible channel revealed a much weaker signal at 9 days, which also appeared to vary chromatically and temporally, leading to the conclusion that the origin of the 9-day signal was more likely related to stellar activity than to a planetary presence. Here we conduct a detailed reanalysis of all available RV time-series-including a set of 31 previously unpublished HARPS measurements-to investigate the effects of time-sampling and of simultaneous modelling of planetary plus activity signals on the existence and origin of the curious 9-day signal. We conclude that the 9-day signal is real and was initially seen to be suppressed in the CARMENES data due to a small number of anomalous measurements, although the exact cause of these anomalies remains unknown. Investigation of the signal's evolution in time with wavelength and detailed model comparison reveals that the 9-day signal is most likely planetary in nature. Using this analysis, we reconcile the conflicting HARPS and CARMENES results and measure precise and self-consistent planet masses of m p,b = 8.63 ± 1.35 and m p,c sin i c = 5.62 ± 0.84 Earth masses. This work, along with the previously published RV papers on the K2-18 planetary system, highlights the importance of understanding the time-sampling and of modelling the simultaneous planet plus stochastic activity, particularly when searching for sub-Neptune-sized planets with radial velocities.
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https://hal.archives-ouvertes.fr/hal-03104142
Contributor : Xavier Bonfils <>
Submitted on : Friday, January 8, 2021 - 5:39:56 PM
Last modification on : Monday, January 11, 2021 - 10:23:23 AM

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R. Cloutier, N. Astudillo-Defru, R. Doyon, Xavier Bonfils, J.-M. Almenara, et al.. Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES. Astronomy and Astrophysics - A&A, EDP Sciences, 2019, 621, pp.A49. ⟨10.1051/0004-6361/201833995⟩. ⟨hal-03104142⟩

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