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

R. Cloutier (1, 2) , N. Astudillo-Defru (3) , R. Doyon (4, 2) , Xavier Bonfils (5) , J.-M. Almenara (6) , F. Bouchy (6) , X. Delfosse (5) , T. Forveille (5) , C. Lovis (6) , M. Mayor (6) , K. Menou , F. Murgas (5) , F. Pepe (6) , N. Santos (7) , S. Udry (6) , A. Wünsche (5)

1 UQAR - Université du Québec à Rimouski
2 iREX - Institut de Recherche sur les Exoplanètes
3 Universidad de Concepción - University of Concepcion [Chile]
4 UdeM - Université de Montréal
5 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble
6 ObsGE - Observatoire Astronomique de l'Université de Genève
7 IASTRO - Instituto de Astrofísica e Ciências do Espaço

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 Here we conduct a detailed reanalysis of all available RV time-series-including a set of 31 previously unpublished HARPS measurements-to investigate the effects of time-sampling and of simultaneous modelling of planetary plus activity signals on the existence and origin of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.

Mots clés: techniques; radial velocities -planets and satellites; fundamental parameters -planets and satellites; detectionmethods; data analysis -planets and satellites; individual; K2-18

Domaines: Astrophysique [astro-ph]

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, 2019, 621, pp.A49. ⟨10.1051/0004-6361/201833995⟩. ⟨hal-03104142⟩