J/A+A/621/A49ivo://CDS.VizieR/J/A+A/621/A49doi:10.26093/cds/vizier.36210049CDSCloutier R.Astudillo-Defru N.Doyon R.Bonfils X.Almenara J.M.,Bouchy F.Delfosse X.Forveille T.Lovis C.Mayor M.Menou K.,Murgas F.Pepe F.Santos N.C.Udry S.Wuensche A.2020-01-17T12:18:25Z2019-01-14T15:29:45ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frmultiple-starsexoplanetsradial-velocityIn 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 Here we conduct a detailed re-analysis of all available RV time-series -- including a set of 31 previously unpublished HARPS measurements -- to investigate the effects of time-sampling and of simultaneous modelling of planetary plus activity signals on the existence and origin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.2019A&A...621A..49Chttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/621/A49CatalogResearchIsServedByTAP VizieR generic servicerelated-toJ/A+A/608/A35 : K2-18 HARPS time-series (Cloutier+, 2017)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/621/A49https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/621/A49http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/621/A49https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/621/A49https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/621/A49http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/621/A49GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapdefaultJ/A+A/621/A49/tablea1K2-18 HARPS time-seriesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintTimeTimestamp (BJD-2450000)dtime.epochdoubleRVRadial velocitym/sphys.veloc;pos.heliocentricfloate_RVRadial velocity uncertaintym/sstat.errorfloatblueRVRadial velocity from blue ordersm/sphys.veloc;pos.heliocentricfloate_blueRVRadial velocity from blue orders uncertaintym/sstat.errorfloatredRVRadial velocity from red ordersm/sphys.veloc;pos.heliocentricfloate_redRVRadial velocity from red orders uncertaintym/sstat.errorfloatNaDSodium doublet activity indexmeta.code;spect.indexdoublee_sigNaDSodium doublet activity index uncertaintystat.errordoubleHalphaH-alpha activity indexspect.index;em.line.Halphadoublee_HalphaH-alpha activity index uncertaintystat.errordoubleFWHMFull width at half maximum of the cross-correlation functionspect.line.widthfloatBISBi-sector inverse slope of the cross-correlation functionphys.velocfloat