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 Here we conduct a detailed reanalysis of all available RV time-series - including a set of 31 previously unpublished HARPS measurements - to investigate the effects of time-sampling and of simultaneous modelling of planetary plus activity signals on the existence and origin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 mp,b = 8.63 ± 1.35 and mp,c sinic = 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.
Based on observations made with the HARPS instrument on the ESO 3.6 m telescope under the programme IDs 191.C-0873(A) and 198.C-0838(A) at Cerro La Silla (Chile).Full Table A.1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/621/A49.- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- January 2019
- DOI:
- 10.1051/0004-6361/201833995
- arXiv:
- arXiv:1810.04731
- Bibcode:
- 2019A&A...621A..49C
- Keywords:
-
- techniques: radial velocities;
- planets and satellites: fundamental parameters;
- planets and satellites: detection;
- methods: data analysis;
- planets and satellites: individual: K2-18;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 19 pages, 10 figures (including 9 interactive figures when viewed in Adobe Acrobat), accepted for publication in Astronomy &