J/A+A/621/A49 K2-18 HARPS time-series (Cloutier+, 2019)
Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES.
Cloutier 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.
<Astron. Astrophys. 621, A49 (2019)>
=2019A&A...621A..49C 2019A&A...621A..49C (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Exoplanets ; Radial velocities
Keywords: techniques: radial velocities -
planets and satellites: fundamental parameters -
planets and satellites: detection - methods: data analysis -
planets and satellites: individual: K2-18
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 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unpublished HARPS measurements -- to investigate the effects of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 mp,b=8.63±1.35 and
mp,csin(ic)=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.
Description:
HARPS spectroscopic time-series of K2-18 including radial velocities
and spectroscopic activity indices.
Objects:
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RA (2000) DE Designation(s)
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11 30 14.52 +07 35 18.3 K2-18 = EPIC 201912552
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 95 106 K2-18 HARPS time-series
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See also:
J/A+A/608/A35 : K2-18 HARPS time-series (Cloutier+, 2017)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 11 F11.6 d Time Timestamp (BJD-2450000)
13- 18 F6.2 m/s RV Radial velocity
20- 23 F4.2 m/s e_RV Radial velocity uncertainty
25- 30 F6.2 m/s blueRV Radial velocity from blue orders
32- 36 F5.2 m/s e_blueRV Radial velocity from blue orders uncertainty
38- 43 F6.2 m/s redRV Radial velocity from red orders
45- 49 F5.2 m/s e_redRV Radial velocity from red orders uncertainty
51- 57 F7.5 --- NaD Sodium doublet activity index
59- 65 F7.5 --- e_sigNaD Sodium doublet activity index uncertainty
67- 73 F7.5 --- Halpha H-alpha activity index
75- 81 F7.5 --- e_Halpha H-alpha activity index uncertainty
83- 87 F5.3 --- FWHM Full width at half maximum of the
cross-correlation function
89- 95 F7.3 --- BIS Bi-sector inverse slope of the
cross-correlation function
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Acknowledgements:
Ryan Cloutier, cloutier(at)astro.utoronto.ca
(End) Ryan Cloutier [UofT, Canada], Patricia Vannier [CDS] 02-Nov-2018