J/A+A/608/A35 K2-18 HARPS time-series (Cloutier+, 2017)
Characterization of the K2-18 multi-planetary system with HARPS:
A habitable zone super-Earth and discovery of a second, warm super-Earth on a
non-coplanar orbit.
Cloutier R., Astudillo-Defru N., Doyon R., Bonfils X., Almenara J.M.,
Benneke B., Bouchy F., Delfosse X., Ehrenreich D., Forveille T., Lovis C.,
Mayor M., Menou K., Murgas F., Pepe F., Rowe J., Santos N.C., Udry S.,
Wuensche A.
<Astron. Astrophys. 608, A35 (2017)>
=2017A&A...608A..35C 2017A&A...608A..35C (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Planets ; Radial velocities
Keywords: techniques: radial velocities - methods: statistical -
planets and satellites: detection -
planets and satellites: fundamental parameters -
planets and satellites: individual: K2-18
Abstract:
The bright M2.5 dwarf K2-18 (Ms=0.36M☉, Rs=0.41R☉) at
34 pc is known to host a transiting super-Earth-sized planet orbiting
within the star's habitable zone; K2-18b. Given the superlative nature
of this system for studying an exoplanetary atmosphere receiving
similar levels of insolation as the Earth, we aim to characterize the
planet's mass which is required to interpret atmospheric properties
and infer the planet's bulk composition. We have obtained precision
radial velocity measurements with the HARPS spectrograph. We then
coupled those measurements with the K2 photometry to jointly model the
observed radial velocity variation with planetary signals and a
correlated stellar activity model based on Gaussian process
regression. We measured the mass of K2-18b to be 8.0±1.9M☉
with a bulk density of 3.3±1.2g/cm3 which may correspond to a
predominantly rocky planet with a significant gaseous envelope or an
ocean planet with a water mass fraction ≳50%. We also find strong
evidence for a second, warm super-Earth K2-18c
(mp,csin(ic)=7.5±1.3M☉) at approximately nine days with a
semi-major axis ∼2.4 times smaller than the transiting K2-18b. After
re-analyzing the available light curves of K2-18 we conclude that
K2-18c is not detected in transit and therefore likely has an orbit
that is non-coplanar with the orbit of K2-18b although only a small
mutual inclination is required for K2-18c to miss a transiting
configuration; |Δi|∼1-2°. A suite of dynamical integrations
are performed to numerically confirm the system's dynamical stability.
By varying the simulated orbital eccentricities of the two planets,
dynamical stability constraints are used as an additional prior on
each planet's eccentricity posterior from which we constrain eb<0.43
and ec<0.47 at the level of 99% confidence. The discovery of the
inner planet K2-18c further emphasizes the prevalence of multi-planet
systems around M dwarfs. The characterization of the density of K2-18b
reveals that the planet likely has a thick gaseous envelope which,
along with its proximity to the solar system, makes the K2-18
planetary system an interesting target for the atmospheric study of an
exoplanet receiving Earth-like insolation.
Description:
HARPS time-series containing 75 measurements of the K2-18 radial
velocities, Ca II H+K Mt. Wilson S index, H-alpha index, full width at
half maximum of the cross-correlation function, and the bi-sector
inverse slope of the cross-correlation function.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea2.dat 58 75 HARPS time-series of K2-18
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Objects:
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RA (2000) DE Designation(s)
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11 30 14.51 +07 35 18.2 K2-18 = EPIC 201912552
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Byte-by-byte Description of file: tablea2.dat
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Bytes Format Units Label Explanations
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1- 11 F11.6 d BJD Barycentric Julian Date (BJD-2450000)
14- 19 F6.2 m/s RV Radial velocity
22- 25 F4.2 m/s sigma Radial velocity measurement uncertainty
27- 33 F7.3 --- Sindex ?=-99 S index
36- 42 F7.5 --- Halpha Halpha index
45- 49 F5.3 --- FWHM FWHM shape parameter of the CCF
52- 58 F7.3 --- BIS Bi-sector inverse slope (BIS) shape
parameter of the CCF
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Acknowledgements:
Ryan Cloutier, cloutier(at)astro.utoronto.ca
(End) Ryan Cloutier [UofT, Canada], Patricia Vannier [CDS] 04-Dec-2017