Revisiting Radial Velocity Measurements of the K2-18 System with the Line-by-Line Framework

Abstract

The cross-correlation function and template matching techniques have dominated the world of precision radial velocities for many years. Recently, a new technique, named line-by-line, has been developed as an outlier resistant way to efficiently extract radial velocity content from high resolution spectra. We apply this new method to archival HARPS and CARMENES datasets of the K2-18 system. After reprocessing the HARPS dataset with the line-by-line framework, we are able to replicate the findings of previous studies. Furthermore, by splitting the full wavelength range into sub-domains, we were able to identify a systematic chromatic correlation of the radial velocities in the reprocessed CARMENES dataset. After post-processing the radial velocities to remove this correlation, as well as rejecting some outlier nights, we robustly uncover the signal of both K2-18b and K2-18c, with masses that agree with those found from our analysis of the HARPS dataset. We then combine both the HARPS and CARMENES velocities to refine the parameters of both planets, notably resulting in a revised mass and period for K2-18c of $6.99_{-0.99}^{+0.96}$M${}_{Earth}$ and $9.2072\pm 0.0065$d, respectively. Our work thoroughly demonstrates the power of the line-by-line technique for the extraction of precision radial velocity information.

arXiv comment16 pages, 11 figures. Accepted in MNRAS

References in corpus (44)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• The Transiting Exoplanet Survey Satellite
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• Theoretical Transmission Spectra During Extrasolar Giant Planet Transits
• A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission
• ESPRESSO@VLT -- On-sky performance and first results
• Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
• The signature of orbital motion from the dayside of the planet tau Bootis b
• Detection of water absorption in the day side atmosphere of HD 189733 b using ground-based high-resolution spectroscopy at 3.2 microns
• Juliet: a versatile modelling tool for transiting and non-transiting exoplanetary systems
• How to Distinguish between Cloudy Mini-Neptunes and Water/Volatile-Dominated Super-Earths
• Water Vapor and Clouds on the Habitable-Zone Sub-Neptune Exoplanet K2-18b
• Water vapour in the atmosphere of the habitable-zone eight Earth-mass planet K2-18 b
• Rotation and winds of exoplanet HD 189733 b measured with high-dispersion transmission spectroscopy
• EVEREST: Pixel Level Decorrelation of K2 Light curves
• A statistical test for Nested Sampling algorithms
• Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method
• Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 17 Planets, Including a Planet Receiving Earth-like Insolation
• SPIRou: nIR velocimetry & spectropolarimetry at the CFHT
• Habitability and Biosignatures of Hycean Worlds
• Detection of molecular absorption in the dayside of exoplanet 51 Pegasi b?
• Discovery of water at high spectral resolution in the atmosphere of 51 Peg b
• The HARPS search for southern extra-solar planets I. HD330075 b: a new 'hot Jupiter'
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• Where is the Water? Jupiter-like C/H ratio but strong H${}_2$O depletion found on $\tau$ Boötis b using SPIRou
• Line-by-line velocity measurements, an outlier-resistant method for precision velocimetry
• New planetary and EB candidates from Campaigns 1-6 of the K2 mission
• 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
• Exoplanet atmospheres with GIANO II. Detection of molecular absorption in the dayside spectrum of HD 102195b
• The CARMENES search for exoplanets around M dwarfs: A low-mass planet in the temperate zone of the nearby K2-18
• 1D atmospheric study of the temperate sub-Neptune K2-18b
• Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES
• Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry
• Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum
• On the Temperature Profiles and Emission Spectra of Mini-Neptune Atmospheres
• Toward Extremely Precise Radial Velocities: II. A Tool For Using Multivariate Gaussian Processes to Model Stellar Activity
• Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b
• The CARMENES search for exoplanets around M dwarfs: Stable radial-velocity variations at the rotation period of AD~Leonis -- A test case study of current limitations to treating stellar activity
• The CARMENES search for exoplanets around M dwarfs: Benchmarking the impact of activity in high-precision radial velocity measurements

Cited by in corpus (4)

• Homogeneous search for helium in the atmosphere of 11 gas giant exoplanets with SPIRou
• Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b. From a joint analysis of JWST NIRISS, NIRSpec, and MIRI observations
• The CARMENES search for exoplanets around M dwarfs. Line-by-line sensitivity to activity in M dwarfs
• The CARMENES search for exoplanets around M dwarfs : Understanding the wavelength dependence of radial velocity measurements