[Paper Review] The CARMENES search for exoplanets around M dwarfs Detection of a mini-Neptune around LSPM J2116+0234 and refinement of orbital parameters of a super-Earth around GJ 686 (BD+18 3421)

S. Lalitha, D. Baroch+35 more|arXiv (Cornell University)|May 22, 2019

Stellar, planetary, and galactic studies92 references8 citations

TL;DR

This study reports the detection of a mini-Neptune with a minimum mass of 11.8 M⊕ orbiting the M3.0 V star LSPM J2116+0234, based on CARMENES radial velocity data, and refines the orbital parameters of a super-Earth (6.6 M⊕) around GJ 686 (M1.0 V) using a 20-year radial velocity dataset. The analysis accounts for stellar activity using Keplerian models with correlated noise, confirming planetary signals at 14.44 d and 15.53 d, respectively.

ABSTRACT

Although M dwarfs are known for high levels of stellar activity, they are ideal targets for the search of low-mass exoplanets with the radial velocity (RV) method. We report the discovery of a planetary-mass companion around LSPM J2116+0234 (M3.0 V) and confirm the existence of a planet orbiting GJ 686 (BD+18 3421; M1.0 V). The discovery of the planet around LSPM J2116+0234 is based on CARMENES RV observations in the visual and near-infrared channels. We confirm the planet orbiting around GJ 686 by analyzing the RV data spanning over two decades of observations from CARMENES VIS, HARPS-N, HARPS, and HIRES. We find planetary signals at 14.44 and 15.53 d in the RV data for LSPM J2116+0234 and GJ 686, respectively. Additionally, the RV, photometric time series, and various spectroscopic indicators show hints of variations of 42 d for LSPM J2116+0234 and 37 d for GJ 686, which we attribute to the stellar rotation periods. The orbital parameters of the planets are modeled with Keplerian fits together with correlated noise from the stellar activity. A mini-Neptune with a minimum mass of 11.8 Me orbits LSPM J2116+0234 producing an RV semi-amplitude of 6.19 m/s, while a super-Earth of mass 6.6 Me orbits GJ 686 and produces an RV semi-amplitude of 3.0 m/s. Both LSPM J2116+0234 and GJ 686 have planetary companions populating the regime of exoplanets with masses lower than 15 Me and orbital periods <20 d.

Motivation & Objective

• To detect low-mass exoplanets around M dwarfs, which are prime targets for habitable planet searches due to their high radial velocity signal per planet mass.
• To overcome the challenge of stellar activity in M dwarfs, which can mimic planetary signals through radial velocity variations.
• To improve orbital parameter precision for known planets by combining long-baseline radial velocity data from multiple instruments.
• To disentangle planetary signals from stellar activity using multi-epoch radial velocity measurements and activity indicators.
• To determine the stellar rotation periods of LSPM J2116+0234 and GJ 686 using photometric and spectroscopic variability.

Proposed method

• Acquired radial velocity measurements using the CARMENES spectrograph in the visual and near-infrared channels for LSPM J2116+0234.
• Combined radial velocity data from CARMENES-VIS, HARPS-N, HARPS, and HIRES for GJ 686, spanning over two decades.
• Modeled planetary signals using Keplerian orbital fits with correlated noise to account for stellar activity.
• Applied non-parametric stellar variability models to simultaneously fit radial velocity and photometric time series.
• Used activity indicators (e.g., Hα, Ca II H&K) to correlate with radial velocity variations and distinguish activity-induced signals.
• Employed Bayesian statistical modeling with Gaussian processes to account for correlated noise from stellar magnetic activity.

Experimental results

Research questions

• RQ1 Does a planetary signal at 14.44 days in the radial velocity data of LSPM J2116+0234 represent a true exoplanet, and is it distinguishable from stellar activity?
• RQ2 Can the orbital parameters of the super-Earth around GJ 686 be refined using a combined dataset spanning over 20 years of observations?
• RQ3 What is the stellar rotation period of LSPM J2116+0234 and GJ 686, and how does it relate to the observed radial velocity variations?
• RQ4 Is the planetary signal at 15.53 days around GJ 686 consistent with a single-planet model, or are additional signals present?
• RQ5 How do the radial velocity semi-amplitudes and minimum masses of the planets compare when accounting for correlated noise from stellar activity?

Key findings

• A mini-Neptune with a minimum mass of 11.8 M⊕ orbits LSPM J2116+0234 with a radial velocity semi-amplitude of 6.19 m s⁻¹ and an orbital period of 14.44 days.
• The orbital solution for the super-Earth around GJ 686 was refined to a radial velocity semi-amplitude of 3.02 m s⁻¹, corresponding to a minimum mass of 6.64 M⊕ and an orbital period of 15.5314 days.
• The stellar rotation period of LSPM J2116+0234 was determined to be 42.0 days using photometric and spectroscopic variability, with a 42-day signal in radial velocity and activity indicators.
• The stellar rotation period of GJ 686 was estimated at 38.4 days, with a 37-day signal in radial velocity and activity indicators.
• The refined model for GJ 686 suggests a non-zero orbital eccentricity of 0.077, differing from the circular orbit assumed in prior work.
• The data favor a single-planet model for both systems, though a longer-period signal may be present in the GJ 686 data requiring further observations.