Revisiting the HD 21749 Planetary System with Stellar Activity Modeling

TL;DR

This study refines the planetary parameters of the HD 21749 system by modeling stellar activity with Gaussian Processes, leading to more accurate mass and density estimates for its planets, especially HD 21749b.

Contribution

The paper introduces a new approach using Gaussian Process regression to disentangle stellar activity from RV signals, improving planetary mass and density measurements.

Findings

HD 21749b has a mass of 20.0±2.7 M⊕ and a density of 4.8+2.0−1.4 g/cm³.

Stellar activity significantly affects RV measurements, causing potential underestimation of planetary masses.

Proper modeling of stellar activity is crucial for accurate exoplanet characterization.

Abstract

HD 21749 is a bright ($V=8.1$ mag) K dwarf at 16 pc known to host an inner terrestrial planet HD 21749c as well as an outer sub-Neptune HD 21749b, both delivered by TESS. Follow-up spectroscopic observations measured the mass of HD 21749b to be $22.7\pm2.2\ M_{\oplus}$ with a density of $7.0^{+1.6}_{-1.3}$ g~cm$^{-3}$, making it one of the densest sub-Neptunes. However, the mass measurement was suspected to be influenced by stellar rotation. Here we present new high-cadence PFS RV data to disentangle the stellar activity signal from the planetary signal. We find that HD 21749 has a similar rotational timescale as the planet's orbital period, and the amplitude of the planetary orbital RV signal is estimated to be similar to that of the stellar activity signal. We perform Gaussian Process (GP) regression on the photometry and RVs from HARPS and PFS to model the stellar activity signal. Our new models reveal that HD 21749b has a radius of $2.86\pm0.20\ R_{\oplus}$, an orbital period of $35.6133\pm0.0005$ d with a mass of $M_{b}=20.0\pm2.7\ M_{\oplus}$ and a density of $4.8^{+2.0}_{-1.4}$ g~cm$^{-3}$ on an eccentric orbit with $e=0.16\pm0.06$, which is consistent with the most recent values published for this system. HD 21749c has an orbital period of $7.7902\pm0.0006$ d, a radius of $1.13\pm0.10\ R_{\oplus}$, and a 3$\sigma$ mass upper limit of $3.5\ M_{\oplus}$. Our Monte Carlo simulations confirm that without properly taking stellar activity signals into account, the mass measurement of HD 21749b is likely to arrive at a significantly underestimated error bar.