Utilizing Photometry from Multiple Sources to Mitigate Stellar Variability in Precise Radial Velocities: A Case Study of Kepler-21

TL;DR

This study combines photometry from Kepler, TESS, and radial velocity data from Keck and WIYN to improve the characterization of Kepler-21 and its exoplanet, demonstrating effective activity mitigation and refining system parameters.

Contribution

It introduces a comprehensive method integrating multiple photometric sources with RV data to mitigate stellar activity effects in exoplanet characterization.

Findings

Kepler photometry effectively removes correlated noise from RVs.

TESS photometry can outperform Kepler in activity mitigation in some cases.

Kepler-21 b is not a water world, and a long-period super-Jupiter candidate is identified.

Abstract

We present a new analysis of Kepler-21, the brightest (V = 8.5) Kepler system with a known transiting exoplanet, Kepler-21 b. Kepler-21 b is a radius valley planet ($R = 1.6\pm 0.2 R_{\oplus}$) with an Earth-like composition (8.38$\pm$1.62 g/cc), though its mass and radius fall in the regime of possible "water worlds." We utilize new Keck/HIRES and WIYN/NEID radial velocity (RV) data in conjunction with Kepler and TESS photometry to perform a detailed study of activity mitigation between photometry and RVs. We additionally refine the system parameters, and we utilize Gaia astrometry to place constraints on a long-term RV trend. Our activity analysis affirms the quality of Kepler photometry for removing correlated noise from RVs, despite its temporal distance, though we reveal some cases where TESS may be superior. Using refined orbital parameters and updated composition curves, we rule out a ``water world" scenario for Kepler-21 b, and we identify a long period super-Jupiter planetary candidate, Kepler-21 (c).