HD 38973 b -- a cold Saturn orbiting a Sun-like star

TL;DR

This paper reports the detection and characterization of a cold Saturn-mass exoplanet orbiting a Sun-like star using radial velocities and astrometric data, demonstrating the power of combining these methods.

Contribution

It introduces a method to constrain exoplanet masses by combining radial-velocity data with astrometric non-detections, providing robust mass estimates.

Findings

Detected a long-period companion with a minimum mass in the sub-Jovian regime.

Derived a true mass of approximately 0.24 Jupiter masses for the companion.

Showed that astrometric non-detections can effectively constrain exoplanet masses.

Abstract

We report the detection of a long-period companion to the nearby solar-type star HD\,38973 using precision radial-velocity measurements. The radial-velocity data reveal a coherent Keplerian signal with a period of $\sim$3000~days and moderate eccentricity, yielding a minimum mass in the sub-Jovian regime. We complement the radial-velocity analysis with astrometric constraints from the \emph{Hipparcos--Gaia} Catalog of Accelerations (HGCA). Although no significant proper-motion anomaly is detected for HD\,38973, the absence of an astrometric signal provides an informative upper limit on the companion mass. By combining the radial-velocity posterior with the HGCA likelihood, we rule out high-mass solutions at low inclinations and derive a robust upper bound on the true companion mass. We find the best-fitting true mass to be $0.240_{-0.040}^{+0.102}\,M_{\rm Jup}$, on an orbit with period $2733^{+210}_{-190}$ days, making HD\,38973b a likely cold Saturn. This study highlights the diagnostic power of astrometric non-detections when combined with precision radial velocities, demonstrating that meaningful constraints on companion masses can be obtained even in the absence of a detected astrometric signal.