High-Contrast Imaging of Intermediate-Mass Giants with Long-Term Radial Velocity Trends

TL;DR

This study combines long-term radial velocity data with high-contrast imaging to identify and characterize companions around intermediate-mass giants, revealing co-moving companions and assessing their role in observed RV trends.

Contribution

It provides direct imaging detections of companions around giants with RV trends and evaluates their impact on planetary eccentricities, advancing understanding of stellar and planetary system evolution.

Findings

Detected co-moving companions to three giants.

Excluded massive companions at certain separations for three targets.

Kozai mechanism explains high eccentricities of some inner planets.

Abstract

A radial velocity (RV) survey for intermediate-mass giants has been operated for over a decade at Okayama Astrophysical Observatory (OAO). The OAO survey has revealed that some giants show long-term linear RV accelerations (RV trends), indicating the presence of outer companions. Direct imaging observations can help clarify what objects generate these RV trends. We present the results of high-contrast imaging observations or six intermediate-mass giants with long-term RV trends using the Subaru Telescope and HiCIAO camera. We detected co-moving companions to $\gamma$ Hya B ($0.61^{+0.12}_{-0.14} M_\odot$), HD 5608 B ($0.10 \pm 0.01 M_\odot$), and HD 109272 B ($0.28 \pm 0.06 M_\odot$). For the remaining targets($\iota$ Dra, 18 Del, and HD 14067) we exclude companions more massive than 30-60 $M_\mathrm{Jup}$ at projected separations of 1arcsec-7arcsec. We examine whether these directly imaged companions or unidentified long-period companions can account for the RV trends observed around the six giants. We find that the Kozai mechanism can explain the high eccentricity of the inner planets $\iota$ Dra b, HD 5608 b, and HD 14067 b.