Asteroseismology of iota Draconis and Discovery of an Additional Long-Period Companion

TL;DR

This study uses asteroseismology and radial velocity data to refine the properties of the giant star iota Draconis, discovering an additional long-period companion and providing insights into planetary dynamics around evolved stars.

Contribution

It presents the first asteroseismic analysis of iota Draconis and reports the discovery of a long-period sub-stellar companion using combined radial velocity and astrometric data.

Findings

Refined stellar parameters with 2% radius, 6% mass, 28% age accuracy.

Detected an additional long-period companion with a ~68-year orbit.

Mass of the new companion suggests it is on the planet-brown dwarf boundary.

Abstract

Giant stars as known exoplanet hosts are relatively rare due to the potential challenges in acquiring precision radial velocities and the small predicted transit depths. However, these giant host stars are also some of the brightest in the sky and so enable high signal-to-noise follow-up measurements. Here we report on new observations of the bright (V ~ 3.3) giant star $\iota$ Draconis ($\iota$ Dra), known to host a planet in a highly eccentric ~511 day period orbit. TESS observations of the star over 137 days reveal asteroseismic signatures, allowing us to constrain the stellar radius, mass, and age to ~2%, ~6%, and ~28%, respectively. We present the results of continued radial velocity monitoring of the star using the Automated Planet Finder over several orbits of the planet. We provide more precise planet parameters of the known planet and, through the combination of our radial velocity measurements with Hipparcos and Gaia astrometry, we discover an additional long-period companion with an orbital period of ~$68^{+60}_{-36}$ years. Mass predictions from our analysis place this sub-stellar companion on the border of the planet and brown dwarf regimes. The bright nature of the star combined with the revised orbital architecture of the system provides an opportunity to study planetary orbital dynamics that evolve as the star moves into the giant phase of its evolution.