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

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 ratio follow-up measurements. Here, we report on new observations of the bright (V ∼ 3.3) giant star ι Draconis (ι 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 radialvelocity 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 36 60 yr. Mass predictions from our analysis place this substellar 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.

Revisiting the Red Giant Branch Hosts KOI-3886 and ι Draconis. Detailed Asteroseismic Modeling and Consolidated Stellar Parameters

“…To deal with the target's saturated nature, a large custom aperture was adopted and a background model was applied to account for the spatially varying background light. Full details on the light-curve preparation are presented in Hill et al (2021). .…”

“…ι Dra, known for two decades to host a planet 24 in a highly eccentric, 511 days period orbit (Frink et al 2002;Zechmeister et al 2008;Kane et al 2010), was observed by TESS over five noncontiguous sectors (each sector is 27.4 days long) during the second year of its nominal mission. In Hill et al (2021), we presented the results of continued radial-velocity (RV) monitoring of ι Dra over several orbits of its known planet. The newly acquired RV observations allowed the detection of curvatures in the previously identified RV trend, which was interpreted as likely being caused by an outer companion.…”

“…The presence of planets orbiting KOI-3886 and ι Dra (putative in the former case) is what originally prompted the seismic analyses-conducted separately, although not in a strictly independent manner-of these two red giant stars. In this follow-up work, we give a full account of the seismic analyses underpinning Lillo-Box & Ribas (2021) and Hill et al (2021), while making an incursion into the wider problem of detailed asteroseismic modeling of red giants. Evolutionary Notes.…”

“…To that end, we carry out a comprehensive stellar characterization by employing two independent and well-established modeling pipelines (Li et al 2020;Ong et al 2021a), hence further allowing the (interpipeline) systematics of the inferred stellar parameters-arising from the use of different model grids, input physics, and analysis methodologies-to be estimated. Critically, we include methodological documentation and discussion that had been deferred to this work from Lillo-Box & Ribas (2021) and Hill et al (2021), where results from a single modeling pipeline (namely, that of Li et al 2020) were only briefly presented.…”

TESS Asteroseismic Analysis of HD 76920: The Giant Star Hosting an Extremely Eccentric Exoplanet

“…Thanks to the high-quality of TESS photometry and large sky coverage, oscillations are expected to be detected in hundreds of thousands of solar-like oscillators (Campante et al 2018;Huber 2018;Schofield et al 2019), including several hundred asteroseismic exoplanet hosts (Campante et al 2016b). Detections of oscillations by TESS in previously known exoplanet-host stars have been reported by several works (e.g., Campante et al 2019;Jiang et al 2020b;Nielsen et al 2020;Hill et al 2021;Huber et al 2022), following on the discovery of the first planet transiting a star in which oscillations could be measured by TESS (Huber et al 2019). These extraordinary synergies between asteroseismology and exoplanetary science significantly improve our understanding of planet systems outside of the solar system and provide insight into the occurrence rates of exoplanets as a function of their host stars' property and evolutionary state, as well as the planets' mass, size, and orbital architecture.…”