The K2-ESPRINT Project V: a short-period giant planet orbiting a subgiant star

TL;DR

This paper reports the discovery of K2-39b, a short-period giant planet orbiting a subgiant star, providing insights into planetary survival and evolution around evolved high-mass stars.

Contribution

It presents the first detailed characterization of a short-period giant planet orbiting a subgiant star, including mass, radius, and density measurements, and discusses implications for planetary survival.

Findings

K2-39b has a 4.6-day orbit, making it the shortest-period planet around a subgiant.

The planet's density suggests a gaseous composition.

A long-term RV trend indicates possible additional companions.

Abstract

We report on the discovery and characterization of the transiting planet K2-39b (EPIC 206247743b). With an orbital period of 4.6 days, it is the shortest-period planet orbiting a subgiant star known to date. Such planets are rare, with only a handful of known cases. The reason for this is poorly understood, but may reflect differences in planet occurrence around the relatively high-mass stars that have been surveyed, or may be the result of tidal destruction of such planets. K2-39 is an evolved star with a spectroscopically derived stellar radius and mass of $3.88^{+0.48}_{-0.42}~\mathrm{R_\odot}$ and $1.53^{+0.13}_{-0.12}~\mathrm{M_\odot}$, respectively, and a very close-in transiting planet, with $a/R_\star = 3.4$. Radial velocity (RV) follow-up using the HARPS, FIES and PFS instruments leads to a planetary mass of $50.3^{+9.7}_{-9.4}~\mathrm{M_\oplus}$. In combination with a radius measurement of $8.3 \pm 1.1~\mathrm{R_\oplus}$, this results in a mean planetary density of $0.50^{+0.29}_{-0.17}$ g~cm$^{-3}$. We furthermore discover a long-term RV trend, which may be caused by a long-period planet or stellar companion. Because K2-39b has a short orbital period, its existence makes it seem unlikely that tidal destruction is wholly responsible for the differences in planet populations around subgiant and main-sequence stars. Future monitoring of the transits of this system may enable the detection of period decay and constrain the tidal dissipation rates of subgiant stars.