K2-139 b: a low-mass warm Jupiter on a 29-day orbit transiting an active K0 V star

Abstract

We announce the discovery of K2-139 b (EPIC 218916923 b), a transiting warm-Jupiter ($T_{\mathrm{eq}}$=547$\pm$25 K) on a 29-day orbit around an active (log $R_{\mathrm{HK}}^{\prime }$ = $-$4.46 $\pm$ 0.06) K0 V star in K2 Campaign 7. We derive the system's parameters by combining the K2 photometry with ground-based follow-up observations. With a mass of~$0.387_{-0.075}^{+0.083}{M}_{\mathrm{J}}$ and radius of $0.808_{-0.033}^{+0.034}{R}_{\mathrm{J}}$, K2-139 b is one of the transiting warm Jupiters with the lowest mass known to date. The planetary mean density of $0.91_{-0.20}^{+0.24}$ $\mathrm{gc}{\mathrm{m}}^{-3}$ can be explained with a core of $\sim$50 $M_{\oplus }$. Given the brightness of the host star ($V$ = 11.653 mag), the relatively short transit duration ($\sim$5 hours), and the expected amplitude of the Rossiter-McLaughlin effect ($\sim 25\mathrm{m}{\mathrm{s}}^{-1}$ ), K2-139 is an ideal target to measure the spin-orbit angle of a planetary system hosting a warm Jupiter.

12 pages, 5 tables, 5 figures, accepted for publication in MNRAS