HD 1397b: A Transiting Warm Giant Planet Orbiting A V = 7.8 mag Subgiant Star Discovered by TESS

Abstract

We report the discovery of a transiting planet first identified as a candidate in Sector 1 of the Transiting Exoplanet Survey Satellite (TESS), and then confirmed with precision radial velocities. HD 1397b has a mass of {M}{{P}} ={0.367}-0.023+0.022 {M}{{J}}, a radius of {R}{{P}}={1.023}-0.013+0.013 {R}{{J}}, and orbits its bright host star (V = 7.8 mag) with an orbital period of 11.5366+/- 0.0003 d on a moderately eccentric orbit (e={0.216}-0.026+0.027). With a mass of {M}star ={1.257}-0.029+0.029 {M}☉ , a radius of {R}star ={2.341}-0.019+0.022 {R}☉ , and an age of 4.46+/- 0.25 Gyr, the solar-metallicity host star has already departed from the main sequence. We find evidence in the radial velocity measurements of a secondary signal with a longer period. We attribute it to the rotational modulation of stellar activity, but a long-term radial velocity monitoring would be necessary to discard if this signal is produced by a second planet in the system. The HD 1397 system is among the brightest ones currently known to host a transiting planet, which will make it possible to perform detailed follow-up observations in order to characterize the properties of giant planets orbiting evolved stars.