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

摘要

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 - + M 0.367 0.023 0.022 J , a radius of = - + R R 1.023 P 0.013 0.013 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 M 1.257 0.029 0.029 ? , a radius of = - + R R 2.341 0.019 0.022 , 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.