HD 1397b: a transiting warm giant planet orbiting a V = 7.8 mag sub-giant 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. HD1397b has a mass of ${\mathrm{M}}_{\mathrm{P}}$ = $0.335_{-0.018}^{+0.018}$ ${\mathrm{M}}_{\mathrm{J}}$, a radius of ${\mathrm{R}}_{\mathrm{P}}$ = $1.021_{-0.014}^{+0.015}$ ${\mathrm{M}}_{\mathrm{J}}$, and orbits its bright host star ($V=7.8$ mag) with an orbital period of $11.53508\pm 0.00057$ d, on a moderately eccentric orbit ($e$ = 0.210 $\pm$ 0.038). With a mass of ${\mathrm{M}}_{\star }$ = $1.284_{-0.016}^{+0.020}$ ${\mathrm{M}}_J$, a radius of ${\mathrm{R}}_{\star }$ = 2.314${}_{-0.042}^{+0.049}$ ${\mathrm{R}}_{\mathrm{J}}$, and an age of 4.7 $\pm$ 0.2 Gyr, the solar metallicity host star has already departed from the main sequence. We find evidence in the radial velocity measurements for a long term acceleration, and a $P\approx 18$ d periodic signal that we attribute to rotational modulation by stellar activity. The HD1397 system is among the brightest systems 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.

15 pages, submitted to AJ