TOI-257b (HD 19916b): a warm sub-saturn orbiting an evolved F-type star
We report the discovery of a warm sub-Saturn, TOI-257b (HD 19916b),
based on data from NASA's Transiting Exoplanet Survey Satellite
(TESS). The transit signal was detected by TESS and confirmed to
be of planetary origin based on radial velocity observations. An
analysis of the TESS photometry, the MINERVA-Australis, FEROS,
and HARPS radial velocities, and the asteroseismic data of the
stellar oscillations reveals that TOI-257b has a mass of M_P
= 0.138 ± 0.023 M_J (43.9
± 7.3 M_⊕ ), a radius of
R_P = 0.639 ± 0.013 R_J (7.16
± 0.15 R_⊕ ), bulk
density of 0.65^+0.12_-0.11 (cgs), and period
18.38818^+0.00085_-0.00084 days . TOI-257b orbits
a bright (V = 7.612 mag) somewhat evolved late F-type star with
M_* = 1.390 ± 0.046 M_sun ,
R_* = 1.888 ± 0.033 R_sun ,
T_eff = 6075 ± 90 K , and vsin i =
11.3 ± 0.5 km s^-1. Additionally, we find
hints for a second non-transiting sub-Saturn mass planet on a
∼71 day orbit using the radial velocity data.
This system joins the ranks of a small number of exoplanet host
stars (∼100) that have been characterized with
asteroseismology. Warm sub-Saturns are rare in the known sample
of exoplanets, and thus the discovery of TOI-257b is important
in the context of future work studying the formation and
migration history of similar planetary systems.