TOI-257b (HD 19916b): a warm sub-saturn orbiting an evolved F-type star

Abstract

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 MP = 0.138 ± 0.023 $\rm {M_J}$ (43.9 ± 7.3 M⊕), a radius of RP = 0.639 ± 0.013 $\rm {R_J}$ (7.16 ± 0.15 R⊕), bulk density of $0.65^{+0.12}_{-0.11}$ (cgs), and period $18.38818^{+0.00085}_{-0.00084}$ $\rm {days}$. TOI-257b orbits a bright (V = 7.612 mag) somewhat evolved late F-type star with M* = 1.390 ± 0.046 $\rm {M_{sun}}$, R* = 1.888 ± 0.033 $\rm {R_{sun}}$, Teff = 6075 ± 90 $\rm {K}$, 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.

TOI-3362b: A Proto Hot Jupiter Undergoing High-eccentricity Tidal Migration

“…Minerva-Australis is an array of four identical 0.7 m telescopes linked via fiber feeds to a single KiwiSpec echelle spectrograph at a spectral resolving power of R ≈ 80,000 over the wavelength region of 5000-6300 Å. The array is wholly dedicated to radial-velocity follow-up of TESS planet candidates (e.g., Nielsen et al 2019;Brahm et al 2020;Addison et al 2021). Simultaneous wavelength calibration is provided via two calibration fibers illuminated by a quartz lamp through an iodine cell.…”

A Transiting Warm Giant Planet around the Young Active Star TOI-201

“…MINERVA-Australis is an array of four PlaneWave CDK700 telescopes located in Queensland, Australia, fully dedicated to the precise RV follow-up of TESS candidates (e.g., Addison et al 2020Addison et al , 2021Jordán et al 2020). The four telescopes can be simultaneously fiber-fed to a single KiwiSpec R4-100 highresolution (R = 80,000) spectrograph (Barnes et al 2012;Addison et al 2019).…”

HD 183579b: a warm sub-Neptune transiting a solar twin detected by TESS

“…MINERVA-Australis is an array of four PlaneWave CDK700 telescopes located at the Mt Kent Observatory in Queensland, Australia, fully dedicated to the precise RV follow-up of TESS candidates (e.g. Jordán et al 2020;Addison et al 2021Addison et al , 2020. The four telescopes can be simultaneously fibre-fed to a single KiwiSpec R4-100 high-resolution (R = 80 000) spectrograph (Barnes et al 2012;Addison et al 2019).…”