A Hot Saturn Orbiting An Oscillating Late Subgiant Discovered by TESS

Abstract

We present the discovery of , the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS ) for which asteroseismology of the host star is possible. TOI-197 (HIP 116158) is a bright (V = 8.2 mag), spectroscopically classified subgiant which oscillates with an average frequency of about 430 µHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2-minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (R = 2.943 ± 0.064R ), mass (M = 1.212 ± 0.074M ) and age (4.9 ± 1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a "hot Saturn" (R p = 9.17 ± 0.33R ⊕ ) with an orbital period of ∼ 14.3 days, irradiance of F = 343 ± 24F ⊕ , moderate mass (M p = 60.5 ± 5.7M ⊕ ) and density (ρ p = 0.431 ± 0.062 g cm −3 ). The properties of show that the host-star metallicity -planet mass correlation found in sub-Saturns (4 − 8R ⊕ ) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ∼ 15%, TOI-197.01 is one of the best characterized Saturn-sized planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology.

Gemini-GRACES high-quality spectra of Kepler evolved stars with transiting planets I. Detailed characterization of multi-planet systems Kepler-278 and Kepler-391

“…The stellar parameters of TOI-197 (T eff = 5080 ± 90 K, log g = 3.60 ± 0.08 dex, M = 1.21 ± 0.07 M , and R = 2.94 ± 0.06 R ) are very similar to those of Kepler-278 and Kepler-391. Moreover, the measured ∆ν value of 28.94 ± 0.15 µHz in TOI-197 (Huber et al 2019), indicating that the star has just started its ascent on the RGB (Mosser et al 2014), is also very similar to that of Kepler-278.…”

“…A possibility could be the use of different stellar models (e.g., MESA vs. PARSEC) to obtain the stellar parameters, although it is not evident why there are no differences for Kepler-391. Additionally, as demonstrated by Huber et al (2019) for TOI-197, uncertainties in the stellar parameters can be underestimated by the use of single, instead of multiple, model grids that do not allow to take systematic errors into account.…”

“…However, only a few late subgiants and early red giants (∼15) are known to host transiting planets, which highlight the difficulty to detect planetary transits at this evolutionary stage. Recently, Huber et al (2019) presented the first oscillating late subgiant star, TOI-197, with a transiting planet discoverd by TESS. The stellar parameters of TOI-197 (T eff = 5080 ± 90 K, log g = 3.60 ± 0.08 dex, M = 1.21 ± 0.07 M , and R = 2.94 ± 0.06 R ) are very similar to those of Kepler-278 and Kepler-391.…”

“…On the other hand, planets such as those orbiting Kepler-391 and Kepler-278 are probably too small or too far from their hosts to fall into the regime of significant inflation (e.g., Van Eylen et al 2016). For example, with F inc ∼ 340 F ⊕ and R p = 9.16 R Jup , the planet TOI-197.01 (Huber et al 2019) is located at the base of a "inflation sequence", a region in the radius-incident flux diagram (Fig. 14) from which the planet radius increases with stellar irradiation (see, e.g., Demory & Seager 2011).…”

“…14) from which the planet radius increases with stellar irradiation (see, e.g., Demory & Seager 2011). While TOI-197.01 would just be starting to be re-inflated (Huber et al 2019), there are other planets around RGB stars on this inflation sequence, including the giant planets K2-97b and K2-132b whose radii have already become re-inflated as result of stellar evolution (Grunblatt et al 2016(Grunblatt et al , 2017.…”