A Hot Saturn Orbiting An Oscillating Late Subgiant Discovered by TESS

TL;DR

This paper reports the discovery and detailed characterization of a hot Saturn-sized exoplanet orbiting an oscillating subgiant star, utilizing TESS data and asteroseismology to precisely determine stellar and planetary properties.

Contribution

It is the first TESS-discovered transiting planet with asteroseismic analysis of its host star, providing precise measurements of the star and planet, and insights into planet density and host star-planet correlations.

Findings

The host star is a subgiant with radius ~2.94 Rsun, mass ~1.21 Msun, and age ~4.9 Gyr.

The planet is a hot Saturn with radius ~9.17 Rearth, period ~14.3 days, and density ~0.43 g/cc.

The host star's metallicity-planet mass correlation does not extend to larger radii.

Abstract

We present the discovery of TOI-197.01, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. TOI-197 (HIP116158) is a bright (V=8.2 mag), spectroscopically classified subgiant which oscillates with an average frequency of about 430 muHz 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 (2.943+/-0.064 Rsun), mass (1.212 +/- 0.074 Msun) 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" (9.17+/-0.33 Rearth) with an orbital period of ~14.3 days, irradiance of 343+/-24 Fearth, moderate mass (60.5 +/- 5.7 Mearth) and density (0.431+/-0.062 gcc). The properties of TOI-197.01 show that the host-star metallicity - planet mass correlation found in sub-Saturns (4-8 Rearth) 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.

Tables

Table 1. Table 1: Extracted oscillation frequencies and mode identifications for TOI-197.

$f\left(\mu \text{Hz}\right)$	${\sigma }_f\left(\mu \text{Hz}\right)$	$l$
413.12	0.29	1
420.06	0.11	0
429.26	0.14	1
436.77	0.24	1
445.85	0.21	2
448.89	0.21	0
460.16	0.33	1
463.81	0.43	1
477.08	0.31	1
478.07	0.35	0

Table 2. Table 2: Host Star Parameters

Basic Properties
Hipparcos ID	116158
TIC ID	441462736
$V$ Magnitude	8.15
TESS Magnitude	7.30
$K$ Magnitude	6.04
SED & Gaia Parallax
Parallax, $\pi$ (mas)	$10.518\pm 0.080$
Luminosity, $L$ ($L_{\odot }$)	$5.15\pm 0.17$
Spectroscopy
Effective Temperature, $T_{\text{eff}}$  (K)	$5080\pm 90$
Metallicity, [Fe/H] (dex)	$-0.08\pm 0.08$
Projected rotation speed, $v\sin i$ (km s-1)	$2.8\pm 1.6$
Asteroseismology
Stellar Mass, $M_{\star }$ ($M_{\odot }$)	$1.212\pm 0.074$
Stellar Radius, $R_{\star }$ ($R_{\odot }$)	$2.943\pm 0.064$
Stellar Density, ${\rho }_{\star }$ (gcc)	$0.06702\pm 0.00067$
Surface gravity, $\log g$ (cgs)	$3.584\pm 0.010$
Age, $t$ (Gyr)	$4.9\pm 1.1$

Table 3. Table 3: High-precision Radial Velocities for TOI-197

Time (BJD)	RV (m/s)	${\sigma }_{\text{RV}}$ (m/s)	Instrument
2458426.334584	4.258	11.260	SONG
2458426.503655	6.328	11.270	SONG
2458427.575230	-12.667	3.000	FEROS
2458428.547576	17.328	18.540	SONG
…	…	…	…
2458443.535340	-14.667	3.600	CORALIE
2458443.541210	-3.067	3.800	CORALIE
2458443.714865	-6.815	0.780	HIRES
2458443.825283	-4.375	0.720	HIRES
…	…	…	…
2458482.562290	19.433	2.000	HARPS
2458483.541710	16.133	2.000	HARPS
2458483.553240	19.233	2.000	HARPS
2458483.564690	16.233	2.000	HARPS

Table 4. Table 4: Planet Parameters

Parameter	Best-fit	Median	84%	16%
Model Parameters
${\gamma }_{\mathrm{HIRES}}$	4.8	5.4	$+$1.6	$-$1.6
${\gamma }_{\mathrm{SONG}}$	1.1	0.2	$+$1.5	$-$1.5
${\gamma }_{\mathrm{FEROS}}$	-15.4	-15.7	$+$1.2	$-$1.2
${\gamma }_{\mathrm{CORALIE}}$	-5.4	-5.0	$+$1.2	$-$1.2
${\gamma }_{\mathrm{HARPS}}$	8.1	8.8	$+$1.5	$-$1.5
${\sigma }_{\mathrm{HIRES}}$	2.71	2.68	$+$0.85	$-$0.80
${\sigma }_{\mathrm{SONG}}$	2.06	2.11	$+$0.91	$-$0.89
${\sigma }_{\mathrm{FEROS}}$	3.49	3.47	$+$0.75	$-$0.71
${\sigma }_{\mathrm{CORALIE}}$	1.88	2.50	$+$0.75	$-$0.64
${\sigma }_{\mathrm{HARPS}}$	2.41	2.69	$+$0.75	$-$0.63
$z$ (ppm)	199.4	199.1	$+$10.6	$-$10.7
$P$ (days)	14.2762	14.2767	$+$0.0037	$-$0.0037
$T_0$ (BTJD)	1357.0135	1357.0149	$+$0.0025	$-$0.0026
$b$	0.744	0.728	$+$0.040	$-$0.049
$R_p/R_{\star }$	0.02846	0.02854	$+$0.00084	$-$0.00071
$e\cos \omega$	-0.054	-0.028	$+$0.063	$-$0.061
$e\sin \omega$	-0.099	-0.096	$+$0.029	$-$0.030
K (m/s)	14.6	14.1	$+$1.2	$-$1.2
${\rho }_{\star }\left({\rho }_{\odot }\right)$	0.06674	0.06702	$+$0.00052	$-$0.00052
u1	0.12	0.35	$+$0.36	$-$0.24
u2	0.71	0.44	$+$0.30	$-$0.44
Derived Properties
$e$	0.113	0.115	$+$0.034	$-$0.030
$\omega$	-118.7	-106.0	$+$34.7	$-$31.1
$a$ (AU)	0.1233	0.1228	$+$0.0025	$-$0.0026
$a/R_{\star }$	9.00	8.97	$+$0.27	$-$0.27
$i$ (${}^{\text{o}}$)	85.67	85.75	$+$0.36	$-$0.35
$R_p\left(R_{\oplus }\right)$	9.16	9.17	$+$0.34	$-$0.31
$R_p\left(R_J\right)$	0.835	0.836	$+$0.031	$-$0.028
$M_p\left(M_{\oplus }\right)$	63.4	60.5	$+$5.7	$-$5.7
$M_p\left(M_J\right)$	0.200	0.190	$+$0.018	$-$0.018
${\rho }_p$ (gcc)	0.455	0.431	$+$0.064	$-$0.060

Full text

planets and satellites: individual (TOI-197) — stars: fundamental parameters — techniques: asteroseismology, photometry, spectroscopy — TESS — planetary systems

††software: Astropy (Astropy Collaboration et al., 2018), Matplotlib (Hunter, 2007), DIAMONDS (Corsaro & De Ridder, 2014), isoclassify (Huber et al., 2017), EXOFASTv2 (Eastman, 2017), ktransit (Barclay, 2018)

1 Introduction

Asteroseismology is one of the major success stories of the space photometry revolution initiated by CoRoT (Baglin et al., 2006) and Kepler (Borucki et al., 2010). The detection of oscillations in thousands of stars has led to breakthroughs such as the discovery of rapidly rotating cores in subgiants and red giants, as well as the systematic measurement of stellar masses, radii and ages (see Chaplin & Miglio, 2013, for a review). Asteroseismology has also become the “gold standard” for calibrating more indirect methods to determine stellar parameters such as surface gravity ($\log g$) from spectroscopy (Petigura et al., 2017a) and stellar granulation (Mathur et al., 2011; Bastien et al., 2013; Kallinger et al., 2016; Corsaro et al., 2017; Bugnet et al., 2018; Pande et al., 2018), and age from rotation periods (gyrochronology, e.g. García et al., 2014; van Saders et al., 2016).

A remarkable synergy that emerged from space-based photometry is the systematic characterization of exoplanet host stars using asteroseismology. Following first asteroseismic studies of exoplanet host stars using radial velocities (Bouchy et al., 2005; Bazot et al., 2005), the Hubble Space Telescope (Gilliland et al., 2011) and CoRoT (Ballot et al., 2011b; Lebreton & Goupil, 2014), Kepler enabled the systematic characterization of exoplanets with over 100 detections of oscillations in host stars to date (Huber et al., 2013a; Lundkvist et al., 2016). In addition to the more precise characterization of exoplanet radii and masses (Ballard et al., 2014), the synergy also enabled systematic constraints on stellar spin-orbit alignments (Chaplin et al., 2014b; Benomar et al., 2014; Lund et al., 2014; Campante et al., 2016a) and statistical inferences on orbital eccentricities through constraints on the mean stellar density (Sliski & Kipping, 2014; Van Eylen & Albrecht, 2015; Van Eylen et al., 2019).

The recently launched NASA TESS Mission (Ricker et al., 2014) is poised to continue the synergy between asteroseismology and exoplanet science. Using dedicated 2-minute cadence observations, TESS is expected to detect oscillations in thousands of main-sequence, subgiant and early red-giant stars (Schofield et al., 2018), and simulations predict that at least 100 of these will host transiting or non-transiting exoplanets (Campante et al., 2016b). TESS host stars are on average significantly brighter than typical Kepler hosts, facilitating ground-based measurements of planet masses with precisely characterized exoplanet hosts from asteroseismology. While some of the first exoplanets discovered with TESS orbit stars that have evolved off the main sequence (Wang et al., 2019; Brahm et al., 2018; Nielsen et al., 2019), none of them were amenable to asteroseismology using TESS photometry. Here, we present the characterization of TESS Object of Interest 197 (TOI-197, HIP 116158) system, the first discovery by TESS of a transiting exoplanet around a host star in which oscillations can be measured.

2 Observations

2.1 TESS Photometry

TESS observed TOI-197 in 2-minute cadence during Sector 2 of Cycle 1 for 27 days. We used the target pixel files produced by the TESS Science Processing Operations Center (Jenkins et al., 2016) as part of the TESS alerts on November 11 2018111https://doi.org/10.17909/t9-wx1n-aw08 (catalog https://doi.org/10.17909/t9-wx1n-aw08). We produced a light curve using the photometry pipeline222https://tasoc.dk/code/ (Handberg et al., in prep.) maintained by the TESS Asteroseismic Science Operations Center (TASOC, Lund et al., 2017), which is based on software originally developed to generate light curves for data collected by the K2 Mission (Lund et al., 2015).

Figure 1a shows the raw light curve obtained from the TASOC pipeline. The coverage is nearly continuous (duty cycle $\sim$ 93%), with a $\sim$ 2 day gap separating the two spacecraft orbits in the observing sector. Two $\sim$ 0.1 % brightness dips, which triggered the identification of TOI-197.01 as a planet candidate, are evident near the beginning of each TESS orbit (see upward triangles in Figure 1a). The structure with a period of $\sim$ 2.5 d corresponds to instrumental variations due to the angular momentum dumping cycle of the spacecraft.

To prepare the raw light curve for an asteroseismic analysis, the current TASOC pipeline implements a series of corrections as described by Handberg & Lund (2014), which includes removal of instrumental artefacts and of the transit events using a combination of filters utilizing the estimated planetary period. Future TASOC-prepared light curves from full TESS data releases will use information from the ensemble of stars to remove common instrumental systematics (Lund et al, in prep.). Alternative light curve corrections using transit removal and gap interpolation (García et al., 2011; Pires et al., 2015) yielded consistent results. The corrected TASOC light curve is shown in Figure 1b. Figure 1c shows a power spectrum of this light curve, revealing the clear presence of a granulation background and a power excess from solar-like oscillations near $\sim$ 430 $\mu$Hz, both characteristic of an evolved star near the base of the red-giant branch.

2.2 High-Resolution Spectroscopy

We obtained high-resolution spectra of TOI-197 using several facilities within the TESS Follow-up Observation Program (TFOP), including HIRES (Vogt et al., 1994) on the 10-m telescope at Keck Observatory (Maunakea, Hawai‘i), the Hertzsprung SONG Telescope at Teide Observatory (Tenerife) (Grundahl et al., 2017), HARPS (Mayor et al., 2003), FEROS (Kaufer et al., 1999), Coralie (Queloz et al., 2001) and FIDEOS (Vanzi et al., 2018) on the MPG/ESO 3.6-m, 2.2-m, 1.2-m, and 1-m telescopes at La Silla Observatory (Chile), Veloce (Gilbert et al., 2018) on the 3.9-m Anglo-Australian Telescope at Siding Spring Observatory (Australia), TRES (Fürész, 2008) on the 1.5-m Tillinghast reflector at the F. L. Whipple Observatory (Mt. Hopkins, Arizona), and iSHELL (Rayner et al., 2012) on the NASA IRTF Telescope (Maunakea, Hawaii). All spectra used in this paper were obtained between Nov 11 and Dec 30 2018 and have a minimum spectral resolution of $R\approx 44000$. FEROS, Coralie, and HARPS data were processed and analyzed with the CERES package (Brahm et al., 2017a), which performs the optimal extraction and wavelength calibration of each spectrum, along with the measurement of precision radial velocities and bisector spans via the cross-correlation technique. Most instruments have been previously used to obtain precise radial velocities to confirm exoplanets, and we refer to the publications listed above for details on the reduction methods.

To obtain stellar parameters, we analyzed a HIRES spectrum using Specmatch (Petigura, 2015), which has been extensively applied for the classification of Kepler exoplanet host stars (Johnson et al., 2017; Petigura et al., 2017a). The resulting parameters were $T_{\textrm{eff}}\>=5080\pm 70$ K, \mbox{\log g}=3.60\pm 0.08 dex, \mbox{\rm{[Fe/H]}}=-0.08\pm 0.05 dex and $v\sin{i}=2.8\pm 1.6$ km s*-1*, consistent with an evolved star as identified from the power spectrum in Figure 1c. To account for systematic differences between spectroscopic methods (Torres et al., 2012) we added 59 K in $T_{\textrm{eff}}\>$ and 0.062 dex in $\rm{[Fe/H]}$ in quadrature to the formal uncertainties, yielding final values of $T_{\textrm{eff}}\>=5080\pm 90$ K and \mbox{\rm{[Fe/H]}}=-0.08\pm 0.08 dex. Independent spectroscopic analyses yielded consistent results, including an analysis of a HIRES spectrum using ARES+MOOG (Sousa, 2014; Sousa et al., 2018), FEROS spectra using ZASPE (Brahm et al., 2017b), TRES spectra using SPC (Buchhave et al., 2012) and iSHELL spectra using BT-Settl models (Allard et al., 2012).

2.3 Broadband Photometry & Gaia Parallax

We fitted the spectral energy distribution (SED) of TOI-197 using broadband photometry following the method described by Stassun & Torres (2016). We used NUV photometry from GALEX, $B_{T}V_{T}$ from Tycho-2 (Høg et al., 2000), $BVgri$ from APASS, $JHK_{S}$ from 2MASS (Skrutskie et al., 2006), W1–W4 from WISE (Wright et al., 2010), and the $G$ magnitude from Gaia (Evans et al., 2018). The data were fit using Kurucz atmosphere models, with $T_{\textrm{eff}}\>$, $\rm{[Fe/H]}$ and extinction ($A_{V}$) as free parameters. We restricted $A_{V}$ to the maximum line-of-sight value from the dust maps of Schlegel et al. (1998). The resulting fit yielded $T_{\rm eff}=5090\pm 85$ K, ${\rm[Fe/H]}=-0.3\pm 0.3$ dex, and $A_{V}=0.09\pm 0.02$ mag with reduced $\chi^{2}$ of 1.9, in good agreement with spectroscopy. Integrating the (de-reddened) model SED gives the bolometric flux at Earth of $F_{\rm bol}=1.88\pm 0.04\times 10^{-8}$ erg s cm*-2*. An independent SED fit using 2MASS, APASS9, USNO-B1 and WISE photometry and Kurucz models yielded excellent agreement, with $F_{\rm bol}=1.83\pm 0.09\times 10^{-8}$ erg s cm*-2* and $T_{\textrm{eff}}\>=5150\pm 130$ K. Additional independent analyses using the method by Mann et al. (2016) and PARAM (Rodrigues et al., 2014, 2017) yielded bolometric fluxes and extinction values that are consistent within 1 $\sigma$ with the values quoted above.

Combining the bolometric flux with the Gaia DR2 distance allows us to derive a nearly model-independent luminosity, which is a valuable constraint for asteroseismic modeling (see Section 3.3). Using a Gaia parallax of $10.518\pm 0.080$ mas (adjusted for the $0.082\pm 0.033$ mas zero-point offset for nearby stars reported by Stassun & Torres 2018) with the two methods described above yielded \mbox{L_{\star}}=5.30\pm 0.14L_{\odot} (using $F_{\rm bol}=1.88\pm 0.04\times 10^{-8}$ erg s cm*-2*) and \mbox{L_{\star}}=5.13\pm 0.13L_{\odot} (using $F_{\rm bol}=1.83\pm 0.09\times 10^{-8}$ erg s cm*-2*). We also derived a luminosity using isoclassify (Huber et al., 2017)333https://github.com/danxhuber/isoclassify, adopting 2MASS $K$-band photometry, bolometric corrections from MIST isochrones (Choi et al., 2016) and the composite reddening map mwdust (Bovy et al., 2016), yielding \mbox{L_{\star}}=5.03\pm 0.13L_{\odot}. Our adopted luminosity was the mean of these methods with an uncertainty calculated by adding the mean uncertainty and scatter over all methods in quadrature, yielding \mbox{L_{\star}}=5.15\pm 0.17L_{\odot}.

2.4 High-Resolution Imaging

TOI-197 was observed with the NIRC2 camera and Altair adaptive optics system on Keck-2 (Wizinowich et al., 2000) on UT 25 November 2018. Conditions were clear but seeing was poor (0.8–2”). We used the science target as the natural guide star and images were obtained through a $K$-continuum plus KP501.5 filter using the narrow camera (10 mas pixel scale). We obtained eight images (four each at two dither positions), each consisting of 50 co-adds of 0.2 sec each, with correlated double-sampling mode and four reads. Frames were co-added and we subtracted an average dark image, constructed from a set of darks with the same integration time and sampling mode. Flat-fielding was performed using a dome flat obtained in the $K^{\prime}$ filter. “Hot” pixels were identified in the dark image and corrected by median filtering with a $5\times 5$ box centered on each affected pixel in the science image. Only a single star appears in the images. We performed tests in which “clones” of the stellar image reduced by a specified contrast ratio were added to the original image. These show that we would have been able to detect companions as faint as $\Delta K=5.8$ mag within 0.4” of TOI-197, 3.8 mag within 0.2”, and 1.8 mag within 0.1”.

Additional NIRC2 observations were obtained in the narrow-band $Br-\gamma$ filter $(\lambda_{o}=2.1686;\Delta\lambda=0.0326\mu$m) on UT 22 November 2018. A standard 3-point dither pattern with a step size of $3\arcsec$ was repeated twice with each dither offset from the previous dither by $0.5\arcsec$. An integration time of 0.25 seconds was used with one coadd per frame for a total of 2.25 seconds on target, and the camera was used in the narrow-angle mode. No additional stellar companions were detected to within a resolution of $\sim 0.05\arcsec$ FWHM. The sensitivities of the final combined AO image were determined following Ciardi et al. (2015) and Furlan et al. (2017), with detection limits as faint as $\Delta Br-\gamma=7.4$ mag within 0.4”, 6.1 mag within 0.2”, and 3.2 mag within 0.1”.

The results from NIRC2 are consistent with Speckle observations using HRCam (Tokovinin et al., 2010) on the 4.1 m SOAR telescope444https://exofop.ipac.caltech.edu/tess/target.php?id=441462736. Since the companion is unlikely to be bluer than TOI-197, these constraints exclude any significant dilution (both for oscillation amplitudes and the depth of transit events).

3 Asteroseismology

3.1 Global Oscillation Parameters

To extract oscillation parameters characterizing the average properties of the power spectrum, we used several automated analysis methods (e.g. Huber et al., 2009; Mathur et al., 2010; Mosser et al., 2012a; Benomar et al., 2012; Kallinger et al., 2012; Corsaro & De Ridder, 2014; Lundkvist, 2015; Stello et al., 2017; Campante, 2018; Bell et al., 2019), many of which have been extensively tested on Kepler data (e.g. Hekker et al., 2011; Verner et al., 2011). In most of these analyses, the power contributions due to granulation noise and stellar activity were modeled by a combination of power laws and a flat contribution due to shot noise, and then corrected by dividing the power spectrum by the background model. The individual contributions and background model using the method by Huber et al. (2009) are shown as dashed and solid red lines in Fig. 1c, and a close-up of the power excess is shown in Fig. 2a.

Next, the frequency of maximum power ($\nu_{\textrm{max}}$) was measured either by heavily smoothing the power spectrum or by fitting a Gaussian function to the power excess. Our analysis yielded \nu_{\textrm{max}}=430\pm 18\,\mbox{\muHz}, with uncertainties calculated from the scatter between all fitting techniques. Finally, the mean oscillation amplitude per radial mode was determined by taking the peak of the smoothed, background-corrected oscillation envelope and correcting for the contribution of non-radial modes (Kjeldsen et al., 2008b), yielding $A=18.7\pm 3.5$ ppm. We caution that the $\nu_{\textrm{max}}$ and amplitude estimates could be significantly biased by the stochastic nature of the oscillations. The modes are not well resolved, as demonstrated by the non-Gaussian appearance of the power spectrum and the particularly strong peak at 420 $\mu$Hz.

5555Échelle diagrams are constructed by dividing a power spectrum into equal segments with length $\Delta\nu$ and stacking one above the other, so that modes with a given spherical degree align vertically in ridges (Grec et al., 1983). Departures from regularity arise from sound speed discontinuities and from mixed modes, and thus probe the interior structure of a star.

Global seismic parameters such as $\nu_{\textrm{max}}$ and amplitude follow well-known scaling relations (Huber et al., 2011; Mosser et al., 2012b; Corsaro et al., 2013), which allow us to test whether the detected oscillations are consistent with expectations. Figure 3 compares our measured $\nu_{\textrm{max}}$ and amplitude with results for $\sim$1500 stars observed by Kepler (Huber et al., 2011). We observe excellent agreement, confirming that the detected signal is consistent with solar-like oscillations. We note that the oscillations in the TESS bandpass are expected to be $\sim$ 15 % smaller than in the bluer Kepler bandpass, which is well within the spread of amplitudes at a given $\nu_{\textrm{max}}$ observed in the Kepler sample. The result confirms that the redder bandpass of TESS only has a small effect on the oscillation amplitude, supporting the expected rich yield of solar-like oscillators with TESS 2-minute cadence observations (Schofield et al., 2018).

3.2 Individual Mode Frequencies

The power spectrum in Fig. 2a shows several clear peaks corresponding to individual oscillation modes. Given that TESS instrument artifacts are not yet well understood, we restricted our analysis to the frequency range 400–500 $\mu$Hz where we observe peaks well above the background level.

To extract these individual mode frequencies, we used several independent methods ranging from traditional iterative sine-wave fitting, i.e., pre-whitening (e.g. Lenz & Breger, 2005; Kjeldsen et al., 2005; Bedding et al., 2007), to fitting of Lorentzian mode profiles (e.g. Handberg & Campante, 2011; Appourchaux et al., 2012; Mosser et al., 2012b; Corsaro & De Ridder, 2014; Corsaro et al., 2015; Vrard et al., 2015; Davies & Miglio, 2016; Roxburgh, 2017; Handberg et al., 2017; Kallinger et al., 2018), including publicly available code such as DIAMONDS6††6https://github.com/EnricoCorsaro/DIAMONDS. We required at least two independent methods to return the same frequency within uncertainties and that the posterior probability of each peak being a mode was $\geq 90\,\%$ (Basu & Chaplin, 2017). A comparison of the frequencies returned by different fitters showed very good agreement, at a level smaller than the uncertainties for all the reported modes. For the final list of frequencies we adopted values from one fitter who applied pre-whitening (HK), with uncertainties derived from Monte Carlo simulations of the data, as listed in Table 1.

To measure the large frequency separation $\Delta\nu$, we performed a linear fit to all identified radial modes, yielding \Delta\nu=28.94\pm 0.15\mbox{\muHz}. Figure 2b shows a greyscale échelle diagram5 using this $\Delta\nu$ measurement, including the extracted mode frequencies. The $l=1$ modes are strongly affected by mode bumping, as expected for the mixed mode coupling factors for evolved stars in this evolutionary stage. The offset $\epsilon$ of the $l=0$ ridge is $\sim\,1.5$, consistent with the expected value from Kepler measurements for stars with similar $\Delta\nu$ and $T_{\textrm{eff}}\>$ (White et al., 2011).

3.3 Frequency Modeling

We used a number of independent approaches to model the observed oscillation frequencies, including different stellar evolution codes (ASTEC, Cesam2K, GARSTEC, Iben, MESA, and YREC, Christensen-Dalsgaard, 2008; Morel & Lebreton, 2008; Scuflaire et al., 2008; Weiss et al., 2008; Iben, 1965; Paxton et al., 2011, 2013, 2015; Choi et al., 2016; Demarque et al., 2008), oscillation codes (ADIPLS, GYRE and Pesnell, Christensen-Dalsgaard, 2008; Townsend & Teitler, 2013; Pesnell, 1990) and modeling methods (including AMP, ASTFIT, BeSSP, BASTA, PARAM, Creevey et al., 2017; Silva Aguirre et al., 2015; Serenelli et al., 2017; Rodrigues et al., 2014, 2017; Deheuvels & Michel, 2011; Yıldız et al., 2016; Ong & Basu, 2019; Tayar & Pinsonneault, 2018; Lebreton & Goupil, 2014; Ball & Gizon, 2017; Mosumgaard et al., 2018). Most of the adopted methods applied corrections for the surface effect (Kjeldsen et al., 2008a; Ball & Gizon, 2017). Model inputs included the spectroscopic temperature and metallicity, individual frequencies, $\Delta\nu$, and the luminosity (Section 2.3). To investigate the effects of different input parameters, modelers were asked to provide solutions using both individual frequencies and only using $\Delta\nu$, with and without taking into account the luminosity constraint. The constraint on $\nu_{\textrm{max}}$ was not used in the modeling since it may be affected by finite mode lifetimes (see Section 3.1).

Overall, the modeling efforts yielded consistent results, and most modeling codes were able to provide adequate fits to the observed oscillation frequencies. The modeling confirmed that only one of the two closely-spaced mixed modes near $\sim$ 460 $\mu$Hz is likely real, but we have retained both frequencies in Table 1 for consistency. An échelle diagram with observed frequencies and a representative best-fitting model is shown in Figure 4.

Independent analyses confirmed a bimodality splitting into lower-mass, older models ($\sim\,1.15M_{\odot}$, $\sim$ 6 Gyr) and higher-mass, younger models ($\sim\,1.3M_{\odot}$, $\sim$ 4 Gyr). Surface rotation would provide an independent mass diagnostic (e.g. van Saders & Pinsonneault, 2013), but the insufficiently constrained $v\sin i$ and the unknown stellar inclination mean that we cannot decisively break this degeneracy. Combining an independent constraint of \mbox{\log g}=3.603\pm 0.026 dex from an autocorrelation analysis of the light curve (Kallinger et al., 2016) with a radius from $L$ and $T_{\textrm{eff}}\>$ favors a higher-mass solution (\mbox{M_{\star}}=1.27\pm 0.13M_{\odot}), but may be prone to small systematics in the $\nu_{\textrm{max}}$ scaling relation (which was used for the calibration). To make use of the most observational constraints available, we used the set of nine modeling solutions which used $T_{\textrm{eff}}\>$, $\rm{[Fe/H]}$, frequencies and the luminosity as input parameters. From this set of solutions, we adopted the self-consistent set of stellar parameters with the mass closest to the median mass over all results. A more detailed study of the individual modeling results will be presented in a follow-up paper (Li et al., in prep).

For ease of propagating stellar parameters to exoplanet modeling (see next section), uncertainties were calculated by adding the median uncertainty for a given stellar parameter in quadrature to the standard deviation of the parameter for all methods. This method has been commonly adopted for Kepler (e.g. Chaplin et al., 2014a) and captures both random and systematic errors estimated from the spread among different methods. For completeness, the individual random and systematic error estimates are \mbox{R_{\star}}=2.943\pm 0.041\rm{(ran)}\pm 0.049\rm{(sys)}\,R_{\odot}, \mbox{M_{\star}}=1.212\pm 0.052\rm{(ran)}\pm 0.055\rm{(sys)}\,M_{\odot}, \mbox{\rho_{\star}}=0.06702\pm 0.00019\rm{(ran)}\pm 0.00047\rm{(sys)\,gcc}, and $t=4.9\pm 0.6\rm{(ran)}\pm 0.9\rm{(sys)}$ Gyr. This demonstrates that systematic errors constitute a significant fraction of the error budget for all stellar properties (in particular stellar age), and emphasize the need for using multiple model grids to derive realistic uncertainties for stars and exoplanets. The final estimates of stellar parameters are summarized in Table LABEL:tab:stellar, constraining the radius, mass, density and age of TOI-197 to $\sim$ 2 %, $\sim$ 6 %, $\sim$ 1 % and $\sim$ 22 %.

4 Planet Characterization

To fit the transits observed in the TESS data we used the PDC-MAP light curve provided by the TESS Science Processing and Operations Center (SPOC), which has been optimized to remove instrumental variability and preserve transits (Smith et al., 2012; Stumpe et al., 2014). To optimize computation time we discarded all data more than 2.5 days before and after each of the two observed transits. We have repeated the fit and data preparation procedure using the TASOC light curve and found consistent results.

A total of 107 radial velocity measurements from five different instruments (see Section 2.2 and Table 3) were used to constrain the mass of the planet. No spectroscopic observations were taken during transits, and hence the measurements are unaffected by the Rossiter-McLaughlin effect ($\sim$ 2.3 m s*-1* based on the measured $v\sin i$ and $R_{p}/R_{\star}$). To remove variations from stellar oscillations, we calculated weighted nightly means for all instruments which obtained multiple observations per night. We performed a joint transit and radial-velocity fit using a Markov Chain Monte Carlo algorithm based on the exoplanet modeling code ktransit (Barclay, 2018), as described in Chontos et al. (2019). We placed a strong Gaussian prior on the mean stellar density using the value derived from asteroseismology (Table LABEL:tab:stellar) and weak priors on the linear and quadratic limb darkening coefficients, derived from the closest $I$-band grid points in Claret & Bloemen (2011), with a width of 0.6 for both coefficients. We also adopted a prior for the radial-velocity jitter from granulation and oscillations of $2.5\pm 1.5$ m s*-1*, following Yu et al. (2018) (see also Tayar et al., 2018), and a $1/e$ prior on the eccentricity to account for the linear bias introduced by sampling in $e\cos{\omega}$ and $e\sin{\omega}$ (Eastman et al., 2013). Independent zeropoint offsets and stellar jitter values for each of the five instruments that provided radial velocities. Independent joint fits using EXOFASTv2 (Eastman et al., 2013) yielded consistent results.

Figures 5 and 6 show the radial velocity timeseries, phase-folded transit and RV data, and the corresponding best-fitting model. Table 4 lists the summary statistics for all planet and model parameters. The system is well described by a planet in a 14.3 day orbit, which is nearly equal in size but $\sim$ 35% less massive than Saturn (\mbox{R_{\rm p}}=\mbox{0.836\pm 0.031}\,R_{\rm J}, \mbox{M_{\rm p}}=\mbox{0.190\pm 0.018}\,M_{\rm J}), with tentative evidence for a mild eccentricity ($e=0.11\pm 0.03$). The long transit duration ($\sim$ 0.5 days) is consistent with a non-grazing ($b\approx 0.7$) transit given the asteroseismic mean stellar density, providing further confirmation for a gas-giant planet orbiting an evolved star. The radial velocity data do not show evidence for any other short-period companions. Continued monitoring past the $\sim$4 orbital periods covered here will further reveal details about the orbital architecture of this system.

5 Discussion

TOI-197.01 joins an enigmatic but growing class of transiting planets orbiting stars that have significantly evolved off the main sequence. Figure 7 compares the position of TOI-197 within the expected population of solar-like oscillators to be detected with TESS (panel a) and within the known population of exoplanet host stars. Evolutionary states in Figure 7b have been assigned using solar-metallicity PARSEC evolutionary tracks (Bressan et al., 2012) as described in Berger et al. (2018)6††6see also https://github.com/danxhuber/evolstate. TOI-197 sits at the boundary between subgiants and red giants, with the measured $\Delta\nu$ value indicating that the star has just started its ascent on the red-giant branch (Mosser et al., 2014). TOI-197 is a typical target for which we expect to detect solar-like oscillations with TESS, predominantly due to the increased oscillation amplitude, which are well known to scale with luminosity (Kjeldsen & Bedding, 1995). On the contrary, TOI-197 is rare among known exoplanet hosts: while radial velocity searches have uncovered a large number of planets orbiting red giants on long orbital periods (e.g. Wittenmyer et al., 2011), less than 15 transiting planets are known around red-giant stars (as defined in Figure 7b). TOI-197 is the sixth example of a transiting planet orbiting a late subgiant / early red giant with detected oscillations, following Kepler-91 (Barclay et al., 2013), Kepler-56 (Steffen et al., 2012; Huber et al., 2013b), Kepler-432 (Quinn et al., 2015), K2-97 (Grunblatt et al., 2016) and K2-132 (Grunblatt et al., 2017; Jones et al., 2018).

Transiting planets orbiting evolved stars are excellent systems to advance our understanding of the effects of stellar evolution on the structure and evolution of planets (see e.g. Veras, 2016, for a review). For example, such systems provide the possibility to test the effects of stellar mass, evolution and binarity on planet occurrence (e.g. Johnson et al., 2010; Schlaufman & Winn, 2013; Stephan et al., 2018), which are still poorly understood. Furthermore, the increased irradiance on the planet caused by the evolution of the host star has been proposed as a means to distinguish between proposed mechanisms to explain the inflation of gas-giant planets beyond the limits expected from gravitational contraction and cooling (Hubbard et al., 2002; Lopez & Fortney, 2016). Recent discoveries by the K2 mission have indeed yielded evidence that planets orbiting low-luminosity RGB stars are consistent with being inflated by the evolution of the host star (Grunblatt et al., 2016, 2017), favoring scenarios in which the energy from the star is deposited into the deep planetary interior (Bodenheimer et al., 2001).

Based on its radius and orbital period, TOI-197 would nominally be classified as a warm Saturn, sitting between the well-known population of hot Jupiters and the ubiquitous population of sub-Neptunes uncovered by Kepler (Figure 8a). Taking into account the evolutionary state of the host star, however, TOI-197 falls at the beginning of the “inflation sequence” in the radius-incident flux diagram (Figure 8b), with planet radius strongly increasing with stellar incident flux (Kovács et al., 2010; Demory & Seager, 2011; Miller & Fortney, 2011; Thorngren & Fortney, 2018). Since TOI-197.01 is currently not anomalously large compared to the observed trend and scatter for similar planets (Figure 8b) and low-mass planets are expected to be more susceptible to planet reinflation (Lopez & Fortney, 2016), TOI-197 may be a progenitor of a class of re-inflated gas-giant planets orbiting RGB stars.

If confirmed, the mild eccentricity of TOI-197.01 would be consistent with predictions of a population of planets around evolved stars for which orbital decay occurs faster than tidal circularization (Villaver et al., 2014; Grunblatt et al., 2018). Moreover, combining the asteroseismic age of the system with the possible non-zero eccentricity would allow constraints on the tidal dissipation in the planet, which drives the circularization of the orbit. Using the formalism by Mardling (2011) (see also Gizon et al., 2013; Davies et al., 2016; Ceillier et al., 2016), the current constraints would imply a minimum value of the planetary tidal quality factor $Q_{p;{\rm min}}\approx 3.2\times 10^{4}$, below which the system would have been already circularized in $\sim$ 5 Gyr. Compared to the value measured in Saturn ($Q\approx 1800$, Lainey et al., 2017), this would demonstrate the broad diversity of dissipation observed in giant planets. Since tidal dissipation mechanisms vary strongly with internal structure (see e.g. Guenel et al., 2014; Ogilvie, 2014; André et al., 2017), this may also contribute to understanding the internal composition of such planets. We caution, however, that further RV measurements will be needed to confirm a possible non-zero eccentricity for TOI-197.01.

The precise characterization of planets orbiting evolved, oscillating stars also provides valuable insights into the diversity of compositions of planets through their mean densities. TOI-197.01 falls in the transition region between Neptune and sub-Saturn sized planets for which radii increase as $R_{P}\approx M_{P}^{0.6}$, and Jovian planets for which radius is nearly constant with mass (Weiss et al., 2013; Chen & Kipping, 2017, Figure 9). Recent studies of a population of sub-Saturns in the range $\sim$ 4–8 $R_{\oplus}$ also found a wide variety of masses, approximately 6–60 $M_{\oplus}$, regardless of size (Petigura et al., 2017b; Van Eylen et al., 2018). Furthermore, masses of sub-Saturns correlate strongly with host star metallicity, suggesting that metal-rich disks form more massive planet cores. TOI-197.01 demonstrates that this trend does not appear to extend to planets with sizes $>8$ $R_{\oplus}$, given its mass of $\sim$ 60 $M_{\oplus}$ and a roughly sub-solar metallicity host star (\mbox{\rm{[Fe/H]}}\approx-0.08 dex). This suggests that Saturn-sized planets may follow a relatively narrow range of densities, a possible signature of the transition in the interior structure (such as the increased importance of electron degeneracy pressure, Zapolsky & Salpeter, 1969) leading to different mass-radius relations between sub-Saturns and Jupiters. We note that TOI-197.01 is one of the most precisely characterized Saturn-sized planets to date, with a density uncertainty of $\sim$ 15%.

6 Conclusions

We have presented the discovery of TOI-197.01, the first transiting planet orbiting an oscillating host star identified by TESS. Our main conclusions are as follows:

• •

TOI-197 is a late subgiant / early red giant with a clear presence of mixed modes. Combined spectroscopy and asteroseismic modeling revealed that the star has just started its ascent on the red giant branch, with \mbox{R_{\star}}=\mbox{2.943\pm 0.064}R_{\odot}, \mbox{M_{\star}}=\mbox{1.212\pm 0.074}M_{\odot} and near-solar age ($4.9\pm 1.1$ Gyr). TOI-197 is a typical oscillating star expected to be detected with TESS, and demonstrates the power of asteroseismology even with only 27 days of data.

• •

The oscillation amplitude of TOI-197 is consistent with ensemble measurements from Kepler. This confirms that the redder bandpass of TESS compared to Kepler only has a small effect on the oscillation amplitude (as expected from scaling relations, Kjeldsen & Bedding, 1995; Ballot et al., 2011a), supporting the expected yield of thousands of solar-like oscillators with 2-minute cadence observations in the nominal TESS mission (Schofield et al., 2018). A detailed study of the asteroseismic performance of TESS will have to await ensemble measurements of noise levels and amplitudes.

• •

TOI-197.01 is a “hot Saturn” (F=\mbox{343\pm 24}\mbox{F_{\oplus}}, \mbox{R_{\rm p}}=\mbox{0.836\pm 0.031}\,R_{\rm J}, \mbox{M_{\rm p}}=\mbox{0.190\pm 0.018}\,M_{\rm J}) and joins a small but growing population of close-in, transiting planets orbiting evolved stars. Based on its incident flux, radius and mass, TOI-197.01 may be a precursor to the population of gas giants that undergo radius re-inflation due to the increased irradiance as their host star evolves up the red-giant branch.

• •

TOI-197.01 is one the most precisely characterized Saturn-sized planets to date, with a density measured to $\sim$ 15%. TOI-197.01 does not follow the trend of increasing planet mass with host star metallicity discovered in sub-Saturns with sizes between 4-8\,\mbox{R_{\oplus}}, which has been linked to metal-rich disks preferentially forming more massive planet cores (Petigura et al., 2017b). The moderate density (\mbox{\rho_{\rm p}}=\mbox{0.431\pm 0.062} g cm*-3*) suggests that Saturn-sized planets may follow a relatively narrow range of densities, a possible signature of the transition in the interior structure leading to different mass-radius relations for sub-Saturns and Jupiters.

TOI-197 provides a first glimpse at the strong potential of TESS to characterize exoplanets using asteroseismology. TOI-197.01 has one the most precisely characterized densities of known Saturn-sized planets to date, with an uncertainty of $\sim$ 15%. Thanks to asteroseismology the planet density uncertainty is dominated by measurements of the transit depth and the radial velocity amplitude, and thus can be expected to further decrease with continued transit observations and radial velocity follow-up, which is readily performed given the brightness (V=8) of the star. Ensemble studies of such precisely characterized planets orbiting oscillating subgiants can be expected to yield significant new insights on the effects of stellar evolution on exoplanets, complementing current intensive efforts to characterize planets orbiting dwarfs.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawai‘ian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We thank Andrei Tokovinin for helpful information on the Speckle observations obtained with SOAR. D.H. acknowledges support by the National Aeronautics and Space Administration through the TESS Guest Investigator Program (80NSSC18K1585) and by the National Science Foundation (AST-1717000). A.C. acknowledges support by the National Science Foundation under the Graduate Research Fellowship Program. W.J.C., W.H.B., A.M., O.J.H. and G.R.D. acknowledge support from the Science and Technology Facilities Council and UK Space Agency. H.K. and F.G. acknowledge support from the European Social Fund via the Lithuanian Science Council grant No. 09.3.3-LMT-K-712-01-0103. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (Grant DNRF106). A.J. acknowledges support from FONDECYT project 1171208, CONICYT project BASAL AFB-170002, and by the Ministry for the Economy, Development, and Tourism’s Programa Iniciativa Científica Milenio through grant IC 120009, awarded to the Millennium Institute of Astrophysics (MAS). R.B. acknowledges support from FONDECYT Post-doctoral Fellowship Project 3180246, and from the Millennium Institute of Astrophysics (MAS). A.M.S. is supported by grants ESP2017-82674-R (MINECO) and SGR2017-1131 (AGAUR). R.A.G. and L.B. acknowledge the support of the PLATO grant from the CNES. The research leading to the presented results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP72007-2013) ERC grant agreement no 338251 (StellarAges). S.M. acknowledges support from the European Research Council through the SPIRE grant 647383. This work was also supported by FCT (Portugal) through national funds and by FEDER through COMPETE2020 by these grants: UID/FIS/04434/2013 & POCI-01-0145-FEDER-007672, PTDC/FIS-AST/30389/2017 & POCI-01-0145-FEDER-030389. T.L.C. acknowledges support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 792848 (PULSATION). E.C. is funded by the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 664931. V.S.A. acknowledges support from the Independent Research Fund Denmark (Research grant 7027-00096B). D.S. acknowledges support from the Australian Research Council. S.B. acknowledges NASA grant NNX16AI09G and NSF grant AST-1514676. T.R.W. acknowledges support from the Australian Research Council through grant DP150100250. A.M. acknowledges support from the ERC Consolidator Grant funding scheme (project ASTEROCHRONOMETRY, G.A. n. 772293). S.M. acknowledges support from the Ramon y Cajal fellowship number RYC-2015-17697. M.S.L. is supported by the Carlsberg Foundation (Grant agreement no.: CF17-0760). A.M. and P.R. acknowledge support from the HBCSE-NIUS programme. J.K.T. and J.T. acknowledge that support for this work was provided by NASA through Hubble Fellowship grants HST-HF2-51399.001 and HST-HF2-51424.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. T.S.R. acknowledges financial support from Premiale 2015 MITiC (PI B. Garilli). This project has been supported by the NKFIH K-115709 grant and the Lendület Program of the Hungarian Academy of Sciences, project No. LP2018-7/2018. Based on observations made with the Hertzsprung SONG telescope operated on the Spanish Observatorio del Teide on the island of Tenerife by the Aarhus and Copenhagen Universities and by the Instituto de Astrofísica de Canarias. Funding for the TESS mission is provided by NASA’s Science Mission directorate. We acknowledge the use of public TESS Alert data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. This research has made use of the Exoplanet Follow-up Observation Program website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This paper includes data collected by the TESS mission, which are publicly available from the Mikulski Archive for Space Telescopes (MAST).