We present the bright (Vmag= 9.12), multi-planet system TOI-431, characterised with photometry from TESS, LCOGT, NGTS and Spitzer, and radial velocities from HARPS, iSHELL, FEROS, and MINERVA-Australis.
TOI-431 b is a super-Earth with radius of 1.28 Rearth, a mass of 2.96 Mearth, and a density of 7.70 gcm-3; TOI-431 d is a sub-Neptune with radius of 3.29 Rearth, a mass of 8.82 Mearth, and a density of 1.36 gcm-3. They have periods of 0.49 and 12.46 days respectively. We find a third planet candidate, TOI-431 c, in the HARPS radial velocity data, but it is not seen to transit in the TESS light curves. It has an msini of 2.90 Mearth, and a period of 4.85 days.
TOI-431 d likely has an extended atmosphere and is one of the best-known TESS discoveries for atmospheric characterisation, while the super-Earth TOI-431 b may be a stripped core. These planets straddle the radius gap, presenting an interesting case-study for atmospheric evolution, and TOI-431 b is a prime TESS discovery for the study of rocky planet phase curves.
TOI-431 b and d also both contribute to the TESS Level-1 mission goal to measure the masses and radii of at least 50 planets with radii smaller than 4 Rearth.
The second page starts with an introduction: The Kepler Space Telescope allowed us to do statistical studies on the exoplanet population for the first time. This includes the identification of the "photoevaporation valley," a bi-modality in the radius distribution of small planets, explained by photoevaporation of close-in Planetary Atmospheres. The Transiting Exoplanet Survey Satellite (TESS) is building on Kepler's legacy: now at the end of its primary two-year mission, there have been over 2000 TESS Objects of Interest (TOIs) released. We present here the discovery of TOI-431b, c, and d; planets b and d were discovered by TESS and confirmed via extensive follow-up. Planet c was found in RV data from HARPS.
On pages 2 and 3, the observations made of the TOI-431 system are described: it was observed in 2 sectors by the TESS telescope, a double-transit event was seen by the Spitzer space telescope, 2 partial transits were seen by the LCOGT, a global ground-based telescope network, and NGTS also observed a partial transit. Radial velocities were taken with the HARPS, iSHELL, FEROS and MINERVA-Australis spectrographs. We have high resolution imaging from SOAR HRCam, Gemini NIRI, Gemini Zorro, and Keck NIRC2, in order to search for nearby sources that can contaminate the TESS photometry, resulting in an underestimated planetary radius, or that can be the source of astrophysical false positives, such as background eclipsing binaries, which we rule out. We also use 2 methods to derive the stellar atmospheric parameters. The system has been observed by WASP-South to monitor its stellar variability, from which we find a rotation period of 30.5 days, corroborated by further monitoring with NGTS.
Figures for the observations are given:
Fig 1 shows the TESS data: the raw photometry, the detrended photometry, and models of the planets TOI-431b and d, which are seen to transit. Phase folds of all the planets are also given; the phase fold of TOI-431c shows that it does not transit in the TESS data.
Fig 2 shows the Spitzer data: the raw photometry, and the photometry detrended by Pixel Level Decorrelation (or “PLD”), which also shows the double transit of TOI-431b and d.
Fig 3 shows some partial transits, 2 from LCOGT and 1 from NGTS. All are of TOI-431d.
Fig 4 shows the RV data, including detrending of the HARPS data, the planet models, and phase folds of the RV data for each planet.
The poster then describes the joint fit model used to model the planets: in brief, we use the Python package called exoplanet by Foreman-Mackey to fit the photometry from TESS, LCOGT, NGTS and Spitzer simultaneously with the HARPS RVs, and detrend stellar activity using gaussian processes (“GPS”).
Finally, the last page gives our results as 2 tables of the system, stellar, and planetary fit parameters. We give a brief discussion of the architecture of the system, the radius valley (a figure shows TOI-431b sits below the valley, and TOI-431d sits above, making the system an interesting test-bed for atmospheric evolution), and we discuss the composition and envelopes of TOI-431b and d using a Mass-Radius plot: the former is a super-Earth with a negligible H-He envelope, while TOI-431d sits above the pure-water curve and has a significant volatile layer of H-He and/or water.