We present the bright (V-mag = 9.12), multiplanet system TOI-431, characterized with photometry and radial velocities (RVs). We estimate the stellar rotation period to be 30.5 +/- 0.7 d using archival photometry and RVs. Transiting Exoplanet Survey Satellite (TESS) objects of Interest (TOI)-431b is a super-Earth with a period of 0.49 d, a radius of 1.28 +/- 0.04 R-circle plus, a mass of 3.07 +/- 0.35 M-circle plus, and a density of 8.0 +/- 1.0 g cm(-3); TOI-431 d is a sub-Neptune with a period of 12.46 d, a radius of 3.29 +/- 0.09 R-circle plus, a mass of M-circle plus, and a density of 1.36 +/- 0.25 g cm(-3). We find a third planet, TOI-431c, in the High Accuracy Radial velocity Planet Searcher RV data, but it is not seen to transit in the TESS light curves. It has an Msin i of M-circle plus, and a period of 4.85 d. TOI-431d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterization, while the super-Earth TOI-431b may be a stripped core. These planets straddle the radius gap, presenting an interesting case-study for atmospheric evolution, and TOI-431b is a prime TESS discovery for the study of rocky planet phase curves.

Given their location on the Hertzsprung-Russell (H-R) diagram, thoroughly characterized subgiant stars can place stringent constraints on a wide range of astrophysical problems. Accordingly, they are prime asteroseismic targets for the Transiting Exoplanet Survey Satellite (TESS) mission. In this work, we infer stellar properties for a sample of 347 subgiants located in the TESS Continuous Viewing Zones, which we select based on their likelihood of showing asteroseismic oscillations. We investigate how well they can be characterized using classical constraints (photometry, astrometry) and validate our results using spectroscopic values. We derive luminosities, effective temperatures, and radii with mean 1 sigma random (systematic) uncertainties of 4.5% (2%), 33 K (60 K), and 2.2% (2%), as well as more model-dependent quantities such as surface gravities, masses, and ages. We use our sample to demonstrate that subgiants are ideal targets for mass and age determination based on H-R diagram location alone, discuss the advantages of stellar parameters derived from a detailed characterization over widely available catalogs, show that the generally used 3D extinction maps tend to overestimate the extinction for nearby stars (distance less than or similar to 500 pc), and find a correlation that supports the rotation-activity connection in post-main-sequence stars. The complementary roles played by classical and asteroseismic data sets will open a window to unprecedented astrophysical studies using subgiant stars.

The chemical abundances of planet-hosting stars offer a glimpse into the composition of planet-forming environments. To further understand this connection, we make the first ever measurement of the correlation between planet occurrence and chemical abundances for ten different elements (C, Mg, Al, Si, S, K, Ca, Mn, Fe, and Ni). Leveraging data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and Gaia to derive precise stellar parameters (sigma(R star) approximate to 2.3%, sigma(M star) approximate to 4.5%) for a sample of 1018 Kepler Objects of Interest, we construct a sample of well-vetted Kepler planets with precisely measured radii (sigma(Rp) approximate to 3.4%). After controlling for biases in the Kepler detection pipeline and the selection function of the APOGEE survey, we characterize the relationship between planet occurrence and chemical abundance as the number density of nuclei of each element in a star's photosphere raised to a power, beta. varies by planet type, but is consistent within our uncertainties across all ten elements. For hot planets (P = 1-10 days), an enhancement in any element of 0.1 dex corresponds to an increased occurrence of approximate to 20% for super-Earths (R-p = 1-1.9 R-circle plus) and approximate to 60% for sub-Neptunes (R-p = 1.9-4 R-circle plus). Trends are weaker for warm (P = 10-100 days) planets of all sizes and for all elements, with the potential exception of sub-Saturns (R-p = 4-8 R.). Finally, we conclude this work with a caution to interpreting trends between planet occurrence and stellar age due to degeneracies caused by Galactic chemical evolution and make predictions for planet occurrence rates in nearby open clusters to facilitate demographics studies of young planetary systems.

Stellar radial-velocity (RV) jitter due to surface activity may bias the RV semiamplitude and mass of rocky planets. The amplitude of the jitter may be estimated from the uncertainty in the rotation period, allowing the mass to be more accurately obtained. We find candidate rotation periods for 17 out of 35 TESS Objects of Interest (TOI) hosting <3 R-circle plus, planets as part of the Magellan-TESS survey, which is the first-ever statistically robust study of exoplanet masses and radii across the photoevaporation gap. Seven periods are >= 3 sigma detections, two are >= 1.5 sigma, and eight show plausible variability, but the periods remain unconfirmed. The other 18 TOIs are nondetections. Candidate rotators include the host stars of the confirmed planets L 168-9 b, the HD 21749 system, LTT 1445 A b, TOI 1062 b, and the L 98-59 system. Thirteen candidates have no counterpart in the 1000 TOI rotation catalog of Canto Martins et al. We find periods for G3-M3 dwarfs using combined light curves from TESS and the Evryscope all-sky array of small telescopes, sometimes with longer periods than would be possible with TESS alone. Secure periods range from 1.4 to 26 days with Evryscope-measured photometric amplitudes as small as 2.1 mmag in g'. We also apply Monte Carlo sampling and a Gaussian process stellar activity model from exoplanet to the TESS light curves of six TOIs to confirm the Evryscope periods.

Stellar rotation is a complex function of mass, metallicity, and age and can be altered by binarity. To understand the importance of these parameters in main-sequence stars, we have assembled a sample of observations that spans a range of these parameters using a combination of observations from The Transiting Exoplanet Survey Satellite (TESS) and the Kepler Space Telescope. We find that while we can measure rotation periods and identify other classes of stellar variability (e.g., pulsations) from TESS light curves, instrument systematics prevent the detection of rotation signals longer than the TESS orbital period of 13.7 days. Due to this detection limit, we also use rotation periods constrained using rotational velocities measured by the APOGEE spectroscopic survey and radii estimated using the Gaia mission for both TESS and Kepler stars. From these rotation periods, we (1) find we can track rotational evolution along discrete mass tracks as a function of stellar age, (2) find we are unable to recover trends between rotation and metallicity that were observed by previous studies, and (3) note that our sample reveals that wide binary companions do not affect rotation, while close binary companions cause stars to exhibit more rapid rotation than single stars.

We present the validation of a transiting low-density exoplanet orbiting the M2.5 dwarf TOI 620 discovered by the NASA Transiting Exoplanet Survey Satellite (TESS) mission. We utilize photometric data from both TESS and ground-based follow-up observations to validate the ephemerides of the 5.09 day transiting signal and vet false-positive scenarios. High-contrast imaging data are used to resolve the stellar host and exclude stellar companions at separations greater than or similar to 0.'' 2. We obtain follow-up spectroscopy and corresponding precise radial velocities (RVs) with multiple precision radial velocity (PRV) spectrographs to confirm the planetary nature of the transiting exoplanet. We calculate a 5 sigma upper limit of M (P) < 7.1 M (circle plus) and rho (P) < 0.74 g cm(-3), and we identify a nontransiting 17.7 day candidate. We also find evidence for a substellar (1-20 M (J) ) companion with a projected separation less than or similar to 20 au from a combined analysis of Gaia, adaptive optics imaging, and RVs. With the discovery of this outer companion, we carry out a detailed exploration of the possibilities that TOI 620 b might instead be a circum-secondary planet or a pair of eclipsing binary stars orbiting the host in a hierarchical triple system. We find, under scrutiny, that we can exclude both of these scenarios from the multiwavelength transit photometry, thus validating TOI 620 b as a low-density exoplanet transiting the central star in this system. The low density of TOI 620 b makes it one of the most amenable exoplanets for atmospheric characterization, such as with the James Webb Space Telescope and Ariel, validated or confirmed by the TESS mission to date.