We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS 2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these, and the singly-periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor 6 smaller than that in the B filter which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 µmag, respectively, in two of the best characterised non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars.

We present the first asteroseismic results for δ Scuti and γ Doradus stars observed in Sectors 1 and 2 of the TESS mission. We utilize the 2-min cadence TESS data for a sample of 117 stars to classify their behaviour regarding variability and place them in the Hertzsprung–Russell diagram using Gaia DR2 data. Included within our sample are the eponymous members of two pulsator classes, γ Doradus and SX Phoenicis. Our sample of pulsating intermediate-mass stars observed by TESS also allows us to confront theoretical models of pulsation driving in the classical instability strip for the first time and show that mixing processes in the outer envelope play an important role. We derive an empirical estimate of 74 per cent for the relative amplitude suppression factor as a result of the redder TESS passband compared to the Kepler mission using a pulsating eclipsing binary system. Furthermore, our sample contains many high-frequency pulsators, allowing us to probe the frequency variability of hot young δ Scuti stars, which were lacking in the Kepler mission data set, and identify promising targets for future asteroseismic modelling. The TESS data also allow us to refine the stellar parameters of SX Phoenicis, which is believed to be a blue straggler.

We search for evidence of the cause of the exoplanet radius gap, i.e., the dearth of planets with radii near 1.8R ⊕. If the cause were photoevaporation, the radius gap should trend with proxies for the early-life high-energy emission of the planet-hosting stars. If, alternatively, the cause were core-powered mass loss, no such trends should exist. Critically, spurious trends between the radius gap and stellar properties arise from an underlying correlation with instellation. After accounting for this underlying correlation, we find that no trends remain between the radius gap and stellar mass or present-day stellar activity as measured by near-UV emission. We dismiss the nondetection of a radius gap trend with near-UV emission because present-day near-UV emission is unlikely to trace early-life highenergy emission, but we provide a catalog of Galaxy Evolution Explorer near-UV and far-UV emission measurements for general use. We interpret the nondetection of a radius gap trend with stellar mass by simulating photoevaporation with mass-dependent evolution of stellar high-energy emission. The simulation produces an undetectable trend between the radius gap and stellar mass under realistic sources of error. We conclude that no evidence, from this analysis or others in the literature, currently exists that clearly favors either photoevaporation or core-powered mass loss as the primary cause of the exoplanet radius gap. However, repeating this analysis once the body of well-characterized <4 R ⊕ planets has roughly doubled could confirm or rule out photoevaporation.

Knowing the high-energy radiation environment of a star over a planet's formation and evolutionary period is critical in determining if that planet is potentially habitable and if any biosignatures could be detected, as UV radiation can severely change or destroy a planet's atmosphere. Current efforts for finding a potentially habitable planet are focused on M stars, yet K stars may offer more habitable conditions due to decreased stellar activity and more distant and wider habitable zones (HZ). While M star activity evolution has been observed photometrically and spectroscopically, there has been no dedicated investigation of K-star UV evolution. We present the first comprehensive study of the near-UV, far-UV, and X-ray evolution of K stars. We used members of young moving groups and clusters ranging in age from 10 -625 Myr combined with field stars and their archived GALEX UV and ROSAT X-ray data to determine how the UV and X-ray radiation evolve. We find that the UV and X-ray flux incident on a HZ planet is 5 -50 times lower than that of HZ planets around early-M stars and 50 -1000 times lower than those around late-M stars, due to both an intrinsic decrease in K dwarf stellar activity occurring earlier than for M dwarfs and the more distant location of the K dwarf HZ.