TOI-1431b/MASCARA-5b: A Highly Irradiated Ultrahot Jupiter Orbiting One of the Hottest and Brightest Known Exoplanet Host Stars

We present the discovery of a highly irradiated and moderately inflated ultrahot Jupiter, TOI-1431b/MASCARA-5b (HD 201033b), first detected by NASA's Transiting Exoplanet Survey Satellite mission (TESS) and the Multisite All-Sky Camera (MASCARA). The signal was established to be of planetary origin through radial velocity measurements obtained using SONG, SOPHIE, FIES, NRES, and EXPRES, which show a reflex motion of K= 294.1 +/- 1.1 m s(-1). A joint analysis of the TESS and ground-based photometry and radial velocity measurements reveals that TOT-1431b has a mass of M-p = 3.12 +/- 0.18 M-J (990 +/- 60 M-circle plus), an inflated radius of R-p = 1.49 +/- 0.05 R-J (16.7 +/- 0.6 R-circle plus), and an orbital period of P = 2.650237 +/- 0.000003 days. Analysis of the spectral energy distribution of the host star reveals that the planet orbits a bright (V= 8.049 mag) and young (0.29(-0.19)(+0.32) Gyr) Am type star with T-eff = 7690(-250)(+400) K, resulting in a highly irradiated planet with an incident flux of < F > = 7.24(-0.64)(+0.68)x 10(9) erg s(-1) cm(-2) (5300(-470)(+500)S(circle plus)) and an equilibrium temperature of T-eq = 2370 +/- 70 K. TESS photometry also reveals a secondary eclipse with a depth of 127(-5)(+4) ppm as well as the full phase curve of the planet's thermal emission in the red-optical. This has allowed us to measure the dayside and nightside temperature of its atmosphere as T-day = 3004 +/- 64 K and T-night = 2583 +/- 63 K, the second hottest measured nightside temperature. The planet's low day/night temperature contrast (similar to 420 K) suggests very efficient heat transport between the dayside and nightside hemispheres. Given the host star brightness and estimated secondary eclipse depth of similar to 1000 ppm in the K band, the secondary eclipse is potentially detectable at near-IR wavelengths with ground-based facilities, and the planet is ideal for intensive atmospheric characterization through transmission and emission spectroscopy from space missions such as the James Webb Space Telescope and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey.